Atmospheric Environment 44 (2010) 2920e2926
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Atmospheric Environment
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Short-term effect of increasing nitrogen deposition on CO2, CH4 and N2O
fluxes in an alpine meadow on the Qinghai-Tibetan Plateau, China
Chunming Jiang a, b, d, *, Guirui Yu a, Huajun Fang a, Guangmin Cao c, Yingnian Li c
a
Synthesis Research Center of Chinese Ecosystem Research Network, Key Laboratory of Ecosystem Network Observation and Modeling,
Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, PR China
b
Graduate University of the Chinese Academy of Sciences, Beijing 100039, PR China
c
Northwest Plateau Institute of Biology, Chinese Academy of Sciences, Xining 810008, PR China
d
Laboratory of Nutrients Recycling, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 October 2009
Received in revised form
23 March 2010
Accepted 24 March 2010
An increasing nitrogen deposition experiment (2 g N m2 year1) was initiated in an alpine meadow on
the Qinghai-Tibetan Plateau in May 2007. The greenhouse gases (GHGs), including CO2, CH4 and N2O, was
observed in the growing season (from May to September) of 2008 using static chamber and gas chromatography techniques. The CO2 emission and CH4 uptake rate showed a seasonal fluctuation, reaching
the maximum in the middle of July. We found soil temperature and water-filled pore space (WFPS) were
the dominant factors that controlled seasonal variation of CO2 and CH4 respectively and lacks of correlation between N2O fluxes and environmental variables. The temperature sensitivity (Q10) of CO2
emission and CH4 uptake were relatively higher (3.79 for CO2, 3.29 for CH4) than that of warmer region
ecosystems, indicating the increase of temperature in the future will exert great impacts on CO2 emission
and CH4 uptake in the alpine meadow. In the entire growing season, nitrogen deposition tended to
increase N2O emission, to reduce CH4 uptake and to decrease CO2 emission, and the differences caused
by nitrogen deposition were all not significant (p < 0.05). However, we still found significant difference
(p < 0.05) between the control and nitrogen deposition treatment at some observation dates for
CH4 rather than for CO2 and N2O, implying CH4 is most susceptible in response to increased nitrogen
availability among the three greenhouse gases. In addition, we found short-term nitrogen deposition
treatment had very limited impacts on net global warming potential (GWP) of the three GHGs together
in term of CO2-equivalents. Overall, the research suggests that longer study periods are needed to verify
the cumulative effects of increasing nitrogen deposition on GHG fluxes in the alpine meadow.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Anthropogenic nitrogen deposition
CO2
CH4
N2O
Q10
Water-filled pore space
Alpine meadow
Qinghai-Tibetan plateau
1. Introduction
Anthropogenic nitrogen deposition, mainly originating from
fertilizer application, fossil fuel combustion and legume cultivation,
drastically increased since the industrial revolution (Matson et al.,
2002) and the increasing trend is considered to be enhanced in
the next few decades (Galloway et al., 2004). The increase of
nitrogen availability will exert great influences on the ecosystem
processes, including stimulating plant growth, changing species
composition and diversity and altering greenhouse gas (GHG) fluxes
(Matson et al., 2002). The effect of anthropogenic nitrogen deposition on GHG fluxes has caused great concerns because GHGs
* Corresponding author at: Institute of Geographical Sciences and Natural
Resources Research, Chinese Academy of Sciences, Beijing 100101, PR China.
E-mail address: [email protected] (C. Jiang).
1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2010.03.030
(CO2, CH4 and N2O) in the atmosphere play an important role in
regulating the global climate (Conrad, 1996; Dalal and Allen, 2008).
Soils are important sources or sinks of the GHGs (Conrad, 1996).
CO2 emission is through root respiration, rhizomicrobial respiration
and soil organic matter decomposition; CH4 is produced by methanogens and consumed by methanotrophs; Nitrification and denitrification are the main processes causing N2O production from
soils (Liu and Greaver, 2009). Environmental factors governing GHG
production and consumption include availability and amount
of substrates, temperature, soil water content, ion deficiencies and
toxicities, atmospheric nitrogen deposition and so on (Dalal and
Allen, 2008). Experimental additions of nitrogen to soil typically
result in decreased CO2 emission and CH4 consumption and increased
N2O emission (Burton et al., 2004; Mo et al., 2008; Mosier et al., 1991;
Scheer et al., 2008), although contrary effects and lacks of response
also have been reported (Ambus and Robertson, 2006; Bodelier and
Laanbroek, 2004; Bradford et al., 2001; Hall and Matson, 1999).
C. Jiang et al. / Atmospheric Environment 44 (2010) 2920e2926
Simulated atmospheric nitrogen deposition experiments have
conducted in many natural ecosystems. However, the impact of
nitrogen deposition on the production or consumption of GHGs is not
well understood and has not been assessed in the alpine meadow on
the Qinghai-Tibetan Plateau. The plateau covers nearly one-quarter of
the area of China and represents one of the largest alpine grasslands in
the world (He et al., 2006). More valuably, the alpine meadow of
Qinghai-Tibetan Plateau, which is very far away from the industrial
area and remains relatively undisturbed by humans, receives much
smaller of atmospheric nitrogen deposition (wet deposition is
only 0.46 g N m2 year1, the data is not published) relative to the
nitrogen pollution regions in Eastern United States and Western
Europe (typically greater than 2.5 g N m2 year1) (Holland et al.,
2005; MacDonald et al., 2002). Consequently, the alpine meadow on
the Qinghai-Tibetan Plateau is an ideal region in which to verify the
actual consequences of increasing nitrogen deposition for GHG fluxes.
Moreover, the experiment of increasing nitrogen deposition on the
alpine meadow can contribute to reducing the uncertainty of QinghaiTibetan plateau in global GHG budgets. The specific objects of the
study are: (1) to quantify short-term effects of increasing nitrogen
deposition on GHG fluxes in the alpine meadow; (2) to investigate the
responses of GHG fluxes to changes of environmental factors.
2. Materials and methods
2.1. Site description
The research was conducted in an alpine meadow, located in
Haibei Alpine Meadow Ecosystem Research Station, Northwest
Plateau Institute of Biology, Chinese Academy of Sciences (37 370 N,
1011200 E and at an altitude of 3250 m above sea level). The local
climate is characterized by strong solar radiation with long, cold
winters, and short, cool summers. The average annual air temperature was 1.7 C. The mean, maximum and minimum of averaged
air temperature were 8.7, 15.6 and 2.5 C, respectively, in summer
and 13.2, 2.2 and 22.1 C, respectively, in winter. Annual mean
precipitation is 580 mm, and about 80% of precipitation is
concentrated in the growing season from May to September
(Li et al., 2004). The soil is a clay loam with an average thickness of
65 cm. The soils, which are classified as Mat Cry-gelic Cambisols
according to the Chinese national soil survey classification system
(Chinese Soil Taxonomy Research Group, 1995), are wet and high in
organic matter. Characteristics of the soils in the experimental plots
are listed in Table 1 (Zhang and Cao, 1999). The plant community in
this alpine meadow is dominated mainly by Kobresia humulis, Stipa
aliena, Elymus nutans, Saussurea superba, Gentiana straminea and
Potentilla nivea (Wang et al., 1998).
2.2. Experimental treatment
An increasing nitrogen deposition experiment (2 g N m2 year1)
was set up in the alpine meadow in May 2007. Six plots (each with
3 m 3 m dimension) were assigned to two treatments (increasing
2921
nitrogen deposition and control) with three replications. The treatments were laid out randomly. NH4Cl was used in the experiment.
Nitrogen application was initiated at the onset of the experiment and
continued throughout the GHG observation period. One year’s
amount of added nitrogen was divided into two equal portions: one
portion in the growing season (from May to September) and the
other portion in the non-growing season. Over the growing season,
nitrogen was added at the first day of every month as 6 equal doses.
Duo to the severe weather condition in the non-growing season,
nitrogen was all applied in one time in December. During each
application, NH4Cl was weighed, dissolved in 1L water, and applied
to each plot using a sprayer. Two passes were made across each plot
to ensure an even distribution of fertilizer. The control plot received
1L water without nitrogen.
2.3. GHG fluxes and environmental parameters measurements
Fluxes of CO2, N2O and CH4 and were measured by using opaque,
static, manual stainless steel chambers and gas chromatography
techniques (Wang and Wang, 2003). The chamber is an open-bottom
a square box (50 50 50 cm) and equipped with a fan installed on
the top wall of each chamber to make turbulence when chamber was
closed. Outside of the chamber was covered with white plastics foam
to reduce the impact of direct radiative heating during sampling.
In April 2008, a stainless steel base (50 50 20 cm) with a water
groove on top was installed at each of the six plots. Gas samples were
taken between 09:00 and 11:00 h local time four times a month from
May to September in 2008. In the sample date, six chambers were
placed over the bases filled with water in the groove to ensure airtightness and the gas samples were taken simultaneously in the six
plots. Gas sample inside the chamber was taken for every 10 min
over a 30 min period by using 100 ml plastic syringes (total of four
samples). CO2, N2O and CH4 concentrations of gas samples were
analyzed with gas chromatography (HP Series 4890D, Hewlett
Packard, USA) within 24 h following gas sampling. The gas chromatography was equipped with an electron capture detector for N2O
analysis and a flame ionization detector for CH4 and CO2 analysis.
The gas chromatography configurations for analyzing concentrations of CO2, N2O and CH4 were according to the method of
Wang and Wang (2003). The calculation of GHG flux followed the
description of Scheer et al. (2008). Negative flux values indicate GHG
uptake from the atmosphere, and positive flux values indicate GHG
emissions to the atmosphere.
In order to evaluate the net global warming impact of the three
GHGs together caused by increasing nitrogen deposition, the
cumulative fluxes of CO2, CH4 and N2O during the observation
period was estimated according to the method of Gu et al. (2007)
and then the net global warming potential (GWP) of the three
GHGs was calculated (Nykanen et al., 1995).
Air temperature (Ta), 5 cm soil temperature (Ts) and 10 cm
volumetric soil moisture (%) were monitored at each chamber
during gas sample collection. Ta and Ts ( C) was measured using
a digital thermometer (SN2202, China). Soil volumetric soil
Table 1
Characteristics of the alpine meadow soila.
Depth
(cm)
Organic
matter (%)
C/N
pH
CaCO3 (%)
CEC
(cmol kg1)
Total N (%)
Hydrolyzable
N (mg kg1)
Total P (%)
Available P
(mg kg1)
Total K (%)
Available K
(mg kg1)
0e10
10e20
20e50
50e70
70e110
12.07
8.64
3.09
1.29
3.54
10.2
10.6
10
11.4
11.3
7.3
7.7
8.3
8.5
8.4
0
0
4.29
4.75
6.41
29.88
31.02
16.44
14.94
25.32
0.419
0.392
0.173
0.063
0.181
103.5
106.8
79
32.9
58.7
0.09
0.088
0.08
0.093
0.069
7.3
3.6
0.2
7.5
1.6
2.14
2.08
2.18
2.14
e
315
187.3
97
70.5
116.1
a
Data are cited from Zhang and Cao (1999).
2922
C. Jiang et al. / Atmospheric Environment 44 (2010) 2920e2926
moisture (%, m3 H2O m3 soil) was determined using a time domain
reflectometry (MPKit, China). The volumetric soil moisture was
transformed to water-filled pore space (WFPS): WFPS ¼ volumetric
soil moisture/(1-bulk density/2.65).
2.4. Statistical analyses
The distributions of GHG fluxes and environmental parameters
in the growing season were all tested for normality with the ShapiroeWilk test. ManneWhitney U-tests were used to analyze
differences of GHG fluxes between control and increasing nitrogen
deposition plots (values in every sample date and cumulative
values in the growing season). To explore the effect of sample date
and the changes of treatment effect with sample date, repeatedmeasures ANOVA was also employed with sample date as the
repeated factor. Correlation analyses and stepwise linear regression
analyses were used to examine the relationships between GHG
fluxes and the measured environmental variables. The van’t Hoff
equation (y ¼ aexp(bTs)) was established to calculate the temperature sensitivity (Q10¼ exp(10b)) of GHG fluxes to the changes of Ts.
SPSS13.0 was used for all statistical analyses.
3.2. CO2 flux
The alpine meadow soil was a source of CO2 and displayed clear
seasonal pattern. The magnitude of CO2 increased very sharply in
the middle of June and the maximum flux occurred in the middle of
July (Fig. 2a). The CO2 fluxes were positively correlated with Ta
(p < 0.001) and Ts (p < 0.001), and negatively correlated with WFPS
(p < 0.001). The WFPS was excluded in the stepwise linear
regression, indicating temperature was the main controlling factor
for temporal variation of CO2 emissions in the alpine meadow.
The Q10 of CO2 emissions was 3.79 (Table 2). The nitrogen deposition did not affect the seasonal varying pattern of CO2. Before
August, the CO2 fluxes were nearly identical between control and
nitrogen deposition plots and nitrogen deposition tended to
suppress the CO2 emissions in August and September (Fig. 2a). In
the entire growing season, there was no significant difference (p ¼
0.82) of CO2 emissions between the control and nitrogen deposition
3. Results and discussion
3.1. Environmental variables
Ta, Ts and WFPS showed clear seasonal patterns in the alpine
meadow. Ta and Ts reached maximum (about 15 C) (Fig. 1a) while
WFPS reached minimum (below 30%) (Fig. 1b) in the middle of July.
The variation of Ts was more stable than that of Ta. The mean values
of the growing season were 10.3 C for Ta, 9.7 C for Ts and 55.6% for
WFPS. The total precipitation of the growing season was 269.1 mm.
Fig. 1. Air temperature, 5 cm soil temperature, WFPS of 10 cm soil and rainfall in the
alpine meadow in the growing season.
Fig. 2. CO2 (a), N2O (b), and CH4 (c) fluxes from the alpine meadow as affected by the
increasing nitrogen deposition (ND) in the growing season. Error bars indicate standard
errors (n ¼ 3) of mean. Asterisk (*) indicates significant difference between control (CK)
and increasing nitrogen deposition plots (ManneWhitney U-test, p 0.05).
C. Jiang et al. / Atmospheric Environment 44 (2010) 2920e2926
2923
Table 2
Relationship between GHG exchanges and soil/climate variables measured in the alpine meadow on the Qinghai-Tibetan Plateau, China. Pearson correlation coefficients,
stepwise liner regression equations and van’t Hoff equations are presented.
Correlation analysis (Pearson)
CO2 flux
N2O flux
CH4 flux
Stepwise linear regression
van’t Hoff
Ta
Ts
WFPS
Equation
Adj. r2
Equation
Q10
0.75***
0.04
0.63**
0.91***
0.01
0.74***
0.7***
0.18
0.9***
y ¼ 65.4Ts þ 21.5Ta306.7
e
y ¼ 1.036 WFPS-94.1
0.86***
e
0.79***
y ¼ 143exp(0.133Ts)
e
y ¼ 11exp(0.119Ts)
3.79
e
3.29
The air temperature and 5 cm soil temperature are Ta and Ts respectively, and water-filled pore space is WFPS. The level of significance of correlations and regression models
are, **P 0.01 and ***P 0.001.
plots and the mean CO2 fluxes of nitrogen deposition and control
plots were 535.7 45.74 mg m2 h1and 548.5 47.35 mg m2 h1,
respectively. The interactive effect between sample date and
nitrogen deposition on CO2 flux was also not observed (p ¼ 0.73)
(Table 3).
The magnitudes of CO2 flux ranged from 222 mg m2 h1 to
897 mg m2 h1 in our study, which were comparable to previous
studies in the alpine meadow (Cao et al., 2004; Hu et al., 2008) and
in a temperate steppe on the Inner Mongolia (Niu et al., 2008) and
was markedly larger than magnitudes of CO2 flux in the alpine
grassland of Wudaolang on the Qinghai-Tibetan Plateau (Pei et al.,
2003). Temperature (especially soil temperature) was the dominant
environmental variable that controlled the seasonal change of CO2
flux in our study, which has been documented in many other
studies (Fang and Moncrieff, 2001; Lloyd and Taylor, 1994). Q10, the
temperature sensitivity of respiration, was 3.79 in the alpine
meadow, which agreed very well with the result of Kato et al.
(2004), who quantified the temperature sensitivity of nighttime
CO2 flux using the eddy covariance method in the same alpine
meadow. The temperature sensitivity of respiration has made
intensive study in global scale and across China, including various
kinds of ecosystems (Chen and Tian, 2005; Peng et al., 2009; Raich
and Schlesinger, 1992; Tjoelker et al., 2001; Zheng et al., 2009). The
Q10 values in global scale varied from 1.3 to 3.3 and had a median
value of 2.4 (Raich and Schlesinger, 1992). Our result is at the high
end value of global scale, indicating the increasing of temperature
in the future will exert great impacts on the ecosystem respiration
of the alpine meadow. WFPS is another important factor affecting
CO2 emission. Positive or negative effects of WFPS on soil CO2 flux
have been reported (Davidson et al., 1998, 2000; Franzluebbers
et al., 2002; Sotta et al., 2006). Xu and Qi (2001) found a splitting
point of soil moisture of 19% (volumetric soil moisture), above
which the effect of soil water content on CO2 emission turned from
positive to negative. In our study, CO2 flux was negatively correlated with WFPS, probably because overabundant soil water
content inhibited plant root andmicrobeactivity or blocked CO2
diffusing from soil to atmosphere. Alternatively, decrease of CO2
emissions with increase of WFPS could be an artifact of changing
soil temperatures whose effect was not separated in this analysis
(Kato et al., 2004). A multi-factor controlled experiment is needed
to identify WFPS, soil temperature and their interactive effect on
CO2 flux in the alpine meadow (Niu et al., 2008).
Before August, the CO2 fluxes were nearly identical between
control and nitrogen deposition plots and nitrogen deposition
suppressed CO2 emissions by only 7% in August and September. The
lack of response of CO2 fluxes to short-term nitrogen deposition
agreed with the finding of Mo et al. (2008) and Burton et al. (2004).
With the lasting of the nitrogen deposition experiment, decreasing
magnitude of CO2 emissions may be even more pronounced
(Bowden et al., 2004). There are two mechanisms explaining
reduction of CO2 emission after nitrogen addition: (1) Because
a large fraction of soil respiration is related to nitrogen assimilation
of plants, with the increasing of nitrogen availability, energetic
costs of nitrogen assimilation may be reduced (Bowden et al.,
2004); (2) Microbial respiration could be decreased in response
to nitrogen additions indicated by that the decomposition rates of
litter and soil organic matter were suppressed (Berg and Matzner,
1997; Zak et al., 2008).
3.3. N2O flux
Variation of N2O fluxes did not show clear seasonal pattern.
There were two peaks of N2O fluxes in the growing season: one at
the end of May, the other at the beginning of August (Fig. 2b).
We did not find correlation between the N2O fluxes and environmental variables (Table 2). Nitrogen deposition tended to increase
N2O emission from middle of July to middle of August (Fig. 2b).
In this study we observed a very high inherent spatial variability
of N2O flux (Röver et al., 1999; Velthof et al., 1996) in the experiment plots, with an average coefficient of variation (CV) of 518%,
which was obviously larger than that of CO2 (15.7%) and CH4
(33.8%). The results suggest that increasing the number of spatial
replicates is very necessary in the N2O observation so that we could
estimate the N2O flux and identify the effect of nitrogen deposition
more exactly. The magnitudes of N2O fluxes in the alpine meadow
was in the range of 2.05 ug m2 h1 to 5.65 ug m2 h1, which
were in the very low part of the range of temperate grassland (Du
et al., 2006; Glatzel and Stahr, 2001; Mosier et al., 1991; Wang et al.,
2005) and tropical pasture (Garcia-Montiel et al., 2001; Passianoto
et al., 2003; Steudler et al., 2002). N2O production is through
nitrification and denitrification, in which microorganism use the
mineral nitrogen (NHþ
4 and NO3 ) as substrate (Conrad, 1996).
The low N2O emission is due to the limited mineral nitrogen in the
alpine meadow soil caused by low mineralization rates of organic
nitrogen (Cao and Zhang, 2001). The negative N2O fluxes were
observed in the growing season, especially from the middle of
August to September. This phenomenon has been reported by
many studies (Clayton et al., 1997; Kellman and Kavanaugh, 2008;
Rosenkranz et al., 2006; Vieten et al., 2009). Rosenkranz et al.
(2006) assumed that in the case of shortage in nitrate supply
denitrifying bacteria might use atmospheric N2O as an alternative
electron acceptor to nitrate. Chapuis-Lardy et al. (2007) also
pointed out that N2O uptake seemed to be favored by low mineral
nitrogen and large moisture contents in soil from analysis a large
number of literatures. The two peaks of N2O in the growing season
were showed by other researches in German temperature grassland (Glatzel and Stahr, 2001) and subalpine meadow of Colorado
(Filippa et al., 2009). The first peak in the end of May could be
attributed to the thawing process of alpine soil, during which the
increasing of N2O emissions resulted from a sudden increase of
bioavailable carbon and nitrogen (Ludwig et al., 2006; Møkved
et al., 2006) or a quick release of N2O trapped by ice layer
(Goodroad and Keeney, 1984; Teepe et al., 2001). The absence of
N2O emission from middle of June to middle of July was probably
because that the mineral nitrogen was almost used up by plants.
This speculation could be supported by the evidence that the
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C. Jiang et al. / Atmospheric Environment 44 (2010) 2920e2926
Table 3
Repeated-measures ANOVA of CO2, N2O and CH4 fluxes as affected by increasing nitrogen deposition (ND) in the growing season.
df
Between subjects
ND
Within subjects
Date
Date ND
CO2 flux
N2O flux
CH4 flux
MS
F
p
MS
F
P
MS
F
p
1
4911
0.038
0.855
0.936
0.105
0.762
993.6
3.1
0.15
19
19
336 076
2514.2
103.7
0.776
0.001
0.728
18.197
5.608
1.71
0.527
0.053
0.942
1742.8
44.15
39.58
1
0.001
0.468
biggest biomass accumulation rate coincided with the lowest level
of mineral nitrogen in the soil at this period (Zuo and Le, 1980).
From middle of July to middle of August, we observed the second
N2O emission peak, resulting from the plants ceased to grow and
mineral nitrogen turned to increase. The largest discrepancy of N2O
emission between the control and nitrogen deposition plots was
detected in this period implied that the increased N2O emission
might be transformed from the added NH4Cl.
Lacks of correlation between N2O fluxes and environmental
variables were observed in the alpine meadow (Table 3), which has
been reported in the temperate forest ecosystems (Brumme et al.,
1999) and in the alpine grassland (Pei et al., 2003). This phenomenon might be because the N2O production process was very
complex and affected by many factors (Groffman et al., 2000).
A weak positive correlation between N2O flux and WFPS (r ¼ 0.18)
and the occurrence of second N2O flux peak coinciding with the
increase of WFPS (N2O emission peak mainly at high WFPS above
60%) suggested precipitation was a important factor controlling
N2O flux and denitrification might be the main source of N2O in the
alpine meadow because denitrification is an anoxic process, during
which O2 availability is the dominant regulating factor (Bollmann
and Conrad, 1998), while Xu et al. (2003) found nitrification was
the main source for N2O production in the Inner Mongolian semiarid steppe.
We did not found high N2O emission pulses (larger than 500 ug
m2 h1) illustrated in previous nitrogen fertilizer experiments
(added nitrogen ranged from 5 gN m2 year1 to 24 gN m2 year1)
(Mosier et al., 1991; Scheer et al., 2008; Zhang et al., 2007).
Our results could be supported by the conclusion of Bradford et al.
(2001), in which he called these nitrogen fertilizer experiments
(large amounts of nitrogen added on one or a few occasions) “agricultural fertilizer” type additions and he suggested simple extrapolation of the results of agricultural fertilizer studies to predict the
effect of elevated atmospheric nitrogen deposition (lower amounts
of nitrogen input and much more frequent) on GHG flux was inappropriate. No difference of N2O flux between control and nitrogen
deposition plots was detected in our short-term experiment
(Table 4), which was consistent with the results of a realistic levels of
nitrogen deposition experiment (1w3 gN m2 year1) for 2 years
(Ambus and Robertson, 2006). In the long-term nitrogen addition
experiments (Hall and Matson, 1999; Zak et al., 2008), their results
indicated decades time of atmospheric deposition of nitrogen
were needed before the nitrogen-limited ecosystems reached the
nitrogen-saturated status (Aber et al., 1998) in which a large amount
of nitrogen would be lost through leaching and nitrogen trace gas
emissions, because at the early stage of the experiments, added
nitrogen did not exceed the combined plant and microbial demands
for growth (Ambus and Robertson, 2006).
3.4. CH4 flux
The alpine meadow soil acted as a sink of CH4. The CH4 uptake
gradually increased from the beginning of the growing season,
reached maximum uptake value in July and then gradually declined
(Fig. 2c). There was positive correlation between CH4 uptake and Ta
(p < 0.01) or Ts (p < 0.001), and negative correlation between CH4
uptake and WFPS (p < 0.001). Among the three environment variables, WFPS was the dominant factor that controlled the CH4 uptake
in the alpine meadow from the stepwise linear regression analysis.
The Q10 of CH4 oxidation was 3.29 (Table 2). From the middle of June,
the CH4 uptake capacities in the nitrogen deposition plots were
always smaller than that in control. Some of these differences were
significant (p < 0.05) from the statistical analysis (Fig. 2c). The value
of CH4 uptake for nitrogen deposition plots was 15.3% smaller than
that for control plots in the entire growing season, however, this
discrepancy was not significant (p ¼ 0.12) (Table 4). There was no
interactive effect between sample date and nitrogen deposition for
CH4 uptake during the investigation period (p ¼ 0.47) (Table 3).
CH4 uptakes in the alpine meadow varied from 1.4 ug m2 h1 to
71.9 ug m2 h1, which were comparable to the range of CH4 uptake
in the temperature grassland in Colorado (Mosier et al., 1991) and in
Inner Mongolia (Wang et al., 2005). The Q10 of CH4 oxidation was
3.29, which was substantially larger than that in the temperature
forest soil (ranged from 1 to 2) (Bowden et al., 1998; King and
Adamsen, 1992; Macdonald et al., 1997). This comparison indicated soil methane consumption in the alpine meadow might be
more sensitive to changes in global temperature than in the
temperature regions. Higher effect of WFPS than temperature on
the uptake of CH4 was consistent with other studies (Bowden et al.,
1998; Bradford et al., 2001) because CH4 was consumed by methanotrophs in aerobic condition (Conrad, 1996; Mancinelli, 1995)
and the uptake activity was mainly controlled by the diffusion rate
of CH4 into the active of methanotrophic zone in mineral soil
(Steinkamp et al., 2001).
Generally, the nitrogen addition could inhibit the uptake of
CH4 (Bodelier and Laanbroek, 2004; Mancinelli, 1995) in soil. The
Table 4
Comparison of the GWP of CO2, N2O and CH4 fluxes (kg ha1) as affected by increasing nitrogen deposition (ND) in the growing season.
N2O_CO2-equivalent
CO2
Mean
ND
19 671 (1680)
Control
20 141 (1739)
GP
p
Mean
GP
CH4_CO2-equivalent
p
0.82
16.1
12.2 (2.1)
GP
Total
p
26.1 (1.7)
14.1 (5.5)
2.3
Mean
15.3
0.51
30.8 (2.1)
Mean
GP
p
2.3
0.83
19 660 (1685)
0.12
20 123 (1738)
The global warming potential (GWP) of CO2 is 1, while for CH4 is 23 times that of CO2 and for N2O is 296. GP indicates greater percent (%) of GHG emission in increasing
nitrogen deposition plots (ND) than control. Values between brackets are standard errors of the mean (n ¼ 3). Significant differences (p-values) between control and increasing
nitrogen deposition plots are analyzed by ManneWhitney U-test.
C. Jiang et al. / Atmospheric Environment 44 (2010) 2920e2926
underlying mechanisms are: (1) NHþ
4 is a competitive inhibitor of
CH4 oxidation due to lack of specificity of methane monooxygenase
(MMO) in methanotroph; (2) hydroxylamine and nitrite produced
by methanotrophic ammonia oxidation are toxic to methanotrophic bacteria; (3) osmotic stress caused by added nitrogen salt
can suppress the activity of methanotroph (Saari et al., 2004). In the
observation period, nitrogen deposition only decreased the CH4
oxidation by 15.3%, which was much smaller than the nitrogen
fertilizer experiments (typically ranged from 60% to 80%) (Adamsen
and King, 1993; Castro et al., 1995; Macdonald et al., 1996; Mosier
et al., 1991). This is because: (1) the amount of added nitrogen in
our experiment was much smaller than that of the nitrogen
fertilizer experiments; (2) nitrogen immobilization capacity in the
alpine meadow soil was very high, which could protect methanotroph from exposure to NHþ
4 (Steinkamp et al., 2001). In our
experiment, the decreasing magnitude of CH4 oxidation was much
larger than that of CO2 emission (15.3% for CH4, only 2.33% for CO2),
which was consistent with the results of Saari et al. (2004).
Moreover, we detected significant difference (p < 0.05) between
the control and nitrogen deposition treatment at some observation
dates for CH4 rather than for CO2 and N2O, implying that methanotroph might be more susceptible to the nitrogen deposition
than other soil microbes relating CO2 and N2O emissions (Saari
et al., 2004).
3.5. Greenhouse effect of GHGs as affected by nitrogen deposition
The emission of 1 kg of N2O to the atmosphere is 296 times more
effective than 1 kg of CO2, while 1 kg of CH4 is 23 times more
effective than 1 kg of CO2 (Liu and Greaver, 2009). By this, the GWP
of the three GHGs was calculated to identify the effect of short-term
nitrogen deposition treatment on global warming. Our results
indicated that CO2 was the overwhelmingly dominant GHG in
terms of its GWP, and nitrogen deposition treatment had limited
impacts on each individual GHG and net GWP of the three GHGs
together in the alpine meadow (Table 4).
4. Conclusions
The GHG (CO2, CH4 and N2O) were observed in the growing
season in the alpine meadow of Qinghai-Tibetan Plateau. The results
showed the temperature sensitivity (Q10) of CO2 emission and CH4
uptake were relatively higher (3.79 for CO2, 3.29 for CH4) than that of
warmer region ecosystems and soil temperature and WFPS were
the dominant factors that controlled seasonal variation of CO2 and
CH4 respectively. Increasing nitrogen deposition decreased the
CH4 absorption and increased the N2O emission but did not strongly
affected CO2 flux. Among the three greenhouse gases, CH4 was most
susceptible in response to increased nitrogen availability. There were
no interactive effects between sample date and nitrogen deposition
on GHG fluxes in the observation periods. In addition, the short-term
increasing nitrogen deposition (2 g N m2 year1) had very small
impacts on global warming potential (GWP) in term of CO2-equivalents. Overall, our study demonstrated that short-term effect of
moderate nitrogen deposition on GHG fluxes was less obvious
than nitrogen fertilizer experiments and longer study periods were
needed to verify the cumulative effects of increasing nitrogen
deposition on GHG fluxes in the alpine meadow.
Acknowledgments
We gratefully acknowledge the Haibei alpine meadow ecosystem
research station for help with logistics and access permission to the
study site. We thank Lancuo Wa, JinlongWa and Jianshe Ge for field
2925
assistance. We are grateful to anonymous reviewers for their valuable comments on earlier versions of the manuscript. This study was
financially supported by National Key Research and Development
Program (No. 2010CB833500), Natural Science Foundation of China
(No. 30590381) and Knowledge Innovation Project of Chinese
Academy of Sciences (No. KZCX2-YW-432).
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