FEMS Microbiology Ecology 53 (2005) 33–40
www.fems-microbiology.org
Degradability of dissolved soil organic carbon and nitrogen
in relation to tree species
Oili Kiikkilä *, Veikko Kitunen, Aino Smolander
Finnish Forest Research Institute, P.O. Box 18, FIN-01301 Vantaa, Finland
Received 19 May 2004; received in revised form 17 August 2004; accepted 19 August 2004
First published online 2 November 2004
Abstract
The degradability and chemical characteristics of water-extractable dissolved organic carbon (DOC) and nitrogen (DON) from
the humus layer of silver birch (Betula pendula Roth), Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.)
stands were compared in short-term incubation of soil solutions. For all extracts the degradation of DOC and DON was low
(12–17% loss) and increased in the order: birch, spruce and pine. In the humus layer under pine a relatively larger pool of rapidly
degrading dissolved soil organic matter (DOM) was indicated by the [3H]thymidine incorporation technique, which measures the
availability of DOM to bacteria. The degradation of DOC was explained by a decrease in the hydrophilic fraction. For DON, however, both the hydrophilic and hydrophobic fractions tended to decrease during incubation. No major differences in concentrations
of hydrophilic and hydrophobic fractions were detected between tree species. Molecular size distribution of DOC and DON, however, revealed slight initial differences between birch and conifers as well as a change in birch extract during incubation. The depletion of very rapidly degrading fractions (e.g., root exudates and compounds from the litter) may explain the low degradability of
DOM in the humus layer under birch.
2004 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies.
Keywords: Degradation; DOC; DON; Chemical characteristics; Tree species
1. Introduction
Dissolved soil organic matter (DOM) contains organic compounds from rainwater and throughfall, root exudates, microbial metabolites, and decomposed litter.
DOM continuously undergoes various processes such
as precipitation, mechanical filtering, adsorbtion, ionexchange and degradation. DOM represents a range of
C- and N- containing molecules that vary in degradability [1,2]. One way to characterize DOM is to fractionate
it into acids, bases and neutrals with hydrophobic and
hydrophilic groups [1,3,4]. The hydrophilic neutral frac*
Corresponding author. Tel.: +358 010 211 2598; fax: +358 010 211
2206.
E-mail address: Oili.Kiikkila@metla.fi (O. Kiikkilä).
tion, which includes the carbohydrates, has been reported to be the most degradable fraction [4,5] and the
hydrophobic acid fraction the least degradable fraction
[1,4]. DOM can also be characterized according to
molecular size distribution, which has been studied very
little [6,7]. However, we might expect that smaller DOM
molecules or units would be degraded preferentially [6].
Birch (Betula pendula Roth or B. pubescens Ehrh.), as
compared to spruce (Picea abies (L.) Karst.) and pine
(Pinus sylvestris L.), has a reputation in forest history
as a tree species that improves soil conditions [8]. Soils
under deciduous trees have often shown higher mineralization of C than soils under coniferous trees [7,9–12].
Explanations given for the differences between birch
soils and coniferous soils have included differences in
ground vegetation cover, chemical composition of leaf
0168-6496/$22.00 2004 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies.
doi:10.1016/j.femsec.2004.08.011
34
O. Kiikkilä et al. / FEMS Microbiology Ecology 53 (2005) 33–40
litter, vertical distribution pattern of roots, root activities such as root exudation, physical structure of soil,
and microclimatic conditions [8]. A major pathway for
the mineralization of C and N in soils is often thought
to be the turnover of dissolved organic carbon (DOC)
and nitrogen (DON) [13]. The DOC and DON concentrations and the C and N mineralization were all higher
in the humus layer under birch than under pine [7]. Extracts of deciduous litter [14] and humus layer [7,15] had
higher concentrations of DOC than did extracts of
coniferous litter and the humus layer, respectively. However, the degradability of DOM can also contribute significantly to C and N mineralization [6]. Very little is
known about the degradability of DOM in forest soils
or about its significance in, e.g., regulating N
availability.
Conifer litter contains more recalcitrant, hydrophobic aromatic compounds [16], whereas leaf litter contains more easily degradable hydrophilic compounds:
sugars, amino acids and aliphatic acids [14]. These differences in litter composition are not directly reflected in
dissolved soil organic matter, because the composition
of DOM is also affected by, e.g., the decomposition rate
of different compounds. No major differences in the proportions of hydrophilic and hydrophobic compounds,
or in molecular size distribution of DOM were observed
in the humus layer under different tree species [7,15].
Depending on the specific compounds included in the
hydrophilic or hydrophobic fractions, however, the degradation of DOM can differ between the tree species
[14].
The aim of this study was to compare the degradability of water-extractable DOC and DON in the humus
layer under silver birch and two conifers, Scots pine
and Norway spruce, and to explain this degradability
according to the chemical characteristics of DOC and
DON. Degradability was assessed in an incubation
experiment by measuring loss of DOC and DON, and
the availability of DOM to bacteria by the [3H]thymidine incorporation technique. Chemical characteristics
of DOM were studied by fractionation of DOC and
DON into hydrophilic and hydrophobic compounds,
and into four groups with different molecular sizes.
2. Materials and methods
2.1. Study site
The study stands, which had originally been a
homogenous Norway spruce stand, were adjacent 70year-old stands in Kivalo, northern Finland (6620 0 N/
2640 0 E), that were dominated by silver birch, Scots pine
or Norway spruce. The soil type was podzol, and the site
type was Hylocomium-Myrtillus [17]. Three study plots
(25 m · 25 m) were placed in each stand. The birch stand
was a pure single-species stand, but the coniferous
stands also contained species other than the dominant
one. The history and characteristics of the tree stands
have been described earlier in more detail [7].
2.2. Soil sampling
Soil samples (core diameter 58 mm) were taken systematically on 9th October 2002, three weeks after the
litter fall of birch, from the humus layer of all plots.
After the green plant material was removed, the samples
were sieved through a 4.0 mm mesh and stored in plastic
bags at 2 C. The content of organic matter was measured as loss-on-ignition at 550 C.
2.3. Incubation experiment
Soil samples (for birch and spruce 600 ml, for pine
900 ml to obtain similar concentrations of C) were shaken in 1.5 l ultra-pure water for 2 h (200 rpm). After centrifugation at 28,000g for 20 min, the suspensions were
finally filtered through 0.2 lm PES membrane as described previously [7]. The extracts were stored in a freezer until incubation.
Before incubation, all extracts were diluted to a concentration of 23 mg l1 DOC (SD = 2, n = 9). The extracts (360 ml) were placed in autoclaved 1 l glass
bottles sealed with rubber septa. pH was adjusted to
the level of the extract with the highest pH (5.1) with
0.1 M NaOH. The mixture of all soils, equal to the
amount of organic matter (3 g wet weight), was crushed
in 100 ml water, and 0.5 ml of the suspension was added
to the sample bottles as inoculum. The amount of DOC
added with the inoculum was 2 mg l1. The bottles were
shaken (80 rpm) in the dark at 20 C. On the first days,
the CO2 evolution was measured daily by gas chromatography (HP 6890) and twice a week thereafter. After
the samples were measured, they were aerated. After
15 days, half of the extracts were frozen until further
analysis. The rest of the extracts were incubated for 30
days after NH4Cl was added to obtain a C:N ratio of
10. The samples were incubated with two laboratory
replicates. CO2 evolution was determined for both samples. Other measurements were performed on the combined samples of the two replicates. Molecular size
distribution was determined and the samples were fractionated into hydrophilic and hydrophobic groups initially and after the 15-day incubation.
The initial and remaining concentrations of DOC and
DON were determined as described previously [7] and
the degradability was expressed as the percentual loss
from the initial concentration of DOC (DOCloss%) or
DON (DONloss%). Molecular size distributions of
DOC and DON were determined by ultrafiltration
(The Amicon 8050 stirred cell system) using AmiconÕs
diaflow-membranes YM100, YM10 and YM1 with
O. Kiikkilä et al. / FEMS Microbiology Ecology 53 (2005) 33–40
nominal weight cut-offs at 100, 10 and 1 kDa, respectively. Of the load volume (50 ml), about 70% was filtered. DOC and DON were fractionated into
hydrophilic and hydrophobic groups using the procedure described previously [3,18]. The pH was adjusted
to 2, and 50 ml of the extract was passed through an
XAD-8 (Amberlite XAD-8, Merck) resin column to adsorb the hydrophobic compounds.
To assess the availability of DOM to bacteria the
[3H]thymidine incorporation technique [19,20] was applied to measure the rate of the bacterial growth. Bacterial suspension was prepared by shaking (250 rpm) 3 g
(wet weight) of a mixture of all soils, equal to the
amount of organic matter, in 100 ml of ultrapure water
for 1 h. The bacterial suspension was centrifuged for 10
min (750g) and filtered through quartz wool. Then 8 ml
of the extract (TdRsample) or distilled water (TdRwater)
was added to 6 ml of the bacterial suspension. Preliminary experiments (3, 20, 48 and 96 h) were made in order
to determine the time required to reach the peak in the
[3H]thymidine incorporation rate (TdR) after the addition of substrate to the bacterial suspension. TdR was
measured after the extract was shaken (100 rpm) at 20
C for 23 h. 1.4 ml was taken into microcentrifuge tubes,
3.5 ll methyl[3H]thymidine (25 Ci mmol1, 925 GBq
mmol1) was added and the samples were incubated
for 2 h at 22 C. Then 70 ll ice-cold 100% TCA was
added and kept at 4 C for 30 min. The samples were
centrifuged (17,400g) for 10 min and the supernatant
was aspirated. The two washing steps consisted of adding 1.4 ml of cold 5% TCA and ice-cold 80% ethanol.
After centrifugation and removal of the supernatant,
0.2 ml of 1 M NaOH was added, and the samples were
maintained at 90 C for 1 h. A 1 ml scintillation cocktail
was added, and the radioactivity was measured using a
liquid scintillator. Availability of DOM to bacteria
was calculated by dividing TdRsample by TdRwater
(TdR). TdR values >1 mean that DOM was a substrate
for bacteria, the higher the value the better the substrate.
Values <1 mean that DOM inhibited bacteria.
2.4. Data analyses
Paired samples t test was used to determine significant differences between the initial and after incubation
values. One-way ANOVA followed by an LSD test was
used to determine the differences between the tree species both before and after the incubation. Pearson correlation coefficients were calculated. To normalize the
distribution of the variables logarithmic transformations
were made to TdR and the C:N ratio data. Statistical
significance for all tests was set at p < 0.05.
Data on molecular size distribution was explored
with non-metric multidimensional scaling (MDS), which
considers the rank order of distances, using a proportional Bray–Curtis coefficient for computing the pair-
35
wise distances. Program package PC-ORD [21] was
used. Before MDS, the values were double square-root
transformed (y0.25) to down weight the influence of very
abundant size classes.
3. Results
The concentration of total water-extractable DOC,
when calculated on the basis of soil organic matter
(o.m.), was significantly higher in the humus layer under
birch (1300 ± 70 mg kg1 soil o.m., means ± SE, n = 3)
and spruce (1100 ± 50 mg kg1 soil o.m.) than under
pine (710 ± 40 mg kg1 soil o.m.). The total amount
of DOC calculated per unit soil organic matter degraded
during the incubation tended to be slightly higher in
birch and spruce soils (150 ± 20 mg kg1 soil o.m.) than
in pine soil (120 ± 10 mg kg1 soil o.m.). During incubation, when the initial DOC concentrations were adjusted
to be equal (23 mg l1), the proportional degradation of
DOC (DOCloss%), however, was highest in pine
(17.2 ± 1.2%) and lowest in birch soil (11.8 ± 1.0%)
(Fig. 1(a)). N addition increased the degradation of
DOC, which was finally highest in spruce and lowest
in birch soil (Fig. 1(a)). Rather similar results were observed when degradation of DOC was monitored as
CO2C production (Fig. 2). DOCloss% correlated moderately with CO2–C loss (r = 0.70, n = 18; after incubation and N addition and incubation).
The concentration of total water-extractable DON
was higher in birch (48 ± 3 mg kg1 soil o.m.) and
spruce soil (55 ± 6 mg kg1 soil o.m.) than in pine soil
(28 ± 1 mg kg1 soil o.m.). The proportional degradation of DON (DONloss%) followed the same order as
the degradation of DOC, being highest in pine soil
and lowest in birch soil (Fig. 1(b)). After N addition
the degradation was highest in spruce soil and lowest
in birch soil. The C:N ratio of DOM was initially at
the same level in all soils (Table 1). After N addition
and incubation, the C:N ratio of the spruce extract
was significantly higher than that of birch and pine soils;
thus after N addition the degradation of DON increased
most in the spruce extract.
The concentrations of both the hydrophilic and
hydrophobic fractions of DOC were initially at the same
level in all soils (Fig. 3). The correlation between DOCloss% and the initial proportion in the hydrophilic fraction of the total DOC was moderate (r = 0.71, n = 9). In
the hydrophilic fraction the decrease detected in birch
(14%) and pine soils (35%) was significant. The concentration of the hydrophobic fraction remained about the
same during incubation.
The concentration of the hydrophilic fraction of
DON did not differ significantly between the tree species, but the hydrophobic fraction was significantly
higher in birch soil than in pine soil (Fig. 3). DONloss%
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O. Kiikkilä et al. / FEMS Microbiology Ecology 53 (2005) 33–40
30
b*
20
a*
100
*
*
DONloss%
DOCloss%
*
ab*
10
*
80
60
40
*
a*
ab*
b*
20
0
0
15 d incubation
(a)
N addition and
incubation
Birch
15 d incubation
(b)
Spruce
N addition and
incubation
Pine
Fig. 1. Proportional degradation (mean and SE) of: (a) DOC (DOCloss%) and (b) DON (DONloss%) after 15 d incubation, and after N addition
and incubation in the humus layers under birch, spruce and pine. Significant differences between the means for different tree species are marked with
different letters. Significant differences in the concentrations of DOC and DON between the initial and after incubation values are marked with *.
did not correlate with the concentrations of either of the
fractions. The hydrophilic fraction of DON decreased
4% in birch soil, 31% in pine soil, and 25% in spruce soil,
but the decreases were not significant. The hydrophobic
Fig. 2. Proportional degradation of DOC (mean and SE) measured as
CO2 evolution during the incubation in the humus layers under birch,
spruce and pine.
fraction of DON also decreased slightly in all soils but
decreased significantly only in birch soil (20%). The
C:N ratio of the hydrophilic fraction was much lower
(ca. 10) than that of the hydrophobic fraction (32–44).
The C:N ratio of the hydrophobic fraction of birch
was significantly lower than that of spruce or pine.
The C:N ratio of the hydrophilic fraction remained at
the same level during incubation, while the C:N ratio
of the hydrophobic fraction tended to increase (not significantly) in all soils (Table 1).
Initially, the availability of DOM to soil bacteria
(TdR) was clearly highest in pine soil (Fig. 4). During
incubation TdR decreased clearly and significantly in
pine soil (44–77%) and to a lesser extent in spruce soil
(12–31%, not significant) and little or not at all in birch
soil, indicating a relatively large pool of rapidly degrading DOM in pine soil. After N addition and further
incubation, TdR was very low, even negative in pine,
and had decreased significantly in all soils (Fig. 5).
The correlation between the change in TdR and DOCloss% (r = 0.76, n = 18; after incubation and after N
addition and incubation) was moderate. TdR correlated
Table 1
Mean and standard error (n = 3) of the C:N ratio of the total DOM and the hydrophilic and hydrophobic compounds of DOM extracted from the
humus layer under birch, spruce, and pine
C:N
Initial
15 d incubation
N addition and incubation
Total
Birch
Spruce
Pine
23.1 (1.5)
26.6 (0.9)
24.5 (1.4)
22.9 (1.4)a
28.0 (0.9)b
26.4 (1.4)ab
33.7 (4.6)a
65.2 (12.9)b*
34.3 (4.8)a
Hydrophilic compounds
Birch
Spruce
Pine
10.1 (2.0)
11.5 (0.3)
10.1 (1.0)
10.4 (2.6)
12.1 (1.6)
9.5 (1.0)
Nd
Nd
Nd
Hydrophobic compounds
Birch
Spruce
Pine
32.3 (0.5)a
41.5 (2.7)ab
43.7 (4.9)b
38.7 (1.2)a
48.3 (4.1)ab
51.3 (1.2)b
Nd
Nd
Nd
The C:N ratio was determined initially, after 15 d incubation, and after N addition and incubation. Means followed by a different letter differ
significantly within tree species (LSD, p < 0.05). Nd, not determined.
* n = 2.
O. Kiikkilä et al. / FEMS Microbiology Ecology 53 (2005) 33–40
Hydrophilic
Hydrophobic
18
*
6
*
4
2
DOC (mg/l)
DOC (mg/l)
8
12
6
0
0
(a)
Birch
Spruce
Pine
(b)
Birch
Pine
0.8
DON (mg/l)
0.8
DON (mg/l)
Spruce
Hydrophobic
Hydrophilic
0.6
0.4
0.2
a
0.6
A*
0.4
ab AB
b
B
0.2
0.0
(c)
37
0.0
Birch
Spruce
Pine
Initial
(d)
Birch
Spruce
Pine
15 d incubation
Fig. 3. Concentrations (means and SE) of hydrophilic and hydrophobic compounds of DOC and DON in the humus layers under birch, spruce and
pine initially and after 15 d incubation. Significant differences between the means for different tree species are marked with different letters (capital
letters after-incubation). Significant differences between the initial and after 15 d incubation values are marked with *.
10.0
b
Spruce
*
*
1.0
Initial
15 d incubation N addition and
incubation
Birch
Spruce
Initial
TdR
*
15 d incubation
Pine
*
DIM2
a
Birch
a
Pine
0.1
Fig. 4. Availability of DOM to bacteria (mean and SE) (TdR) initially,
after 15 d incubation and N addition and incubation. Significant
differences between the means for different tree species are marked with
different letters. Significant differences between initial and afterincubation values are marked with *. Note the logarithmic scale.
positively with the proportions of hydrophilic DOC
(r = 0.63, n = 17; initial and after incubation) and hydrophilic DON (r = 0.54, n = 17) of the total DOC or DON,
respectively.
MDS of the molecular size distribution of DOC and
DON initially separated birch from conifers and during
incubation showed a change in birch soil along dimension 2 (Fig. 5). After incubation, all plots were located
close together, indicating that the molecular size distribution was very similar in all soils. The initial difference
between birch and conifers seemed to be explained by
two size classes. The 10–100 kDa size class of both the
DOC and DON of birch soil increased (not significantly)
during incubation. For DOC the increase was from
DIM1
Fig. 5. Multidimensional scaling ordination along the first two
dimensions (DIM 1 and DIM 2) of the molecular size distribution
with respect to incubation and tree species.
7.0 ± 2.7 to 11.1 ± 0.9 mg l1 and for DON from
0.19 ± 0.07 to 0.34 ± 0.06 mg l1. Another difference
was in size class 1–10 kDa, which initially was significantly higher in birch than in conifers and in birch soil
decreased significantly from 9.4 ± 3.3 to 4.3 ± 0.4 mg
l1 DOC and from 0.29 ± 0.07 to 0.14 ± 0.03 mg l1
DON. Initially the C:N ratios of the 1–10 and 10–100
kDa size classes were higher (33–48) than those of the
<1 or >100 kDa size classes (10–28).
4. Discussion
The degradation of DOM, that is, mineralization and
microbial incorporation, was low in all soils; during the
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O. Kiikkilä et al. / FEMS Microbiology Ecology 53 (2005) 33–40
15-day incubation, only 12–17% of the DOC and 12–
25% of the DON was degraded. Earlier studies have reported 9–93% degradability of forest soil-derived DOC
measured by various methods of sampling and incubation [13,16]. DOC can be divided into recalcitrant,
slowly degradable (stable), easily degradable (semi-labile), and very rapidly degradable (labile) fractions [6].
Labile DOC has seldom, possibly never, been measured
completely in degradation studies [22]. Higher concentration of total water-extractable DOC was reported in
the humus layer under birch than under pine [7], which
was also the case in this study. We also expected the
amount of DOC degraded during the 15-day incubation,
which we might call semi-labile DOC, to be higher in
birch soil extract than in pine soil extract. However, it
was only slightly higher. Moreover, the proportional
degradability of DOC (DOCloss%) was clearly lower
in birch soil than in pine soil. However, we measured
only part of the annual DOM flux in the field.
Several points support the possibility that the laboratory incubation used here gave degradability values lower than the actual values in the field, especially for the
humus layer under birch. First, the amount of stable
DOM extracted from the humus layer under birch
may be higher than that of pine. This was supported
by the result that by the end of incubation the availability of DOM to bacteria (TdR) had decreased sharply in
pine soil but hardly at all in birch soil. The stable DOM
could also be more degradable in the humus layer under
birch than under pine [16]. Second, fresh leaf litter has
been suggested to be the primary source of DOM [23].
Part of the semi-labile DOM originating from litter
may have been depleted in the field late in the autumn
(three weeks after the leaf fall of birch) or during
processing of the soil, especially in the humus layer under birch. Previously, the hydrophilic fraction was found
to be depleted and the hydrophobic fraction to be largest in the autumn [24]. This was supported by the fractionation results. The easily degradable hydrophilic DOC
and DON tended to be slightly lower, whereas the more
recalcitrant hydrophobic DON was higher under the humus layer of birch than under that of pine. Third, labile
DOM may be a larger pool of DOM than the stable or
semi-labile DOM. The degradation of amino acids,
which were calculated to constitute 10–20% of the total
DON, was extremely rapid, turning over 20 times a day
[25]. Root exudates such as carbohydrates and amino
acids have been suggested to be the primary source of labile DOM in forest soils [26]. Birch soil contains more
root exudates than conifer soils do [27], and the composition of root exudates can differ between tree species
[28]. Therefore, the measures of DOM in laboratory
incubation may differ considerably from the field situation, especially in birch soil.
In accordance with earlier studies [1,4], the degradability of DOC was explained by the loss of the hydrophi-
lic fraction. Hydrophilic compounds are, in general,
more degradable than hydrophobic compounds; but
they can also include recalcitrant compounds such as
some hydrophilic acids [4,14]. Therefore, only moderate
correlations were expected between the initial proportion of hydrophilic compounds of total DOC and DOCloss% or TdR. With regard to DON, both the
hydrophilic and hydrophobic fractions appeared to degrade slightly.
Molecular size distribution showed only slight differences between birch and conifers. In conifer soils during
incubation all size classes decreased about the same
amount. In birch soil a decrease was detected in the
smaller molecular size (1–10 kDa) and an increase in
the larger molecular size (10–100 kDa) class of both
DOC and DON. Thus, birch apparently differed from
the conifers. Very little is known about the link between
the degradability and the molecular size distribution of
DOM. Molecular size may be only a secondary attribute
controlling DOM degradability, the primary factor
being structural characteristics [6,29]. On the other
hand, labile or semi-labile compounds of low molecular
size are assumed to be released continuously from
decaying plant materials [30] or microorganisms [31].
The availability of DOM to bacteria as measured
with the [3H]thymidine incorporation technique, seems
to be compatible with other measures of DOM degradation. The correlation between the change in TdR and
DOCloss% was moderate. TdR measurement gave a larger pool of degradable DOM than did the other measures of degradability, which is in accordance with
previous results [5]. Thus, the thymidine incorporation
technique can be applied in studying the availability of
DOM to bacteria. Fungi might be more important than
bacteria in DOM decomposition in soil. However, because in soils the degradation is enhanced by factors
such as highly active biofilms on particle surfaces, the
degradability of DOM with solution assay reflects
mainly bacterial activity [31].
The C:N ratio of the hydrophobic fraction tended to
increase slightly during incubation (Table 1), thus showing relatively greater decomposition of DON than of
DOC. No preference was detected in the hydrophilic
fraction, which, however, was clearly more N-rich than
the hydrophobic fraction. Few studies have investigated
the degradation of DON in soil. The DON of forest soil
percolates was found to be as refractory as the DOC [1],
whereas in agricultural soil greater decomposition of
DON than DOC was observed [32]. The differences
can be partly explained by the different C:N values. In
the forest soil percolates the C:N ratio was 43–55 [1],
in the agricultural soil it was 13 [32], while we obtained
21–24.
Addition of nitrogen increased the degradability of
DOC in pine soil and especially that of spruce soil,
but to a lesser extent that of birch soil. The addition
O. Kiikkilä et al. / FEMS Microbiology Ecology 53 (2005) 33–40
of mineral N also seemed to increase the degradability of
initial DON because for all soils the C:N ratio increased.
After N addition the degradability of DON seemed to
increase most in spruce soil. This is not surprising since
initially the C:N ratio was highest in spruce soil, which
thus seemed to suffer most from N deficiency.
In conclusion, the higher microbial biomass and
activity in soil under birch than under pine, which has
been shown previously, could not be explained by differences in the degradability of DOM or by the availability
of DOM to bacteria. The lowest degradability of DOM
and availability of DOM to bacteria were detected under birch. However, only part of the annual DOM flux
in the field was measured. More research on stable
DOM, DOM originating from litter of trees and ground
vegatation, and highly labile DOM, such as root exudates, is needed.
Acknowledgements
We are grateful to Prof. Eino Mälkönen and Pekka
Välikangas for establishing the field experiments, to
Anneli Rautiainen for excellent laboratory work and,
to Anne Siika for the figures and to Dr. Joann von Weissenberg for checking the English language of this paper.
The Academy of Finland supported this work financially (Grant No. 541221).
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