Effects of acid deposition on 150 forest stands in the Netherlands
Input output budgets for sulphur, nitrogen, base cations and aluminium
W. de Vries
P.C. Jansen
Report 69.3
DLO Winand Staring Centre, Wageningen (The Netherlands), 1994
ABSTRACT
De Vries, W. and P.C. Jansen, 1994.Effects of acid deposition on 150forest stands in the Netherlands; 3. Input output budgets for sulphur, nitrogen, base cations and aluminium. Wageningen
(The Netherlands), DLO Winand Staring Centre. Report 69.3 60 pp.; 12Figs; 21 Tables; 38 Refs;
1 Annex.
Input fluxes of sulphur, nitrogen, base cations and aluminium to 148 Dutch forest stands were
derived from results of an atmospheric deposition model (sulphur and nitrogen) and by
interpolating available data at several weather stations (aluminium and base cations). Output fluxes
from the rootzone and the unsaturated zone were obtained by multiplying an estimated
precipitation excess for each stand with measured element concentrations in soil solution at 60-100
cm depth (all 148 stands) and in upper groundwater (only 71 stands),respectively. A comparison
of input and output fluxes showed that on average sulphur behaved as a tracer whereas nitrogen
was strongly retained in the soil. The net acid input was mainly buffered by aluminium
mobilization. However, base cations dominated in the soil solution and in groundwater due to
large inputs from the atmosphere.
Keywords: sulphur, nitrogen, aluminium, budget, soil acidification, eutrophication, weathering
ISSN 0927-4537
©1994 DLO Winand Staring Centre for Integrated Land, Soil and Water Research (SC-DLO),
P.O. Box 125, NL-6700 AC Wageningen (The Netherlands).
Phone: 31 837074200;fax: 31 837024812.
The DLO Winand Staring Centre is continuing the research of: Institute for Land and Water
Management Research (ICW), Institute for Pesticide Research, Environment Division (IOB),
Dorschkamp Research Institute for Forestry and Landscape Planning, Division of Landscape
Planning (LB), and Soil Survey Institute (STIBOKA).
No part of this publication may be reproduced or published in any form or by any means, or
stored in a data base or retrieval system, without the written permission of the DLO Winand
Staring Centre.
Project 7241
[Rep69.3.HM/05.94]
Contents
page
Preface
9
Summary
11
1 Introduction
1.1 Input-output budgets for Dutch forests
1.2 Aim of the present research
1.3 Contents of the report
2 Methods
2.1 Choice and characterization of the locations
2.2 Sampling and chemical analysis of soil and groundwater
15
3 Atmospheric deposition
3.1 Sulphur and nitrogen
3.2 Base cations, chloride and aluminium
15
16
17
19
19
22
23
23
25
4 Chemical composition of soil solution and groundwater
4.1 Chloride and sodium
4.2 Sulphur and nitrogen
4.3 Base cations
4.4 Aluminium and pH
27
5 Hydrological conditions
5.1 Precipitation, évapotranspiration and precipitation excess
5.2 Residence time of the percolation water
5.3 Relation between precipitation excess and budgets for chloride and
sodium
33
33
34
6 Input-Output budgets
6.1 Budgets of sulphur and nitrogen
6.1.1 The rootzone
6.1.2 The unsaturated zone
6.2 Budgets of base cations and aluminium
6.2.1 The rootzone
43
43
43
45
47
47
27
28
29
30
38
6.2.2 The unsaturated zone
49
7 Discussion and conclusions
53
References
Annex
On interception and évapotranspiration of the forest stands
55
59
Tables
1 Number of forest stands per tree species where the chemical composition
of soil solution and groundwater was measured
20
2 Distribution of the 150 forest stands over soil type
20
3 Distribution of the 150 forest stands over groundwaterlevel class
21
4 Filtering factors for the deposition of S0 4 , N0 3 and NH4 on different
types of forests
25
5 Effect of tree species on the median concentration of CI and Na in the
soil solution (148 stands) and in upper groundwater (71 stands)
27
6 Effect of tree species on the median Na/Cl ratio in the soil solution (148
stands) and in upper groundwater (71 stands)
28
7 Effect of tree species on the median concentration of S0 4 , N0 3 and NH4
in the soil solution (148 stands) and in upper groundwater (71 stands)
29
8 Effect of tree species on the median concentration of Ca, Mg and Kin
the soil solution (148 stands) and in upper groundwater (71 stands)
30
9 Effect of tree species on the median Al concentration and the median pH
in the soilsolution (148 stands) and in upper groundwater (71 stands)
31
10 Estimated average spring level for the groundwaterlevel classes occurring
at the 148 'soil solution' stands
35
11 Waterretention characteristics of three major soil types in the 148 soil
solution stands
35
12 Estimated watercontent in the rootzone (0-80 cm below the soil surface)
and the unsaturated zone (75 cm below the spring groundwaterlevel) of
three major soil types in the 148 soil solution stands
36
13 Estimated residence times of percolation water in the rootzone and
unsaturated zone as a function of tree species, soil type and
groundwaterlevel class
36
14 Wet deposition of major ions in De Bilt (RIVM/KNMI, 1988)
37
15 Actual évapotranspiration and precipitation excess of the tree species
occurring at the 148 'soil solution' stands and the 71 'groundwater'
stands
40
16 Median annual deposition (in), leaching (out) and their difference (in-out)
of CI and Na for the rootzone and the unsaturated zone for different tree
species1*
41
17 Median annual deposition (in), leaching from the rootzone (out) and their
difference (in-out) of S0 4 , N, N0 3 and NH4 for the tree species occurring
at the 148 'soil solution' stands
45
18 Median annual deposition (in), leaching from the unsaturated zone (out)
and their difference (in-out) of S04, N, N0 3 and NH4 for the tree species
occuring at hte 71 'groundwater' stands
47
19 Median annual deposition (in), leaching from the rootzone (out) and their
difference (in-out) of Al, K, Ca and Mg for the tree species occurring at
the 148 'soil solution' stands
49
20 Median annual deposition (in), leaching from the unsaturated zone (out)
and their difference (in-out) of AI, K, Ca and Mg for the tree species
occurring at the 71 'groundwater' stands
51
21 Summary of the net budgets (input-output) of major ions for the rootzone
and unsaturated zone
54
Figures
1 Distribution of sampling locations over the Netherlands
21
2 Inverse cumulative frequency distributions of the total deposition of S0 4
(A), N (B), NH4 (C) and N0 3 (D) on the forest stands where the chemical
composition of soil solution (148) and groundwater (71) was measured
24
3 Inverse cumulative frequency distributions of the bulk deposition of CI
(A), Na (B), Ca (C) and Mg (D) on the forest stands where the chemical
soil (148) and groundwater composition (71) was measured
26
4 Inverse cumulative frequency distributions of the calculated precipitation
excess of the tree species occurring at the 148 'soil solution' stands
34
5 Wet CI deposition in Eelde in 1989 and 1990
37
6 Wet S0 4 deposition in Eelde in 1989 and 1990
38
7 Inverse cumulative frequency distributions of the deposition and leaching
of CI (A) and Na (B) from the rootzone at the 148 'soil solution' stands 39
8 Inverse cumulative frequency distributions of the deposition and leaching
of CI (A) and Na (B) from the unsaturated zone at the 71 'groundwater'
stands
39
9 Inverse cumulative frequency distributions of the deposition and leaching
of S0 4 (A), N (B), N0 3 (C) and NH4 (D) from the rootzone at the 148
'soil solution' stands
44
10 Inverse cumulative frequency distributions of deposition and leaching of
S0 4 (A), N (B), N0 3 (C) and NH4 (D) from the unsaturated zone at the
71 'groundwater' stands
46
11 Inverse cumulative frequency distributions of the deposition and leaching
of Al (A), K (B), Ca (C) and Mg (D) from the rootzone at the 148 'soil
solution' stands
48
12 Inverse cumulative frequency distributions of the deposition and leaching
of Al (A), K (B), Ca (C) and Mg (D) from the unsaturated zone at the 71
'groundwater' stands
50
Preface
To examine the impact of atmospheric deposition on non-calcareous soils of Dutch
forests, a survey of the chemical soil and soil solution composition below 150 forest
stands, including seven major tree species, was carried out by the DLO Winand
Staring Centre. This research was financially supported by the Ministry of
Agriculture, Nature Management and Fisheries and the Ministry of Housing, Physical
Planning and Environment of the Netherlands.
The results of the research are presented in four reports with the common title:
Effects of acid deposition on 150forest stands in the Netherlands, with the following
subtitles:
1 Chemical composition of the humus layer, mineral soil and soil solution;
2 Relationships between forest vitality characteristics and the chemical composition
of foliage, humus layer, mineral soil and soil solution;
3 Input output budgets for sulphur, nitrogen, base cations and aluminium;
4 Predictions of the chemical composition of foliage, mineral soil, soil solution and
groundwater on a national scale.
In this report, number 3,results are given of element budgets for both the rootzone
and the unsaturated zone. In this context, use was made of research results on the
quality oftheupper groundwater at 71 of these sites assessed bythe National Institute
of Public Health and Environmental Protection.
We thankfully acknowledge all colleagues who assited in site characterisation and
soil sampling i.e. A.H. Booy, D. Eilander, H. van het Loo, P. Mekkink and R.
Zwijnen, in analyzing the various soil samples, i.e. M.M.T. Meulenbrugge, A.
Louwerse and W. Balkema, and in dataprocessing, i.e. J.C. Voogd. Furthermore, we
thank Ir.L.J.M. Boumans and Dr. J.W. Erisman of the National Institute of Public
Health and Environmental Protection who supplied us the data on chemical
groundwater composition and atmospheric deposition of sulphur and nitrogen,
respectively.
Summary
The aim of this report is to give a rough estimate of input-output budgets for S, N,
(NH4, N0 3 ), Ca, Mg, K, Na and Al, of Dutch forest soils based on a nation-wide
assessment of deposition and leaching data. Nearly all former research activities on
input-output budgets have been limited to a number of intensively monitored forest
sites.
To gain insight in element leaching a national assessment has been made of the
chemical composition of the soil solution and phreatic groundwater of forest
ecosystems. The chemical composition of the phreatic groundwater was determined
by the National Institute of Public Health and Environmental Protection. The study
concerned the quality of the upper groundwater at 156 sites with forests or a heather
vegetation on sandy soils. The samples were taken per grid of 0.5 x 0.5 km. In each
grid 10 watersamples were taken every 50 meters along a transect. Samples were
withdrawn from a tube, placed till one meter below the groundwater table, of which
the last 0.5 m was perforated The sampling took place in the period 1 October 1989 15 april 1990. The RIVM locations were used for the selection of 150 sites for soil
moisture sampling by the DLO Winand Staring Centre. Sites were restricted to major
tree species (Scots pine, black pine, douglas fir, Norway spruce, Japanese larch, oak
and beech) on non-calcareous soils. Soil samples, from which the soil moisture was
withdrawn, were taken of the mineral (top)soil layer (0-30 cm) and the (sub)soil
layer (60-100 cm). Per location the sample of each mineral layer consisted of a
composite sample of 20 subsamples. The soil was sampled in 1990 in the period
February 15 to May 16. This period is supposed to be representative for the annual
flux weighted solute concentration. Only 71 of the sites coincided with sites where
groundwater was sampled as well.
The budgets of the elements were derived by subtracting input (deposition) and output
(leaching). S 0 4 and N (NH 4 , N0 3 ) deposition values were obtained from the results
of a model for 5 km x 5 km grids for 1989, multiplied by filtering factors since the
deposition of these ions on forests is higher due to filtering activity. Bulk deposition
data of 22 weatherstations for the period 1978 -1986 were used to estimate the bulk
deposition of Ca, Mg, Na, K and Cl. Based on the ratio between Na in throughfall
and bulk deposition the total deposition of base cations and CI on the forest stands
was calculated. Leaching of elements from the rootzone and unsaturated zone
respectively, was calculated by multiplying element concentrations in the layer
60-100 cm and in groundwater with the precipitation excess. Compared to the layer
60-100 cm, concentrations in the upper groundwater were generally lower for NH 4 ,
N 0 3 CI, Ca, K, Na, Al and H (increased pH), whereas it was nearly comparable for
S 0 4 and Mg. The precipitation excess was calculated by subtracting the interception
and the actual évapotranspiration of each type of forest from the precipitation of
the gridcell in which the site is situated. For the precipitation the average of the 30
years period 1950-1980 was used. Interception was calculated as a percentage of
the precipitation.
11
To investigate whether the measured concentration of the soil solution represents
an average annual concentration, the expected residence time of soil water was
calculated for the various tree species as a function of soil type, by dividing an
estimated moisture content in the rootzone, by the estimated precipitation excess.
Since the estimated residence times for water in the rootzone were mostly less than
one year, especially for the deciduous trees, we investigated how the precipitation
excess should be adjusted such that the median CI deposition coincided with the
median CI leaching for each of the seven tree species. This caused a remarkable
decrease in the average precipitation excess of Japanese larch, beech and oak, i.e.
the tree species where concentrations are likely to deviate from annual average values
due to short residence times. In order to obtain reliable leaching fluxes adjusted
precipitation excesses were thus used for the rootzone whereas original values were
used for the unsaturated zone.
For all stands the S0 4 budget was -29 molc ha"1yr"1in the rootzone and -186 molc
ha* yr* in the unsaturated zone. Overall S0 4 thus seems tobehave like atracer. The
estimated large leaching under oak is likely to be unreliable due to lateral seepage.
The median leaching of N from the rootzone (834 molc ha* yr"1) was reduced by
80% compared to the median N deposition (4159 molc ha"1yr"1). The contribution
of N0 3 to the deposition of Nwas relative small (971 molcha"1yr"1).However, the
median leaching of N0 3 (700 molc ha"1 yr"1) almost equalled the median N0 3
deposition. NH4 contributes most to the deposition of N (3188 molc ha"1 yr"1), but
a lot of it disappeared due to (preferential) uptake and nitrification. The median net
retention was 3054 molc ha_1yr"1.The output fluxes of N from the unsaturated zone
were generally smaller than from the rootzone. NH4 almost disappeared and the
leaching of N0 3 was also less than at the depth of the rootzone (490 molc ha"1yr"1).
Together 90% disappeared as compared to the deposition.
Thedeposition of Al is neglectable.Therefore the entire leaching of this ion is caused
by Al mobilization. Al leaching from the rootzone was relatively small (less than
1000molc ha"1yr_1)below deciduous trees, including Japanese larch and relatively
large for coniferous trees, especially douglas fir (more than 2000 molc ha! yr"1).For
all stands the median mobilization was 976 molc ha"1 yr_1. Al leaching from the
unsaturated zone was nearly similar, i.e. a median mobilization of 1054 molc ha"1
yr"1.Leaching of K, Ca and Mg was mostly larger than the deposition of these ions,
because of weathering and ion-exchange. Median values for the net mobilization
of K, Ca and Mg in the rootzone and the unsaturated zone were 221 and 253 molc
ha"1yr"1,which is close to the average weathering rate of non-calcareous sandy soils
in the Netherlands. Striking was the large mobilization of Ca and Mg under oaks,
butthis is likely tobe influenced by seepage since most oak stands occurred on wet
sites.
From this study we concluded that in Dutch forest soils (i) S0 4 generally behaves
like a tracer, (ii) N is still largely retained and complete N saturation hardly occurs
yet, (iii) Al mobilization is the major buffermechanism in the soil, whereas base
cation input from the atmosphere largely neutralizes S0 4 and N0 3 input and (iv)
the acidification (Al mobilization) is much larger below coniferous trees, especially
12
douglas fir and Norway spruce, than below deciduous trees (including the Japanese
larch).
13
1 Introduction
Impacts of elevated deposition levels of SOx, NOx and NHX on the chemical
composition of forest soils, and its effect on forest vitality, has received much
attention in the Netherlands during the last decade (Heij and Schneider, 1991).
Impacts of atmospheric deposition on the soil have been derived by the determination
of input-output budgets of major ions, i.e. sulphate (S04), nitrate (N03), ammonium
(NH4), aluminium (Al), base cations (Ca, Mg, K, Na) and protons (H). This was
done to gain insight in (1) the fate of S and N in the ecosystem and (2) the
buffermechanism in the soil to neutralize the acid input associated with it. However
\ nearly all research activities havebeen carried out on a limited number of intensively
Imonitored forest sites.These sites are not representative for Dutch forest regarding
jj deposition level, stand characteristics such as tree species and site characteristics
Isuch as soil type. The aim of this report is to give rough estimates of input-output
"budgets for S, N,Ca, Mg, K, Na and Al of major tree species based on a nation-wide
assessment of deposition and leaching data.
1.1 Input-output budgets for Dutch forests
f
«A
.1*
Research on soil acidification and N accumulation at various forest sites in the
Netherlands has mainly been carried out within the Dutch Priority Program on
Acidification that started in 1985.An overview of the results of seventeen budget
studies thus carried out has been given in Van Breemen and Verstraten (1991).
Research results include nine intensively and eight extensively monitored sites. In
the intensively monitored sites input fluxes were derived from fortnightly or monthly
measurements of the chemical composition of throughfall water, multiplied by the
throughfall flux. Output fluxes were derived by multiplying monthly measurements
of the soil solution composition at various depths with simulated unsaturated soil
water fluxes (Van Grinsven et al., 1987). In the eight extensively monitored sites
(all douglas stands) input fluxes were also derived from monthly measurements of
throughfall (Kleijn et al., 1989).However, the soil solution composition was only
measured four times i.e. inJune, september and december 1986 and in april 1987.
Unlike the intensively monitored sites, where the soil solution was extracted with
porous cups permanently installed in the field, soil solution was extracted by
centrifugation from a previously sampled soil.This was done to enable measurements
in mid-summer and to put more emphasis on spatial variability inthe field. Results
on the soil and soil solution composition have been given in Kleijn and De Vries
(1987) and Kleijn et al. (1989).Output fluxes were derived by multiplying the annual
average precipitation excess for these stands (Reurslag et al., 1990) by the soil
solution composition measured in april. This was done since analyses of the data
of several intensively measured sites indicated that the concentration in early spring
(march, april) is generally most representative for the annual flux weighted solute
concentration.
i M,
15
Results showed that the average S0 4 input equalled the average S0 4 output,
suggesting that the forests are sulfur-saturated. N saturation occurred at five sites.
On average about 1.5kmol (± 20 kg) N ha"1yr"1was removed from the soil, either
by uptake or denitrification, or retained in organic matter by immobilization.
Furthermore, on average 70% of the amount of S0 4 and N0 3 leaching from the
system was accompanied by Al, indicating that Al mobilization is a major
buffermechanism in non-calcareous sandy forest soils in the Netherlands (Van
Breemen and Verstraten, 1991; Heij et al., 1991). The dominating role of Ali *, mobilization in the investigated forest sites was due to the low base saturation of
rJ' ! the adsorption complex (nearly always less than 20%).Laboratory experiments with
?l
,J*- V -!s o ^ s a m p l e s from these sites showed that the Al-pool responsible for buffering was
"9V
V ^ ' i V- ff" * m a m l y limited to organically bound Al and amorphous Al-hydroxides (Mulder et
«j/* s j, | al., 1989).This implies that major physio-chemical soil changes are presently taken
-®
* place by the depletion of this limited pool.
J
"if
I
,¥*
1.2 Aim of the present research
Soil acidification research conducted sofar has greatly increased ourknowledge about
the present impact of atmospheric deposition on non-calcareous sandy forest soils.
However the sites that were studied are not representative for the Dutch forests. First
of all, most sites were located in areas with intensive animal husbandry with a bias
towards high N loads. Secondly, the tree species studied was mainly douglas fir,
since the Dutch Priority Program on Acidification was particularly focused on this
tree. However douglas fir isnot representative for needleforest asfar as hydrological
characteristics are concerned. The high needle amount (and canopy coverage) causes
a large rate of interception evaporation thus increasing soil solution concentrations
due to low water fluxes (De Vries and Kros, 1989).It also causes a relatively large
input of elements by dry deposition. Finally, both soil type and groundwater level
of most intensively studied sites are not representative for sandy forest soils in the
Netherlands. They are either too loamy or too wet.
In order to overcome the various limitations of the research carried out so far a
national assessment has been made of the chemical composition of the soil solution
and phreatic groundwater of forest ecosystems. The soil solution composition was
measured in 150stands for seven major tree species (Scots pine, black pine, douglas
fir, Norway spruce, Japanese larch, oak and beech) on non-calcareous sandy soils.
The limitation to these soil types is because they are most sensitive to N accumulation
and most Dutch forests (about 85%) are located on these soiltypes (De Vries et al.,
1989). Forest vitality characteristics were known for the stands as they are part of
the forest vitality study by the State Forestry Service (e.g. IKC-NBLF, 1992).
^
<^- j The chemical composition of the phreatic groundwater was determined by the
1
National Institute of Public Health and Environmental Protection. The study
»Kxj concerned the quality of the upper groundwater at.156 sites with forests or aheather
V y**
vegetation on sandy soils. 71 of these sites coincided with sites where soil moisture
was sampled.
16
I
J
i
JMajor aims of the research are the determination of the regional variability in soil
Iand soil solution composition in relation to deposition level, stana^cTîaracteristics
|and site characteristics. Results thus obtained are given in De Vries and Leeter§ _;* WlJA
(1993). Another important aim was the assessment of a relationship between soil
and soil solution composition and forest vitality characteristics for major tree species ^
as reported in Hendriks et al. (1993). Here we report the results of input-output
budgets for sulphur, N, Al and base cations of the 150 forest stands and apart also
of the 71 stands where moisture and groundwater was analyzed. This was done to
gain insight in the influence of deposition level and tree species on element budgets.
Element budgets for the rootzone and the unsaturated zone were calculated by
subtracting the output (leaching) fluxes at the bottom of these zones from the input
(deposition) fluxes via the surface. Output fluxes were obtained by multiplying the
concentration of the elements in the soil moisture and upper groundwater with the
precipitation excess.
1.3 Contents of the report
A description of the methods that were used to gain insight in the soil and
groundwater quality below forest stands is given in chapter 2. This includes the
selection of the locations, and the sampling procedure and chemical analyses of soil
moisture and upper groundwater. The input (deposition) fluxes of the different ions
are given in chapter 3. The chemical composition of the soil solution and groundwater
and the precipitation excess, which form the basic information to derive the output
fluxes, are given in chapters 4 and 5, respectively. In chapter 5 the relation of the
precipitation excess with the budgets of CI and Na is also discussed because it is
assumed that the waterbalance and the balances of the inert ions CI and Na matches.
In chapter 6 the final results are given, i.e. the input- output budgets, of S, N, base
cations and Al for the rootzone and the unsaturated zone. Finally conclusions and
a discussion are given in chapter 7.
17
f
• — »
i' ,ï
Hj
2 Methods
2.1 Choice and characterization of the locations
Choice
In infiltration areasthe chemical composition of soil moisture and groundwater under
forests is largely determined by the quantity and quality of the precipitation that
infiltrates via the surface and the interaction with the soil and the vegetation. Stand
characteristics such as tree species, canopy coverage, structure of the stand, height
of the trees and position and distance to the edge are of major importance for the
1 the total deposition on forest and precipitation excess of forests. In choosing the
locations, allowance for these factors was made in order to isolate their influences.
\t
f$
To select locations for groundwater sampling, RIVM used adatabase of
0.5 km x 0.5 km grids, containing information about soil use and the total N
deposition. The atmospheric Ndeposition was calculated by using the TREND model
(Van Jaarsveldf and Onderdelinden, 1993).It"was"sïïppösedTthat the soil use causes
differences in the emission and deposition of N. Taking into account that grids had
to be representative for the Netherlands and that they had to show as much as
possible differences inN deposition levels, 168locations were selected. Preferentially,
sites were chosen where throughfall was monitored orwhere the vitality of thetrees
was established in 1988 and/or 1989.More information on the selection procedure
has been given by Boumans and Beltman (1991).
The RIVM locations formed the starting-point for the selection of 150sites for soil
moisture sampling by the DLOWinand Staring Centre. However, some additional
requirements that were made,i.e. restriction to major tree species (Scots pine, black
pine, douglas fir, Norway spruce, Japanese larch, oak and beech) and ensuring a large
range in soil type and groundwater level caused that only 89 of the 150 locations
that were selected coincided withRTVMlocations.More information onthe selection
procedure has been given by de Vries and Leeters (1993). Of the 89 locations that
are in common 18 were dropped when processing the data, mostly because other
tree^srjecies dominated at the transect alonjgjwhkhj^yj^sampled the groundwater.
Furthermore, tKë*chemical corlposition of the soil solution of two locations was
doubtful.
V^. ^ r € K JÙfa^4ÊfhÂ^ * * * * c6 ***'*» 6
Table 1gives an overview of the distribution of the tree species over the locations
and Figure 1 gives the distribution of the locations over the Netherlands.
"«*i
e?
Characterization
Site characteristics, such as soil type and groundwaterlevel, and stand characteristics,
such as canopy coverage, tree height, distance to -and soil use at the edge of the
forest and the presence of forest roads were listed. Results on the distribution of the
tree species over soil type and groundwaterclass are given in Table 2and 3. The
complete dataset and acomprehensive description of the sampling procedure has
been given by De Vries and Leeters (1993).
19
Table 1
Number offorest stands per tree species where the chemical composition
of soil solution and groundwater was measured
Tree species
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
Number of stands
Soil solution
Groundsrater1'
original dropped final
original
442)
dropped final
15
15
30
15
0
0
0
1
0
1
0
44
15
16
14
15
29
15
23
10
10
11
11
22
2
5
1
1
1
2
7
1
18
9
9
10
9
15
1
150
2
148
89
18
71
15
162>
" At these sites the chemical soil solution composition was measured as well.
2)
At one stand the major tree species appeared to be Douglas fir instead of
Scots pine (as expected).
Table 2
Soil type
Distribution of the 150forest stands over soil type
Number of stands
Scots
pine
Haplic Arenosol1) 14
Gleyic Podzol2)
21
Cambic Podzol
9
Fimic Anthrosol
1
0
Umbric Gleysol3'
0
fDystric Gleysol
Black
pine
Douglas Norway Japanese Oak
fir
spruce larch
Beech
6
9
0
0
0
0
1
7
3
3
1
0
0
7
2
2
3
1
0
11
0
0
3
0
0
13
1
1
0
0
3
10
2
3
7
4
i „ l^ u " Including Gleyic Arenosols
2)
*,;',}-•
Including Carbic Podzols
e
3)
I '
Including organic rich soils
Most tree species occurred on Gleyic and Carbic Podzols, especially Norway spruce
and Japanese larch and to a lesser extent on Arenosols. Scots pine and black pine
occurred almost exclusively onthese 'nutrient-poor' soiltypes.Douglas fir has greater
needs for nutrients (and moisture) and occurred more often on 'nutrient-rich' soil
types such as the Cambic Podzol and Fimic Anthrosol. Oak and beech occurred on
nearly all soil types (cf Table 2).
Table 3 showsthatthe coniferous trees mainly occurred on well-drained soils whereas
the deciduous trees also occurred relatively often on poorly-drained soils. In these
soils removal of N by denitrification may play an important role.
20
A
Ç=^
soil solution stands
A [ soil solution and
groundwater stands
Fig. 1 Distribution of sampling locations over the Netherlands
Table 3 Distribution of the 150forest stands over groundwaterlevel class
Groundwaterlevel Number of stands
ciass
II+III
V
IV+VI
VII
Scots
pine
Black
pine
Douglas Norway Japanese Oak
fir
spruce larch
2
5
3
35
0
2
2
11
1
0
3
11
1
1
6
6
1
0
3
11
4
5
11
9
Beech
2
2
2
9
21
2.2 Sampling and chemical analysis of soil and groundwater
p
t)A
^ç
Sampling
The soil was sampled in the period 15february 1990 - 16 may 1990. This period
is based on results of the intensive soil solution in Hackfort (van Breemen et al.,
1988). Results indicated that the concentration in early spring (march, april) is
generally most representative for the average annual flux weighted solute
concentration.
Soil samples were taken of the humus layer and three mineral soil layers, i.e.
0-30 cm, 30-60 cm and 60-100 cm. Samples of each mineral layer consisted of one
composite sample of 20 subsamples. The sample points were chosen along a steady
pattern in a square of 20x 20meters inthe middle of the forest stand. Soil moisture,
was withdrawn from the samples of twomineral layers ,i.e. 0-30 cm and 60-100 cm.
Phreatic groundwater was sampled by the RIVM per grid of 0.5 km x 0.5 km. In
each grid 10 samples were taken every 50 meters along a transect in the biggest
united area of forest and other nature grounds (heather). When the transect turned
to be shorter than 450 meters, the remaining samples were taken along the middle
leadline of the transect. At least 3 groundwater samples were taken in a forest stand
where the vitality was observed and where the DLO Winand Staring Centre also
collected soil samples. Sampling took place in the period 1October 1989 - 15 april
1990.At each sampling site a hole was bored till one meter below the water table
in which a tube was placed of which the last 0.5 mwas perforated. The water-sample
was withdrawn from the tube.Detailed information has been given by Boumans and
Beltman (1991).
Chemical analysis
Soil moisture was released from the soilsamples by using acentrifugation method.
Next the water waspressed through afilter with aporesize of 0.45um and the acidity
(pH) and electric conductivity was measured. In diluted samples the concentrations
of Si,Al,Fe,Ca,Mg, K, Na,NH4, N03, Cl, S0 4 , H2P04 and dissolved organic carbon
(DOC) were measured (cf. de Vries and Leeters, 1993).
The groundwater samples were sucked by vacuum through a filter with a poresize
of 0.2 mm within 24 hours after sampling. Next the water was analyzed for AI, Ca,
Mg, K, Na, NH4, N0 3 , S0 4 , Cl, DOC, electric conductivity and acidity (cf. Boumans
and Beltman, 1991).
22
3 Atmospheric deposition
Human activities cause an increase of the deposition of particular sulphur and N,
while other ions, like Cl,Na and Mg are strongly influenced by the distance to the
sea. Next to this,considerable changes in deposition can occur at short time intervals,
mainly due to the variability of the precipitation and the direction and velocity of
the wind.
W1
fc; J'. W**
iJ
3.1 Sulphur and nitrogen
" '
ddM^
The concentration of NHX,NOx and SOx that was measured at several weather stations
and stations of the National Air Quality Monitoring/Network formed the inception
for an empirical model (Erisman, 1991) through vWiichthe wet and dry deposition
of these elements for 5km x 5km grids was calculated. Results for 1989,the year
preceding the sampling of the soil moisture and groundwater (Erisman, 1991) were
recalculated to averages for 10km x 10km grids since data for thebase cations were
available on this gridsize (cf Section 3.2).
Cumulative frequency diagrams of the total deposition of sulphur (S04) and nitrogen
(NH4and N0 3 ) on the 148 stands where soil moisture and the 71 stands where also
the upper groundwater was sampled, are given in Figure 2. The frequency distribution
of the 71 stands agreed quite well with that of the 148 stands, which means that the
distribution of both groups over the Netherlands isfairly similar (cf Figure 1).Only
the number of stands with a high deposition level of N0 3 is under-represented in
the group with 71 stands. High deposition levels of N0 3 are mostly found near
industrial zones.The cumulative frequency distribution of S0 4 deposition shows that
at 50% of the locations the deposition lies between 1000 and 1200 molc ha_1yr"1.
Those locations are situated in the northeastern part of the Netherlands. At the other
locations the deposition is greater, up to 2000 molc ha"1 yr"1. The NHX deposition
varied from 1500up to 5000 molc ha"1yr"1, with the highest amounts in areas with
intensive animal husbandry. The range in the NOx deposition at more than 90 %of
both groups of locations is small and lies between 1050 and 1250 molc ha"1yr"1.
The deposition given in Figure 2 equals the total average deposition on the gridcells
where the forest stands are situated. However, the deposition of most ions on forests
is higher due to filtering activity. To account for this effect, total deposition values
weremultiplied byfiltering factors (Table 4).Thesefactors were taken from DeVries
(1991) who derived these data by comparing throughfall data in42 forested stands
with deposition estimates derived by the TREND model.
23
Cumulative frequency
Cumulative frequency
(*)
71stands
1000
1250
1500
1750
2000
2000
2500
3000
3500
4000
4500
5000
Ndeposition (mol.ha1.yr1 )
S0 4 deposition (mol.ha' .yr' )
Cumulative frequency
Cumulative frequency
(%)
D
148stands
71stands
1000
2000
3000
4000
5000
NH< deposition (moLha' .yr' )
1000
1100
1200
1300
1400
1500
N0 5 déposition (mol.ha' .yr'1 )
Fig. 2 Inverse cumulative frequency distributions of the total deposition of S04 (A), N
(B), NH4 (C) and N03 (D) on theforest stands where the chemical composition of
soil solution (148) and groundwater (71) was measured
24
Table 4
Filtering factors for the deposition of SO^ N03 and NH4 on different types of
forests
Forest type
Scots pine, Black pine
Douglas fir, Norway spruce
Japanese larch, Oak, Beech
Filtering factor (-)
so 4
N03
NH4
1.40
1.60
1.15
0.85
1.00
0.70
1.30
1.50
1.10
3.2 Base cations, chloride and aluminium
At several places in the Netherlands the quality of theprecipitation is measured. From
these data the average concentration of several elements, including base cations and
CIwas calculated for theperiod 1978-1986 (KNMI/RIVM, 1988).These data were
used to estimate the bulk deposition of calcium (Ca),magnesium (Mg), sodium (Na),
potassium (K) and chloride (CI) for grids of 10 km x 10 km, using interpolation
techniques (De Leeuw, RIVM,pers.comm.).The bulk deposition includes the wet
deposition and the soluble fraction of the dry deposition ontheprecipitation collector.
Cumulative frequency diagrams of the bulk deposition of CI,Na, Ca and Mg on the
148 stands where soil moisture was sampled and the 71 stands where also the upper
groundwater was sampled, are given in Figure 3.The deposition of Alis negligible
and of K very small (in most stands 30 molc ha"1yr"1) and therefore not given.
As with N and Sthe frequency distribution of the 71 stands agreed quite well with
that of the 148 stands.The bulk deposition of CIis high; in 10%of the stands even
1000-2000 molc ha"1yr"1.These are all situated in the northern province Friesland.
Thedeposition decreases in south eastern direction. These differences are connected
with the distance to the sea. The same sea spray effect explains the differences in
Na and Mg deposition. The Na deposition differs from 300 to 1300 molc ha"1yr"1
with a median of 650 and the Mg deposition from 80 to 350 molc ha"1yr"1 with a
median of 140.The highest deposition-rates of Ca are not only found near the coast,
but also in the extreme south of the Netherlands due to occurence of limestone
quarries. In the middle and in the northeastern part, where about 50 %of the stands
are situated, the Ca deposition is less than 200 molc ha_1 yr"1. The deposition of K
is small, in most stands 30 molc ha_1yr"1.
The total deposition of base cations on forests is mostly much higher than the bulk
deposition, especially near forest edges (Draaijers et al., 1992).It also depends on
the tree species, density and height of the forest (Stuijfzand, 1984;Verstraten et.al.,
1984). Insight in the effect of dry deposition can be derived from the ratio between
Na in throughfall and bulk deposition assuming that canopy interactions for Na are
negligible, i.e. throughfall equals total depostion (Bredemeier, 1988).Using data on
Na in throughfall and bulk deposition in 42 forests, de Vries (1991) derived a ratio
of 2.0 for Japanese larch, oak andbeech; 2.5 for black pine and Scots pine and 2.75
for Norway spruce and douglas fir. Total deposition of base cations and CIon each
forest stand was thus calculated by multiplying the bulk deposition with these factors.
25
Cumulative frequency
Cumulative frequency
(ft)
(ft)
B
7Islands
500
1000
500
1500
1000
1500
J
Nadeposition (mol.ha' .yr )
Cldeposition (mol.ha' .yr' )
Cumulative frequency
Cumulative frequency
(%)
(ft)
D
71 stands
200
250
300
Cadeposition (molc.ha"'.yr ' )
71stands
50
100
150
200
250
300
Mgdeposition (moLha' .yr'' )
Fig. 3 Inverse cumulative frequency distributions of the bulk deposition of CI (A), Na
(B), Ca (C) and Mg (D) on theforest stands where the chemical soil (148) and
groundwater composition (71) was measured
26
4 Chemical composition of soil solution and groundwater
Leaching of elements from therootzone andunsaturated zone, respectively,was
calculated bymultiplying the element concentration inboth compartments withthe
precipitation excess. Regarding theelement concentration, data were availablefor
the soil solution inthelayers 0-30cmand60-100 cmin spring 1990 andfor the
groundwater intheupper 50-100 cmbelow thephreatic surface in autumn1989.
Soil solution data forthe layer 60-100 cm were used tocalculate leaching fromthe
rootzone, whereas thegroundwater data were used todothe same for thewhole
unsaturated zone. It wasassumed that theelement concentrations in the various
periods equalled theflux weighed average annual concentration (cf. section2.2).
For groundwater this islikely tobeabetter assumption than forthesoil solution
which varies considerable over the year dueto seasonal differences in litterfall,
deposition, hydrology, etc. This aspect isfurther elaborated inChapter5.
4.1 Chloride and sodium
Table 5gives themedian CIandNa,concentrations fortheentire data setand for
singletree species.The concentration insoil moisture was much higherthaninupper
groundwater. For CIthe concentration difference between soil moisture at60-100 cm
and upper groundwater varied from afactor 1.1(beech) upto3.1 (Norway spruce)
and forNafrom 1.0 (beech) to2.3(black pine). These differences imply thatthe
CIand Na concentrations donot represent the annual average concentration either
in soil moisture below therootzone orinthe groundwater orinboth compartments.
This problem isdiscussed further inchapter 5.2, where water- and CIbalancesare
compared.
Table 5
Effect of tree species on the median concentration of CIand Na in the soil
solution (148 stands) and in upper groundwater (71 stands)
Tree species
CI concentration (molc m
•
3
Na concentration (molcm :3)
)
0-30cm
60-100 cm
groundwater
0-30 cm
60-100cm
groundwater
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
1.10 (0.95)1'
1.15 (0.86)
2.32 (1.89)
1.89 (1.80)
1.63 (1.35)
1.03 (0.93)
1.06
-2>
0.88 (0.87)
0.84 (0.84)
1.58 (1.58)
1.99 (1.74)
1.25 (1.22)
1.07 (0.93)
1.07
-2)
0.38
0.35
0.62
0.64
0.59
0.72
0.99
0.70 (0.64)
0.73 (0.61)
1.26 (1.25)
1.15 (1.02)
0.97 (0.74)
0.70 (0.55)
0.69
-2)
0.69 (0.69)
0.69 (0.69)
1.17 (1.18)
1.13 (0.94)
0.87 (0.84)
0.72 (0.59)
0.83
-2)
0.35
0.30
0.60
0.53
0.56
0.61
0.80
Total
1.29 (1.15)
1.06 (1.06)
0.58
0.77 (0.74)
0.78 (0.72)
0.51
Values between brackets are median concentrations at the 71locations where the
groundwater composition was measured aswell
Not enough data
27
The difference in concentration between the various tree species was considerable.
The CIand Na concentration was high in soil moisture under douglas fir and Norway
spruce, which is explainable by the small precipitation excess and therelatively high
deposition. The opposite is expected for deciduous trees and the needle-loosing
Japanese larch. However, the CI and Na concentration under the pine forests was
much smaller. In the groundwater even much lower concentrations were found.
CIis an almost inert element apart from a small uptake by the vegetation. Na is also
relatively inert, although it can take part in weathering and cation exchange.
Consequently, the (soil solution) concentration of both ions is expected to be related
to the precipitation excess and the deposition. The ratio between Na and CI in sea
water is about 0.86. TheNa/Cl ratio's were lower in soil moisture and groundwater
especially in the upper layer whereas a similar ratio (median of 0.88) was found for
groundwater (Table 6). Apparently CI recycles via uptake and litterfall and/or Na
plays a role in the ion exchange, in particular in the upper layer. However, these two
processes cannot really explain the relatively low ratios in the subsoil.
Table 6
Effect of tree species on the median Na/Cl ratio in the soil solution (148
stands) and in upper groundwater (71 stands)
Tree species
Na/Cl ratio (-)
0-30 cm
1
60-100 cm
groundwater
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
0.64 (0.67) '
0.63 (0.71)
0.54 (0.66)
0.61 (0.57)
0.60 (0.55)
0.68 (0.59)
2)
0.65
0.78 (0.79)
0.82 (0.82)
0.74 (0.75)
0.57 (0.54)
0.70 (0.69)
0.67 (0.63)
2)
0.78
0.92
0.86
0.97
0.83
0.95
0.85
0.81
Total
0.60 (0.64)
0.74 (0.68)
0.88
11
2)
Values between brackets are median ratios at the 71 locations where the
groundwater composition was measured as well
Not enough data
4.2 Sulphur and nitrogen
In Table 7 the median concentration of S0 4 , N0 3 and NH4in the soil solution and
upper groundwater are given. The S0 4 concentration at different depths was rather
stable for most tree species. Only douglas fir andNorway spruce showed a decreasing
concentration. Furthermore, it is striking that the median S0 4 concentration in the
layer 60-100 cm was exceptional high at the oak-locations where the groundwater
composition was measured as well.
28
Table 7
Effect of tree species on the median concentration of SO^ N03 and NH4 in
the soil solution (148 stands) and in upper groundwater (71 stands)
Tree species
S 0 4 concentration (molc m"3)
0-30 cm
60-100 cm
Scots pine
0.97 (1.04)1*
Black pine
0.81 (0.77)
Douglas fir
2.45 (1.53)
Norway spruce 1.99 (1.92)
Japanese larch 1.01 (0.76)
Oak
0.81 (0.75)
Beech
0.63
- 2)
1.00 (1.12)
0.90 (0.88)
1.81 (1.11)
1.78 (1.78)
0.94 (0.87)
1.22 (2.18)
0.94
- 2)
Total
1.10 (1.08)
0.97 (0.97)
N 0 3 concentration (molc m"3)
ground- 0-30 cm
60-100 cm groundwater
water
0.93
0.48 (0.42) 0.53 (0.59) 0.49
0.70
0.30 (0.25) 0.34 (0.25) 0.14
0.95
0.93 (0.88)
1.28 (1.20) 0.79
1.20
0.48 (0.44) 0.47 (0.41) 0.16
0.92
0.65 (0.59) 0.49 (0.47) 0.21
1.04
0.57 (0.64) 0.26 (0.25) 0.12
1.14
0.20
- 2) 0.17
- 2) 0.00
1.04
0.53 (0.54)
0.48 (0.53)
0.24
NH4 concentration (mol m'3)
Scots pine
0.21 (0.22)
Black pine
0.19 (0.19)
Douglas fir
0.61 (0.47)
Norway spruce 0.47 (0.45)
Japanese larch 0.22 (0.22)
Oak
0.12 (0.12)
Beech
0.14
-2)
0.10 (0.09)
0.10 (0.11)
0.10 (0.09)
0.08 (0.09)
0.09 (0.10)
0.07 (0.07)
0.09
-2)
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Total
0.09 (0.09)
0.00
11
2)
0.18 (0.20)
Values between brackets are median concentrations at the 71 locations where the
groundwater composition was measured as well
Not enough data
The NH4 concentration decreased with depth, most likely due to uptake and
nitrification. In the upper groundwater NH4 completely disappeared. The N0 3
concentration increased between the layers 0-30 and 60-100 cm under stands of pine
and (Douglas) fir, but decreased,just like the other stands,on its way to groundwater.
This might be due to denitrification.
4.3 Base cations
The median Ca, Mg and Kconcentrations are given in Table 8. Except for oak, the
Ca concentration decreased with depth. Apparently, deeper in the soil profile Ca is
absorbed and probably exchanged by Al. The increase in Ca concentration under
oak stands, which is sometimes 3-5 times greater in upper groundwater than in the
upper soil solution, may be due to seepage, since oak occurs relatively often on wet
soils (cf Table 3).The Mg concentration under the different species remained rather
stable, also under oak. The concentration was lowest under black pines,but in general
the differences between the tree species were modest. The Kconcentration was small,
differed not much between the tree species and decreased in all cases with depth.
29
Table 8
Effect of tree species on the median concentration of Ca, Mg and K in the soil
solution (148 stands) and in upper groundwater (71 stands)
Tree species
Ca concentration (molc m'3)
Mg concentration (molc m •3)
60-100 cm
ground'• 0-30 cm
water
60-100 cm
groundwater
0.39 (0.41)1'
Scots pine
Black pine
0.37 (0.34)
0.92 (0.72)
Douglas fir
Norway spruce 0.58 (0.59)
Japanese larch 0.45 (0.44)
Oak
0.51 (0.58)
Beech
0.27
- 2)
0.29 (0.42)
0.40 (0.30)
0.37 (0.33)
0.47 (0.47)
0.38 (0.38)
0.70 (1.16)
0.19
- 2)
0.27
0.22
0.19
0.33
0.32
1.03
0.09
0.20 (0.19)
0.18 (0.18)
0.53 (0.46)
0.39 (0.34)
0.29 (0.22)
0.27 (0.23)
0.19
- 2)
0.20 (0.20)
0.15 (0.15)
0.33 (0.30)
0.39 (0.39)
0.24 (0.23)
0.41 (0.45)
0.23
- 2)
0.34
0.11
0.22
0.42
0.35
0.47
0.25
Total
0.39 (0.41)
0.26
0.25 (0.25)
0.23 (0.21)
0.31
0-30 cm
0.44 (0.45)
K concentration (molc m
3
)
0.19 (0.18)
Scots pine
Black pine
0.17 (0.17)
0.21 (0.17)
Douglas fir
Norway spruce 0.18 (0.17)
Japanese larch 0.14 (0.14)
Oak
0.24 (0.22)
Beech
0.19
-2)
0.11 (0.09)
0.08 (0.08)
0.13 (0.10)
0.09 (0.07)
0.09 (0.09)
0.11 (0.10)
0.07
-2)
0.05
0.04
0.05
0.05
0.05
0.06
0.05
0.19 (0.18)
0.10 (0.09)
0.05
Total
11
21
Values between brackets are median concentrations at the 71 locations where the
groundwater composition was measured as well
Not enough data
Due to a recycling effect the concentration of Ca, Mg and K will be influenced in
the upper layer by litterfall whileuptake also takes place deeper in the soil profile.
This effect clearly occured for Ca and K.
4.4 Aluminium and pH
Table 9 shows the median pH and Al concentration. The pH increased in all cases
with depth. The Al concentration did not show a comparable decrease. Between the
layer 0-30 cm and 60-100 cm the concentration increased. This is due to the fact
that the soil solution in the topsoil was undersaturated with respect to all minerals
containing Al (e.g. gibbsite). The Al concentration thus increased with depth because
of the mobilization of Al(OH)3, causing apH increase. The lowest Al concentrations
were found under oaks and the highest Al concentrations under firs and spruces.
Deeper in the soil profile the Al concentration was generally in equilibrium with
Al hydroxide and here the opposite occurred: the pH increased and the Al
concentration decreased due to base cation weathering. Only for black pine, there
was an increase in Al concentration between the layer 60-100 cm and the upper
groundwater.
30
Table 9
Effect of tree species on the median Al concentration and the median pH in the
soil solution (148 stands) and in upper groundwater (71 stands)
Tree species
Al concentration (molc m f3)
0-30cm
PH
ground'- 0-30cm
water
60-100cm
groundwater
Scots pine
0.71 (0.75)1» 0.68 (0.80)
Black pine
0.50 (0.53)
0.60 (0.63)
Douglas fir
1.43 (1.43)
1.77 (1.60)
Norway sprue« : 1.05 (1.05)
1.43 (1.43)
Japanese larch 0.70 (0.69)
0.85 (0.85)
Oak
0.40 (0.40)
0.35 (0.35)
Beech
0.44
- 2) 0.27
- 2)
0.53
0.78
1.14
1.10
0.31
0.14
1.45
3.61 (3.62)
3.70 (3.76)
3.41 (3.53)
3.43 (3.43)
3.58 (3.61)
3.70 (3.62)
3.77
- 2)
3.89 (3.86)
3.95 (3.95)
3.79 (3.81)
3.80 (3.79)
3.89 (3.89)
4.02 (4.01)
3.98
- 2)
4.45
4.37
4.27
4.32
4.48
4.43
4.24
Total
0.54
3.61 (3.59)
3.90 (3.88)
4.37
1]
2>
0.63 (0.69)
60-100cm
0.58 (0.67)
Values between brackets are median concentrations at the 71 locations
the groundwater composition was measured as well
Not enough data
where
31
5 Hydrological conditions
As stated before (chapter 4), element leaching was calculated by multiplying the
measured element concentration in spring in the soil solution and in the upper
groundwater with the precipitation excess. This was based on the assumption that
the concentrations that are measured represent flux weighed annual average
concentrations. The precipitation excess was calculated from the annual precipitation
minus the annual interception and actual annual évapotranspiration.First an estimate
was made of annual precipitation excesses for the seven tree species included in
this study on the basis of literature data on average precipitation, interception and
transpiration (Section 5.1). Next a calculation was made about the residence time
of soil water to investigate whether it is likely that the data on element concentrations
reflect the effect of the deposition of 1989, the year preceding the sampling and
represent annual average values (Section 5.2). Information thus obtained was used
to adjust several precipitation excesses by using CI concentration data and assuming
that CI behaves as a tracer (Section 5.3).
5.1 Precipitation, évapotranspiration and precipitation excess
To calculate element budgets, information is required about the precipitation of the
year that preceded the sampling. However, during the period October 1989 -March
1990 an amount of 350 -400 mmrain has fallen, a quantity that equals the average
value for the period 1950 - 1980.Since adata set for this period was available for
gridcells of 10 km x 10 km (interpolated from data of 280 weather stations;
Hootsmans and Van Uffelen, 1991),it was used to calculate the average precipitation
on the forest stands.
In order to obtain the precipitation excess,the interception and the actual evaporation
and transpiration have to be known. Literature data about interception, evaporation
and transpiration of different types of forests are often not comparable (Nonhebel,
1987; Mulder, 1985; Gash, 1979; De Visser en De Vries, 1989; Hendriks et al.,
1990). Based on the review in the Annex the following, interception percentages
of the precipitation were selected: 25%for beech, oak and Japanese larch; 30% for
black pine and Scots pine; 40% for douglas fir and 45%for Norway spruce. Data
used for the sum of average actual transpiration and evaporation were 325 mm for
beech, black pine, Scots pine and Japanese larch and 350mmfor douglas fir, Norway
spruce and oak (cf Annex).
The precipitation excess was calculated by subtracting the interception and the actual
evaporation and transpiration of each tree species from the precipitation of the grid
square in which the site is situated. Cumulative frequency distributions of the
precipitation excesses thus calculated for the tree species occurring at the 148 'soil
solution' stands (Fig.4) show that theranges for the different tree species were small.
33
Cumulative Frequency
Norway
spruce
(%)
100
\ 'x
80
\ \
Y\
60
'Ai
'1
\\
\\\
fir
Black
pine
l\
\
'
40
Douglas
l\
y
1 'i
Scots
pine
Oak
\\ft *i
20
V
\
0
50
100
150
200
250
300
Beech
350 ~ ~ '
JapanCSe
Larch
Precipitation Excess (mm.yr ' )
Fig. 4 Inverse cumulative frequency distributions of the calculated precipitation excess
of the tree species occurring at the 148 'soil solution' stands
This isbecause only regional differences in precipitation caused adifference in the
precipitation excess of a tree species. Consequently, the frequencies of the tree
species at the 71 'groundwater' stands were quite similar. Actually, the range will
be greater because of differences in évapotranspiration induced by differences in
canopy coverage, tree height, vitality, soil cover, available moisture, etc. However,
it is supposed that the median that will be used to calculate the leaching is fairly
reliable. Further Fig. 4 shows that the calculated precipitation excesses were smallest
below Norway spruce, and Douglas fir, greatest below beech and Japanese larch and
intermediate below Scots pine and black pine and oak. The precipitation excesses
given above were used for a first approximation of the element leaching from the
rootzone and unsaturated zone, respectively.
5.2 Residence time of the percolation water
To answer the question whether the measured concentrations, in the soil solution
and in the upper groundwater represent an average annual concentration, and if so
for which year, the expected residence time of soil water was calculated for the
various tree species on major soil types. The residence time (yr) was estimated by
dividing an estimated moisture content (mm) in the rootzone and in the whole
unsaturated zone respectively (mm), by the estimated precipitation excess (mmyr"1).
For the rootzone a soil layer of 80cm was used and for the unsaturated zone alayer
with a thickness from the surface till 75 cm below the phreatic groundwaterlevel
(middle of the sampled layers).
Calculations were made for three major soil types, i.e. fine loamy sand, fine sand
poor in loam and coarse sand. To estimate the water content it was assumed that
(i) the top soil is 40 cm thick, (ii) the groundwaterlevel during sampling equalled
34
the average spring level and (iii) fieldcapacity occurs up to 100 cm above the
groundwaterlevel (above that level a soil water profile was supposed). Average spring
levels were related to groundwaterlevel classes as given in Table 10 (Van der Sluis,
1982). The distribution of groundwaterlevel classes is given inTable 3. Relatively
many deciduous stands were located on fine, loamy sand and onwet soils, whereas
most coniferous stands were located on coarse sand and on dry soils
(groundwaterclass VII), especially the pine stands (cf Table 3).
Table 10 Estimated average spring level for the groundwaterlevel classes occurring at
the 148 'soil solution ' stands
Groundwaterlevel
Groundwaterlevel (cm)
class
average
highest
II
III
V
VI
VII
<40
<40
<40
40-80
>80
average
lowest
spri
50-80
80-120
>120
>120
>160
35
45
55
100
150
The watercontent was derived from the waterretention curves that belong to the
standard soils (Table 11).Results are given in Table 12.Comparison of these data
with the (original) precipitation excesses (cf Table 14) gives an indication of the
residence time of the water as given in Table 13.
Table 11 Waterretention characteristics of three major soil types in the 148 'soil
solution' stands
Soil type
Occurrence in
Water retention characteristics1'
148 stands (%)
fine sand, loamy
fine sand, poor in
coarse sand
11
18
loam 63
12
topsoil
subsoil
b3
bl 1 '
bl
o3
ol
o5
For the actual water retention characteristics related to these codes we refer to
Wösten et al., 1987
35
Table 12 Estimated watercontent in the rootzone (0-80 cm below the soil surface) and
the unsaturated zone (75 cm below the spring groundwaterlevel) of three major
soil types in the 148 'soil solution stands'
Soil type
Estimated watercontent (mm)
Gt V1'
Gt VI
root
zone
Fine sand, loamy
300
Fine sand, poor in loam 267
Coarse sand
260
11
unsaturated
zone
485
455
437
Gt VII
root unsaturated root unsaturated
zone zone
zone zone
255 579
213 680
195 524
129 593
151 461
112 497
Gt is groundwaterlevel class
Table 13 Estimated residence times ofpercolation water in the rootzone and unsaturated
zone as afunction of tree species, soil type and groundwaterlevel class
Soil type
Gt
Traveltime (yr)
Pine/Oak
Fine sand,
loamy
Fine sand,
poor in loam
Course sand
V
VI
VII
V
VI
VII
V
VI
VII
1
Douglas ßr
Norway spruce
Larch/Beech
RZ *
uz *
RZ
UZ
RZ
UZ
RZ
UZ
1.3
1.1
1.0
1.2
0.9
0.6
1.2
0.7
0.5
2.2
2.7
3.1
2.1
2.4
2.7
2.0
2.1
2.3
2.5
2.1
1.7
2.2
1.6
1.1
2.1
1.2
0.9
3.7
4.4
5.2
3.5
4.0
4.5
3.3
3.5
3.8
3.2
2.7
2.3
2.9
2.1
1.4
2.8
1.6
1.2
5.2
6.2
7.3
4.9
5.6
6.4
4.7
5.0
5.3
1.1
1.0
0.9
0.9
0.7
0.6
0.8
0.5
0.4
1.7
2.1
2.4
1.6
1.9
2.1
1.6
1.7
1.8
1
" RZ is rootzone and UZ is unsaturated zone
Residence times in the unsaturated zone were nearly almost always more than two
years (Table 13). Consequently, the influence of deposition fluctuation on
groundwater quality will be small. Besides the sample formed a mix of 50 cm of
the saturated zone that, assuming a poresize of 0.36, contains 180 mm water. For
some tree species this is more than the annual precipitation excess. Consequently,
it is likely that the concentration in groundwater represents an annual average value.
Unlike the unsaturated zone,the calculated residence time in the rootzone was nearly
always less than one year for all tree species on moderately to well drained soils (Gt
VI and VII) except for douglas fir and Norway spruce (Table 13).Below the latter
tree species, the watercontent in the rootzone will be descended from at least the
winter before. However, the sampled soil moisture below stands of pine, Japanese
larch, oak and beech on soils with groundwaterlevel classes VI and VU, will generally
be infiltrated during the preceding months, i.e. the autumn and winter of 1989/1990.
This implies that the measured concentrations will certainly not represent annual
average values below these tree species. This is specifically true for CI and Na. It
is known that sea-spray effects occur particularly during the autumn/winter period.
This is illustrated by the deposition of CIin Eelde, aplace situated in the northeastern
part of the Netherlands (Fig. 5). It shows that the deposition in the winter of
36
1989/1990 was 2.5 times the average of nearly 5 mmolc m"2 month"1. Na and to a
lesser extent Mg show comparable variations in deposition as shown in Table 14.
Table 14 Wet deposition of major ions in De Bilt (RIVM/KNMI, 1988)
Wet deposition (molc ha"1yr"1)
Period
Winters 1978-1986
Summers 1978-1986
Na
K
Mg
Ca
NH4
N03
so 4
CI
510
230
17
16
57
32
49
63
460
380
190
220
270
240
600
260
It is likely that the concentrations of S0 4 , N0 3 and NH4will also deviate from annual
average values, although the deposition of Sand N varies much less in time (cf Table
14).For Sthis is also illustrated in Fig. 6 (cf Fig. 5).However, use of the original
precipitation excess from the rootzone calculated for the various tree species might
not give reliable leaching estimates of S, N,BC and Al, with thepossible exception
of douglas fir and Norway spruce. Consequently, we investigated how the
precipitation excess changed while assuming that seasonal changes in S0 4 , N0 3 , BC
and Al are equal tothose of CI.This is notunlikely since hydrological changes have
a larger influence ondifferences in concentrations throughout the year than deposition
changes, especially at greater depth. The tracer behaviour of CI was thus used to
estimate the precipitation excess.
CI
Cldeposition
(mol.ha' )
2001
100"
1
...•mA Ife —-T- i .i 1 i ..•. ....
J F M A M J
J A S O N D J
1989
I
F M A M J
J A S O N D
1990
Fig. 5 Wet Cl deposition in Eelde in 1989 and 1990
37
SOt deposition
(molt.ha' )
ïoo80
60
* i
40"
20
0 "-
il
i
I
ill
„u».
JF M A M J J A S O N D J F M A M J
1989
"11
I
J A S O N D
1990
Fig. 6 Wet S04 deposition in Eelde in 1989 and 1990
5.3 Relation between precipitation excess and budgets for chloride
and sodium
In Figure 7 the cumulative frequency distributions of CI deposition and CI leaching
of the 148 'moisture' sites are given and in Figure 8 the same is given for the 71
'groundwater' sites. Total deposition was calculated from the wet deposition,
according the method discussed in chapter 3.2 and leaching of CI and Na was
calculated by multiplying the precipitation excess with the concentration in the soil
moisture at 60 - 100 cm and the upper groundwater. In these figures the cumulative
frequency distribution for CIleaching is also given after adjusting the precipitation
excess in such away, that the median deposition coincides with the median leaching
for each of the seven tree species.
Figure 7 shows that leaching of CI and Na from the rootzone, calculated with the
original precipitation excess, exceeds CI and Na deposition. Inversely, calculated
CIand Na leaching from the unsaturated zone is less than CI and Na deposition (Fig.
8). Fitting the medians of the deposition and the leaching of CI and Na from the
rootzone by changing the average precipitation excess hardly influenced the
precipitation excesses of the coniferous tree species, but it caused a large decrease
in the precipitation excesses of the deciduous trees, including the needle shedding
Japanese larch (Table 15). This corresponds with the high concentrations of CI
38
Cumulative frequency
Cumulative frequency
deposition
deposition
leaching
(original)
leaching
(adjusted)
X
0
1000
2000
3000
4000
0
5000
1000
2000
3000
4000
5000
1
Nafluxes (moKha .yr')
Cl fluxes (moLha' .yr')
Fig. 7 Inverse cumulative frequency distributions of the deposition and leaching of Cl
(A) and Na (B)from the rootzone at the 148 'soil solution ' stands
Cumulative frequency
Cumulative frequency
(%)
deposition
deposition
leaching
(original)
leaching
(adjusted)
1000
2000
3000
4000
5000
Clfluxes (moLha' .yr"1)
0
1000
2000
3000
4000
5000
Nafluxes (moLna' .yr'1)
Fig. 8 Inverse cumulative frequency distributions of the deposition and leaching of Cl
(A) and Na (B)from the unsaturated zone at the 71 'groundwater' stands
and Na in the soil solution under these tree species (Section 4.1) induced by the
influence of sea-spray in autumn-winter as explained in Section 5.2.
39
It is not clear whether the dry summer of 1989,with 100 mm less rainfall than the
annual average amount, also attributed to this fact. Apparently it didn't increase the
concentration of CI and Na in the soil solution under the other tree species. Since
ion concentrations below deciduous tree species will not represent annual average
values, the adjusted precipitation excesses most likely produces more reliable results
for the leaching of S, N, BC and Al from the rootzone.
Table 15 Actual évapotranspiration and precipitation excess of the tree species occurring
at the 148 'soil solution' stands and the 71 'groundwater' stands
Tree species
Soil
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
Precipitation excess
(mm yr"1)
Actual évapotranspiration
(mm yr 1 )
2)
Groundwater
Soil
Groundwater
O"
A
O
A
O
A
O
A
325
325
350
350
325
350
325
319
365
341
337
480
450
440
325
335
350
350
325
350
325
50
68
148
117
368
390
216
225
122
93
276
230
271
222
185
131
106
121
130
156
216
214
131
93
280
219
-
491
471
333
326
237
179
3
>
3)
_3)
11
0 = Original values
A = Adjusted values based on a fitted CI budget
31
Not enough data
2)
The precipitation excesses that were adjusted with the concentration in the
groundwater, approached the original values for oak and larch (beech has not enough
locations) but the values for Douglas fir, Norway spruce and especially the pine
became unexplainably high. It is unlikely that the CI deposition values used are
related to the concentration in groundwater and therefore the original precipitation
excesses are likely tobe morereliable. Consequently, adjusted precipitation excesses
wereused for the rootzone whereas the original values wereused for the unsaturated
zone.Net budgets for CIand Na (Table 16),however, giveunreliable results for the
unsaturated zone when precipitation excesses are not adjusted.
40
Table 16 Median annual deposition (in), leaching (out) and their difference (in-out) of
CIand Nafor the rootzone and the unsaturated zone for different tree species^
CI budget (molc ha ' yr"1)
Na budget (molc ha"1yr"1)
in
out
in
out
in-out
Rootzone:
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
1855
1655
2035
2035
1480
1480
1480
1803
2013
1906
1772
3421
2580
2738
1538
1350
1752
1756
1280
1697
1280
1523
1615
1428
1040
2365
1697
2112
(1558)
15 (-20)
(1319) -265 (31)
(1533) 324 (219)
(1187) 716 (569)
(1005) -1085 (275)
(981) -417 (299)
(1117) -832 (163)
total
1605 2144(1669) -539 (0)
Tree species
Unsaturated zone
Scotspine
Blackpine
Douglasfir
Norwayspruce
Japaneselarch
Oak
Beech2'
total
in-out
(1855)
52
(1655) -358
(2035) 129
(2035) 263
(1480) -1941
(1480) -1100
(1480) -1258
1855 891 (1855) 964
1827 850 (1827) 977
2035 816 (2035) 1219
2035 591 (2035) 1444
1480 1746 (1480) -266
1480 1768 (1480) -288
1660
-
-
941 (1846)
-
(0)
(0)
(0)
(0)
(0)
(0)
(0)
13201634(1204) -314 (116)
(0)
(0)
(0)
(0)
(0)
(0)
-
719 (186)
" Valuesbetweenbracketsareadjustedvaluesaccordingto
foreachtree species
2)
Notenoughdata
1538 644 (1469) 894 (69)
1499 838 (1772) 661(-273)
1760 733 (1946) 1027(-186)
1756 502 (1725) 1254 (31)
1280 1560 (1302) -280 (-22)
1280 1314 (1098)
-34(182)
1444
-
-
803 (1511)
641 (-67)
afittedCIbalance
41
6 Input-Output budgets
Element input is defined here as the deposition of an element that enters the soil
and element output as the amount that percolates with the precipitation excess to
lower soil layers. Positive differences between input and output fluxes indicate
removal from the soil (e.g. uptake by the vegetation, denitrification) or storage in
the soil profile (immobilization, adsorption). Negative differences indicate
mobilization from the soil profile (e.g. weathering, mineralization, desorption). Equal
input and output budgets normally occur with inert elements and elements with which
the soil is saturated.
6.1 Budgets of sulphur and nitrogen
6.1.1 The rootzone
The cumulative frequency distributions of the input- and output fluxes of S and N
of 148forest stands (Figure 9), show that the variation in deposition of S0 4 and N0 3
was small and of NH4 wide whereas the opposite was true for leaching. Leaching
data in Figure 9 were only based on precipitation excesses that were adjusted on the
basis of a fitted CIbalance (cf Table 15).The medians of the input and output fluxes
and the net budgets of the separate tree species (Table 17) show that use of the
original precipitation excesses lead to a large removal of S0 4 from the soil for the
deciduous trees (including Japanese larch) which is very unlikely. Using precipitation
excesses that were calculated according to a fitted CI balance (5.3) the difference
between median input and output fluxes was small for each tree species. S0 4 thus
seems to behave like a tracer.
The median leaching of N was reduced by 80%compared to its median deposition,
using the adjusted values for the precipitation excess.The decrease was smallest for
Douglas fir (63%) and greatest for Norway spruce (90%).The contribution of N0 3
to the deposition of N was relatively small but the average leaching of N0 3 almost
equalled its deposition. Most likely, the occurrence of nitrification compensates
denitrification anduptake. The differences between the tree species, however, were
considerable. NH4contributed most to N deposition, but hardly toNleaching. Nearly
all NH4 was removed by immobilization, nitrification and (preferential) NH4uptake.
43
Cumulative frequency
Cumulative frequency
(%)
deposition
leaching
0
1000
2000
3000
4000
5000
1
0
1000
2000
3000
4000
5000
Nfluxes (mcJ.ha'' .yr'1 )
1
SO^ fluxes (moLha .yr" )
Cumulative frequency
Cumulative frequency
(*)
D
deposition
deposition
leaching
leaching
v.
0
1000
2000
3000
4000
5000
NO, fluxes (moI_.ha' .yr ' )
0
1000
2000
3000
4000
5000
NHt fluxes (mol.ba' .yr'1)
Fig. 9 Inverse cumulative frequency distributions of the deposition and leaching of S04
(A), N (B), N03 (C) and NH4 (D)from the rootzone at the 148 'soil solution'
stands
44
Table 17 Median annual deposition (in), leaching from the rootzone (out) and their
difference (in-out) of SO^ N, N03 and NH4for the tree species occurring at
the 148 'soil solution ' stands^
Tree species
S 0 4 budget (molc ha * yr'
N budget (m( )lc ha J yr 1 )
)
in
out
in-out
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
2013
1926
2646
1798
1286
1283
1403
2001
1924
2493
1822
1024
1771
1415
12
(1935)
(78)
(2371)
2 (-445)
(2339) 153 (307)
(1592) -24 (206)
(2509) 244 (-1223)
(2972) -488 (-1689)
(2569) -12 (-1166)
Total
1744 1773 (2355)
-29
N 0 3 budget (molc ha"1yr"
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
Total
11
999 665 (649) 334
999 981 (1185)
18
1227 1756 (1648) -529
1113 363 (303) 750
769 540 (1277) 229
769 368 (621) 401
852 227 (489) 625
971
700
(999)
271
in
(-611)
l
out
in-out
4092 892 (871)
4954 1157 (1405)
5034 1885 (1768)
4510 453 (382)
3206 651 (1540)
3428 462 (788)
3473 347 (706)
3200(3221)
3797 (3549)
3149 (3266)
4057 (4128)
2555(1666)
2966 (2640)
3126 (2767)
834 (1187)
3325(2972)
4159
NH4 budget (molc ha'1 yr 1 )
)
(350)
(-186)
(-421)
(810)
(-508)
(148)
(363)
3093
3955
3807
3397
2437
2659
2621
227
176
129
90
111
94
120
(222)
(220)
(120)
(79)
(263)
(167)
(217)
2866 (2871)
3779 (3735)
3678 (3687)
3307 (3318)
2326(2174)
2565 (2492)
2501 (2404)
(-28)
3188
134
(188)
3054 (3000)
Values between brackets are calculated with the orginal precipitation excess (not
adjusted) according to a fitted CI balance.
6.1.2 The unsaturated zone
Budgets for S and N in the unsaturated zone are given in Figure 10and Table 18.
Compared to the roötzone, the output fluxes of S0 4 from the unsaturated zone were
somewhat smaller below the coniferous trees, whereas the leaching under Japanese
larch and oak was much larger (when the adjusted precipitation excesses were used
for the rootzone; cf Table 17 and 18). However, it is likely that for the latter tree
species S0 4 in groundwater not only originates from leaching from the rootzone but
also from lateral seepage (wet sites).
The output fluxes of N from the unsaturated zone were mostly smaller than from
the rootzone. NH4 almost disappeared due to nitrification and also the leaching of
N0 3 was less than at the depth of the rootzone. The greatest leaching took place
under Norway spruces and the smallest under black pines.
45
Cumulative frequency
Cumulative frequency
(%)
0
1000
2000
3000
4000
deposition
deposition
leaching
leaching
0
5000
1000
2000
3000
4000
5000
Nfluxes (moKha'.yr'1)
S0 4 fluxes (mol.ba'.yr ' )
Cumulative frequency
Cumulative frequency
(%)
(%)
deposition
deposition
leaching
leaching
40 '
\
0
1000
2000
3000
4000
5000
1
NO, fluxes (moLha' .yr' )
Fig. 10
46
0
\
1000
2000
3000
4000
5000
NH4 fluxes (molo.ha' .yr'1 )
Inverse cumulative frequency distributions of deposition and leaching of S04
(A), N (B), N03 (C) and NH4 (D)from the unsaturated zone at the 71
'groundwater' stands
Table 18 Median annual deposition (in), leaching from the unsaturated zone (out) and
their difference (in-out) of SO^, N, N03 and NH4for the tree species occurring
at the 71 'groundwater' stands
Tree species1'
S 0 4 budget (molc ha 1 yr J )
N budget (molc] ha'1 yr'1)
in
out
in-out
in
out
in-out
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
2198
2093
1789
1817
1267
1283
1802
1828
1228
1098
2723
2628
396
265
561
719
-1456
-1400
4092
4988
4691
4624
3027
3514
303
986
890
147
587
292
3789
4002
3801
4477
2440
3222
Total
1763
1949
-186
4328
490
3838
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Total
N 0 3 budget (molc ha 1 yr l )
NH 4 budget (molc ha x yr_1)
999
1001
1122
1113
774
769
303
986
890
144
587
292
696
15
232
969
187
477
3093
3987
3569
3511
2253
2745
0
0
0
3
0
0
3093
3987
3569
3508
2253
2745
999
490
509
3329
0
3329
11
For beech no budgets are given because only one value was available
6.2 Budgets of base cations and aluminium
6.2.1 The rootzone
The input and output fluxes and the net budgets of Al and the base cations, K, Ca
and Mg for each tree species are given in Table 19. In Figure 11 the cumulative
frequency distributions of the 148 stands are given.
Al deposition is neglectable. Therefor the entire leaching of this ion is caused by
the mobilization of Al (hydroxide). This occurs when other buffering mechanisms
are exhausted and the acidity increases. Figure IIA shows that the range in Al
leaching was large. Al leaching was small under beech and oak, and large under
douglas fir and Norway spruce (Table 19).
Ca leaching and on less account also that of K and Mg is likely to be greater than
the deposition of these ions in acid soils because of weathering and ion-exchange.
In most cases this appeared to be the case. Striking is the great difference between
the output and input of Ca, and to a lesser extent of Mg at the oak stands. Some
locations with oaks have high water tables with probable influences of deep
groundwater (seepage).Another explanation is the possibly larger Ca (and Mg) input
on oak stands which generally border agricultural land.
47
Cumulative frequency
Cumulative frequency
(%)
A
B
deposition
leaching.
leaching.
V
O
1000
2000
3000
4000
5000
1
250
500
1
750
1000
1
Alleaching (moLba .yr" )
Kfluxes (moLha .yr' )
Cumulative frequency
Cumulative frequency
(%)
(%)
D
deposition
deposition
leaching.
leaching.
\
0
1000
2000
3000
4000
5000
Cafluxes (moLha' .yr' )
Fig. 11
48
0
1000
2000
3000
4000
5000
1
Mgfluxes (molJu .yr' )
Inverse cumulative frequency distributions ofthe deposition and leaching of
Al (A), K (B), Ca (C) and Mg (D)from therootzone atthe 148 'soil solution'
stands
Table 19 Median annual deposition (in), leaching from the rootzone (out) and their
difference (in-out) of Al, K, Ca and Mgfor the tree species occurring at the
148 'soil solution 'stands'*
Tree species
Al budget (molc ha'1
1
yr 1 )
K budget (molcha"1
yr 1 )
in
in-out
in
in-ou t
out
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
0
0
0
0
0
0
0
Total
0
1042
1273
2284
1533
897
422
523
(1018)
(1528)
(2140)
(1337)
(2220)
(778)
(830)
-1042
-1273
-2284
-1533
-897
-422
-523
976 (1210)
Ca budget (molc ha"
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
563
475
536
454
320
430
380
863 (837)
556 (668)
512 (480)
496 (435)
470 (1073)
943 (1650)
303 (524)
Total
457
530
(770)
1
out
(-1018)
H528)
(-2140)
(-1337)
(-2220)
(-778)
(-830)
80
75
83
83
60
60
64
179
194
138
98
109
130
119
(174)
(243)
(128)
(86)
(234)
(260)
(203)
-976 (-1210)
75
151
(203)
-99
-119
-55
-15
-49
-70
-55
-76 (-128)
yr 1 )
Mg budget (molc ha i-1 yr1: )
-300 (-274)
-81 (-193)
24
(56)
-42
(19)
-150 (-753)
-513 (-1220)
-77 (-144)
363
313
385
385
280
280
280
320 (310)
334 (412)
430 (400)
369 (306)
257 (629)
525 (909)
382 (641)
308
380
-73
(-313)
(479)
(-94)
(-168)
(-45)
(-3)
(-174)
(-200)
(-139)
43
(53)
-21 (-99)
-45 (-15)
16
(79)
23 (-349)
-245 (-629)
-102 (-361)
-72 (-171)
" Values between brackets are calculated with the original precipitation excess
(not adjusted according to a fitted CI balance).
6.2.2 The unsaturated zone
The output fluxes of AI, K, Ca and Mg that were calculated for the unsaturated zone
were quite comparable to those that were calculated for the rootzone (cf. Fig. 11and
Fig. 12).Al leaching was nearly equal for black pine, Japanese larch and oak, larger
for Scots pine and lower for Douglas fir and Norway spruce (Table 20).In the last
case this might be due to precipitation of Al-hydroxide, induced by BC weathering,
and by exchange of Al with adsorbed Ca and other base cations deeper in the soil
profile. Compared totherootzone median Ca leaching of all stands was nearly equal,
Mg leaching was larger and K leaching was less in the unsaturated zone. The
calculated leaching of K, Ca and Mg from the unsaturated zone under Douglas fir
and Norway spruce was even smaller than the deposition of theseions.Striking, again
is the extremely large Ca leaching below oak stands which is definitely influenced
by seepage at wet sites.
49
Cumulative frequency
Cumulative frequency
(%)
B
deposition
leaching.
leaching .
X,
\
0
1000
2000
3000
4000
5000
250
500
Alleaching (moLha' .yr')
750
Kfh»es (moLha' .yr ')
Cumulative frequency
Cumulative frequency
(%)
0
1000
2000
3000
4000
5000
Cafluxes (moLba' .yr' )
Fig. 12
50
1000
D
deposition
deposition
leaching.
leaching.
0
1000
2000
3000
4000
5000
Mgfluxes (moKha' .yr')
Inverse cumulative frequency distributions ofthe deposition and leaching of
Al (A), K(B), Ca (C) and Mg (D)from theunsaturated zone atthe71
'groundwater' stands
Table 20 Median annual deposition (in), leaching front the unsaturated zone (out) and
their difference (in-out) of Al, K, Ca and Mgfor the tree species occurring at
the 71 'groundwater' stands
Tree species1'
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
Al budget (molc ha* yr')
K budget (molc ha J yr ] )
in
in
0
0
0
0
0
0
Total
out
out
in-out
1670
1283
1553
918
864
386
-1670
-1283
-1553
-918
-864
-386
75
75
83
83
60
60
86
133
63
47
135
139
-11
-58
20
36
-75
-79
1054
-1054
75
98
-23
Ca budget (molc ha"1yr'1)
in-out
Mg budget (molc ha'1 yr'1)
Scots pine
Black pine
Douglas fir
Norway spruce
Japanese larch
Oak
563
512
495
447
320
440
516
564
264
306
832
2119
47
-52
231
141
-512
-1679
363
350
385
385
280
280
269
796
269
341
1007
1143
94
-446
116
44
-727
-863
Total
454
520
-66
320
484
-164
" For beach no budgets are given because only one value was available
51
7 Discussion and conclusions
Results of input-output budgets for seven major Dutch forests types are given in
this report as medians to mimic the large uncertainty in deposition and leaching
estimates of the elements. However, even these results may strongly be influenced
by methodological errors, i.e.:
1. Concentrations inMarch and April are only an approximation of the flux-weighted
annual average concentration. Especially for S0 4 , N0 3 and NH4, correlations
between concentrations in these months and annual average concentrations appear
to be rather weak (R2adjbetween 0.42 and 0.60). For Al and Ca, the correlation
is better but here the regression coefficients indicate that Ca concentrations in
March and April are likely to be higher than the annual average concentration
whereas the opposite is true for Al (cf De Vries et al., 1994);
2. Centrifuge soil solutions deviate from lysimeter soil solutions.For example base\ J',. ff^
cation (Ca) concentrations in soil solution obtained by centrifugation (soil survey| A?
sites) are generally higher than base cation (Ca) concentrations in soil solution | yy,fß
obtained by suction cups, whereas the opposite is true for Al (Zabowski and f . v S
Ugolini, 1990; Verhagen and Diederen, 1991). These two aspects may have j v'""*' "''"
caused that the Al mobilization is underestimated whereas BC mobilization is
^.tf yt/f,<J)
overestimated;
3. The use of a Cl-corrected precipitation excess, to correct for concentration
deviations from annual average values, may cause errors in leaching estimates
inforest-soil combinations with a short soil water residence time in therootzone.
CIdeposition ishighly variable over the year and high CIconcentrations in the
subsoil may there be caused by a considerable CI deposition in the beginning of
1990. Calculated residence times for the various tree species were mostly more
than one year for the coniferous trees, whereas it was often less than one year
for deciduous trees. The correction in precipitation excess for deciduous trees
may thus be too large causing an underestimation in leaching estimates. However,
combining the original precipitation excesses with measured S0 4 concentrations
would result in a large removal of S0 4 from the soil for the deciduous trees
(including Japanese larch), which is very unlikely;
4. The deposition is derived from grid squares for which yearly sums have been
determined. For some ions seasonal variations are great. The filtering factors that
are used to obtain the bulk deposition from the total or wet deposition are based
upon scant research through which particularly the filtering factors for the base
cations are roughly assessed;
5. The groundwater is withdrawn 50-100 cmbelow the groundwaterlevel and forms
a mix of the precipitation excess of often more than a year. The point of time
that it infiltrated via the soil surface was several years before. The sampled
groundwater therefore seems a reliable indication of the average leaching.
However, at greater depth the chance increases that the samples are influenced
by the horizontal groundwaterflux and thus represent a greater area.
Despite theuncertainties involved, the summary of thebudgets of all species (Table
21), warrants the general conclusions that:
1. S0 4 behaves as a tracer;
53
A.
I / k i A\ê-^
v
L
2. N is strongly retained (especially NH 4 );
3. Al mobilization is the dominant buffer mechanism and
4. most interactions occurred in the rootzone.
The increased N (NH 4 and N0 3 ) retention between rootzone and groundwater is
conform the expectation (denitrification). For all cations, i.e. Al, K, Ca and Mg, an
^increased mobilization was expected between rootzone and groundwater due to
weathering, but this was only the case for Al and Mg. This is, however, due to the
reliability of data (see above).
J rvj'i -f/ Table 21 Summary of the net budgets (input-output) of major ionsfor the rootzone and
unsaturated zone
»y- •
,-0 'j*d*
Element
so 4
•u*-
N
N03
NH4
Al
K
Ca
Mg
Net budgets (molc ha'1 yr 1 )
Rootzone1'
Unsaturated
zone2'
Unsaturated - Rootzone
zone
-29
3325
271
3054
-976
-76
-73
-72
-186
3838
509
3329
-1054
-23
-66
-164
-157
513
238
275
-78
-53
7
-92
11
Values according to a fitted CI balance
Values calculated with original precipitation excesses
21
The uncertainties involved affect reliable conclusions about input-output budgets for
each tree species. For example, the (extremely) large leaching fluxes of S 0 4 and base
cations (especially Ca and Mg) from the unsaturated zone of the deciduous trees
(especially oak) are likely influenced by seepage since a relatively large part of these
tree species is located on wet soils. One conclusion that can be drawn however is
that Al mobilization both in the rootzone and unsaturated zone, clearly decreases
going from spruce trees (Douglas fir, Norway spruce) to pine trees (Scots pine, black
pine) to deciduous trees (Japanese larch, oak and beech). This is conform the
expectation since atmospheric deposition generally decreases in the same direction
due to a decrease in forest filtering induced by a lower canopy coverage.
54
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58
Annex On interception and évapotranspiration of the forest
stands
Interception
Using models, Nonhebel (1987) determined the interception and transpiration of
different types of forests at five places inthe Netherlands: De Kooy,Eelde, DeBilt,
Vlissingen and Zuid Limburg. For the transpiration he distinguished sand and clay
soils.The interception as percentage of theprecipitation over the yearremained rather
stable for the different types of forest, although there were differences up to 18%
between the two models he used (Mulder, 1985 and Gash, 1979).In Table 1.1the
averages of both models for six forest types are given for places in the eastern parts
of the Netherlands during the period 1974-1978.
Table LI Calculated average interception lossfor different tree species during the period
1974-1978 as percentage of the annual precipitation at three places in the
eastern part of the Netherlands (from Nonhebel, 1987)
Tree species
Scots pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
Interception (%)
De Bilt
Eelde
Zuid -Limburg
Average
33
55
56
22
19
18
38
61
60
27
24
22
35
57
56
24
21
20
35
58
57
24
21
20
The interception percentages for spruce and douglas fir that are given in Table1.1
are very high. It implies that in a year with an average amount of precipitation
(780 mm) there is only about 325 mm available for evaporation and transpiration
which seems unlikely. A much lower average interception percentage for douglas
fir was derived byDe Visser and de Vries (1989) who computed water balances for
different tree species with the model SWATRE (Belmans et al., 1983). For Scots
pine, douglas fir and deciduous forests they found an annual average interception
loss of 39%, 33%and 23%of the precipitation, respectively. Based on their data
and on a literature survey of Hiege (1985) we used the following interception
percentages: 20%for oak, 25%for Japanese larch, 30%for beech, black and Scots
pine, 40% for douglas fir and 45% for Norway spruce (cf De Vries, 1991).
Evapotranspiration
In Table 1.2 results are given of the sum of actual evaporation and transpiration of
several tree species as computed by Nonhebel (1987).He found that differences in
transpiration between forests on sand and clay were small (about 15 mm yr"1).
Transpiration of forest stands on sandy soils was reduced at a moisture deficit of
more than 70 mm. During the considered period 1974-1978 this only occurred in
1976.However, the differences between the succeeding years were small. This was
confirmed by Hendriks et al. (1990) with measured data for American oaks.
59
Table 1.2 The average annual évapotranspiration for different tree species on sand
during the period 1974-1978 (from Nonhebel, 1987)
Tree species
Scots pine
Douglas fir
Norway spruce
Japanese larch
Oak
Beech
Evapotranspiration (mm yr 1 )
De Bilt
Eelde
Zuid •Limburg
Average
189
237
300
206
286
211
170
208
267
192
267
195
183
233
314
198
293
202
181
226
294
199
282
203
De Visser and de Vries (1989) computed much larger values for actual evaporation
and transpiration. Furthermore, they found considerable differences in actual
transpiration for different soil types and groundwaterlevel classes. For example,
transpiration values for adry, coarse sand and a wet, fine sand varied between 225
and 375 mm for Scots pine, between 250 and 375 mm for Douglas fir and between
265 and 380 mm for a deciduous stand. The actual evaporation varied from 55 to
75 mm. Although Nonhebel did not give an explanation for the small transpiration
amounts, thedifferences areprobably partly due to assumed hydrological conditions.
The transpiration amounts of Visser and de Vries fit better with actual measured data
(Hendriks et al., 1990).Based on their results weused the following data for the sum
of the average actual transpiration and evaporation: 325 mmfor beech, black pine,
Scots pine and Japanese larch and 350 mm for Douglas fir, Norway spruce and oak.
60