Download Carbon Balance in an Alpine Steppe in the Qinghai

Journal of Integrative Plant Biology 2009, 51 (5): 521–526
Carbon Balance in an Alpine Steppe in the Qinghai-Tibet
Plateau
∗
Zhi-Yong Pei1 , Hua Ouyang2 , Cai-Ping Zhou2 and Xing-Liang Xu2
(1 The Administrative Center for China’s Agenda 21, Beijing 100038, China;
2
Key Laboratory of Ecosystem Network Observation and Modelling, Institute of Geographic Sciences and Natural Resources Research, the Chinese
Academy of Sciences, Beijing 100101, China)
Abstract
Carbon fluxes were measured using a static chamber technique in an alpine steppe in the Qinghai-Tibet Plateau from July
2000 to July 2001. It was shown that carbon emissions decreased in autumn and increased in spring of the next year, with
higher values in growth seasons than in winters. An exponential correlation (E carbon = 0.22(exp(0.09T) + ln(0.31P + 1)),
R2 = 0.77, P < 0.001) was shown between carbon emissions and environmental factors such as temperature (T) and
precipitation (P). Using the daily temperature (T) and total precipitation (R), annual carbon emission from soil to the
atmosphere was estimated to be 79.6 g C/m2 , 46% of which was emitted by microbial respiration. Considering an average
net primary production of 92.5 g C/m2 per year within the 2 year experiment, alpine steppes can take up 55.9 g CO 2 -C/m2
per year. This indicates that alpine steppes are a distinct carbon sink, although this carbon reservoir was quite small.
Key words: alpine grassland; carbon balance; carbon flux; net primary productivity; Qinghai-Tibet Plateau.
Pei ZY, Ouyang H, Zhou CP, Xu XL (2009). Carbon balance in an alpine steppe in the Qinghai-Tibet Plateau. J. Integr. Plant Biol. 51(5),
521–526.
Available online at www.jipb.net
It is well known that increases in atmospheric concentrations of greenhouse gases are responsible for global warming
(Tett et al. 1999; Crowley 2000). Among all the greenhouse
gases in the atmosphere, carbon dioxide (CO 2 ) is the most
abundant gas and contributes almost 50% of all greenhouse
forcing (Lashof and Ahuja 1990; Rodhe 1990). The concentration of atmospheric CO 2 has increased from 280 parts per
million by volume (p.p.m.v.) since pre-industrial times (pre-1800)
to 380 p.p.m.v., increasing at the rate of 3 p.p.m.v. per year
(Neftel et al. 1985; Friedli et al. 1986; Fans et al. 1998; Monnin
et al. 2001). Besides anthropogenic changes, a large amount
of carbon (C) is fixed into the biosphere from the atmosphere
via plant photosynthesis and subsequently released from biota
Received 25 Sept. 2008
Accepted 20 Nov. 2008
Supported by the National Basic Research Program (2005CB422005),
and the Pre-studies Project of National Basic Research Program
(2005CCA05500).
∗
Author for correspondence.
Tel: +86 10 6488 9813;
Fax: +86 10 6488 9813;
E-mail: <[email protected]>.
C 2009 Institute of Botany, the Chinese Academy of Sciences
doi: 10.1111/j.1744-7909.2009.00813.x
to the atmosphere by respiration or burning of plants. So, the
release of CO 2 from terrestrial biota has contributed significantly
to the atmospheric CO 2 concentration (Sommerfeld et al. 1993).
The Qinghai-Tibet Plateau is the so-called “third pole” of the
earth with a mean altitude of more than 4000 m above sea
level. Great uplift of the plateau created and then strengthened
the South Asian monsoon, and has a tremendous impact on
the development of physical environments and ecosystems
(Zheng and Zhu 2000). Due to the topographic features and
the characteristics of the atmospheric circulation, typical alpine
zones of forests, meadows, steppe and deserts appear in
succession from southeast to northwest in the plateau (Zheng
et al. 1979).
Alpine steppe ecosystems cover a large area of the whole
plateau. They are very sensitive to climate change and anthropogenic activities. Several studies have explored the C
exchange in alpine meadows and shrubs in the Qinghai-Tibet
Plateau (Xu et al. 2006; Zhao et al. 2007; Hu et al. 2008), but few
have been carried out in alpine steppes. The objectives of the
present study were: (i) to measure CO 2 flux between soil and
atmosphere in the field; (ii) to establish the relationship between
C emission and environmental factors such as temperature
and precipitation; and (iii) to estimate the C balance in alpine
steppes. Integrating these data, we hope to discover the role of
alpine steppes in the C budget.
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Results
Soil physical and chemical properties
The alpine steppe soil at the study site had a light texture (sandy
loam), and the other physical and chemical characteristics of
surface soil are presented in Table 1. Higher C storage was
found in the 10–20 cm depth of this sandy loam soil. Soil moisture increased with depth because of the strong evaporation
in this region. Soil pH decreases gradually with increasing soil
depth.
the non-growth seasons. The highest mean CO 2 emission
occurred in August. Root respiration and microbial respiration
were the main source of soil CO 2 . In the present study, CO 2
emissions had a pattern similar to the variation of temperatures.
It was shown that the variation in CO 2 emissions was positively
related to air temperatures (Figure 2).
In addition, a combined effect of temperature and precipitation
on soil CO 2 emission was found in this study. As a result, the
regression formula between experimental soil respiration and
climate temperature as well as precipitation was calculated as
follows:
E carbon = 0.22(exp(0.09T) + ln(0.31P + 1))
Vegetation characteristics
× (R2 = 0.77, n = 17, P < 0.001)
The biomass was a bit lower than other grasslands in the
plain regions. The ratio of biomass between below-ground and
above-ground was more than 14:1. The C concentration in the
green part was higher than litter and root parts (Table 2).
(1)
Where T is the daily mean temperature during the experiment
day, and P is the total precipitation during the experimental day.
Based on Equation 1, continuous CO 2 emissions from 2000 to
2001 were estimated and shown in Figure 3. Annual C emission
was 79.97 g C/m2 for 2000 and 79.24 g C/m2 for 2001.
Net primary production
C balance
Based on Table 2, the above-ground net primary productivity
(NPP) was estimated to be 17.32 g C/m2 per year in 2000
and 22.53 g C/m2 per year in 2001. Because below-ground net
primary productivity occupied almost one third of the whole root
biomass for alpine meadows (GM Cao, pers. comm., 2004),
the below-ground NPP in alpine steppes was estimated to be
61.75 g C/m2 per year in 2000 and 83.38 g C/m2 per year in
2001.
CO 2 emission
CO 2 emissions showed a clear seasonal variation, decreasing
in autumn and increasing in spring (Figure 1). CO 2 emissions
during the growth seasons were much higher than those during
Table 1. Soil characteristics in alpine steppe at Wudaoliang
Depth (cm) Moisture (%)
pH
Organic C (%)
Total N (%)
0–10
3.61±1.06
9.02 ± 0.23
0.15 ± 0.05
0.04 ± 0.005
10–20
5.05 ± 2.82
9.03 ± 0.16
0.15 ± 0.02
0.04 ± 0.004
20–30
7.28 ± 2.15
8.90 ± 0.21
0.32 ± 0.04
0.07 ± 0.004
Table 2. Vegetation characteristics in alpine steppe at Wudaoliang
Biomass (g/m2 )
Plant C (%)
Total N (%)
44.55 ± 16.25
38.88 ± 1.31
1.02 ± 0.13
2000
Green
Litter
1.24 ± 0.39
34.44 ± 4.64
0.70 ± 0.11
Root
668.75 ± 227.3
27.70 ± 3.97
0.86 ± 0.16
Green
56.48 ± 17.84
39.90 ± 3.08
1.99 ± 0.35
Litter
8.46 ± 1.44
36.93 ± 6.13
0.94 ± 0.24
Root
1117.14 ± 141.94
22.39 ± 3.87
0.99 ± 0.10
2001
Carbon exchanges between atmosphere, land and the oceans
play an important role in regulating the global carbon cycle and
balance. The net carbon exchange of terrestrial ecosystems
is the result of a balance between uptake (photosynthesis) and
loss (respiration). Soil respiration is derived from both root respiration and the respiration of soil organic matter by heterotrophs
(Andrews et al. 1999). According to some previous studies, the
CO 2 production by respiration activity in soil originating from
soil organic matter (microbial respiration) contributes 45%–48%
of soil respiration in grassland ecosystems (Kelting et al. 1998;
Rochette et al. 1999; Hu et al. 2008). So in our study area,
annual CO 2 emission derived from microbial respiration was
estimated to be about 36.79 g/m2 in 2000, and 36.45 g/m2
in 2001. In natural ecosystems, carbon inputs via litterfall and
fine root turnover are the final plant primary production. In our
experiment, estimates of the NPP during the 2 year experiment
were 79.07 and 105.91 g/m2 per year. This indicates that alpine
steppes can take up 42.28 g CO 2 -C/m2 in 2000 and 69.46 g
CO 2 -C/m2 2001.
Discussion
In these kind of alpine steppes, the ratio of biomass between
below-ground and above-ground was over 14:1, higher than
other plain areas (12:1, Chen and Wang 2000). The vegetation
root system here was stronger than in the plain area due to
low soil water content. NPP in the alpine steppe was about
79.50 g C/m2 in 2000 and 109.02 g C/m2 in 2001. These
values were lower than temperate steppes in Inner Mongolia (189.2 g C/m2 per year, Xiao et al. 1996). A possible
Carbon Balance in an Alpine Steppe 523
0.8
Carbon emissions (g/m2 per day)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
10
25
/J
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/A 000
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Time
Figure 1. Soil respiration (g C/m2 per day) in alpine steppe at Wudaoliang on the measuring days in 2000 and 2001.
Carbon emmision (g/m2 per day)
1.2
1.0
R2 = 0.78, P<0.001
0.8
0.6
0.4
0.2
0
–25
–20
–15
–10
–5
0
5
10
15
20
Temperature (°C)
Figure 2. Relationship between soil respiration (g C/m2 per day) and air temperatures (◦ C) in alpine steppe of Wudaoliang.
explanation is that plant growth in alpine steppes is limited by
lower temperature and less precipitation, as well as barren soils.
Total soil respiration from alpine steppes at Wudaoliang was
estimated to be 79.97 g C/m2 in 2000 and 79.24 g C/m2 in
2001, respectively. These values were lower than those from
temperate grasslands in Inner Mongolia (Dugas et al. 1999;
Dong et al. 2000; Mielnick et al. 2001) and from alpine meadows
in the Qinghai-Tibet Plateau (Wu et al. 2005; Zhang et al. 2005).
Two possible explanations can be proposed. First, biomass, soil
substrate as well as climatic factors influence soil respiration.
It has been shown that annual soil respiration rates correlate
well with the biomass of different vegetation biomes (Raich and
Schlesinger 1992). Besides, numerous studies have showed
that soil respiration is positively related to soil total C and N
content (Frank and Groffman 1998). In the present study the
biomass, soil organic C and total N, annual temperature, and
annual precipitation were lower than other grasslands (Dong
et al. 2000; Mielnick et al. 2001; Ross et al. 2001). This partly
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No. 5 2009
1.0
Carbon fluxes (g C/m2 per day)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
365
2000–2001
730
Figure 3. Estimated soil respiration (g C/m2 per day) in alpine steppe at Wudaoliang, calculated from daily air temperature and precipitation based
on Equation 1.
accounts for why soil respiration rates in the alpine steppe are
lower than other grasslands. Second, soil pH values have an
impact on soil respiration, for example, Xu and Qi (2001) showed
that CO 2 efflux was negatively correlated with soil pH values.
In the present study the soil pH value was a little higher than in
other grasslands.
The C balances in our study area were 42.28 g C/m2 in 2000
and 69.46 g C/m2 in 2001, with an average of 55.47 g C/m2 per
year. This indicates that alpine steppe was a CO 2 sink, although
net C sequestration was very small. The mysterious missing
C sink has become a great issue in ecological sciences for
decades. Recent studies have suggested that the Northern
Hemisphere land areas (especially the forests) are functioning as a significant C sink (Fang et al. 2001; Schimel et al.
2001). Tropical land areas were approximately in balance with
respect to C exchange, implying a C sink that offset emissions
due to tropical deforestation. However, Schindler (1999) suggested that the missing sink may be several smaller sinks.
Our study area is also located in the northern hemisphere, and
contributes as a small reservoir. Alpine steppe ecosystems may
also be important in C uptake in addition to forest ecosystems
in the northern hemisphere.
Materials and Methods
Site description
The study site was located in Wudaoliang, Qinghai Province,
China (35.13◦ N, 93.05◦ E). The altitude of the study site is 4 767
m above sea level. The area has no human activity, so it is
an ideal place to investigate the mechanism of carbon flux.
This area is characterized by a sub-frigid and semiarid climate.
Average monthly air temperature was below 0 ◦ C except during
the growth season, and mean annual temperature was −5.6 ◦ C
(Sun and Zheng 1998). Annual mean precipitation ranged from
200 mm to 400 mm, with 84% occurring in the growth season
(from June to September). The soil type was the alpine steppe
soil. Permafrost can be found in this region. The ecosystem was
classified as an alpine steppe, and the study site was dominated
by Stipa purpurea (Zheng et al. 1979). The leaf area index was
about 0.63 (TX Luo, pers. comm., 2004).
Soil and plant sampling
Three plots were randomly selected at the research site. The
distances between each plot were more than 10 m. Vegetation
samples were collected before gas sampling. Soil samples
(3.2 cm diameter core, from soil layers of 0–10 cm, 10–20 cm
and 20–30 cm) and root samples were collected at the end of the
experiment during the growth season. The plant samples (both
above and below ground) were all dried at 60 ◦ C over 48 h and
weighed for the biomass. These samples were then ground
into powder for measuring organic carbon by digestion with
potassium dichromate and back-titrating with 0.025 M ferrous
ammonium sulfate (Kalembasa and Jenkinson 1973) and total
nitrogen by Kjeldahl digestion (Bremner 1965). Soil moisture
was determined by an oven dry method at 60 ◦ C for more than
48 h. Soil pH was measured using a glass electrode at a 1:2
soil-to-water ratio. Organic carbon and total nitrogen of soil were
measured using the same method as the plant samples.
Carbon Balance in an Alpine Steppe 525
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