Measuring Soil Denitrification and
Mineralization in Riparian Zone
Author: Kajsa-Stina Kalin
International Summer Water
Resources Research School
VVRF05
LTH, Sweden and Xiamen
University, China
Instructor: Professor Wenzhi Cao
Research assistant: Meng Luwei
Co student: Xiao Dong Li
2008-07-11
Abstract
The aim of the experiment was to determine mineralization and denitrification in the
riparian zone. Eutrophication caused by over enrichment with nitrogen and
phosphorus is a widespread problem in rivers, lakes and coastal oceans. Research
has indicated that the planting and/or preservation of riparian buffer zones can be an
effective means of reducing non-point pollution from agricultural fields.
The soil samples were collected in Fujian province, southeast China, close to a small
town called Nanjing. They were exclusively collected in the bamboo forest in the
riparian zone of Wuchuan river. The samples were incubated in the sites June 24th
2008 and recovered July 1st 2008. Standard methods were used to measure NH3-N,
NO3-N, TP, AP, pH, soil moisture and organic matter in the soil samples. Due to
lacking equipment the denitrification rates couldn’t be measured.
The soil moisture ranged between 27.6 % and 16.1 % which are relatively high
values but is explained by the short distance to river. The pH was slightly acidic
which is to be expected since the dominating bedrock in the area is granite and the
main soil type is lateritic red earth and red earth. The organic matter in the soil
ranged in values between 2.98 % and 1.37 %. A study conducted in the same area
for a master thesis by Da Peng Li showed a level of 2.5 %. Based on this, the level of
the organic matter seems to be normal. The experiments to measure TP and AP
failed, most likely because the equipment wasn’t clean enough.
The level of NH3-N decreased during the incubation (except for one differing value)
and the level of NO3-N increased. The nitrogen mineralization rate ranged between
0.97 mg N / kg · d and 1.13 mg N / kg · d. These values are in the same range as the
data collected by Da Peng Li where the mineralization rate ranged between -0.58 mg
N / kg · d and 1.47 mg N / kg · d. The ammonification rate also fitted the pattern with
figures ranging between 0.062 mg N / kg · d and -0.19 mg N / kg · d. The study by Da
Peng Li had figures ranging between -0.83 mg N / kg · d and 1.34 mg N / kg · d. The
nitrification rate ranged between 0.83 mg N / kg · d and 0.89 mg N / kg · d. This is
high compared to the data collected by Da Peng Li where the nitrification rate ranged
between -0.11 mg N / kg · d and 0.59 mg N / kg · d.
The rate of mineralization was quite high, probably due to high temperatures in the
summer (between 25 degree Celcius and 35 degree Celcius). But since there was
limited time and limited experiments, conclusions shouldn’t be drawn.
2
Table of Contents
Abstract ...................................................................................................................... 2
Introduction ................................................................................................................. 4
The nitrogen cycle ...................................................................................................... 4
The riparian zone ........................................................................................................ 6
Introduction of the riparian zone .............................................................................. 6
The influence of vegetation and slope..................................................................... 7
Temporal and seasonal variability ........................................................................... 8
Spatial variability ..................................................................................................... 8
Soil temperature .................................................................................................... 10
Soil moisture ......................................................................................................... 10
Soil pH .................................................................................................................. 10
Level of groundwater table .................................................................................... 10
Carbon availability ................................................................................................. 11
Oxygen availability ................................................................................................ 11
Method...................................................................................................................... 11
Results and Discussion ............................................................................................ 13
Controlling factors ................................................................................................. 14
Magnitude of mineralization, ammonification and nitrification rates ...................... 16
Denitrification ........................................................................................................ 18
Overall discussion ................................................................................................. 19
Conclusions .............................................................................................................. 20
Evaluation ................................................................................................................. 20
References ............................................................................................................... 20
Reports ................................................................................................................. 20
Electronic sources ................................................................................................. 22
Interview................................................................................................................ 22
Appendix................................................................................................................... 23
Group pictures ...................................................................................................... 23
3
Introduction
Eutrophication caused by over enrichment with nitrogen and phosphorus is a
widespread problem in rivers, lakes and coastal oceans (Carpenter, Caraco, Correll,
Howarth, Sharpley, & Smith, 1998). Over enrichment with nitrogen and phosphorus
causes a wide range of problems, including toxic algal blooms, loss of oxygen, fish
kills, loss of aquatic vegetation, degradation of coral reefs and loss of biodiversity.
Thus, nutrient fouling seriously degrades the marine and freshwater resources and
impairs their use for industry, agriculture, recreation, drinking water and other
purposes (Carpenter, Caraco, Correll, Howarth, Sharpley, & Smith, 1998). Martin et
al. write “excess nitrate has deleterious effects for environmental and human health”
(Martin, Kaushik, Trevors, & Whiteley, 1999).
Runoff from agriculture and cities is a major source of phosphorus and nitrogen
entering rivers, lakes, and coastal waters. Acid rain and airborne pollutants generated
by human activities also supply nitrogen to surface waters. (Carpenter, Caraco,
Correll, Howarth, Sharpley, & Smith, 1998) These are examples of non-point source
pollution, i.e. the sources are not discrete places but diffuse areas somewhere on the
earth’s surface. Non-point source pollution is difficult to measure and regulate
because of its dispersed origin and because it varies with season and weather
(Carpenter, Caraco, Correll, Howarth, Sharpley, & Smith, 1998). Yet non-point inputs
have a profound impact of water quality.
The best solution should evidently be to minimize the outlets, but when this doesn’t
happen fast enough other methods to reduce the effect of the discharged nutrients
are researched. Research has indicated that the planting and/or preservation of
riparian buffer zones can be an effective means of reducing pollution from agricultural
fields (Martin, Kaushik, Trevors, & Whiteley, 1999).
The nitrogen cycle
In order to research the possible ways of reducing nitrogen in watersheds it is of
importance to understand how nitrogen can be transported and transformed, by that
making it possible to understand the different paths nitrogen can take in the area
being studied. Therefore the nitrogen cycle will be shortly introduced. Figure 1 shows
the nitrogen cycle.
4
Figure 1 The nitrogen cycle. (Pidwirny, 2008)
The nitrogen cycle represents one of the most important nutrient cycles found in
terrestrial ecosystems. Nitrogen is used by living organisms to produce a number of
complex organic molecules like amino acids, proteins and nucleic acids. (Pidwirny,
2008) Seventy-eight percent of the atmosphere is made up of nitrogen in its gas
phase. Atmospheric nitrogen becomes part of living organisms in two ways. The first
is through bacteria in the soil that form nitrates out of nitrogen in the air. The second
is through lightning. (Gordon, 2005) Nitrogen is also brought to the ecosystem via
human activities such as agriculture, fossil fuel burning and through sewage water.
Nitrogen mineralization is the process where organic nitrogen from decaying plants
and animals (proteins, nucleic acids, etc) is converted to ammonia (NH3) and
ammonium (NH4+). The resultant ammonia can be taken up by microbes and plants
(assimilated) and converted back to organic N (immobilization), or nitrified to nitrate
(NO3-). (Corbin, 1998)
Nitrate is a source of nitrogen that promotes eutrophication. It is a non-point source
pollutant that is intimately linked to agriculture through its association with excess
fertilizer use and the improper disposal of livestock wastes. Nitrate is a highly water
soluble anion who is repelled by most soils and therefore very mobile. (Martin,
Kaushik, Trevors, & Whiteley, 1999) The processes of interest in reducing nutrients
reaching watersheds are primarily denitrification but also microbial immobilisation and
plant consumption.
5
Denitrification takes place during anaerobic conditions. Under low oxygen tension,
nitrate (NO3-) instead of O2, is used by denitrifying bacteria as the terminal electron
acceptor for energy production (Watts & Seitzinger, 2000). Through bacterial
metabolic reduction, nitrate (NO3-) becomes dinitrogen (N2) or nitrous oxide gas
(N2O). Both of these gases then diffuse into the atmosphere and the nitrogen
concentration in the soil is constantly reduced.
Microbial immobilisation is a dissimilatory reduction of NO3- to NH4+ which occurs
under anaerobic conditions. In nitrogen immobilization, ammonia and nitrate are
taken up by microbes and is largely immobilized. The nitrate undergoes nitrate
reduction by a four step process and is converted to ammonia by reactions that are
similar to those that occur in denitrification. The ammonia is then incorporated into
Kreb's cycle intermediates to form amino acids. (Corbin, 1998) Microbial
immobilisation only temporarily reliefs the nutrient load on the soil since nitrification
can bring nitrate back to the ground.
Plant consumption is the uptake of nitrate by plants. The nitrate used by the plant is
however brought back to the soil when the plant dies. While microbial immobilisation
and uptake by vegetation likely have a supporting role in the retention of NO3-, it is
biological denitrification that is the main mechanism by which ground water is
attenuated (Martin, Kaushik, Trevors, & Whiteley, 1999).
Nitrous oxide gas is produced during numerous nitrogen transformations in soils but
on most occasions denitrification and nitrification are the main sources. With the most
commonly used method to measure denitrification rates, acetylene inhibits the
production of dinitrogen gas, thus making nitrous oxide gas the sole product of
denitrification. N2O fluxes are therefore often treated as a way to estimate
denitrification rates. In a study conducted by Simek et al. denitrification rate was
found to be significantly related to N2O emission (Simek, Elhottova, Klimes & Hopkins,
2003).
The riparian zone
The riparian zone is a biogeochemical and hydrological complex environment, which
together with the stream bed can profoundly influence stream chemistry (Cao, 2008).
Of this reason the potential of the riparian zone in mediating the nutrient load on
watersheds will be investigated together with parameters affecting the riparian zone.
Introduction of the riparian zone
The interface between terrestrial and aquatic ecosystems is defined as the riparian
zone. It represents an important ecological component of the landscape. The riparian
zone is a transition compartment between the stream and the catchment and
6
encompasses the vegetated strip of land that extends along streams and rivers. This
location, between uplands and aquatic ecosystems, allows riparian zones the
capability of modifying effects on the aquatic environment (Martin, Kaushik, Trevors,
& Whiteley, 1999).
Figure 2 Schematic picture of the riparian zone (Parkyn & Colley, 2007)
The influence of vegetation and slope
One of the ways the riparian zone influences an adjacent waterway is through its
vegetation. Trees in the riparian zone intercept solar radiation, thereby modifying the
magnitude of solar inputs, and significantly lower stream temperatures. Several instream processes are directly related to stream temperature, for example the amount
of dissolved oxygen and the viscosity of the water. For the aquatic species living in
the stream the temperature is one of the factors that limit habitat suitability. (Martin,
Kaushik, Trevors, & Whiteley, 1999) Riparian vegetation, such as forests and
macrophytes, influences the channel form and stream function by contributing
particulate organic matter and large woody debris, by providing shade, bank stability,
sinks for organic matter, sediment and by regulating nutrient movement and
transformation (Shivoga, et al., 2007).
Shivoga et al. write that undisturbed dense riparian forest and in-stream vegetation
are linked to the lowest levels of phosphate and nitrate in the river Njoro. A gentle
slope of the landscape downwards the stream also contributes with a positive effect
on the nutrients level. A gentle slope allows for longer interaction time. Furthermore
Shivoga et al. recognize that upland land uses are as important as near-stream land
uses. (Shivoga, et al., 2007)
7
Merrill and Benning say that the effect of the riparian zone on water quality differs
dramatically among sites. They therefore suggest that the riparian zone be viewed as
an area with several different ecosystem types rather than a single, uniform swath of
land. Once the different ecosystem types have been identified and mapped, land
managers can use the information to gain insight on spatial variation in riparian
processes, such as ground and surface water, nitrogen inputs and removal. (Merrill &
Benning, 2005)
Temporal and seasonal variability
The seasonal variation in nitrate concentration most commonly shows a pattern with
dormant season maxima and growing season minima. This is thought to be the result
of a reduced uptake by terrestrial vegetation and of increased leaching during the
winter months (Meynendonckx, Heuvelmans, Muys, & Feyen, 2006). Martin et al. and
Merrill & Benning confirm the thesis. In temperate climates, the most concentrated
nitrate discharges occur in the winter months when the bioassimilation of nitrogen by
plants is not possible (Martin, Kaushik, Trevors, & Whiteley, 1999). The denitrification
rates are highest during spring and summer and are most strongly correlated with soil
moisture and availability of anaerobic conditions (Merrill & Benning, 2005). But there
have also been inconsistent seasonal patterns reported with the lowest rates of
denitrification occurring in the summer months at very poorly drained sites (Martin,
Kaushik, Trevors, & Whiteley, 1999).
Simek et al. report that besides the seasonal pattern of small nitrous oxide emission,
hence small denitrification rates in the spring and fall, and high nitrous oxide
emissions in the summer, there is also a tendency towards much more variable
emission rates in the summer. (Simek, Elhottova, Klimes & Hopkins, 2003) Watts and
Seitzinger shed light on the high variability from day to day. They explain this
variability by delayed release of trapped atmospheric nitrogen. Another explanation
for this short-term temporal variability is the so called denitrification ‘hot spots’ that
are created in the soil column during the decomposition of organic matter. (Watts &
Seitzinger, 2000) Simek et al. write that it is a typical feature of nitrous oxide
emissions to have a strong temporal pattern related both to changes in environmental
conditions and management practices (Simek, Elhottova, Klimes & Hopkins, 2003).
Spatial variability
Denitrification is a complex process governed by several factors including oxygen
content, carbon availability, pH and temperature. It has been speculated that these
factors cannot be simultaneously met in deeper sediments. (Martin, Kaushik, Trevors,
& Whiteley, 1999) High spatial variability in soil denitrification rates have been
reported by several authors, among others Folrunso and Rolston (1984), Parkin
(1987) and Parsons et al. (1991). Pavel et al. showed in an experiment taking place
8
in the Virginia coastal plain that only minimal rates of denitrification occur in the
subsurface horizon (25-45 cm below the surface). The mean rates of denitrification
were several times greater for the surface horizons and greatest for the ponded
surface horizon (ponded means that free water covers the soil surface).
Figure 2 The spatial distribution profile as defined in the report by Pavel et al.
In the experiment they let water flow through soils collected in two different types of
surface horizons and one subsurface horizon. Figure 2 shows how the surface
horizons and the subsurface horizon were defined by Pavel et al. It is however
speculated that some of the large differences observed in denitrification rates
between the soil horizons are due to less residence time of the water passing through
the subsurface horizon compared to the residence time of the water passing through
the two surface horizons. Thus substrate availability may have been lower for the
subsurface horizon leading to a reduced denitrifying activity. (Pavel, Reneau Jr,
Berry, Smith, & Mostaghimi, 1996)
However, Whitelaw and Edwards (1980) found that, despite an exceptional decrease
of carbohydrate concentrations with depths, sufficient carbohydrates existed to
support bacterial activity to the researched depth of 30 m. (Martin, Kaushik, Trevors,
& Whiteley, 1999) Similarly Francis et al (1989) found the potential for denitrification
existed in almost all sediments tested to a depth of 289 m.
Watts and Seitzinger have observed that some soil cores demonstrate flux rates
which are several times greater than those of the other cores taken from the same
area. They claim that uneven distribution of organic carbon is one of the reasons
behind spatial variability of denitrification rates. (Watts & Seitzinger, 2000). Pavel et
al. agrees with this hypothesis. The patchy dispersion of organic material in soil is a
9
major factor affecting the variability of denitrification rates between sites (Pavel,
Reneau Jr, Berry, Smith, & Mostaghimi, 1996).
Soil temperature
Pavel et al. report that denitrification rates are significantly higher for soils incubated
at 19,9°C than at 16,4°C or 13,5°C. This follows by denitrifying bacteria in most
cases getting enhanced metabolism at higher temperatures. (Pavel, Reneau Jr,
Berry, Smith, & Mostaghimi, 1996) Almost all metabolic processes are enhanced with
increasing temperature and temperature is thought of as being one of the major
controlling factors for denitrification and mineralization rates. Temperature is the
reason behind high rates of denitrifaction and mineralization occurring in the spring
and summer and accordingly low rates occuring in the winter.
Soil moisture
The denitrification rates are most strongly positively correlated with soil moisture and
availability of anaerobic conditions. High soil moisture content suppresses
nitrification, thereby reducing local nitrate production, and at the same time promotes
denitrification (Merrill & Benning, 2005). A strong positive correlation exists between
the proportion of well drained soils and downstream nitrate concentrations. This can
be explained by the higher infiltration capacity, the shorter residence time of
subsurface water and groundwater and consequently less denitrification taking place.
(Meynendonckx, Heuvelmans, Muys, & Feyen, 2006) Mineralization is on the other
hand strongely negatively correlated with soil moisture. The bacteria conducting
mineralization demand oxygen and since oxygen level tends to decrease when soil
moisture increase, the rate of mineralization decreases accordingly. (Li, 2007)
Soil pH
Denitrifying bacteria can adapt to low soil pH but an overall reduction in denitrification
is to be expected. High denitrification potentials have however been observed in acid
soils by several researchers. (Pavel, Reneau Jr, Berry, Smith, & Mostaghimi, 1996)
Level of groundwater table
A dry near-stream compartment have a damping effect on storm hydrographs and
runoff exports are much lower compared to wet hydrological conditions. Evans et al.
(1999) showed that high stream flows in a peat catchment occurred at times of high
groundwater table. No discharge peaks were associated with low groundwater table.
(Butturini, Bernal, Sabater, & Sabater, 2002)
10
Carbon availability
Organic carbon is a required element for denitrification to occur. Different types of
vegetation vary in their ability to contribute with organic carbon. ‘Hot spots’ of
denitrification activity are associated with patches of organic carbon in the soil.
(Martin, Kaushik, Trevors, & Whiteley, 1999) As organic carbon breaks down, organic
and inorganic N are released and carbon substrates become available. As a result
respiration increases, thereby creating anaerobic conditions and enhancing
denitrification. (Watts & Seitzinger, 2000)
However the composition of the carbon source might also effect denitrification. Soil
organic matter that is readily decomposable and denitrification shows a stronger
correlation than total organic carbon and denitrification. (Pavel, Reneau Jr, Berry,
Smith, & Mostaghimi, 1996)
Oxygen availability
Since the denitrifying bacteria need anaerobic conditions denitrification can only
occur when oxygen concentrations are sufficiently low. Trevors (1985) found
denitrification to be negligible in soil samples with oxygen concentrations above 0.28
µmol mL-1. However when the concentration was 0.20 µmol mL-1 or less
denitrification did take place. (Martin, Kaushik, Trevors, & Whiteley, 1999)
Method
The soil samples were collected in Fujian province, southeast China, close to a small
town called Nanjing, see Figure 4 and Figure 5. They were exclusively collected in
the bamboo forest in the riparian zone of Wuchuan river at a distance of 17 m, 37 m,
52 m and 72 m from the riverside edge of the bamboo forest, see Figure 6. The
samples were incubated in the sites June 24th 2008 and recovered July 1st 2008.
11
Figure 4 Map over Fujian Province. The samples were collected in the area the arrow is
pointing towards.
Figure 5 View from the site where the first sample was collected.
Distance from the riverbank (m)
80
70
Distance (m)
60
50
40
30
20
10
0
0
1
2
3
Sampling site
4
5
Figure 6 The sampling sites and the distance to the riverbank.
Wuchuan river is a tributary of the upper Jiulong River. Rainfall in the area is strongly
influenced by the monsoon system. Red earth and lateritic red earth are the main soil
12
types. Land use is very diverse including horticulture, forestry, vegetables, residential
areas, rice paddies and fishponds.
The sample sites were approximately 4x4 m2 and the soil samples were collected in
plastic bags and PVC pipes (8 cm diameter by 15 cm in length). The plastic bags
were brought back, dried and analyzed June 26th. The PVC pipes were enveloped by
plastic foil on the top and a piece of material on the bottom. The plastic foil was
perforated in order to let air in to the soil. The PVC pipes were buried in the land and
incubated until July 1st when they were retrieved from the sites. Two PVC pipes were
buried in each site, thus a quality control was made. Figure 7 shows the PVC pipes
as incubated in the sites.
Figure 7 The PVC pipes as incubated in the sites.
The soil was dried and sieved to 2 mm. The organic matter was measured by
hydration potassium dichromate oxidation colorimetric method and determined using
an ultraviolet spectrophotometer. The NO3-N and the NH3-N was determined using
the indigo hydroxybenzene blue colorimetric method and analyzed with an ultraviolet
spectrophotometer. Total phosphorus was determined using the antimonomolybdenum colorimetry method.
Soil moisture was determined gravimetrically after drying fresh soil samples collected
at each site for 24 h at 105 degree Celsius. Soil pH was measured through extraction
with water using a calibrated pH meter.
Results and Discussion
In this research project the aim was to measure both soil mineralization and
denitrification rates in riparian zone, thus mapping out the nitrogen fluxes in the area.
Nitrogen is of great importance for the plant- and microorganism society. Both the
lack of nitrogen and the excess of nitrogen might be problematic.
13
Controlling factors
The controlling factors researched were pH, organic matter and soil moisture.
Besides these, total phosphorus and available phosphorus was measured in case of
deviant data. The temperature ought to have been measured, but weren’t. An
approximate estimation is that the temperature ranged between 24 ˚C and 35 ˚C.
Soil moisture (%)
30
25
%
20
15
10
5
0
0
1
2
3
Sampling site
4
5
Figure 8 Soil moisture as measured in the sites i e as a function of the distance to the river
14
Organic matter and pH
6
5
4
Organic matter (%)
pH
3
2
1
0
0
1
2
3
Sampling site
4
5
Figure 9 Organic matter and pH as measured in each sampling site
Table 1 The controlling factors, organic matter, soil moisture and pH, measured in the sites
Sampling site
1
2
3
4
Organic matter (%)
2.976
1.581
1.374
1.885
Soil moisture (%)
27.64
24.29
21.87
16.1
pH
5.5
5.32
4.86
4.76
The soil moisture ranged between 27.6 % and 16.1 %, as seen in Figure 8 and Table
1. These relatively high values are explained by the short distance to river. High soil
moisture often means anaerobic conditions which promotes denitrification but inhibits
nitrification. Davidson et al report that nitrous oxide production from nitrification is
most important on well-drained sites whereas nitrous oxide production from
denitrification is most important on poorly drained sites (Davidson, Swank & Perry,
1986). A typhoon hit the sampling area in June 25th and June 26th. Since the soil
moisture was measured at samples collected June 24th the soil moisture during the
incubation time might actually have been even higher than the ones we measured.
This should have a damping effect on mineralization rate.
Figure 9 and Table 1 shows the pH as measured in the sites. The pH was slightly
acidic which is to be expected since the dominating bedrock in the area is granite
15
and the main soil type is lateritic red earth and red earth. It is said in the study by
Pavel et al that low soil pH usually inhibits the soil microorganisms thereby inhibiting
most of the processes in the nitrogen cycle. However a pH ranging between 5.5 and
4.76 is not acidic enough to inhibit denitrification or mineralization though it might
contribute with a damping effect.
The organic matter in the soil ranged in values between 2.98 % and 1.37 % as seen
in Figure 9 and Table 1. A study conducted in the same area for a master thesis by
Da Peng Li showed a level of 2.5 % (Li, 2007). Based on this, the level of the organic
matter seems to be normal. It is probably just coincidence that the organic matter
showed higher levels in site one, however it adds some uncertainty to the analysis
since the pH in site one also is the highest. A high level of organic matter is normally
linked to a decrease in pH since the process of decomposition of organic matter
makes the soil acidic.
The experiments to measure TP and AP failed, most likely because the equipment
wasn’t clean enough. The received values are not included in the report since they
are not plausible and hence wouldn’t contribute to the analysis.
Magnitude of mineralization, ammonification and nitrification rates
Table 2 Levels of NO3-N and NH3-N measured in the sites in June 24th and July 1st
Sampling
site
1
2
3
4
NO3-N (mg/kg)
June 24
2.57
1.43
0.68
0.31
NO3-N (mg/kg)
July 1
7.96
7.23
6.07
5.75
NH3-N (mg/kg)
June 24
1.555
1.611
1.077
2.2
NH3-N (mg/kg)
July 1
1.955
0.511
0.933
0.988
The level of NH3-N decreased during the incubation (except for one differing value)
and the level of NO3-N increased as seen in Table 2. The levels of NO3-N are small
compared to data collected in Devon, England by Dendooven, Duchateau and
Anderson (1995). They show levels of NO3- ranging between 30.73 mg N/kg and
207.62 mg N/kg. The levels of NO3- are low also compared to data collected in the
same area by Xu Yuyu for his master thesis (2007). He showed levels of 25.5 mg
N/kg in the bamboo forest of Wuchuan catchment. The concentration of NH4+ is also
low compared to the data collected by Xu Yuyu which showed levels of 38.4 mg N/kg
in the area.
16
The nitrogen mineralization rate, the nitrogen ammonification rate and the nitrogen
nitrification rate was calculated using the following equations:
A1 = NH3-N June 24th
A2 = NH3-N July 1st
N1 = NO3-N June 24th
N2 = NO3-N July 1st
d = number of days (in this case 6.5 days)
Nmin = (A2 + N2 –A1 –N1) / d
Namm = (A2 - A1) / d
Nnitr = (N2 - N1) / d
Table 3 Nitrogen mineralization rate, ammonification rate and nitrification rate measured in the
sites between June 24th and July 1st
Sampling site
1
2
3
4
Nmin (mg N/kg *d)
1.13
0.97
0.97
0.99
Namm (mg N/kg *d)
0.062
-0.17
-0.022
-0.19
Nnitr (mg N/kg *d)
0.83
0.89
0.83
0.84
1.2
1
mg N/kg * d
0.8
0.6
N ammonification
N mineralization
N nitrification
0.4
0.2
0
0
1
2
3
4
5
-0.2
-0.4
Sampling site
Figure 10 Nitrogen mineralization rate, ammonification rate and nitrification rate measured in
the sites between June 24th and July 1st
17
Table 4 Comparison of mineralization rates between studies and areas
Study by
Kajsa-Stina Kalin
Da Peng Li
Kristensen et al
Area studied
Wuchuan catchment, China
Wuchuan catchment, China
Denmark
Nmin (mg N/kg*d)
0.93 ～ 1.13
-0.58 ～ 1.47
0.30 ～ 0.70
The nitrogen mineralization rate ranged between 0.97 mg N / kg · d and 1.13 mg N /
kg · d. These values are in the same range as the data collected by Da Peng Li
where the mineralization rate ranged between -0.58 mg N / kg · d and 1.47 mg N / kg
· d. The study by Da Peng Li had figures ranging between -0.83 mg N / kg · d and
1.34 mg N / kg · d. The values are also validated by a study conducted in Denmark
by Kristensen et al (2003). They showed rates of mineralization ranging between
0.30 to 0.70 mg N / kg · d.
The ammonification rate ranged between -0.19 mg N / kg · d and 0.062 mg N / kg · d
and was also validated by the study conducted by Da Peng Li. His figures ranged
between -0.83 mg N / kg · d and 1.34 mg N / kg · d. The nitrification rate ranged
between 0.83 mg N / kg · d and 0.89 mg N / kg · d. This is however high compared to
the data collected by Da Peng Li where the nitrification rate ranged between -0.11
mg N / kg · d and 0.59 mg N / kg · d.
The rate of mineralization was quite high as is expected in summer when the
temperature is high. The temperature is the absolutely dominating factor in the
mineralization process. The levels are however not as high as the ones measured by
Da Peng Li in the same area and the same month. He then received mineralization
rates of 1.47 mg N / kg · d. The rate measured by Da Peng Li was probably higher
due to lower soil moisture content. The soil moisture content measured in this study
was pretty high and the mineralization rate usually shows a weakening tendency with
increasing water content in the soil. However the mineralization rate is highest in site
one, where the soil moisture is highest. This might be an indication that pH is of even
greater importance than soil moisture since the pH in site one is the highest. A high
pH promotes mineralization. Due to limited time and limited experiments it is however
not recommended to draw any conclusions of this extent from our experiments.
Denitrification
Due to closed businesses during the summer holidays the measurements of the
denitrification rates weren’t conducted. The method is however worth discussing. The
method we would have used is called the acetylene (C2H2) inhibition method and is
the most widespread and commonly used. It is based on the principle that C2H2
18
blocks the enzymatic reduction of N2O to N2, thereby allowing the denitrification rate
to be measured based on measurements of N2O (Watts & Seitzinger, 2000).
However it adds an inhibitor to the system and the inhibitor may not always be
effective. A number of problems are associated with the acetylene inhibition method
including uneven penetration of acetylene into soil microsites, incomplete inhibition at
low soil nitrate concentrations and incomplete inhibition in the presence of sulfides.
The acetylene method might also inhibit nitrification which is a huge problem since
nitrification may be a major source of nitrate. (Watts & Seitzinger, 2000) Partitioning
N2O production between different processes is problematic because, most notably,
denitrification is dependent in part at least on nitrification, so the processes can not
be exclusively suppressed. (Simek, Elhottova, Klimes & Hopkins, 2003) The
acetylene inhibition method is overall thought to underestimate the denitrification rate.
Overall discussion
It is often difficult to compare data from different studies because of differences in
methods including storage of soils after sampling and before measurement. For
instance differing temperatures during incubation and incubation under field or
laboratory conditions could affect the resultant data. It is important that different
methods to measure and calculate data are compared to each other and taken in to
account while comparing data from different studies.
Another problem is the high spatial and temporal variability. Usually large chambers
(exceeding 1 m in diameter and height) are recommended to overcome
heterogeneity in spatial flux (Simek, Elhottova, Klimes & Hopkins, 2003). The
existence of temporal changes often needs the measurements to be done frequently,
perhaps as frequently as several times per day (Simek, Elhottova, Klimes & Hopkins,
2003). In our experiment we didn’t take the heterogeneity of the soil into account,
neither the temporal variations. The measurements were conducted at the soil in the
PVC pipes without consideration of soil depth and were only conducted twice. The
most common pattern is a decreasing rate of mineralization and denitrification with
depth. If a more through analysis of the soil mineralization and denitrification rate in
the area is to be made, the soil depth ought to be considered in the analysis. Also the
sampling ought to be made more frequently.
The processes in the nitrogen cycle are intimately linked and often dependent of
each other. It is hard to be sure of how different processes affect and influence each
other. Much work has been done to examine the effect of carbon, nitrate
concentration, moisture content and temperature. Quantification of their role remains
complicated by a high degree of spatial and temporal variability, and the number of
interactions and feedback of the denitrification and mineralization processes
themselves (Ellis, Dendooven & Goulding, 1995). Besides apparent environmental
controllers, temperature, moisture, oxygen concentration, NO3-, available carbon
19
supply and the microbial community, other factors need to be considered when
modelling and predicting nitrogen fluxes in soil.
Conclusions
Since there was limited time and limited experiments, conclusions shouldn’t be drawn.
Evaluation
Unfortunately enough my project couldn’t begin until the second week. Knowing what
I know today I think it would have been a good idea if we would have started the first
week with preparing the standard solutions and reagents. Because we didn’t start our
laboratory work until the second week we had very limited time and the Chinese
students had to conduct one of the experiments during a weekend when I couldn’t
join.
Further more all the methods were in Chinese at the start of our experiments and
none of them were translated to English until the experiments had already been
conducted. This put me in a very vulnerable position and left me without knowledge
of what we were doing. The Chinese students were both very nice to me and tried
their best to explain what we were doing in English, but since their English was
limited I hardly ever knew what we were doing and I couldn’t be of much assistance.
It got better during the weeks though, and in the end of the course communication
was much better, thus I understood what we were doing and could take own
initiatives in the laboratory work.
I think that the students in my project have been very kind and helpful. But there have
been problems with the communication and I think it would be in everybody’s interest
that the students elected for this type of project are comfortable in speaking English.
References
Reports
Butturini, A., Bernal, S., Sabater, S., & Sabater, F. (2002). The influence of riparianhypoheric zone on the hydrological responses in an intermittent stream. Barcelona:
Hydrology and Earth System Sciences, 6(3), 515-525 (2002).
Davidson, E.A., Swank, W.T., Perry, T.O. (1986). Distinguishing between Nitrification
and Denitrification as Sources of Gaseous Nitrogen Production in Soil. North Carolina,
U.S.A: Applied and Environmental Microbiology, Dec 1986, p. 1280-1286
20
Dendooven, L., Duchateau, L., Anderson, J.M. (1995). Gaseous products of the
denitrification process as affected by the antecedent water regime of the soil. Soil
Biol. Biochem. Vol. 28, No. 2, pp. 239-245, 1996
Ellis, S., Dendooven, L., Goulding, K.W.T. (1995). Quantitative assessment of soil
nitrate disappearance and N2O evolution during denitrification. Herts, U.K.: Soil Biol.
Biochem. Vol. 28, No. 4/5, pp. 589-595, 1996
Kristensen, H. L., Debosz K., McCarty, G. W. (2003). Short-term effects of tillage on
mineralization of nitrogen and carbon in soil. Aarslev, Denmark: Soil Biology and
Biochemistry, Vol. 35, Issue 7, July 2003, Pages 979-986
Li, Da Peng (2007). Field studies on Soil Nitrogen and Mineralization in an
Agriculture Catchment, South Fujian Province. Xiamen University, Master Thesis
Martin, T. L., Kaushik, N., Trevors, J., & Whiteley, H. (1999). Review: Denitrification
in temperate climate riparian zones. Water, Air and Soil Pollution 111 , 171-186.
Merrill, A., & Benning, T. (2005). Ecosystem type differences in nitrogen process
rates and controls. Berkeley and San Francisco: Forest Ecology and Management
222 (2006) 145–161.
Meynendonckx, J., Heuvelmans, G., Muys, B., & Feyen, J. (2006). Effects of
watershed and riparian zone characteristics on nutrient. Belgium: Hydrol. Earth Syst.
Sci., 10, 913–922, 2006.
Pavel, E. W., Reneau Jr, R. B., Berry, D. F., Smith, E. P., & Mostaghimi, S. (1996).
Denitrification potential of nontidal riparian wetland soils in the Virginia coastal plain.
Wat. Res. Vol. 30, No 11, pp. 2798-2804, 1996.
Shivoga, W. A., Muchiri, M., Kibichi, S., Odanga, J., Miller, S. N., Baldyga, T. J., et al.
(2007). Influences of land use/cover on water quality in the upper and middle reaches
of River Njoro, Kenya. Njoro, Kenya: Lakes & Reservoirs: Research and
Management 2007 12: 97–105.
Simek, M., Elhottova, D., Klimes, Frantisek., Hopkins, D. W. (2003). Emissions of
N2O and CO2, denitrification measurements and soil properties in red clover and
ryegrass stands. Czech Republic and Scotland, UK: Soil Biology & Biochemistry 36
(2004) 9-21.
Watts, Susan H., Seitzinger Sybil P. (2000). Denitrification rates in organic and
mineral soils from riparian sites: a comparison of N2 flux and acetylene inhibition
methods. New Brunswick, NJ, USA: Soil Biology & Biochemistry 32 (2000) 13831392.
Yuyu, Xu (2007). Study on Soil Denitrification in an Agricultural Catchment in the
Southeastern Area of Fujian Province. Xiamen University, Master Thesis
21
Electronic sources
Carpenter, S., Caraco, N. F., Correll, D. L., Howarth, R. W., Sharpley, A. N., & Smith,
V. H. (1998, 08). Nonpoint Pollution of Surface Waters with Phosphorus and
Nitrogen. Retrieved 05 20, 2008, from The Ecological Society of America (ESA):
http://www.esa.org/science_resources/issues/TextIssues/issue3.php
Corbin, K. (1998 01 05). Biogeochemical cycles. Retrieved 03 07, 2008, from
BIOL/CSES 4684:
http://filebox.vt.edu/users/chagedor/biol_4684/Cycles/Mineralization.html
Gordon, N. (2005, 03). Nitrogen in the Earth System. Retrieved 05 20, 2008, from
The University Corporation for Atmospheric Research, UCAR:
http://www.ucar.edu/news/backgrounders/nitrogen.shtml
Parkyn, S., Colley, R.D. (2007). Riparian management: how well are we doing?
Retrieved 07 10, 2009, from NIWA Science: http://www.niwa.cri.nz/pubs/wa/ma/114/riparian
Pidwirny, M. (2008, 04 17). The Nitrogen Cycle. Retrieved 05 20, 2008, from
PhysicalGeography.net: http://www.physicalgeography.net/fundamentals/9s.html)
Interview
Cao, P. W. (2008, 03 28). (K.-S. Kalin, Interviewer)
22
Appendix
Group pictures
23