How does drainage affect the nitrogen
cycle in organic soils?
Anna-Karin Björsne
Uppsats för avläggande av naturvetenskaplig kandidatexamen i
Miljövetenskap
15 hp
Institutionen för växt- och miljövetenskaper
Göteborgs universitet
Juni 2010
Abstract
Nitrogen (N) is one of the most abundant elements on Earth and its different forms can be
summarized in the N cycle. The increased anthropogenic input of reactive N has altered the
global N cycle with large consequences for sensitive ecosystems where N saturation and overfertilization can be a problem. The aim of this study was to investigate how N transformations
differs between a drained and an undrained peatland, in order to give an increased
comprehension of how water content in soil affects the N fluxes. Samples were taken from
two sites in Västra Götaland, an undrained (UP) and a drained peatland (DP). Pools of NH4+
and NO3- were labelled with the isotope 15N according to the isotope dilution technique, and
other soil properties were measured. The C:N ratio was slightly higher in UP, while the pH
value was lower. UP had a high background content of NH4+, which corresponded to higher
mineralization rates in that soil type. The background content of NO3- was varied between the
replicates, but is generally lower than the NH4+ content. Soil organic matter (SOM) was lower
than expected for DP, only 51 % compared to 95 % in UP. The conclusions of this study are
that the large NH4+ pool and high mineralization rates in UP probably are effects of high
SOM content. The nitrification rates were higher in DP because of the lower water content,
which enables better aeration of the soil. Dissimilatory nitrate reduction to ammonium
(DNRA) stands for 2-6 % of total consumption of NO3-, and there were no significant
differences between the soil types.
Sammanfattning
Kväve (N) är ett av de mest förekommande grundämnena på jorden, och dess olika former
kan sammanfattas i kvävecykeln. Ökad antropogen tillförsel av reaktivt kväve har förändrat
den globala N-cykeln, med stora konsekvenser för känsliga ekosystem där N-mättnad och
övergödning kan utgöra problem. Målet med den här studien var att undersöka skillnaderna
mellan omvandlingar av N i en dränerad (DP) och en icke-dränerad (UP) torvmark, för att ge
ökad förståelse för hur vattenhalten i marken påverkar kvävets flöden. Jordprover togs från
två platser i Västra Götaland, en ursprunglig våtmark och en dränerad. Märkning av proverna
med isotopen 15N gjordes enligt isotoputspädningsmetoden och omvandlingshastigheterna
mättes tillsammans med andra jordegenskaper. C:N-kvoten var något högre i UP medan pH
var lägre. UP hade en hög bakgrundshalt av NH4+ vilket stämde väl överens med högre
mineraliseringshastigheter för den jordtypen. Bakgrundshalten av NO3- var varierande mellan
replikaten, men i allmänhet lägre än halten av NH4+. Halten av organiskt material (SOM) var
lägre än väntat i DP, endast 51 % jämfört med 95 % i UP. Slutsatserna av denna studie är att
de höga NH4+-halterna och mineraliseringshastigheterna i UP beror på hög halt av SOM.
Nitrifikationshastigheterna var högre i DP på grund av den lägre vattenhalten, vilket medför
bättre luftning av jorden. Dissimilativ nitratreduktion till ammonium (DNRA) utgör 2-6 % av
den totala konsumtionen av NO3-, utan skillnader mellan jordtyperna.
1
Table of contents
List of abbreviations
3 1. INTRODUCTION
4 1.1 Nitrogen and its functions in soil
1.2 Measuring transformation rates
1.3 Aims and objectives
4 5 6 2. MATERIALS AND METHODS
7 2.1 Sites
2.2 Sampling
2.3 Laboratory work
2.3.1 Determining soil properties
2.3.2 NH4+ conversions
2.3.3 NO3- conversions
2.3.4 Calculations
7 7 7 8 8 8 9 3. RESULTS
11 3.1
3.2
3.2
3.3
11 12 12 13 Sites comparison
Gross nitrification rates
Gross mineralization rates
DNRA-rates
4. DISCUSSION
14 4.1 Soil properties
4.2 Mineralization and nitrification
4.3 Consumption
14 14 15 5. CONCLUSIONS
16 6. ACKNOWLEDGEMENTS
17 7. REFERENCES
18 APPENDIX
20 20 21 15
Appendix A – Atom % of N
Appendix B – Variables used in calculations
2
List of abbreviations
DNRA – Dissimilatory nitrate reduction to ammonium
DP – Drained peat
FIA – Flow injection analysis
IRMS – Isotope ratio mass spectrometry
N – Nitrogen
SOM – Soil organic matter
UP – Undrained peat
3
1. Introduction
1.1 Nitrogen and its functions in soil
Nitrogen (N) is one of the most abundant elements in the atmosphere, hydrosphere and
biosphere. 99 % of all N consists as N2, a non-reactive gas that the majority of the organisms
on Earth cannot use for assimilation (Robertson and Groffman 2006). The forms of N that
organisms can incorporate are reactive forms e.g. nitrate (NO3-), ammonium (NH4+), and
organic compounds such as urea. These substances are quickly transformed to other forms by
microbial activity and plant uptake (Galloway, Aber et al. 2003). All these different forms of
N and the processes that generate them can be summarized in the N cycle (Fig. 1). The cycle
includes processes like nitrification, which convert NH4+ into NO3-, mineralization, which
transforms organic N into inorganic N and denitrification, where NO3- is converted back to
non-reactive N2. Immobilization is the process where inorganic N is transformed to organic N
by microbial assimilation. There is also another microbial pathway of the N cycle, not as
examined as the others: Dissimilatory nitrate reduction to ammonium (DNRA) transforms
NO3- to NO2- and then to NH4+ under anaerobic conditions (Silver, Herman et al. 2001).
Figure 1. Some of the processes occurring in the N cycle. m is mineralization, n is nitrification and
c is consumption. The dotted lines indicate gas transport.
In most terrestrial ecosystems N is a limiting factor for primary production (Robertson and
Groffman 2006). However, human activities have increased the amount of reactive N on
Earth, and therefore also the N that is available for plant uptake. Between 1860 and 2005 the
creation of reactive N has increased globally from approximately 15 billion tons per year to
187 billion tons per year (Galloway, Townsend et al. 2008). The human production rates of
reactive N are now by far higher than the production in all natural terrestrial ecosystems
(Galloway, Aber et al. 2003). The reasons for this are primarily the industrial fabricated
ammonium, according to the Haber-Bosch process, where N2 gas is converted to NH3 through
a reaction with hydrogen. The process has enabled farming at a larger scale due to higher
yields at lower costs. Furthermore the cultivation of crops that promote N2-fixation, such as
legumes, and the combustion of fossil fuels, which leads to emission of NOX-gases, are other
contributing factors to the increasing amount of N in the ecosystems. However, the rates of
4
denitrification have far from increased at the same rate, which leads to accumulation of
reactive N in the environment. From the soil, N leaks to streams, lakes and coastal areas,
causing eutrophication (Galloway, Aber et al. 2003).
Atmospheric transport and deposition is now the dominant distribution process of reactive N.
In ecosystems not affected by human impact, the N deposition is about 0.5 kg N ha-1 year-1.
Today, the average N deposition rates go beyond 10 kg N ha-1 year-1 in some regions of the
world (Galloway, Townsend et al. 2008; Kreutzer, Butterbach-Bahl et al. 2009). For forests,
this has greater consequences than for agricultural land, since forests are more slowly growing
ecosystems and not fertilized to the same extent as agricultural soils. Due to this many forest
soils has become N saturated with increased leaching and N2O-emissions as a consequence
(Aber, McDowell et al. 1998). Higher amounts of reactive N in the environment lower the
C:N ratio in soil and in forest soils this can lead to higher N2O emissions. N2O is a by-product
of nitrification and denitrification and is a highly potent greenhouse gas that contributes to
global warming. The N2O-fluxes from soils are directly dependent on rates of nitrification and
denitrification, and with a higher amount reactive N in the soil the rates of these processes
will increase (Klemedtsson, von Arnold et al. 2005).
Another large anthropogenic impact on soil is drainage, which has been a way of obtaining
larger areas for forestry and agriculture. Drainage greatly changes the conditions in soil. The
water content of the soil decreases and allows air to enter the soil, and due to better growth
conditions the primary production increases (Ernfors 2009). This changes the carbon balance
in the soil, and as a result of increasing oxygen content the N stored in organic matter is
mineralized and therefore available for nitrification (Kasimir-Klemedtsson, Klemedtsson et
al. 1997).
Many factors indicate that as a result of anthropogenic input the global N cycle has altered,
and the availability and mobility of N has increased (Vitousek, Aber et al. 1997). What the
consequences of this will be for ecosystems all around the world is hard to predict owing to
the complexity of N, with all its different forms. In order to handle these consequences in the
future it is important to get a better comprehension of the N fluxes in soil, and the factors
affecting them.
1.2 Measuring transformation rates
Nitrogen has two stable isotopes, 14N and 15N, with abundances of 99.63 % and 0.37 %
respectively. Since the latter is naturally scarce in the environment it can be used for
monitoring fluxes between N pools, and determine gross transformation rates in soil. Gross
transformation shows more about the fluxes between the pools in the N cycle than net
transformation. The net transformation is the balance between the influx and the outflow of
the pool (Robertson and Groffman 2006), and as long as the pool does not change in size it is
difficult to say something about how large the fluxes between the pools are, since net and
gross transformation rates are not always correlated (Hart, Nason et al. 1994). For example,
Davidson, Hart et al. (1992) compared gross and net mineralization in one young and one old
forest and found that only looking at the net mineralization can be misleading. In their study
the net mineralization of the old forest was almost zero, while much higher in the young
forest. However, when measuring the gross rates they found that they were in fact much
higher in the old forest than in the young.
There are different methods for measuring transformation rates. One is the tracer technique,
where one N pool, such as NH4+, is labelled with 15N and the movement of the isotope is
monitored through the system (Hart and Myrold 1996). Another method, used in this study, is
based on dilution of a 15N labelled N pool. During incubation, there will be an influx and out5
flux of the pool. The influx is diluting the pool with 14N and with the out-flux 15N vanishes
out of the pool (Fig. 2). The dilution is monitored by measuring the 15N as a fraction of the
whole N content at two times, one right after the isotope labelling and one after incubation for
24 hours.
Figure 2. The principles of the isotope dilution technique. The
15
N-content in the pool becomes diluted due to mineralization
or nitrification (m). The out-flux of the pool is consumption (c).
The 15N fraction is measured at both t0 and t1 and rates are
calculated with the equations of Kirkham and Bartholomew
(1954). Figure redrawn from Hart and Myrold (1996).
Kirkham and Bartholomew (1954) presented equations to calculate the rates of influx and
outflow of the N pool, i.e. the mineralization/nitrification and consumption together with
three important assumptions:
(I) Microorganisms in soil do not differentiate between the isotopes 14N and 15N, i.e. the
isotopes behave the same in the soil.
(II) The mineral atoms are not being remineralized, as they are so few compared to the
organic material.
(III) The rates of mineralization/nitrification and consumption are the same during the
incubation period.
This method is widely used for measuring transformation rates and has the advantages that a
flux can be monitored with measurements of only one pool (Hart and Myrold 1996).
1.3 Aims and objectives
The aim of this study was to investigate how the fluxes in the nitrogen cycle differ between a
drained and undrained peat soil. Gross transformation rates of processes such as
mineralization and nitrification in the N cycle were measured to give an increased
understanding of how the N cycle is affected by water content in soil, using the isotope
dilution technique and the equations by Kirkham and Bartholomew (1954) and Silver,
Herman et al. (2001)
6
2. Materials and methods
2.1 Sites
Samples were taken from two sites in Skogaryd, Västra Götaland, on the 4th of April 2010.
The first site is a drained peatland (DP), with planted coniferous woodland, dominated by
Norway spruce (Picea abies). The ground vegetation is sparse and dominated by bryophytes
such as Mnium hornum, Polytricum formosum and Pleurozium schreberi. The peat was
drained in the 1870’s and used for agriculture until 1951 when the current tree stand was
planted (Ernfors 2009). At the time of sampling there was still plenty of snow at this site, and
the soil was frozen in some of the samples. The second site is a bog (undrained peatland =
UP), with Sphagnum, Carex and Erica tetralix growing on top layer.
Figure 3. The sites in Skogaryd. Drained peat (left) and bog site (right).
2.2 Sampling
Three replicates were taken from each site, with a distance of 2 m; each replicate contained
five subsamples. The subsamples were taken in a line, with a distance of 50 cm from each
other. The total sampling area was 2 x 6 m. Subsamples were taken to a depth of 10 cm; the
litter not included, and put in plastic bags.
2.3 Laboratory work
Roots and other larger distinguishable materials were removed from the samples and passed
through a sieve with meshes of 5.6 mm. About 30 g of soil from each sample was extracted
with the double amount of 2 M KCl. The bottles were shaken in an orbital shaker for one hour
at 120 rpm, and the extract was filtrated with Whatman GF/D filter papers. The filter papers
were rinsed with 2 M KCl before filtration. The extract was measured with flow injection
analysis (FIA) to determine the background content of NH4+ and NO3-.
The samples were divided into 24 plastic bottles (four for each sample) with 100 g soil in
each and covered with plastic film. Holes were made in the film to enable air exchange
without drying the soil. Half of the 24 bottles were labelled with 15NH4NO3 and the other half
with NH415NO3. The enrichment of the salts was 98 atom percent and the added amount
3 µg g-1 wet soil. 30 min after labelling half of the samples (12 bottles) from each labelling
were extracted with 100 ml of 2 M KCl according to the same procedure described above.
The other 12 samples were incubated for 24 hours and extracted the following day (Stevens
and Laughlin 1995).
7
The total content of NH4+-N and NO3--N in the extracts after labelling was measured with
FIA. The labelled N pool had to be converted into N2O gas in order to measure the fractions
of the N isotopes with isotope ratio mass spectrometry (IRMS).
2.3.1 Determining soil properties
About 10 g of each soil sample were weighed, dried at 70 degrees for 24 hours and weighed
again, to determine the water content. Soil organic matter (SOM) was determined by
calculating the weight difference of dry soil before and after burning at 550° C for 8 hours.
The C and N content of soil was measured with an elemental analyzer, using dried and milled
samples. The pH was measured in the 2 M KCl extracts.
2.3.2 NH4+ conversions
Saghir, Mulvancy et al. (1993) describes this technique in detail.
The extracts were pipetted into kilner jars together with a spike solution ((NH4)2SO4),
2 M KCl and MgO. A tripod was placed in the jar and on top of that a petri dish with 3 ml of
an absorption solution containing 0.005 M H2SO4 and 0.5 mM CuSO4 (Fig. 4). The lids were
closed and the samples were then incubated at 20°C for five days. After diffusion the
absorption solution was poured in glass vials and dried in 150°C. The vials were thereafter
cooled down to room temperature, sealed with an airtight septum cap and evacuated. 1 ml of
NaOBr was injected with a syringe in the vial. The vial was left for 5 minutes in order to let
the reaction start and ensure that enough N2O was produced. Helium gas was then added to
get atmospheric pressure inside in the vial. The concentration of the produced N2O was
measured with gas chromatography (GC) and the isotope ratio in the samples with IRMS.
Figure 4. Adding MgO to the solution in the jars (left). Closed kilner jars with the tripod and petri
dish inside (right).
2.3.3 NO3- conversions
Stevens and Laughlin (1994) describes this technique in detail.
The extracts were pipetted into 60 ml dark glass bottles together with 2 M KCl-solution, a
spike solution (KNO3) and 2 ml of 0.2 M sulphamic acid (H3NSO3). A Cd/Cu reductor was
added to the bottles, containing of a piece of Cd-foil (10 x 10 x 0.25 mm) formed into a
cylinder. Before addition in the bottles the cylinders were prepared by treatment with 6 ml of
HCl for 1 minute, washed with distilled water for 1 minute, then treated with 0.04 M
CuSO4-solution for 1 minute and finally washed with distilled water.
The bottles were capped with septum caps and evacuated. 2.5 ml of 1 M acetic acid buffer
were added with a syringe through the septa. The bottles were laid flat on an orbital shaker
8
and shaken for 2 hours at 120 rpm. The N2O concentration in the bottles after shaking was
measured with GC (Fig. 5), and the isotope ratio with IRMS.
Figure 5. The produced N2O in the bottles were meaured by IRMS.
2.3.4 Calculations
The following equations were used, developed by Kirkham and Bartholomew (1954) and
presented here as in Davidson, Hart et al. (1991):
(1)
m=
M 0 " M1 ln(H 0 M1 /H1 M 0 )
#
t
ln( M 0 / M1 )
(2)
!
c=
M 0 − M1 ln( H 0 /H1 )
×
t
ln( M 0 / M1 )
where M0 = initial 14+15N pool (µg N g-1 dry soil)
€ 14+15N pool (µg N g-1 dry soil)
M1 = post-incubation
H0 = initial 15N pool (µg N g-1 dry soil)
H1 = post incubation 15N pool (µg N g-1 dry soil)
m = mineralization rate (µg N g-1 soil day-1)
c = consumption rate (µg N g-1 soil day-1)
t = time (day)
9
For samples that were labelled with 15NH4+, the NH4+ pool was used for calculating M and H.
For samples that were labelled with 15NO3-, the NO3- pool was used for M and H, and the
symbol m was replaced by n, which is the rate of nitrification. “Consumption” refers to all
processes that consume the labelled N pool (see 4.3).
The DNRA rates were calculated from methodology described by Silver, Herman et al.
(2001).
(3)
NH 4
MRTNH 4 =
[M0 ]
c NH 4
(4)
€
([
DNRA =
where
€
MRTNH4
NH4
[M0]
cNH4
DNRA
[15NH4%]t0
[15NH4%]t1
Mavg
[15NO3%]avg
15
)
NH 4 %] t 0 − [ 15 NH 4 %] t1 × M avg
MRT × [ NO3%] avg
15
= mean residence time of NH4+ (days)
= initial 14+15N pool of (µg N g-1 dry soil)
= gross consumption rate of NH4+ (µg N g-1 soil day-1)
= rate of DNRA (µg N g-1 soil day-1)
= atom % of 15NH4+ at t0
= atom % of 15NH4+ at t1
= mean 14+15NH4+ pool during the interval (µg N g-1 dry soil)
= mean 15NO3- atom % during the interval
All values used in calculations can be found in Appendix.
10
3. Results
3.1 Sites comparison
Various soil properties for DP and UP are presented in Table 1. The C:N ratio was slightly
higher in the UP, mainly due to a higher C content. pH values were higher in DP. The water
content and SOM were much higher in the UP.
The measurements of the background content of NO3- and NH4+ showed that there is a large
difference between the NH4+ pools in the two soils (Fig. 6). UP has a larger NH4+ content than
DP. There was a large spatial variability in the initial NO3- pool sizes, due to one high value,
which was about nine times higher than the other two for DP. Except for that value the NO3pool sizes do not differ much between the soils. The NO3- pool is generally smaller than the
NH4+ pool, especially for UP where the lowest value for NH4+ is about ten times higher than
the highest for NO3-.
Soil property
C (%)
N (%)
C:N
pHKCl
Water content (%)
SOM (%)
Drained
peatland
Undrained
peatland
37.9 (3.70)
1.67 (0.12)
22.7 (0.57)
3.73 (0.09)
66.5 (0.74)
51.0 (2.60)
47.9 (0.89)
1.68 (0.10)
28.6 (1.19)
3.08 (0.04)
89.6 (2.74)
94.9 (0.21)
Table 1. Average values (SD) of selected soil properties in DP and UP.
Figure 6. Initial pool sizes of NO3- (left) and NH4+ (right) for DP and UP, measured before labelling with
15
N. Values for all three replicates are shown. Note the scale difference between NO3- and NH4+ pool sizes.
11
3.2 Gross nitrification rates
Figure 7. Gross rates of nitrification (left) and consumption (right) of nitrate for DP and UP. Values for all
three replicates are shown. Note the scale difference between nitrification and consumption rates.
Gross nitrification rates are slightly higher in DP (ranges 1.35-4.32) than in UP (0.70-1.69).
For DP the values show great variation, one value is higher than the two others. The
consumption rates of NO3- are higher in UP than in DP. The values for consumption rates
shows great variability but are generally higher than the nitrification rates in both soils. In UP
the difference between the gross nitrification and the consumption rates of NO3- (n-c) are
larger than for DP, average values are -6.59 for UP compared to -1.07 in DP.
3.2 Gross mineralization rates
Figure 8. Gross rates of mineralization (left) and consumption (right) of ammonium for DP and UP. Values for
all replicates are shown for DP, for UP only values from two of the replicates were accurate and shown here.
Note the scale difference between mineralization and consumption rates.
The gross mineralization rates were higher in UP (8.34-15.3) than in DP (4.33-8.03).
Regarding consumption rates of NH4+ there were no significant difference between the two
soils. The mineralization rates for DP were slightly lower than the consumption rates,
although it was great variability in the values. For UP the mineralization rates are higher than
consumption rates, although it was great variability between the values for mineralization.
The rates of mineralization are generally higher than for nitrification in both soil types.
12
3.3 DNRA-rates
Figure 9. The rates of DNRA in DP and UP. Values for all
replicates are shown for DP, for UP only values from two
of the replicates were accurate and shown here.
The DNRA rates were higher in UP than in DP, although the spatial variations in the rates
were large for both soil types. 2-6 % of total consumption of NO3- is DNRA, with no
significant differences between the soil types.
!
13
4. Discussion
4.1 Soil properties
The amount of SOM was surprisingly low for DP, only about 50 % (Table 1). Other studies
made at the same site in Skogaryd indicated SOM about 90 % (Ernfors 2009). This parameter
was measured twice with the same result. Forest soils are heterogeneous and this could be due
to natural spatial variability. The SOM in UP was 95 %, which was expected. Since the SOM
content was so different in the two sampled soils, but was expected to be the same, this had to
be considered when interpreting the results.
4.2 Mineralization and nitrification
The initial pools of NH4+ in UP were considerably higher than in DP, and much higher than
the NO3- pools for both soils. Similar results have been described by Ambus, Mosier et al.
(1992) in a study of a riparian fen, where the NH4+ pool was 5-10 times higher than the NO3pool at a soil depth of 10 cm. Also Westbrook and Devito (2004) has compared the different
initial pools in peatlands and found that the initial pool of NH4+ was ~130 times larger than
the NO3- pool. In the present study the NH4+ pool was 22-27 times larger than the NO3- pool.
There was one replicate for UP with the value of 60.4 µg g-1, while the two others were ~20
µg g-1 (Fig. 6). How the large initial pool of NH4+ in that replicate affected the transformation
rates in the experiments is unfortunately not known as the IRMS value was not accurate (see
Table A1).
The large NH4+ pool correlates well with the higher gross mineralization rates in UP which
probably was a consequence of the higher SOM content. Booth, Stark et al. (2005) concluded
that the C:N ratio and the mineralization rates are negatively correlated, i.e. a low C:N ratio
imply higher mineralization rates. In the present study UP has both a higher C:N ratio and
mineralization rate than DP. However, Booth, Stark et al. (2005) also suggested that the SOM
composition impact the mineralization in soil, possibly through influence on microbial
biomass. There is a strong positive correlation between C and N concentration and microbial
biomass, and as seen in the results the C content in soil was higher for UP (Table 1).
The rates of mineralization were higher in this study than in a study done by Münchmeyer,
Russow et al. (2000), where they compared drained and reflooded peatlands in Germany. The
rates in drained peatlands were 1.42 µg g-1 d-1 compared to average 5.69 µg g-1 d-1 in this
study. For reflooded peatlands the rates were 4.57 µg g-1 d-1 compared to average 12.0 µg g-1
d-1 for UP.
Münchmeyer, Russow et al. (2000) also mentions the loss of 15N from the soil samples after
labelling, which could be due to abiotic reasons such as volatilization and clay fixation.
Volatilization of NH3 can be a problem for alkaline soils, but is in this study probably
negligible, since the pH in the used soils is ~3.5 and the NH4+ adsorption capacity is high
under acidic conditions (Huber, Oberhauser et al. 2002). SOM can fixate N as well as clay,
which could be a source of error since the SOM content in the used soil is high. This is
primarily a problem for NH4+, since the soil particles are negatively charged (Schlesinger
1997). How large the SOM fixation is can be measured using sterilized soils to see how much
of the 15N that is recovered at t0 without microbes in the soil (Davidson, Hart et al. 1991).
This has not been done in this study due to time limitations and thus the affect of cation
exchange of SOM is not known.
The initial pools of NO3- show great variability for DP, particularly one replicate that is nine
times higher than the other two (Fig. 6). Nevertheless, this is related to an almost three times
14
higher nitrification rate compared to the other replicates (Fig. 7). Excluding this extreme
value, the initial pools of NO3- do not differ much between the soils, but regarding the
nitrification rates they are still higher in DP. Booth, Stark et al. (2005) imply that the
nitrification rates are highly dependent on the mineralization rates, since nitrifying microbes
need NH4+ as a substrate. In spite of the fact that the mineralization rates are higher in UP they
are not correlating to the nitrification rates in this study. This is due to the lower water content
in DP that enables better aeration of the soil. As a result of this the gross nitrification rates are
higher in DP, since it is an oxygen demanding process.
4.3 Consumption
In order to estimate the rates of consumption of NO3- and NH4+ it is necessary to add substrate
to the samples. If the natural N pool is small in the samples the addition of substrate can result
in a temporary boost of the microbial activity, which speeds up the measured transformation
rates. It is also possible that the removal of plant roots from the soil makes the inorganic N
more available and hence alters the activity of microbes. Overestimation of the real
consumption rates of NO3- and NH4+ can therefore be a source of error in this type of study
(Hart, Nason et al. 1994).
Since the values of the consumption rates might be uncertain, this method cannot be used to
measure net transformation by taking the difference between gross mineralization and gross
consumption (Davidson, Hart et al. 1992). Kirkham and Bartholomew (1954) did not make a
distinction between the immobilization and consumption of NH4+. This distinction is made
here since immobilization is only one of several possible outcomes for NH4+ in soil. The
others are autotrophic nitrification, clay or SOM fixation, volatilization and leaching losses.
However, under laboratory conditions the microbial assimilation and nitrification are the most
important processes (Di, Cameron et al. 2000).
Overestimation of the consumption can also be a factor affecting the nitrification rates, since a
major fate of consumed NH4+ is nitrification. Adding labelled substrate to the already small
NH4+ pool might have affected the transformation rates, mainly in DP. However, in order to
apply this method labelled substrate has to be added, and several studies indicate that the
isotope dilution technique is a good instrument for measuring microbial processes (Davidson,
Hart et al. 1991; Di, Cameron et al. 2000; Hart and Myrold 1996).
Despite the uncertainties mentioned above, the consumption rates for NH4+ was probably
more accurate than the consumption rates for NO3-, since the initial pools of NO3- in the
samples were smaller. Included in the consumption of NO3- are microbial assimilation,
DNRA and denitrification. DNRA stands for 2-6 % of the total consumption of NO3-, with no
significant differences between the soil types. As a result the higher DNRA rates in UP can
neither be derived from the fact that it is an anoxic process favoured by high water content,
nor that it is thought to be favoured by a high C:N ratio (Fazzolari, Nicolardot et al. 1998).
The explanation is rather that the total consumption rates are generally higher for that soil
type.
15
5. Conclusions
The aim of this study was to compare the gross transformation rates of N between a drained
and an undrained peat soil by using the isotope dilution technique. Although this study might
be too small to draw definitive conclusions about the effect of drainage, these results give
suggestions about the N dynamics in peat soil.
-
The NH4+ pool was more than twice as large in undrained peat as in the drained. This
was also reflected in the gross transformation rates where the mineralization rates in
the undrained peat were higher. This was probably a consequence of the high SOM
content in the undrained peat rather than the water content.
-
The water content of the soil had a larger impact on the nitrification rates. The
nitrification rates were slightly higher in the drained peat, which was due to the lower
water content and the improved aeration of the soil.
-
DNRA stands for 2-6 % of total consumption of NO3-, and there were no significant
differences between the soil types.
16
6. Acknowledgements
Special thanks to
Tobias Rütting,
my supervisor, for your dedication, support whenever needed and good advises.
I’ve learnt a lot!
Katja VanNieuland,
at the University of Gent, for wonderful help and enthusiasm in the lab, and for
introducing us to a lot of good Belgian specialties during our weeks in Gent.
Leif Klemedtsson,
for encouraging conversations and enthusiastic help with a little of everything.
Vivian Aldén and Mats Räntfors
for helping in the lab and for analyzing our samples in Gothenburg.
Robert Björk
for help with the sampling in Skogaryd.
Elin Julén,
for good company during long days in the lab, especially in Gent. The lab work
would have taken twice as long if it weren’t for you!
17
7. References
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19
Appendix
Appendix A – Atom % of 15N
Table A1. Atom % of 15N from NH4+.
Label
Drained peat
Undrained peat
15
NH4+
15
NO3-
t0
t1
t0
t1
55.2
9.60
0.39
0.51
31.3
15.0
0.45
1.79
25.8
10.4
0.43
1.34
24.8
19.0
0.43
2.44
24.5
14.9
0.46
3.98
19.4
-†
0.41
2.49
† This value from the IRMS was not accurate, and therefore not included in the calculations.
Table A2. Atom % of 15N from NO3-.
Label
Drained peat
Undrained peat
15
NH4+
15
NO3-
t0
t1
t0
t1
0.95
7.99
17.53
13.92
6.67
14.80
65.29
40.67
4.29
28.45
66.08
34.58
0.36
0.20
88.48
79.47
0.41
0.26
87.62
80.66
0.48
2.08
87.87
65.75
20
Appendix B – Variables used in calculations
Table B1. Variables used in calculations of nitrification. mineralization and consumption rates
along with their values.
Drained peat
Equation
symbol
Initial 14+15N pool
(µg g-1)
Initial 15N pool
(µg g-1)
Final 14+15N pool
(µg g-1)
Final 15N pool
(µg g-1)
Mineralization rate
(µg g-1 day-1)
Nitrification rate
(µg g-1 day-1)
Consumption rate
(µg g-1 day-1)
Undrained peat
NH4+
NO3-
NH4+
NO3-
4.19
16.9
31.7
10.9
7.52
4.92
28.1
11.7
8.77
4.65
-†
11.5
2.31
2.97
7.85
9.67
2.35
3.21
6.87
10.3
2.26
3.07
-†
10.1
1.58
20.7
33.1
6.05
4.50
1.49
34.4
5.81
8.84
1.13
-†
2.45
0.15
2.87
6.31
4.80
0.67
0.61
5.14
4.69
0.92
0.39
-†
1.61
4.69
-
8.54
-
4.34
-
15.36
-
8.03
-
-†
-
-
4.32
-
0.89
-
1.36
-
0.70
-
1.61
-
1.69
7.29
0.60
7.11
5.77
7.36
4.79
9.02
6.60
7.97
5.13
-†
10.7
M0
H0
M1
H1
m
n
c
† Could not be calculated since the value from the IRMS was not accurate for this replicate.
21
Table B2. Variables used in calculations for DNRA rates and their values.
Equation symbol
Mean residence time of NH4+ (days)
MRTNH4
Initial 14+15N pool of NH4+
(µg N g-1 dry soil)
NH4
Gross consumption rate of NH4+
(µg N g-1 soil day-1)
-1
[M0]
cNH4
-1
Rate of DNRA (µg N g soil day )
15
Atom % of NH4 at t0
Atom % of 15NH4 at t1
DNRA
15
[ NH4%] t0
[15NH4%] t1
Mean 14+15NH4+ pool during the interval
(µg N g-1 dry soil)
Mean 15NO3- atom % during the interval
Mavg
[15NO3%] avg
Drained peat
Undrained peat
0.57
4.46
1.02
3.11
1.10
-†
3.92
29.9
4.60
30.8
7.71
36.5
7.29
7.11
7.36
9.02
7.97
-†
0.04
0.16
0.13
0.40
0.13
-†
0.39
0.43
0.45
0.46
0.43
0.41
0.51
2.44
1.79
3.98
1.34
2.49
2.88
30.0
5.31
29.8
7.87
35.3
15.7
84.0
53.0
50.3
84.1
76.8
† Could not be calculated since the value from the IRMS was not accurate for this replicate.
22