Environ Earth Sci DOI 10.1007/s12665-014-3382-7
ORIGINAL ARTICLE
Soil carbon and nitrogen in the active layers of the permafrost regions in the Three Rivers’ Headstream Guanglu Hu • Hongbing Fang • Guimin Liu • Lin Zhao • Tonghua Wu • Ren Li • Xiaodong Wu
Received: 1 September 2013 / Accepted: 21 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract The pedogenesis, soil organic carbon (SOC), soil inorganic carbon (SIC), hot water-soluble organic carbon (WSOC) and total nitrogen (TN) of the active layers were examined beneath four typical vegetation communities in the permafrost regions in the Three Rivers’ Headstream region in the Qinghai-Tibetan Plateau. In all soil areas, except for in the steppe, the SOC and TN showed rapidly decreasing trends with increasing depth. The highest SOC, WSOC and TN contents were found in the wet meadow, with contents in the eluviate layer being 180.9, 40.2 and 10.9 g kg-1, respectively. In the steppe, the average SOC, WSOC and TN at 180 cm depth were 6.2, 0.67 and 0.59 g kg-1, respectively. The SIC contents showed increasing trends with increasing depth in the soils of the steppe community. The correlation analysis suggested that the moisture and fine particle fractions positively correlated to SOC, TN and WSOC, while bulk density and pH negatively correlated to SOC, TN and WSOC. The SOC and TN were significantly related to bulk density. The SIC was positively correlated with pH but negatively correlated with SOC, TN and WSOC. The C/N ratios were negatively correlated with pH while positively correlated with SOC, TN and fine soil particles. The results suggest that the SOC in the wet meadow soils in the permafrost regions of Qinghai-Tibetan have the largest
G. Hu H. Fang G. Liu School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou, China G. Hu H. Fang L. Zhao T. Wu R. Li X. Wu (&) Cryosphere Research Station on the Qinghai-Tibetan Plateau, State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China e-mail:
[email protected]
potential contributions to the emissions of greenhouse gases and cause future global warming. Keywords Soil organic carbon Total nitrogen Three Rivers’ Headstream Permafrost Soil inorganic carbon Vegetation communities
Introduction Coping with and adapting to climate change is one of the great challenges of the 21st century (IPCC 2011). The increasing concentration of greenhouse gases in the atmosphere has been verified as the main influential factor in global warming (Zhang et al. 2005). The soil organic carbon (SOC) that is stored in the northern permafrost regions is roughly double the amount of carbon currently in the atmosphere (Tarnocai et al. 2009). The SOC in permafrost regions has become a focus of attention because, over the next 100 years, global warming could potentially cause large-scale changes in the carbon dynamics in these regions and consequently affect the global carbon cycle (Schuur et al. 2009). High-elevation regions will also be sensitive to climate change, but our understanding of their carbon dynamics is very limited (Wickland et al. 2001). Carbon flux between the atmosphere and terrestrial ecosystems involves both SOC and SIC. SIC stocks, which are mainly present in arid and semi-arid regions, could function as carbon sinks or sources in the global carbon cycle (Lal and Kimble 2000). Changes in the SIC stocks could also affect the soil pH (Bronick and Lal 2005). Not only are carbon stocks important in permafrost degradation, but soil nitrogen content also plays an important role in the permafrost carbon cycle (Harden et al. 2012). The rate of SOC turnover is strongly linked to nitrogen
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availability. However, little work has been conducted on nitrogen storage within the soil profile in comparison to carbon storage (Crowe et al. 2004). The Qinghai-Tibetan Plateau is an important part of the global terrestrial ecosystem and one of the main regions of low-latitude permafrost in the world. The Qinghai-Tibetan Plateau is one of the most sensitive areas to climate change and has an important role in affecting climate change (Yashiro et al. 2010; Wang and Wu 2013). Large quantities of CO2 can be released from frozen carbon pools as the permafrost thaws, due to increased soil temperatures, improved soil drainage and oxygen availability. Therefore, these SOC pools may play an important role in greenhouse gas emissions as the permafrost degrades due to global warming although the decomposition of SOC was not the only path for the greenhouse gases emission in the permafrost regions (Wu et al. 2013). Because the QinghaiTibetan Plateau is sensitive to climate change (Yashiro et al. 2010), more attention must be given to its SOC. The SOC of the plateau has recently created many concerns. Although the SOC pools were estimated from the global soil database or China’s national soil survey and combined with sparse field samples (Fang et al. 1996; Wang et al. 2002), there is disagreement about the SOC content in the plateau because of the limited sampling points and the quality of the SOC data that has been gathered to date. It has been recognized that most of the SOC is stored in the soils under meadows and wet meadow cover (Wang et al. 2002, 2013), which are prevalent in the eastern portion of the plateau. Despite the importance of the SOC in this area, there are few studies that examine the SOC distribution in the permafrost regions in this area. Because of this, the total nitrogen, the SIC and their possible relationships to environmental conditions remain unknown. Soil organic matter (SOM) consists of various functional pools that are stabilized by specific mechanisms and have certain turnover rates (Huang et al. 2002). During the past few years, numerous fractionation schemes have been developed to separate and analyze SOM fractions (Lutzow et al. 2007). It has been suggested that solubility in water is one attribute of readily bioavailable SOM (Marschner and Kalbitz 2003) and may have important effects on the SOM decomposition rates (Wu et al. 2013). However, little is known about the water soluble SOM in the permafrost regions of the Qinghai-Tibetan Plateau. With the soils collected from the permafrost region in the Eastern Qinghai-Tibetan Plateau, the present study aimed to analyze the SOC, SIC, soil nitrogen and their possible relationships with physicochemical properties according to soil pedology. The resulting data provides insight into the soil carbon and nitrogen dynamics, and their implications on the carbon cycle in permafrost regions of the Qinghai-Tibetan plateau.
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Materials and methods Study site The research was conducted with soil samples from steppe, degraded meadow, meadow and wet meadow locations in the Three Rivers’ Headstream region, which is in the eastern part of the Qinghai-Tibetan Plateau. This region is a large plateau with a mean elevation of 4,500 m above sea level. The annual mean temperature is approximately -2.3 °C and the mean annual precipitation is 397 mm [monthly meteorological data from 1956 to 2002, Chinese national Meteorological Centre (CNMC). This semi-arid area belongs to a continuous permafrost region, although some parts of this area do not have permafrost. Permafrost was found at the three of the four sites; the site under the steppe communities did not have permafrost. The environmental conditions for the soils are shown in Table 1. Sampling and analysis The soils were sampled in the eastern portion of the Qinghai-Tibetan Plateau (Fig. 1) in July 2011. The soil pits were excavated with a mechanical digger and the soil samples were collected according to the genetic horizons. The field bulk density, or the weight of the soil per unit volume, was measured by the clod method and expressed as the field moisture weight for each sample. Samples for moisture determination were stored in aluminum sampling boxes and carefully sealed to prevent changes in soil moisture. The soil moisture content was expressed as the ratio of the mass of water present to the dry weight of the soil sample. Soil pH was determined in a 1:5 suspension of soil. Soil particle size distribution was determined by the wet sieve method. The soils were separated into seven size fractions (\2, 2–50, 50–100, 100–250, 250–500 lm, 500 lm– 1 mm, and 1–2 mm) by a combination of wet sieving and assessing for particle size distribution by a LS-230 laser particle size analyzer. The soil carbon and nitrogen content were determined using an Elemental Analyzer (Vario EL Elementar), then the SIC content was estimated by the equation (SIC = TC-SOC). The soil samples were treated by hydrochloric acid before analyzing for SOC in order to exclude the impact of SIC. To extract the water soluble organic carbon (WSOC), soil samples of known weight were put in 50 ml tubes, and the tubes were capped and placed for 16 h in a hot-water bath at 80 °C. At the end of the extraction period, each tube was shaken for 10 s in a vortex shaker to ensure that WSOC released from the soils.
River terrace 0 180 4,263 34.54746 Stipa spp.,Artemisia sacrorum Ledeb. Steppe
50
M3.5.1 (Hapli-Ustic Isohumosols)
KDDN (Calcic Haplocryepts)
95.51627
1
Talus apron 250 60 4,472 34.9018 ABDG (Aquic Molliturbels) Kobresia tibetica Maximowicz Wet meadow
95
J3.1.3 (Albic Stagni-Udic Isohumosols)
94.78933
4
Talus apron 230 95 4475 34.90331 ABGE (Typic Haploturbels) Kobresia tibetica Maximowicz Meadow
95
M1.1.2 (Permi-Gelic Cambosols)
94.7904
6
Alluvial and flood fan 0 3 110 4422 94.1804 35.0706 Androsace spp. Degraded meadow
50
M1.1.2 (Permi-Gelic Cambosols)
ABGE (Typic Haploturbels)
Pits depth Altitude (m) Latitude (N) Vegetation coverage Dominant species Soil
Table 1 Environmental condition at the sampling sites
Chinese soil taxonomy
US soil taxonomy
Longitude (E)
Slope (°)
Aspect (°)
Topography
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Fig. 1 Sampling sites in the Qinghai-Tibetan Plateau (D degraded meadow, W wet meadow, M meadow, S steppe)
The suspension was then filtrated and analyzed by an elemental analyzer. All the analyses were conducted in triplicate using subsamples. All of the results in the present study are presented as mean values and standard deviations. The SPSS software package 14.0 for Windows was used for statistical analyses.
Results Soil horizons in the different vegetation communities The soil types [according to the Soil Taxonomy (ST, USDA) and the Chinese ST (Gong 1999)] are listed in Table 1, along with their environmental conditions. The vegetation cover in the steppe and the degraded meadow was approximately 50 %, while it was 95 % in the meadow and wet meadow. According to the World Reference Base for Soil Resources (IUSS Working Group WRB 2006), the soils under the degraded meadow, meadow and wet meadow belong to the cryosol order, and the soil under steppe vegetation belongs to the cambisol order. In the soil profile of steppe, all the soil textures were sandy loam. The A horizon, with a soil color of 10YR5/3, was 20 cm, followed by three B layers and then BC and 2C layers. The B layers and BC layer showed a color of 10YR4/4. There was still no frozen layer at a depth of 180 cm. Under the conditions of degraded meadow, where there was poor drainage due to slope, the A layer was 20 cm. From the surface to the bottom layers, the soil texture changed from silt to loam to sandy loam and then to sandy clay loam. The soil color in the upper three layers was 10YR4/2 or 10YR3/2, and the color of the Bg layer was Gley 2 4/10B. The thaw depth was 110 cm.
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Although the soil horizons under the meadow had an A layer of 11 cm, the soil layers from 11 to 75 cm contained materials from the A layer. The soil textures of the soil layers were silt, and the soil color from the upper to the bottom layers were 10YR2/2, 10YR3/3, 10YR3/2, 10YR4/ 4, and 10YR4/4. The thaw depth in this pit was 95 cm. There were two layers of A and a layer of Bw in the soil under wet meadow. The soil texture was silt, and the soil color of the A1, A2 and Bw layers were 10YR2/2, 10YR3/2, and 10YR4/2, respectively. The active layer thickness was 60 cm because visible ice was encountered at this depth. Distribution of SOC, WSOC, SIC, and TN The distribution of SOC, WSOC, SIC and TN along the soil depth in the steppe profile is shown in Fig. 2. The average concentrations of SOC, WSOC, SIC, and TN were 6.2, 0.7, 8.0, and 0.59 g kg-1, (dry weight), respectively. WSOC showed decreasing trends with increasing depth, while the SOC and TN content showed similar variations. The soil with the highest SIC content appeared in the bottom C layer followed by the BC layer. The mean SIC content was slightly higher than that of the SOC. In the degraded meadow profile, the SOC, WSOC and TN clearly decreased from the upper layer to the bottom layers (Fig. 3). The maximum SOC, WSOC and TN in the A layer were 18.9, 1.24, and 1.60 g kg-1, respectively, which were much higher than those of the Bw2 and Bg layers. The mean SIC content of this profile was 9.4 g kg-1, which was largely homogeneous in all of the layers except for a peak recorded in the Bw layer. The SOC and TN were 97.0 and 6.82 g kg-1, respectively, in the A layer under meadow conditions. In the next layer, which contained materials from the A layer, the contents of SOC and TN were relatively high at 20.6 and 1.53 g kg-1, respectively. The WSOC showed a steep decreasing trend with increasing depth. The lowest SIC content was recorded in the A layer, while the highest SIC value appeared in the middle ABbw1 (Fig. 4). As shown in Fig. 5, only three soil horizons were found in the wet meadow soil profile. The SOC, WSOC, SIC and TN showed decreasing trends with increasing depth. The mean contents of SOC, WSOC, and SIC were 115.9, 27.4, and 7.4 g kg-1, respectively. Notably, the SIC content was lowest in the bottom layer of Bw with a value of 0.3 g kg-1. Atomic C/N ratios The atomic ratios of carbon between the SOC to TN (C/N ratios) of the soil layers under steppe conditions were from 12.0 to 13.0 (Fig. 6). In the meadow soil layers, the C/N ratios showed lower values in the A and Bw1 layers with a
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mean value of 13.5. Below these two layers, the average C/N ratios were 18.8. In the meadow soil layers, the C/N ratios were largely similar among the different layers with a mean value of 15.6. The highest C/N ratios were recorded in the bottom of the Bw layer of the wet meadow. The mean C/N ratio of the three wet meadow layers was 18.3. Correlation between the soil carbon, TN and environmental factors Correlations between the SOC, WSOC, SIC, TN, C:N ratios and soil factors at the sites are shown in Table 2. Because there were significant positive correlations among the soil particle fractions of \2, 2–50 and 50–100 lm, these fractions were combined and shown as fine particles. Similarly, the 100–250 and 250–500 lm particles were classified as medium fractions, and the 500 lm–1 mm and 1–2 mm particles were classified as coarse fractions. Statistically significant (P \ 0.01, two-tailed) positive correlations were found between SOC, WSOC, TN and the moisture content. The SOC, WSOC, and TN were significantly negatively correlated with the SIC and pH. The C/N ratios were positively correlated with the SOC, the TN, and the fine soil particle fractions and was negatively correlated with the pH and the medium soil particle fractions. The bulk density was positively correlated with the SIC and pH and showed negative correlations with the SOC, TN, and WSOC. The moisture content was significantly positively correlated to the fine particle fractions, while negatively correlated to the medium particle fractions and bulk densities. A strong positive correlation was observed between the SOC and WSOC (Fig. 7). The linear regression analysis showed that pH, SIC, moisture content, and the soil particle size distribution were excluded from the regression model (the TOC/TN ratio was not included in the analysis because it is a covariable factor of TOC and TN). The bulk density was the most important factor that significantly contributed to the TOC and TN. The best regression models are displayed as follows: SOC = TN =
98:97 bulk + 153:33 6:38 bulk + 10:04
p\0:001 p\0:001
. Discussion SOC are mainly produced as dead plants and animals are decomposed by the soil microbial biomass; therefore, it is reasonable that the SOC contents showed decreasing trends from the upper to the lower soil layers beneath the degraded meadow, meadow and wet meadow communities. In the steppe, the distribution of SOC with depth showed a largely uniform profile with a relatively low value in comparison to other soils. This may be related to relatively
Environ Earth Sci Fig. 2 Distribution SOC (a), WSOC (b), SIC (c) and TN (d) under steppe conditions
Fig. 3 Distribution SOC (a), WSOC (b), SIC (c) and TN (d) under degraded meadow conditions
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Environ Earth Sci Fig. 4 Distribution SOC (a), WSOC (b), SIC (c) and TN (d) under meadow conditions
Fig. 5 Distribution SOC (a), WSOC (b), SIC (c) and TN (d) under wet meadow conditions
higher biogeochemistry intensity in the soil layers because lower moisture content will increase the decomposition of organic carbon (Schlesinger and Andrews 2000). In the permafrost regions of the Qinghai-Tibetan Plateau, the surface SOC contents in the steppe were
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approximately 7.0 g kg-1, and showed a clear decreasing trend with increasing depth (Wu et al. 2012). In the present study, the SOC content of the A layer in the steppe was 6.7 g kg-1. However, there was no obvious decreasing trend in the vertical distribution. Therefore, the SOC
Environ Earth Sci Fig. 6 Atomic TOC/TN ratios of steppe (a), degraded meadow (b), meadow (c), and wet meadow (d) conditions
Table 2 Correlation matrix of soil carbon and total nitrogen and environmental factors SOC SOC
1.00
SIC
-0.65**
SIC
WSOC
TN
0.96**
-0.61**
TN
0.99**
-0.66**
0.94**
1.00
C/N ratio
0.56**
-0.36
0.53*
0.51*
-0.76**
-0.88**
Moisture
Bulk
Moisture
pH
Fine
Medium
Coarse
1.00
WSOC
Bulk
C/N ratio
-0.85**
0.77**
1.00 1.00 -0.40
1.00
0.83**
-0.60**
0.75**
0.85**
0.67**
-0.87**
pH
-0.85**
0.75**
-0.79**
-0.86**
-0.51**
0.89**
Fine
0.40
-0.23
0.42
0.38
Medium
-0.39
-0.16
-0.28
-0.40
-0.53*
0.22
-0.53*
0.29
-0.51*
Coarse
-0.05
0.40
-0.19
-0.02
-0.32
0.15
-0.22
0.12
-0.58**
0.78**
storage in the steppe in the eastern regions of the plateau was higher than that of the western regions. This could be explained by the higher vegetation cover in the eastern regions due to the higher precipitation (397 mm in the eastern region, approximately 100 mm in the western region). The SOC and TN content were similar to that of China’s alpine meadow vegetation in the Qinghai-Tibetan Plateau, where the average content of SOC and TN at a depth of 1 m were approximately 50 g kg-1 (Wang et al. 2002) and 4.3 g kg-1 (Chen et al. 2011), respectively. In the northern circumpolar permafrost region, the average SOC stocks of the upper 1 m layer in the Histels and Turbels soils were 66.6 and 32.2 kg m-2 (Tarnocai et al. 2009), respectively.
-0.33
1.00 -0.80**
1.00
0.67**
-0.37
1.00 1.00 -0.40
1.00
In the present study, the SOC stock in the upper 1 m layer of the steppe, degraded meadow and meadow were estimated to be 8.2, 11.4, and 22.9 kg m-2, respectively. For the upper 60 cm layer of the wet meadow, the SOC stock was 35.0 kg m-2, which was comparable to that of the Histels soils. The SIC in the northern circumpolar permafrost regions received less attention than the SOC. It has been suggested that the SIC content for the upper 1 m layer in the Qinghai-Tibetan Plateau largely ranged from 15.0 to 25.0 kg m-2 (Mi et al. 2008). In the present study, the SIC contents were similar to that of the SOC in the steppe and degraded meadow soils, which could be estimated to be 7.9 and 15.3 kg m-2, respectively. The SIC stocks for the upper 1 m in the meadow soil was 6.8 kg m-2. However,
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Fig. 7 Regression between the SOC and WSOC
the SIC for the upper 60 cm layer in the wet meadow soils was only 0.1 kg m-2. Although there were only 4 soil pits in this study, the results showed different patterns in the distribution and storage of SOC, TN and SIC in the soils of different vegetation type. The results suggested that the distribution of the wet meadow in the permafrost regions in the Qinghai-Tibetan plateau should be highlighted because the wet meadow was not classified as a vegetation type in the previous study (Hou 1982). In the initial phase of SOM decomposition, water soluble compounds and non-lignified carbohydrates are preferentially decomposed and their relative concentrations decrease (Berg 2000). From the evidence of incubation experiments (Wu et al. 2014) and the stable isotopes of organic carbon (Mu et al. 2014), it has been suggested that the WSOC could be regarded as a labile component in the carbon pools. In the present study, the WSOC was significantly positively correlated with the SOC and TN content. However, the decreasing trend of the vertical distribution for WSOC was more obvious than for SOC and TN. For example, the ratios of the SOC of the bottom layer to that of the surface layer in the soils of the steppe, degraded meadow, meadow and wet meadow were 0.91, 0.23, 0.07, and 0.42, respectively. As for WSOC, these ratios were 0.34, 0.19, 0.07, and 0.32, respectively. The rapid decreasing trends along the depth in WSOC suggested that the SOC in the upper soil layers contained more labile carbon pools than the deeper soils. The contribution of SIC at a depth of 1 m to the total terrestrial carbon storage in China was almost equal to the SOC contribution (Li et al. 2007). Precipitation and evaporation are important factors affecting SIC (Mi et al. 2008; Shi et al. 2012). In the present study, SOC and SIC in the soils correlated negatively and their correlation coefficients were -0.65 and -0.66, respectively (p \ 0.001). This could be explained by the higher moisture condition, which results in lower decomposition of SOC and increased primary production (implying higher organic matter input).
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Lower C/N ratios can indicate high microbial activity in soils. The TN densities are significantly positively correlated to SOC densities at most sites. The C/N ratios (atomic ratio) in this study ranged from 11 to 20. Our results were higher than the previous results in the eastern QinghaiTibetan Plateau in which C/N ratios ranged from 5 to 11 (Liu et al. 2012). The higher C/N ratios in the study area in this study are related to the vegetation community. In area of the present study, the C/N ratios of the sites with frozen soil layers were higher than those of under steppe soils, where no frozen layer was recorded. In the permafrost area, the active layer is typically the top layer of soil that thaws during the summer and freezes again during the autumn. A definite conclusion could not be drawn about the permafrost from the measured thaw depth because the thaw depth did not reach its maximum value until early September in this region. The soil pH values affect the SOM formation (Nierop and Verstraten 2003) and decomposition (Wang et al. 1993). Additionally, the soil pH may play a determining role in vegetation succession (Prach et al. 2009) and thus SOC content. As expected, the pH was significantly negatively correlated with the SOC, TN, and WSOC in the present study. The negative relationship between the C/N ratios and the soil pH values (p \ 0.001) was in agreement with a previous study that showed microbial activity had higher rates in older, more acidic landscapes (Whittinghill and Hobbie 2012). As expected, the moisture negatively correlated to soil bulk density (Franzen and Ehlers 1994) and positively correlated to the proportion of fine particles. It should be mentioned that the fine particles proportions were largely positively correlated with the SOC, TN and moisture content. This result could be explained by the fact that fine particles tend to stabilize and retain more organic matter than those with coarser particle contents (Gregorich et al. 1994) and the moisture storage capacity increases with fine particle proportions (Vereecken et al. 1989; Gomez-Plaza et al. 2001). Previous studies have suggested that the soil moisture content and bulk densities were the most important factors in the determination of SOC and TN in the QinghaiTibetan Plateau (Baumann et al. 2009; Yang et al. 2008). In the present study, the step-wise regression results suggested that the bulk density was the most important factor for the SOC and TN contents. The soil moisture and soil particle distribution were excluded in the model. The results of the regression were reasonable because the bulk density is positively correlated with moisture content, soil particle distribution and pH. The emission of greenhouse gases caused by permafrost degradation has accelerated global warming through positive feedback mechanisms. It has been documented that
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significant losses of soil carbon from permafrost thaw is significantly higher than the increased plant carbon uptake (Schuur et al. 2009). In fact, the whole permafrost area in Qinghai-Tibetan Plateau possesses a cold, dry and windy climate. Permafrost degradation would decrease vegetation biomass and thereby change the vegetation types (Wang et al. 2012). It has been demonstrated that the active layer thickness of the permafrost regions may increase by 0.1–0.7 m for the year 2049 and 0.3–1.2 m for the year 2099 (Pang et al. 2012). Therefore, more attention should be paid to the SOC, SIC and TN in this permafrost region in the future. The present study showed the contents of carbon and nitrogen in the soils in the eastern portion of the permafrost regions in the Qinghai-Tibetan Plateau. The results demonstrated that the SOC and TN decreased in the wet meadow, meadow, degraded meadow and steppe. Although the mechanisms of vegetation succession are very complicated, it is generally regarded that permafrost thawing will lead to decreasing moisture content and thus vegetation degradation. It is worth mentioning that the proportion of WSOC to SOC in the wet meadow soils (the ratio of their surface layers is 25.5 %) is higher than those of other vegetation types (10.6 % for meadow, 6.8 % for degraded meadow, and 16 % for steppe). Therefore, the results suggest that the SOC in the wet meadow soils in the permafrost regions of the Qinghai-Tibetan Plateau have the potential to release greenhouse gases and contribute to future global warming.
Conclusions In the present study, the SOC, WSOC, SIC, and TN distribution under four typical vegetation communities were examined. The highest SOC, WSOC and TN in the wet meadow soil horizons of layer A were 180.9, 46, and 10.9 g kg-1, respectively. The SOC, WSOC, and TN contents were higher in the meadow soils than those in the degraded meadow soils. Under the steppe communities, the SOC and TN showed the lowest values of 6.2 and 0.59 g kg-1, respectively, in the 180 cm soil profile, and showed much lower vertical decreasing trends in comparison to those of degraded meadow, meadow and wet meadow. The SIC contents showed comparatively similar values with the SOC in the steppe and were much lower than the SOC in other vegetation communities. The C/N ratios the wet meadow and meadow were higher than those in the degraded meadow and steppe. Higher pH and lower moisture content means higher SIC content and higher SOC decomposition rates. Furthermore, the SOC contents varied greatly under different vegetation communities. The bulk density was the most significant predictor for the distribution of SOC, WSOC, SIC, and TN. The results
suggested that the SOC in wet meadow soils of the permafrost regions of the Qinghai-Tibetan Plateau have the potential to release greenhouse gases to contribute to future global warming. Acknowledgments This work was financially supported by the National Basic Research Program of China (973 Program) (2010CB951402) and the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (41121001). This work was also supported in part by the Fund of the State Key Laboratory of Cryospheric Science (SKLCS-ZZ-2012-03-01) and the National Natural Science Foundation of China (41161082, 41261002, 41101524). We acknowledge the West Light Foundation of the Chinese Academy of Sciences and the Foundation for Excellent Youth Scholars of Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. We gratefully acknowledge the anonymous reviewers, as well as the Editors for their constructive comments.
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