Storage, patterns and controls of soil organic carbon in the alpine shrubland in the Three Rivers Source Region on the Qinghai-Tibetan Plateau

Catena 178 (2019) 154–162

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Storage, patterns and controls of soil organic carbon in the alpine shrubland in the Three Rivers Source Region on the Qinghai-Tibetan Plateau Xiuqing Niea,b,c, Lucun Yanga,b, Fan Lid, Feng Xionga,b,c, Changbin Lia,b,c, Guoying Zhoua,b,

T

⁎

a

Key Laboratory of Tibetan Medicine Research, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining 810008, China Qinghai Key Laboratory of Qing-Tibet Plateau Biological Resources, Xining 810008, China c University of Chinese Academy of Sciences, Beijing 100049, China d Institute of Qinghai Meteorological Science Research, Xining 810008, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Alpine shrubland SOC Controlling factors Pattern Qinghai-Tibetan Plateau

Alpine shrubland ecosystems in the Three Rivers Source Region (TRSR) store substantial soil organic carbon (SOC), but the storage, patterns and control of SOC in those ecosystems have rarely been investigated. In this study, using data from 66 soil profiles surveyed from 22 sites between 2011 and 2013, we estimated the storage and patterns of SOC, and their relationships with climatic factors, elevation, ground cover and slope. Our results showed that SOC storage in the top 100 cm across the TRSR shrubland was 0.68 ± 0.38 Pg C, with an average SOC density (soil carbon storage per area) of 26.21 ± 14.58 kg m−2. Spatially, SOC density increased with longitude and latitude. Vertically, SOC in the topsoil at 30 cm and 50 cm accounted for 56% and 75%, respectively, of the total at 100 cm. SOC density showed a decreasing trend with increasing elevation, but it was greater in regions of higher ground cover. The density had no relationship with either mean annual precipitation or slope. Increasing mean annual temperature had positive effects on SOC density, which is inconsistent with the global trend. With increasing soil depth, however, the effects of temperature on SOC density were not significant. Therefore, in a global warming scenario, increasing temperature gives shrubland considerable C sink potential on the topsoil, and the regions of C sequestration differ as a result of uneven increases in temperature. Hence, further monitoring of dynamic changes is necessary to provide a more accurate assessment of potential C sequestration in TRSR shrubland.

1. Introduction Soil organic carbon (SOC) plays a significant role in the global C cycle because it accounts for between one half to two thirds of total terrestrial organic storage (Wynn et al., 2006). As the largest C pool in the terrestrial ecosystem, SOC has attracted much attention (Wang et al., 2015), and its storage in high-altitude regions is of special interest because of its high stocks (Davidson and Janssens, 2006). The study of SOC is meaningful not only because SOC acts as an important index of soil fertility and quality (Stockmann et al., 2015), but also because in a global warming scenario soil may be a carbon sink, which can offset increasing atmospheric CO2 (Davidson and Janssens, 2006). It has been confirmed that 25–30% of anthropogenic carbon dioxide (CO2) emissions have been absorbed by terrestrial ecosystems over the past 50 years (Reichstein et al., 2013). Many researchers have concluded that SOC stocks have increased with temperature (McGuire

et al., 2016; Yang et al., 2008). Minor changes in SOC storage may result in substantial effects on atmospheric CO2 concentration, which could affect currently warming temperatures (Davidson and Janssens, 2006; Han et al., 2016; Johnston et al., 2004; Schipper et al., 2007; Stockmann et al., 2013; Wang et al., 2016). Some studies have shown that SOC decomposition has increased more than the net primary production with increasing temperature (Nemani et al., 2003; Soleimani et al., 2017), which has resulted in a negative relationship between SOC and temperature (Jobbágy and Jackson, 2000). However, no consensus has been reached on whether increasing temperature has a negative or positive effect on SOC in different regions (Huang et al., 2018; McDaniel et al., 2017). Thus, some researches have been conducted to explore the effects of climate on SOC in the topsoil (0–30 cm) in both shrubland (Nie et al., 2018b) and grasslands on the QinghaiTibetan Plateau (Yang et al., 2008). However, the effects at other soil depths, such as 30–50 cm and 50–100 cm, have not been investigated.

⁎ Corresponding author at: Key Laboratory of Tibetan Medicine Research, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining 810008, China. E-mail address: [email protected] (G. Zhou).

https://doi.org/10.1016/j.catena.2019.03.019 Received 29 October 2018; Received in revised form 1 March 2019; Accepted 10 March 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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Similarly, no firm conclusions about the relationship between SOC and elevation have been reached. Most studies have found that soil C has an increasing trend with elevation (Garten and Hanson, 2006; Maraseni and Pandey, 2014; Tashi et al., 2016; Tsui et al., 2013), but some studies have reported no relationship (Tewksbury and Van Miegroet, 2007) or a decreasing relationship (Kumar et al., 2013). The result from a meta-analysis of global data showed a positive trend between soil C and elevation in the range of 317 m and 3300 m. However, SOC density responses at higher elevations have been underexplored. An accurate estimation of the storage, patterns and controls of SOC is considered of critical importance on the Qinghai-Tibetan Plateau, particularly when considering global warming. Shrubland has been identified as the most uncertain factor in the C sink in China, and information about the SOC of this important biome is scarce (Piao et al., 2009). On the Qinghai-Tibetan Plateau, the SOC in both the alpine wetlands (Ma et al., 2016) and alpine grasslands (Ding et al., 2016; Liu et al., 2016; Yang et al., 2008) has been explored, but SOC storage in the alpine shrubland of the Three Rivers Source Region (TRSR) has not been thoroughly estimated. For example, one study investigated SOC in the TRSR, but it classed the shrubland as alpine grassland (Chang et al., 2014) instead of as an independent type. Therefore, the precise quantification of SOC storage in TRSR alpine shrubland is required to draw credible conclusions about the potential feedback between the terrestrial C cycle and climatic factors. The Qinghai-Tibetan Plateau has experienced major effects from climate change and can induce significant feedback, as its altitude is higher than that of the surrounding land mass (Liang et al., 2013). The TRSR is located in the Qinghai-Tibetan Plateau's interior, and although the environment is extremely fragile, it has very important ecological functions and forms a security barrier for the sustainable development of the middle and lower reaches of three rivers (Luo et al., 2014). Due to its special geographical position, changes in the climate of the TRSR have been greater than those in other regions of the world in recent years (Guo et al., 2015), which has affected the SOC in the area (Mondal et al., 2016; Post et al., 1982; Poulter et al., 2014; Reichstein et al., 2013; Shang et al., 2016). It should be noted that soil C is a resource that is potentially manageable (Six et al., 2002; Torn et al., 2009). Land management can have greater effects on SOC than those associated with responses to climatic variability (Torn et al., 2009). The SOC distribution is expected to provide reference information for shaping policy at the administrative level, which can then protect soil C and enhance C sequestration in the TRSR of the Qinghai-Tibetan Plateau. In this study, we aimed to estimate the storage and controls of SOC in the TRSR shrubland, using 66 soil profiles sampled from 22 sites from 2011 to 2013. Our objectives were to determine (1) how much SOC is stored in the TRSR shrubland, (2) what the spatial and vertical SOC distribution patterns are, (3) how the mean annual temperature (MAT)

and mean annual precipitation (MAP) affect the SOC stock, and (4) identify the relationships among elevation, ground cover, slope and SOC in the TRSR shrubland. 2. Materials and methods 2.1. Study area The TRSR extends between latitudes from 31.65° to 37.02° N and longitudes from 89.40° to 102.45° E. The area is composed of the water source regions of the Yangtze, Yellow and Lancang (Mekong) Rivers and is located in the center of the Qinghai-Tibetan Plateau (Qin, 2014). The specific geographical position of the TRSR has led to it being identified as a national park in China. The most important vegetation types include alpine steppe, alpine meadow and alpine shrubland. As one of the most important biomes in the study area, the shrubland, with an area of 2.59 × 104 km2, is dominated by woody plants such as Rhododendron thymifolium, Potentilla fruticosa, R. capitatum and Sibiraea laevigata biomes (Chinese Academy of Science, 2001), mainly distributed at altitudes between 3600 m and 4500 m. The soil types are mainly alpine steppe soil, gray-cinnamon soil and chestnut soil. Alpine steppes are 7.52 × 104 km2 (Chang et al., 2014), and are widely located at altitudes between 3400 m and 4600 m. The dominant species include Achnatherum splendens, Leymus secalinus, Carex moorcroftii and Stipa purpurea, and chestnut soil and alpine steppe soil are widely distribution in the alpine steppe. Alpine meadows cover 20.18 × 104 km2 (Chang et al., 2014), and are mainly distributed at altitudes between 2300 and 4700 m. The dominant species include Kobresia capillifolia, K. parva, K. graminifolia and K. humilis, and alpine meadow soil is the most widely distribution soil type in alpine meadows. Generally, the soils have characteristics such as thinness, poor water-retention and low fertility (Editorial Committee of the Ecological Environment in the Three Rivers Source Region, 2002; Qin, 2014; Zhao, 2011). The MAT, MAP and mean elevation levels are −5.6 °C to 3.8 °C, 262.2–772.8 mm, and > 4000 m, respectively (Qin, 2014). The geographic, slope, species, ground cover and altitude information at each site are provided in Table S1. 2.2. Field survey and laboratory measurements To estimate the storage of SOC in alpine shrubland, 66 soil profiles were systematically sampled from 22 sites in shrubland biomes across the TRSR during summer (July–August) from 2011 to 2013 (Fig. 1). The method used for sampling was identical to the Technical Manual Writing Group of the Ecosystem Carbon Project (2015). At each site, the geographical position (longitude, latitude, elevation and slope), was determined using a global positioning system (GPS) device (BHC Navigation Company Beijing, China). Three plots of 5 m × 5 m were

Fig. 1. The locations of 22 sites in the Three Rivers Source Region shrubland on the background of a vegetation map of China at a scale of 1:1,000,000 (Chinese Academy of Sciences, 2001). 155

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Table 1 Density and storage of soil organic carbon (SOC) for different vegetation types in the Qinghai-Tibetan Plateau. The results of this study are shown as mean values ± standard deviation, and results of Yang et al. (2008) are shown as estimated values ± uncertainties variances. Vegetation types

Alpine steppe Alpine meadow Alpine shrubland

Area (104 km2)

61.08 51.74 2.59

SOC density (kg m−2)

SOC storage (Pg C)

30 cm

50 cm

100 cm

30 cm

50 cm

100 cm

2.94 ± 0.88 6.17 ± 1.85 12.78 ± 5.74

3.67 ± 0.11 7.51 ± 2.25 18.02 ± 8.33

4.38 ± 1.31 9.25 ± 2.78 26.21 ± 14.58

1.80 ± 0.54 3.19 ± 0.96 0.33 ± 0.15

2.24 ± 0.67 3.89 ± 1.01 0.47 ± 0.22

2.68 ± 0.80 4.68 ± 1.40 0.68 ± 0.38

The SOC density in the alpine shrubland ranged from 1.69 to 25.15 kg m−2 for the 0–30 cm depth, from 2.47 to 33.60 kg m−2 for 0–50 cm, and from 3.23 to 53.87 kg m−2 for 0–100 cm. The mean SOC density of all alpine shrubland sites for the three soil depths (0–30, 0–50 and 0–100 cm) were 12.78 ± 5.74, 18.02 ± 8.33 and 26.21 ± 14.58 kg m−2, respectively (Table 1). The total SOC storage at 0–100 cm across the TRSR shrubland was estimated at 0.68 ± 0.38 Pg C. There was a decreasing trend in SOC density with increasing soil depth (Fig. 3). The top soil in the upper 30 cm and 50 cm accounted for about 56% and 75% of the total SOC, respectively, in the 0–100 cm range (Fig. 4). Over half of the total SOC was stored in topsoil; SOC storage at depths of 0–10, 10–20, and 20–30 cm is given in Table 2.

The alpine grassland SOC data used in this study were from Yang et al. (2008) and Chang et al. (2014). We used the following equations to estimate the SOC density for each soil depth, and further calculated the SOC stocks for different soil depths. These calculations were conducted with the following equations (Eqs. (1)–(2)).

i=1

(2)

3.1. Density and storage of SOC

2.4. Data sources and data analysis

(1 − Ci ) 100

Yang et al. (2008) Yang et al. (2008) This study

3. Results

MAP and MAT data were obtained from the climate database of Qinghai-Plateau for 2000–2010, which had been compiled and spatially interpolated from the records of 50 climatic stations located at altitudes above 3000 m on the Qinghai-Plateau. In addition, the summer mean temperature (July–August) and summer total rainfall were interpolated at each sampling site.

n

78 57 22

where SOCDi, Ti, BDi, and Ci are the SOC density (g cm−3), soil thickness (cm), bulk density (g cm−3) and volume percentage of the fraction > 2 mm at layer i (cm), respectively. SOCS is SOC stock (Pg C), and area is the total surface area (104 km2) of TRSR alpine shrubland. The mean value of 22 sites was used to calculate SOCS in the TRSR alpine shrubland. Ordinary least squares regression analyses were conducted to explore the relationships among SOC density, longitude and latitude, slope and climatic factors (MAT and MAP). A nonparametric test was used to compare SOC density in different ranges of ground cover and elevation. All analyses were performed using R version 3.1.1 (R Development Core Team, R Project for Statistical Computing, Vienna, Austria), and graphs were prepared using SigmaPlot 12.5 (Systat Software, Inc., Point Richmond, CA).

2.3. Climate data

∑ Ti × BDi × SOCi ×

Reference

SOCS = SOCD × Area

selected at each site to represent shrubland communities. The dominant species of shrubland and herbs were recorded in these plots. One plot (1 m length × 1.5 m width × 1 m depth) was excavated at each site, and soil samples were collected from three soil profiles. For each profile, soil samples were collected at depths of 0–10, 10–20, 20–30, 30–50, 50–70 and 70–100 cm. The soil samples from the same depth intervals were then mixed to yield one composite sample, for a total of 132 composite samples (22 sites × 6 soil depths). The bulk density was determined as follows for each layer in the soil profile. Samples were taken using a standard 100 cm3 container (50.46 mm diameter, 50 mm height), and were weighed gravimetrically after 24 h of desiccation at 105 °C in the laboratory. The bulk density was obtained via the oven dry mass of soil over volume (Yang et al., 2007, 2008). The soil samples for C analysis were air-dried through a 2.0 mm sieve, fine roots were removed and the soil was then ground in a ball mill. The SOC was determined by wet oxidation using the Walkley-Black method, which consisted of adding sulfuric acid to a potassium dichromate solution to oxidize the organic matter (Conyers et al., 2011; Yang et al., 2008). Ground cover was measured using visual estimation in the field (Yi et al., 2010; Zhang et al., 2003). A PHS-3C meter (Shanghai Dapu Instrument Company, Shanghai, China) was used to measure the soil pH in a 1:2.5 mixture of soil and water (w/v) (Li et al., 2016).

SOCD =

n

3.2. Patterns and effects of controlling factors on SOC Spatially, SOC density in the top 30 cm increased with latitude (R2 = 0.28, P < 0.05) (Fig. 2a) and longitude (R2 = 0.21, P < 0.05) (Fig. 2b). Similar trends between SOC density and longitudes and latitudes were also observed at depths of 0–50 cm and 0–100 cm (Fig. 2c–f). The MAT showed a significant positive influence on SOC density in the top 30 cm across the TRSR shrubland (R2 = 0.18, P < 0.05)

(1)

Table 2 Density and storage of soil organic carbon (SOC) at depths of 0–10, 10–20, and 20–30 cm for different vegetation types in the Three Rivers Source Region. The results are shown as mean values ± standard deviation. Vegetation types

Alpine meadow Alpine steppe Alpine shrubland Total

Area (104 km2)

20.18 7.52 2.59 30.29

SOC density (kg m−2)

SOC storage (Pg C)

0–10 cm

10–20 cm

20–30 cm

0–10 cm

10–20 cm

20–30 cm

2.93 ± 0.83 1.71 ± 0.51 4.72 ± 2.33 2.77

2.44 ± 0.61 1.57 ± 0.38 4.47 ± 2.21 2.41

2.02 ± 0.53 1.48 ± 0.35 3.58 ± 2.06 2.01

0.59 ± 0.17 0.13 ± 0.04 0.12 ± 0.06 0.84

0.49 ± 0.12 0.12 ± 0.03 0.12 ± 0.06 0.73

0.41 ± 0.11 0.11 ± 0.03 0.09 ± 0.05 0.61

156

n

Reference

21 9 22 52

Chang et al. (2014) Chang et al. (2014) This study This study

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Fig. 2. Relationships of soil organic carbon (SOC) density with latitude (a, c, e) and longitude (b, d, f) in the Three Rivers Source Region shrubland: (a–b) 0–30 cm, (c–d) 0–50 cm, (e–f) 0–100 cm across 22 sites in the Three Rivers Source Region shrubland.

0–50 cm and 0–100 cm, similar relationships between SOC density and climatic factors were observed (Fig. 5c–f). However, the effects of temperature on SOC were weak at soil depths of 30–50 cm (P > 0.05) and 50–100 cm (P > 0.05), which means that SOC at greater soil depths was less affected by climatic factors. Similarly, the summer mean temperature and summer total precipitation had almost the same effects on SOC as the MAT and MAP (Fig. S1), and there was a significant collinearity between summer climatic factors and mean annual climatic factors at the sampling sites (P < 0.001) (Fig. S2). The range of elevation in the TRSR shrubland was between 3133 m and 4296 m (Table S1). The SOC density in the top 30 cm in the TRSR shrubland was the greatest at low elevation (3133–3528 m), while it was the least at high elevation (3529–3906 m) and moderate at medium elevation (3907–4296 m) (Fig. 6a). However, the nonparametric test indicated that this difference was not significant (P > 0.05). A similar trend was also found at 0–50 cm and 0–100 cm (Fig. 6b, c). The range of ground cover in the shrubland was between 35% and 88% (Table S1). The SOC density in the top 30 cm showed an increasing trend with higher ground cover. Specifically, the highest SOC density was in high ground cover (71–88%), while the lowest was in low ground cover (35–53%), and in medium cover (53–71%) the SOC density was moderate (Fig. 7a). The nonparametric test again indicated that this difference was not significant (P > 0.05). SOC density at 0–50 cm and 0–100 cm showed a similar trend to that at 0–30 cm (Fig. 7b, c). The slope had no significant effect on SOC density in the TRSR shrubland (P > 0.05).

Fig. 3. Soil organic carbon (SOC) density in different soil depths across 22 sites in the Three Rivers Source Region shrubland.

(Fig. 5a), whereas SOC remained relatively constant with the MAP (Fig. 5b). Compared with precipitation, temperature had more significant effects on SOC density across the TRSR shrubland. At depths of 157

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Fig. 4. Relationships between soil organic carbon (SOC) density in the surface soil layer and that in the upper 100 cm (a and b), and a comparison of the proportion of SOC density in the surface layer (0–30 cm and 50 cm) to that in the top 100 cm (c) across 22 sites in the Three Rivers Source Region shrubland.

4. Discussion

(Davidson and Janssens, 2006). The high altitude (Tashi et al., 2016) and lower temperatures (Jobbágy and Jackson, 2000) in such ecosystems can promote the accumulation of SOC. On the Qinghai-Tibetan Plateau, the alpine grasslands mainly consist of alpine meadows and alpine steppes, the SOC densities of which were estimated to be 6.17 and 2.94, 7.51 and 3.67 and 9.25 and 4.38 kg m−2 at 0–30, 0–50 and 0–100 cm, respectively (Yang et al., 2008) (Table 1). Our results show that SOC density at the same depths in alpine shrubland on the Plateau were 12.77, 18.02, and 26.21 kg m−2, respectively, which are much

4.1. SOC density of alpine shrubland in the TRSR In this study, we used 66 sets of soil profile data from 22 sites to assess the SOC density in TRSR shrubland. The average SOC density at a soil depth of 100 cm in China is nearly 8.01 kg m−2 (Wu et al., 2003), which is lower than our result of 26.21 kg m−2. SOC storage in highaltitude ecosystems is of particular interest due to its high density

Fig. 5. Relationships of soil organic carbon (SOC) density with mean annual temperature (MAT) (a, c and e) and mean annual precipitation (MAP) (b, d and f) across 22 sites in the Three Rivers Source Region shrubland, (a–b) 0–30 cm, (c–d) 0–50 cm, (e–f) 0–100 cm. 158

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Fig. 6. Soil organic carbon (SOC) density in different ranges of elevation across 22 sites in the Three Rivers Source Region shrubland. Low means 3133–3528 m, medium means 3529–3906 m, and high means 3907–4296 m, (a) 0–30 cm, (b) 0–50 cm, (c) 0–100 cm.

number of soil profiles from the sites analyzed by Chang et al. (2014). The significant spatial heterogeneity of soil can greatly affect SOC density measurements (Chang et al., 2014; Jobbágy and Jackson, 2000; Yang et al., 2007).

higher than those in both alpine meadow and steppe. The higher SOC in alpine shrubland ecosystems indicates that shrubland is an important biome for the estimation of soil C (Piao et al., 2009). A similar result was also observed in Nigerian terrestrial ecosystems (Akpa et al., 2016), where SOC density in shrubland was higher than that in grasslands at different soil depths (3.14 vs 2.27 kg m−2 for 0–30 cm, and 6.86 vs 5.10 kg m−2 for 0–100 cm). This difference in SOC density between the areas may have the following causes. Aboveground biomass is a principal C resource (Wynn et al., 2006). The aboveground biomass production from plants in the shrubland biome (1103 g m−2) (Nie et al., 2016) is larger than that of meadow (100 g m−2) or steppe biomes (43 g m−2) (Yang et al., 2009). The aboveground biomass of herbs in one shrubland ecosystem was found to be 114 g m−2, which was also larger than the aboveground biomass of grasslands on the Qinghai-Tibetan Plateau (Nie et al., 2018a). Thus, compared with alpine grasslands, the larger amount of litter not only from woody plants but also from herbs in shrubland ecosystems (Nie et al., 2017) can contribute to a greater accumulation of SOC (Lorenz and Lal, 2005). Similarly, the belowground biomass of shrubland was found to be 875 g−2 (Nie et al., 2016), which is higher than that in alpine grasslands (331 g m−2) (Yang et al., 2009), and the belowground biomass of herbs in alpine shrubland also contributes to inputting C into soils. Therefore, the C released to the soil in alpine shrubland may exceed that released in alpine grasslands, meaning that SOC density is higher in alpine shrubland than in both alpine meadows and alpine steppes. Although SOC density was found to be higher in the alpine shrubland on the Qinghai-Tibetan Plateau than in both alpine meadows and alpine steppes, the SOC storage in the shrubland was lower (Table 2). The smaller land area of alpine shrubland than of the other two vegetation types accounts for this (Table 2). Chang et al. (2014) classed shrubland as alpine grassland and also estimated the SOC density in TRSR shrubland. In their analysis, the SOC density of the topsoil was 9.36 kg m−2, which was lower than our result of 12.78 kg m−2. This difference could be due to the very limited

4.2. Patterns of SOC density in the TRSR Spatially, the SOC increased with both longitude and latitude. Our result was similar to that for alpine grassland in the TRSR (Chang et al., 2014). A higher SOC density was distributed in northeastern TRSR alpine grasslands (Chang et al., 2014). Climatic conditions varied with longitude and latitude. In our study regions, MAT increased with latitude (R2 = 0.43, P < 0.05; Fig. 8a) and longitude (R2 = 0.27, P < 0.05; Fig. 8b). Thus, the SOC density distribution could be due to increased temperature with increased longitude and latitude. SOC density changes more with latitude than with longitude. Specifically, the increased SOC density was 2.51 kg m−2 with an increased unit in latitude in the top 30 cm, while it was only 1.13 kg m−2 with an increased unit in longitude (Fig. 2a–b). Similar changes among SOC density, longitude and latitude were also found at depths of 0–50 cm and 0–100 cm (Fig. 2c–f). This difference may result from different increased MAT with increased units of latitude and longitude; the increase in MAT was higher with an increased unit of latitude than it was with an increased unit of longitude (Fig. 8a–b). Vertically, SOC in the top 30 cm accounted for approximately 56% of the total SOC in the 100 cm of topsoil in the TRSR shrubland, whereas the same depth accounted for 68% in the Qinghai-Tibetan grasslands (Yang et al., 2008). Therefore, more SOC is distributed in the deeper soils in alpine shrubland than in grassland, which confirms that shrubland has a deeper SOC distribution than grasslands (Lorenz and Lal, 2005). The different distribution patterns of SOC in the topsoil may be a result of the different vegetation types (Jobbágy and Jackson, 2000). Alpine shrubland plants have deeper roots than those of grasslands. Belowground residues, root exudates and root turnover represent

Fig. 7. Soil organic carbon (SOC) density in different range of ground cover across 22 sites in the Three Rivers Source Region shrubland. Low means 35–53%, medium means 53–71%, and high means 71–88%, (a) 0–30 cm, (b) 0–50 cm, (c) 0–100 cm. 159

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Fig. 8. Relationships of mean annual temperature (MAT) with latitude (a) and longitude (b) across 22 sites in the Three Rivers Source Region shrubland.

years (Qin, 2014). This warming trend has significantly positive effects on aboveground vegetation growth in the TRSR alpine shrubland ecosystem (Nie et al., 2018a), which accelerates the occurrence of a substantial C sink in the shrubland. It has been demonstrated that the MAT increased by 1.5 °C from 1961 to 2006. (Qin, 2014). The positive relationship between the MAT and SOC in alpine grasslands (Yang et al., 2008) indicates that TRSR ecosystems may have C sink potential in the global warming scenario. The temperature increases for the Yellow River, Yangtze River and Lantsang regions were 0.33 °C, 0.30 °C and 0.27 °C 10 a−1, respectively (Qin, 2014). Hence, the C sink potential differs with the uneven degree of increased temperature; specifically, the C sequestration potential in the Yellow River region is higher than that in the Yangtze River region, and the lowest is in the Lantsang region. Therefore, for “universal” C sink in the TRSR shrubland, long-term monitoring of dynamic change is necessary to provide a precise evaluation of the C potential among different regions.

direct SOC inputs (Eyles et al., 2015). The SOC storage in the upper 30 cm accounts for nearly half of the total SOC at a depth of 100 cm and plays a major role in the total SOC stock. Previously, it was demonstrated that SOC production is highest near the surface (Hobley and Wilson, 2016). Combined with the SOC stock in the TRSR alpine meadow and steppe (Table 2), we can estimate that the topsoil of the SOC stock in the TRSR ecosystems (alpine shrubland and alpine grassland) was 2.18 Pg C and those at 0–10, 10–20, and 20–30 cm were 0.84, 0.73, and 0.61 Pg C, respectively (Table 2).

4.3. Relationship between SOC density and MAT Increasing temperatures can accelerate the decomposition of SOC and thus decrease SOC density (Burke et al., 1989; Jobbágy and Jackson, 2000). Therefore, temperature is a significant factor in determining SOC density (Reichstein et al., 2013; Schimel et al., 1994). In our study, the SOC density increased significantly with the temperature in the TRSR shrubland. This result was contrary to the global shrubland trend (Jobbágy and Jackson, 2000; Schimel et al., 1994), and also inconsistent with the negative relationship at regional scales between 52°N and 40°S (Huang et al., 2018). SOC decomposition is predominantly limited by temperature, whereas SOC formation via photosynthesis is controlled by numerous factors, such as nutrient availability, water stress and light in temperate and tropical zones (Ouyang et al., 2016). However, our result was in accordance with high-latitude areas (Callesen et al., 2003), and the SOC density increased significantly with MAT. Increasing temperature can stimulate plant productivity inputs to soils on the Qinghai-Tibetan Plateau (Piao et al., 2006), and also contribute to increasing the length of the growing season (Leblans et al., 2017). It should be noted that although increasing temperature can transfer soil C to the atmosphere by accelerating its decomposition (Davidson and Janssens, 2006), in cold regions, such as arctic zones, the C decomposition is strongly constrained by temperature (McGuire et al., 2016). Increased C inputs may surpass temperature-induced C losses in the soil decomposition rate with increasing temperature, thus SOC density tends to increase in the TRSR shrubland. It has been suggested that changes in climate and atmospheric carbon dioxide concentrations modify the C cycle, and that terrestrial ecosystems render large C sinks (Cao and Woodward, 1998). Our results show that when only considering the temperature effects, the SOC increased to 1.42 kg m−2, 2.23 kg m−2 and 4.03 kg m−2 at the depths of 0–30 cm, 0–50 cm and 0–100 cm, respectively, with an increased unit of temperature (Fig. 4a, c, e). However, a simple temperature response function for global climatic change predictions has been found to result in an underestimation in the short term and an overestimation in the long term (Torn et al., 2009). Therefore, longterm monitoring of dynamic change should be conducted for more accurate prediction in the future. In a global warming scenario, the MAT in the TRSR is also increasing at a rate of 0.27–0.33 °C every ten

4.4. Relationship between SOC density and MAP Precipitation has been found to contribute to SOC accumulation by stimulating plant biomass in regions with a water shortage (Jobbágy and Jackson, 2000; Wynn et al., 2006). However, in our study the relationship between SOC density and MAP was uncorrelated, indicating that precipitation may not be a factor for the accumulation of SOC in the TRSR shrubland. The MAT and MAP in the study region were 2.21 °C and 527.38 mm, respectively. Lower temperature is a major limiting factor for the prediction of SOC, and there is sufficient precipitation to satisfy the needs of plant growth in an ecosystem with low temperature, such as alpine shrubland in the TRSR. It should be noted that the range of mean annual evaporation in the TRSR is between 730 mm and 1700 mm (Zhao, 2011), which is higher than the MAP (257.38 mm). However, dew at high altitude and in cold regions can significantly replenish water for vegetation growth (Li, 2013). In water balance analysis, soil water can increase because of dew in the alpine meadows of the TRSR (Li, 2013). Therefore, precipitation may not be a significant restrictive factor for SOC in the TRSR shrubland, due to the common effects of higher MAP, dew and low MAT. 4.5. Relationship between SOC density and elevation Most studies suggest that soil C has an increasing trend with elevation (Garten and Hanson, 2006; Maraseni and Pandey, 2014; Singh et al., 2011). Altitude is a complex factor and can lead to atmospheric changes, such as in temperature, precipitation, O2, CO2, humidity and radiation (Körner, 2007; Laughlin and Abella, 2007). However, in a meta-analysis of global data, temperature was considered the main driver for the C cycle along the altitudinal gradient (Tashi et al., 2016). Temperature was found to be lower with increasing elevation (Körner, 2007), and the positive relationship between SOC density and 160

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temperature partly supports the negative relationship between SOC density and altitude. It should be noted that microbial decomposition rates decrease with increasing elevation (Tashi et al., 2016), which can reduce the output of SOC density by limited decomposition. Thus, the decreasing level of SOC could be partly offset by the suppression of the decomposition of SOC density, and consequently SOC has a decreasing trend along elevation, but the relationship is not significant.

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4.6. Relationship between SOC density and ground cover Climate changes may result in increasing shrubland ground cover (Myers-Smith, 2011), and increases in shrubland ground cover have been observed at high latitude (Naito and Cairns, 2011). Higher ground cover can result from a larger shrubland canopy, and shrubland canopies can promote litter inputs to soils (Cornelissen et al., 2007), thus contributing to SOC accumulation. This is probably why higher SOC density was found in the TRSR shrubland. Aboveground vegetation growth has an increasing trend in that shrubland (Nie et al., 2018a), which could have increased the ground cover and thus stimulated C accumulation. 5. Conclusions The total SOC storage in the top 100 cm of soil in the TRSR shrubland was estimated at 0.68 ± 0.38 Pg C, with an average density of 26.21 ± 14.58 kg m−2. Spatially, SOC density increased with longitude and latitude, and vertically SOC in the topsoil at 30 cm and 50 cm accounted for 56% and 75%, respectively, of the total in 100 cm. Compared with the MAP and slope, the MAT could have a significant influence on the SOC in the topsoil. The SOC had an increasing trend with increasing ground cover, but a decreasing trend with increasing elevation in the TRSR shrubland. Furthermore, in the global warming scenario, the SOC density in the topsoil has great potential to increase across the TRSR shrubland, but this degree differs according to uneven increases temperature in different regions. Hence, future monitoring of dynamic change is necessary to improve the accurate assessment of the potential of the SOC sink in the TRSR shrubland. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2019.03.019. Acknowledgements We thank Dr. Qien Yang for constructive advice on the early version of this manuscript, and Wenzhu Song, Zebing Zhong, Hechun Liu, and Yi Ning for facilitating our field surveys on the Qinghai-Tibetan Plateau and for their laboratory assistance. This study was funded by the National Program on Basic Work Project of China (Grant no. 2015FY11030001-5), the Natural Science Foundation of Qinghai Province (2019-ZJ-910), the International Communication and Cooperation Project of Qinghai Province (2019-HZ-807), and the China Railway Corporation technology research and development Project (2017Z003-B). References Akpa, S.I.C., Odeh, I.O.A., Bishop, T.F.A., Hartemink, A.E., Amapu, I.Y., 2016. Total soil organic carbon and carbon sequestration potential in Nigeria. Geoderma 271, 202–215. Burke, I.C., Yonker, C., Parton, W., Cole, C., Schimel, D., Flach, K., 1989. Texture, climate, and cultivation effects on soil organic matter content in US grassland soils. Soil Sci. Soc. Am. J. 53, 800–805. Callesen, I., Liski, J., Raulund-Rasmussen, K., Olsson, M., Tau-Strand, L., Vesterdal, L., Westman, C., 2003. Soil carbon stores in Nordic well-drained forest soils—relationships with climate and texture class. Glob. Chang. Biol. 9, 358–370. Cao, M., Woodward, F.I., 1998. Dynamic responses of terrestrial ecosystem carbon cycling to global climate change. Nature 393, 249–252. Chang, X., Wang, S., Cui, S., Zhu, X., Luo, C., Zhang, Z., Wilkes, A., 2014. Alpine grassland soil organic carbon stock and its uncertainty in the three rivers source region of the Tibetan Plateau. PLoS One 9, e97140.

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