Opposite patterns of soil organic and inorganic carbon along a climate gradient in the alpine steppe of northern Tibetan Plateau

Opposite patterns of soil organic and inorganic carbon along a climate gradient in the alpine steppe of northern Tibetan Plateau

Catena 186 (2020) 104366 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena Opposite patterns of soi...

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Catena 186 (2020) 104366

Contents lists available at ScienceDirect

Catena journal homepage: www.elsevier.com/locate/catena

Opposite patterns of soil organic and inorganic carbon along a climate gradient in the alpine steppe of northern Tibetan Plateau


Chenjun Du, Yongheng Gao

CAS Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization & Ecological Restoration and Biodiversity Conservation Key Laboratory of Sichuan Province, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China



Keywords: Arid regions Climate change Vertical distribution Soil carbon content C dynamics

Both soil organic carbon (SOC) and inorganic carbon (SIC) are important components of soil carbon storage. However, few studies have attempted to estimate the magnitude and patterns of SIC in arid areas. We measured the concentrations of SOC and SIC in alpine steppe grasslands along a climate gradient in the Tibetan Plateau. At all sites, an opposing trend was found in the distribution of SOC compared with SIC. SOC increased with increasing precipitation, while SIC decreased. Within sites, SOC decreased with soil depth, while SIC increased. Among the variables considered, soil total nitrogen (TN) and soil water content explained the most variability in SOC (r2 = 0.95 for TN, r2 = 0.93 for soil water content), and soil pH had the highest correlation with SIC (r2 = 0.90). This finding indicates that SOC is closely related to biological processes, such as biomass input and litter accumulation, while SIC is determined by abiotic factors such as chemical and physical processes of soil formation. SOC and SIC densities showed that the SOC pool comprised 91% of total carbon (TC) storage in these alpine steppe soils. A decrease in SOC density occurred with depth, but this pattern was the opposite for SIC density in all soil profiles. These findings help to characterize the C cycle in the alpine steppe of the Tibetan Plateau.

1. Introduction In recent decades, a number of studies have assessed C storage locally, regionally, and globally owing to the significant influence of the C cycle on ecological and societal functioning (Batjes, 2006; Batjes, 2014; Bellamy et al., 2005; Lal, 2004). Among the five main global C pools, the oceanic pool (38000 Pg, 1 Pg = 1015 g) is the largest, followed by the geological (5000 Pg), soil (2500 Pg), atmospheric (760 Pg), and biotic (560 Pg) pool (Lal, 2004). Soil is the largest C pool in the terrestrial biosphere, and consists of organic and inorganic components containing C levels 1.7 times higher than that of atmospheric and vegetation C combined (Yang et al., 2012). Globally, the upper 100 cm of soil contains 2157–2293 Pg C, storing 1462–1548 Pg SOC and 695–745 Pg SIC, respectively (Batjes, 2014). Alterations to SOC and SIC stocks are of profound importance because they could significantly affect the global C budget and exacerbate climate change (Bellamy et al., 2005; Bradford et al., 2016). The global SOC stock holds the equivalent of more than 200 years of current fossil fuel C emissions (Mathieu et al., 2015), and SOC pools are most extensive in cool or high latitude/altitude ecosystems (Chen et al.,

2016; Lal, 2004). These SOC pools are a significant nexus in the feedback between the global carbon balance and climate change (Bradford et al., 2016; Salome et al., 2010). For instance, climate warming would stimulate biological decomposition and enhance SOC decomposition rates (Bellamy et al., 2005; Lal, 2004; Mathieu et al., 2015; Salome et al., 2010), reducing the capacity cool-region soils to act as a C sink, and may even transform them to C sources that promote further warming (Schmidt et al., 2011). The global SIC pool occurs mainly in arid and semiarid soils that cover 33% of the surface of the earth (Wu et al., 2009), and is 2–10 times larger than the SOC pool in these regions (Batjes, 2006). SIC is classified as lithogenic inorganic carbon (LIC) and pedogenic inorganic carbon (PIC). LIC is usually inherited from the soil’s parent material and is relatively stable in the short term. PIC is more dynamic and is usually formed by dissolution and precipitation of carbonate parent material. This process is closely dependent on water availability, the partial pressure of CO2, and Ca2+/HCO3− concentrations (Mi et al., 2008; Wu et al., 2009). SIC also plays an important role in the global terrestrial C cycle as an effective atmospheric CO2 sequestration path, where the soil secondary carbonates seize atmospheric CO2 during pedogenic carbonate precipitation (Emmerich,

Corresponding author: No. 9, South Renmin Road, Chengdu 610041, China. E-mail address: [email protected] (Y. Gao).

https://doi.org/10.1016/j.catena.2019.104366 Received 6 June 2019; Received in revised form 7 November 2019; Accepted 10 November 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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than 500 mm (Mi et al., 2008). Due to the unique climate and harsh conditions, disturbance from human activity is low, which makes the area an ideal region for exploring SOC and SIC storage in arid ecosystems. Much is known about the SOC stock within potions of the Tibetan Plateau, and these large pools may be highly sensitive to climate warming (Liu et al., 2012; Shi et al., 2012; Wu et al., 2017; Yang et al., 2008). However, fewer studies have focused on the distributions and drivers of SIC in this area (Mi et al., 2008; Shi et al., 2012; Yang et al., 2012; Yang et al., 2010a). Previous regional studies of SIC were inconsistent, finding that SIC density decreased with mean annual temperature (MAT) and mean annual precipitation (MAP) (Yang et al., 2010a), or that lower MAP corresponded to lower topsoil SIC (Shi et al., 2012). Potential soil C responses to future global changes remain uncertain for the Tibetan Plateau because information on SOC and SIC distributions in alpine steppe, and their relationships with climate variables is lacking. In present study, we investigated SOC and SIC dynamics within the alpine steppe of the Tibetan Plateau with the following goals: (1) to quantify vertical distribution of SOC and SIC along naturally occurring climate gradients to determine the effects of mean annual precipitation and temperature on SIC and SOC, and (2) to evaluate the relationship between SOC/SIC and soil water content and other soil chemical properties (i.e., pH, total nitrogen concentration).

2003). Compared with the more dynamic SOC pool, the SIC pool is regarded as relatively stable and less responsive to climate change. However, SIC in the top 10 cm of soils across grasslands in China has decreased significantly due to chronic soil acidification (Yang et al., 2012). The loss of SIC in these regions could amplify carbon-climate feedbacks (Yang et al., 2012). SOC and SIC are critical C pools in the terrestrial biosphere, and both need to be considered when assessing carbon-climate feedbacks (Mi et al., 2008; Yang et al., 2010a). While SOC is well understood, there are little data available on the magnitude and spatial distributions of SIC (Mi et al., 2008; Wang et al., 2015a,b; Yang et al., 2010a). Furthermore, assessments of both SOC and SIC in arid regions are lacking (Wang et al., 2015a,b). Previous studies mainly focused on SOC and SIC pools in the topsoil (0–10 cm), while little is known about SOC and SIC dynamics below 10 cm (Raheb et al., 2017). In general, SOC increases with precipitation but decreases with soil depth due to biological processes such as vegetation production and litter input (Mathieu et al., 2015; Salome et al., 2010; Shi et al., 2012; Tuo et al., 2018). However, changes in SIC along precipitation gradients and with soil depth are ambiguous due to limited available data. For example, there are reports of decreases (Raheb et al., 2017) and increases (Wang et al., 2010) in SIC with increasing precipitation. The vertical distributions of SIC were observed to be generally stable (Han et al., 2018), increasing (Wang et al., 2015a,b), and decreasing (Wang et al., 2010). The Tibetan Plateau formed from the collision of the Indian and Eurasian continental plates (Zhuang et al., 2010), and is the highest and largest plateau in the world, with inhospitable environmental conditions. The plateau is also known as the “roof of the world”, with an average elevation above 4000 m (Chen et al., 2015; Ding et al., 2017). The northern Tibetan Plateau is mainly covered by alpine steppe, distributed in the relatively arid areas (Fig. 1). In China, 84.0% of SIC storage is distributed in regions with mean annual precipitation lower

2. Materials and methods 2.1. Study sites Soil samples were collected in mid-August 2017 from six sites (80.47oE ~ 91.04oE, 31.51oN ~ 32.35oN) along a climate gradient in the Tibetan Plateau (Fig. 1). In our study, the sites were defined as Gar, Gakyi, Gerts, Nima, Palgon, and Amdo according to the local name. To

Fig. 1. Location of the study plots, vegetation map (1:1000000), and precipitation contours (mm) on the Tibetan Plateau. 2

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Fig. 2. Depth profiles for SOC concentrations for each of the sites. Values indicate mean ± SE.

g cm−3) by using Eq. (1):

reduce errors caused by topographic variation and human disturbance, we selected sampling sites within flat and open terrain away from developed areas, and approximately 2 km from main roads. The six sampling sites were all located in alpine steppe dominated by native grasses such as Stipa purpurea, Oxytropis glacialis, Oxytropis microphylla and Orinus thoroldii (Table S1). The climate is cold and relatively dry, and over 80% of MAP occurs during the growing season (May-September) (Hopping et al., 2018), with relatively drier winters and springs (Chen et al., 2013). The soil types are all classic calcic, which are defined as cambisols according to FAO-UNESCO Soil Map of the World soil taxonomy system (Wu et al., 2003; Yang et al., 2010a).

BD = 0.29 + 1.296 exp (−0.0167 SOC )


where BD is bulk density (g cm−3) for SOC concentration (g kg−1). Then, the SOC density (SOCD, kg m−2) and SIC density (SICD, kg m−2) for each soil layer was determined by BD and their concentrations using Eqs. (2) and (3):

SOCDi = BDi × SOCi × Ti ×

SICDi = BDi × SICi × Ti × 2.2. Sampling and analysis

1 100


1 100

(3) −2

where SOCDi stock is SOC density (kg m ); SICDi stock is SIC density (kg m−2); BDi is soil bulk density (g cm−3), SOCi is SOC concentration (g kg−1); SICi is SIC concentration (g kg−1); and Ti is thickness (cm, 10 cm in present study) at layer i. Estimates of MAP and MAT at each site were spatially interpolated from the records of 119 meteorological observations between 1982 and 2013 (Qin et al., 2018). Generalized linear models (GLM) were used to assess the relationships between SOC/SIC, TC, C:N ratio, SOC:SIC ratio, TN, pH, DOC, TDN, DON, NH4+N, NO3−-N and soil water content of six sampling sites to obtain the best predictors of SOC and SIC. The effects of MAP and MAT on SOC/ SIC and aboveground biomass were also quantified using GLM. All statistical analyses were performed by using R.3.4.3 software (R Development Core Team, 2017).

At each sampling site, aboveground biomass was harvested in 1 × 1 m plots, and a total of 18 biomass samples (3 replicates × 6 sites) were obtained. After clearing the surface litter, a bucket auger with an 8 cm diameter was used to collect soil samples. Profiles were analyzed in 10 cm increments down to 70 cm depth, resulting in 126 composite soil samples (3 replicates × 7 depths × 6 sites) which were stored at 4 °C until analyzed. Aboveground biomass samples were oven-dried at 65 °C for 48 h to a constant weight and weighed to the nearest 0.01 g. Gravimetric soil water content (%) was measured on dried at 105 °C for 48 h. Soil samples were air-dried and passed through a 2 mm sieve to analyze chemical properties after manual removal of roots and fine gravels. SOC and TN were analyzed using an elemental analyzer (Vario EL III, Elementar, Germany). SIC was determined using the pressure calcimeter method. Soil pH was measured in a 1:2.5 soil: water mixture. Dissolved organic carbon (DOC), ammonia nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) concentrations were measured using a continuous flow autoanalyzer (Skalar San++ 8505, Netherlands). TC was calculated as the sum of SOC and SIC. Dissolved organic nitrogen (DON) was calculated as TN - (NH4+-N + NO3−-N). Total dissolved nitrogen (TDN) was calculated as the sum of DON, NH4+-N and NO3−-N. Soil C:N ratio was calculated by dividing SOC by TN.

3. Results 3.1. Distributions of SOC and SIC concentrations SOC concentrations strongly decreased with soil depth at all sites (Fig. 2). SOC decreased with depth more steeply at wetter sites than at drier sites. Within each 10 cm soil layer, SOC increased from Gar to Amdo, consistent with increasing MAP. In contrast, SIC generally increased with soil depth at all sites (Fig. 3). We found that wetter sites had less SIC than drier sites along the sampled climate gradient. Overall, SOC concentrations were about 10 times higher than that of SIC (Figs. 2 and 3). The distributions of TC concentration were similar to SOC (Fig. S1).

2.3. Statistical analyses To evaluate the densities (kg m−2) of SOC and SIC, an empirical function (Yang et al., 2010b) was used to evaluate soil bulk density (BD, 3

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Fig. 3. Depth profiles for SIC concentrations for each of the sites. Values indicate mean ± SE.

precipitation across the alpine steppe of the Tibetan Plateau, indicating that potential SOC accumulation is higher in wetter sites than in drier sites. Our findings are supported by previous studies along precipitation gradients in the central Great Plains of the United States (Klopfenstein et al., 2015), Inner Mongolia in China (Evans et al., 2011), northwestern Iran (Raheb et al., 2017) and in other Tibetan Plateau grasslands (Yang et al., 2008). SOC increases with precipitation because soil water availability is typically the primary limiting factor in plant production, especially in arid regions (Jobbagy and Jackson, 2000), and increasing precipitation generally stimulates plant productivity and litter inputs (Hong et al., 2015; Huang et al., 2016). Stronger correlations between SOC and MAP in topsoil (0–30 cm) compared to deeper soil layers reflect litter inputs from aboveground biomass, as well as belowground biomass which is often shallowly distributed in steppe grasslands (Yang et al., 2009b). We also found that SOC and aboveground biomass significantly decreased with mean annual temperature. This is probably because higher temperatures accelerate evapotranspiration (Wang et al., 2015a,b; Wu et al., 2013), thereby reducing soil water availability and plant production. Temperature can be an important control on SOC stocks through its effects on microbial decomposition (Lu et al., 2013). However, decomposition rates are generally less than biomass production in our sampling sites, owing to the lower annual temperature (Table S1) and seasonally frozen ground with enormous amounts of SOC (Koven et al., 2015; Zou et al., 2017). In summary, relatively high mean annual precipitation and low mean annual temperature increase the storage of SOC in alpine steppe grasslands (Liu et al., 2012; Yang et al., 2008). Regression analysis clarified the relationships between SOC concentration and soil chemical properties. Aside from soil water content, SOC was most strongly related to TN, consistent with other findings from Tibetan Plateau grasslands (Liu et al., 2012). Stoichiometric principles (Elser et al., 2007) such as C:N ratios could explain the positive relationship between SOC and TN. We found a strongly negative correlation between SOC and soil pH, in agreement with a previous study in Mongolian and Tibetan grasslands (Shi et al., 2012). Soil pH generally exerts an indirect effect on SOC through mediating the soil microbial population (Chen et al., 2016), with higher soil pH leading to greater decomposition of SOC (Wu et al., 2017). At the site level, SOC contents and proportional distributions in the topsoil (0–30 cm) are markedly larger than in deeper soil (40–70 cm). Vegetation strongly

3.2. Relationships between SOC/SIC concentrations and chemical properties We observed strong negative correlations between SOC and SIC, and soil pH (Fig. 4). However, SOC was positively correlated to TC, C:N ratio, SOC:SIC ratio, TN, pH, DOC, TDN, DON, NH4+-N, NO3−-N and soil water content. Of the measured variables, TN (r2 = 0.95) and soil water content (r2 = 0.93) were the most strongly related to SOC. SIC responses to each soil chemical parameter were opposite to those of SOC (Fig. 5). Soil pH was the strongest predictor of SIC (r2 = 0.90), and exhibited the only positive correlation. SIC was negatively related to the other soil chemical parameters. The effects of other soil chemical parameters on TC concentrations were similar to those of SOC (Fig. S2). 3.3. Relationships between SOC/SIC concentrations and climate We found that MAP had a positive effect on SOC (Fig. 6, Table 1). The slope and r2 values of MAP correlations decreased with soil depth. In contrast, MAT exhibited a negative effect on SOC, and the slope and r2 values of MAT correlations generally decreased with soil depth. MAP and MAT showed the opposite influence on SIC compared with SOC. 3.4. Distribution pattern of SOC and SIC densities SOC and SIC densities were evaluated by Eqs. (1)–(3) to show the effects of climate gradients on soil C storage. SOC density increased notably with MAP and decreased with MAT (Fig. 7), similar to aboveground biomass (Fig. 9). For each site, SOC density decreased with soil depth (Fig. S3). Likewise, the proportional distribution (%) of SOC density decreased with the depth (Fig. 8). At the site level, SIC density increased with soil depth. The proportional distributions of SIC density increased with depth. Overall, 91% of TC is contributed by SOC, while SIC added only 9% of TC. The effects of climate and soil depth on TC density were similar to those of SOC (Fig. S4, Table S2). 4. Discussions 4.1. Pattern of SOC along a climate gradient We found that SOC concentrations increase with mean annual 4

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Fig. 4. Regressions between SOC concentrations and SIC, TC, C:N ratio, SOC:SIC ratio, TN, pH, DOC, TDN, DON, NH4+-N, NO3−-N and soil water content (a-l). Data for each site were pooled across soil depths. ** P < 0.001.

We used the pressure calcimeter method for calculating the difference between TC and SOC, and a loss-on-ignition (LOI) procedure was used for soil organic matter (SOM) and SIC (Wang et al., 2012). SIC was calculated as the difference between TC and SOC, which could inflate relative errors (Shi et al., 2012). Second, regional differences in soil formation and climatic conditions could contribute to different distribution patterns among studies. Compared to other regions, our sites were located at higher altitudes and had lower MAP (Table S1). Our results showed that MAP had negative effects on SIC concentrations, probably because higher precipitation can accelerate leaching of inorganic carbon from topsoil to deeper soils and groundwater during the rainy season (Mi et al., 2008). This process could lead to a higher SIC by containing higher LIC in drier sites than in wetter sites. Furthermore, very low CO2 partial pressure at the high altitudes of the Tibetan Plateau (Wang et al., 2004), combined with low MAT and soil respiration rates at our sample sites, could increase the formation of PIC as per the following chemical Eq. (5):

affects SOC storage in the upper 100 cm of soils in alpine grasslands of the Tibetan Plateau (Tan et al., 2010; Wu et al., 2017). Shallower root biomass distributions combined with aboveground biomass could account for this phenomenon (Sun and Wang, 2016), previous study found that almost 90% of total root biomass is distributed in the top 0–30 cm of soil (Yang et al., 2009b).

4.2. Pattern of SIC along a climate gradient A decreasing SIC content along the precipitation gradient in our study indicates that there is greater potential SIC accumulation in the drier sites than the wetter sites. This finding is supported by previous studies in continental steppes in Inner Mongolia (Wang et al., 2010), across China (Mi et al., 2008) and in the Tibetan alpine grasslands (Yang et al., 2010a). However, our results contrast with several studies from Inner Mongolia (Wang et al., 2013), northwest Iran (Raheb et al., 2017) and Mongolian grasslands (Shi et al., 2012). Unlike SOC distributions, observed SIC distributions along climate gradients are more variable, and patterns may diverge for several reasons. First, there are various methods for determining SIC, based on quantification by acid dissolution as per the chemical reaction in Eq. (4): CaCO3 + 2H+ → Ca2+ + H2O + CO2

Ca2+ + 2HCO3− ↔ CaCO3 + H2O + CO2


More aboveground plant biomass with increasing precipitation could also cause enhanced accumulation of Ca2+ in plant tissues (Liu et al., 2014). Thus, lower soil Ca2+ in wetter sites could reduce SIC, as presented in Eq. (5). Furthermore, lower CO2 partial pressure would

(4) 5

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Fig. 5. Regressions between SIC concentrations and SOC, TC, C:N ratio, SOC:SIC ratio, TN, pH, DOC, TDN, DON, NH4+-N, NO3−-N and soil water content (a-l). Data for each site were pooled across soil depths. ** P < 0.001.

Fig. 6. Regressions of SOC, SIC and TC concentrations (g kg−1) against MAP (mm) and MAT (°C) for each 10 cm depth intervals (a-f). MAP was log transformed. 6

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Table 1 Summary of slope and r2 values (in bracket) from the regressions of SOC, SIC and TC concentrations (g kg−1) against MAP (mm) and MAT (°C) for each 10 cm depth intervals at six sites. MAP was log transformed. SOC concentration

0–10 cm 10–20 cm 20–30 cm 30–40 cm 40–50 cm 50–60 cm 60–70 cm

SIC concentration

TC concentration

MAP (mm)

MAT (°C)

MAP (mm)

MAT (°C)

MAP (mm)

MAT (°C)

7.197 6.355 2.578 2.098 1.584 0.676 0.729

−3.370 −2.599 −1.052 −0.761 −0.600 −0.184 −0.227

−0.049 −0.045 −0.036 −0.045 −0.077 −0.062 −0.058

0.028 0.020 0.010 0.017 0.027 0.023 0.023

7.148 5.310 2.542 2.053 1.507 0.614 0.671

−3.442 −2.579 −1.042 −0.744 −0.573 −0.161 −0.204

(0.954) (0.889) (0.811) (0.653) (0.840) (0.233) (0.219)

(0.959) (0.905) (0.584) (0.371) (0.521) (0.075) (0.092)

(0.196) (0.214) (0.135) (0.227) (0.496) (0.615) (0.607)

(0.268) (0.180) (0.043) (0.136) (0.308) (0.374) (0.419)

(0.954) (0.885) (0.828) (0.656) (0.842) (0.204) (0.200)

(0.956) (0.903) (0.601) (0.372) (0.527) (0.061) (0.080)

move the equilibrium towards enhanced PIC by reprecipitating carbonate. Thus, the effects of CO2 partial pressure and all of its influencing factors in extreme high-altitude environments should be considered in future studies. Third, we found that MAT was positively correlated to SIC content, in contrast with a study in the Tibetan alpine grasslands (Yang et al., 2010a). The effect of MAT on reprecipitation of carbonates is an outcome of the influence on soil respiration by yielding Ca2+ and Mg2+ (Bronick and Lal, 2005; Mi et al., 2008). Consequently, the lower MAT leads to lower Ca2+ and moves the equilibrium towards less PIC by precipitating carbonate. In addition, higher MAT could lead to higher SIC in this arid area by accelerating evaporation, which contributes to the reprecipitation of PIC as presented in Eq. (5). Of the soil chemical properties analyzed, only soil pH exhibited a significantly positive relationship to SIC. More acidic environments decrease SIC precipitation by enhancing carbonate dissolution, as presented in Eq. (4). We observed that both SOC and soil water content had significantly negative relationships to SIC, consistent with a study in the Loess Plateau (Wang et al., 2016). Higher SOC and soil water content cause less SIC by shifting Eq. (5) towards more carbonate dissolution. When the vertical distributions of SIC were analyzed, its contents and proportional distributions in the topsoil (0–30 cm) were less than in deeper soil (40–70 cm) at all sites, indicating that SIC stock was

Fig. 7. Total SOC, SIC and TC densities (kg m−2) of six sampling sites.

Fig. 8. Proportional distributions (%) of SOC, SIC and TC density in all sites (a-c). Different colors represent different soil depths as shown in other figures. 7

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Fig. 9. Regressions of aboveground biomass against MAP (mm) and MAT (°C) (a-b). MAP was log transformed.

that upper soil layers could represent a substantial regional soil carbon sink in a warming climate (Ding et al., 2017). A slightly wetter climate in the Tibetan Plateau (Chen et al., 2013) caused by increasing precipitation could lead to an increase in the SIC storage in deeper soil layers (Mi et al., 2008; Wu et al., 2009; Yang et al., 2010a). In the last two decades, SIC decreased at a mean rate of 26.8 g m−2 yr−1 in Chinese grasslands due to soil acidification (Yang et al., 2012). The total amount of SOC and SIC in Tibetan Plateau grasslands remains undetermined due to limited available data (Chen et al., 2013). A limitation of the present study was that only the upper 70 cm of the soil profile was studied, owing to the harsh conditions at this latitude. Therefore, a long-term in situ investigation of deeper soil layers along climate gradients should be developed to clarify C dynamics in the future.

distributed in deeper soil layers in the alpine steppe grasslands. First, this pattern may be explained by higher SOC and lower soil pH observed in the topsoil, where SOC decomposition could provide an acidic environment (Wang et al., 2015a,b) that increases carbonate dissolution. Second, the small SIC in the topsoil may be caused by higher soil water content and CO2 partial pressure, which shifts the Eq. (5) towards less PIC formation by carbonate dissolution. Third, higher SIC in the deeper soil layers may be associated with leaching of carbonate from the topsoil to deeper soil layers during the rainy season (Mi et al., 2008; Yang et al., 2010a).

4.3. Implication for C storage in alpine steppe The primary form of C pools in the upper 70 cm of soils in the Tibetan Plateau alpine steppe is SOC rather than SIC. Across our sites, SOC is approximately 10 times as high as SIC. This SOC:SIC ratio is similar to values reported for topsoil in Mongolian and Tibetan grasslands (Shi et al., 2012). Human activities and climate change could significantly affect the biogeochemical cycles of the Tibetan Plateau (Chen et al., 2013). At present, many effective C management systems have been developed to focus to enhance C storage. For instance, programs such as the “Retire-livestock-and-restore-grassland” and “Startup Re-grass Program” were established in the Tibetan Plateau to improve grassland degradation through grazing exclusion (Hu et al., 2016). These ecological engineering projects improved C stocks significantly (Deng et al., 2017; Hu et al., 2016). However, the amount of SOC and SIC maintained under changing land usage is still unknown due to limited data (Li et al., 2016; Liu et al., 2017). Although relatively low SIC concentrations were observed in our sites, the distribution of and potential changes to SIC should not be ignored in future management programs because large SIC stocks occur in deeper soil layers of arid regions, and these may play important role in the global C cycle (Han et al., 2018; Zhang et al., 2015). Global changes such as climate warming, changing precipitation patterns, and increased nitrogen deposition could alter C dynamics in Chinese alpine steppes. While the effects of increased temperatures on C storage in the Tibetan Plateau remain uncertain, increased decomposition rates can offset higher litter inputs to SOC (Yang et al., 2009a,b). However, a recent study indicated

5. Conclusion The patterns of SOC and SIC along climate gradients in the Tibetan Plateau were analyzed within relatively undisturbed alpine steppe sites. SOC increased with increasing MAP, but SIC decreased. This trend in SOC indicates that increasing MAP enhanced the soil organic matter by improving plant production. However, SIC increased with soil depth and showed a negative correlation with MAP, indicating leaching from topsoil to deeper layers in the rainy season. TN and soil water content were the two strongest predictors for SOC, while soil pH was the strongest predictor for SIC. Our data showed that the SOC pool was the major form of total carbon storage in the upper 70 cm of alpine steppe soils. Long-term field investigation and deep soil analysis should be conducted to improve our understanding of the patterns of SOC and SIC in the alpine steppe of the northern Tibetan Plateau.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


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