Ecosystem organic carbon storage and their drivers across the drylands of China

Earth’s drylands store large amounts of organic carbon and thus play an important role in regulating atmospheric CO2 concentrations and mitigating climate change. However, little is known about the magnitude and the drivers of organic carbon stocks because of limited data, in particular in desert ecosystems. To fill this gap, we surveyed 763 plots and collected 5091 soil samples from 170 sites covering grassland and desert ecosystems across the drylands of China. The results show that 10.96 Pg organic carbon is stored in these ecosystems (7.07 Pg in grasslands and 3.89 Pg in deserts). The effects of environmental variables on carbon density were found to be contingent on ecosystem type and soil depth. Mean annual precipitation had a strong positive effect on the spatial distributions of vegetation carbon density in both types of ecosystems. The spatial variations of surface (0-40cm) and subsurface (40-100cm) soil organic carbon (SOC) density in grasslands were mainly correlated with plant productivity and mean annual temperature (MAT), respectively, while in desert ecosystems, they were mainly correlated with soil salinity (electrical conductivity). The spatial sensitivities (linear regression slopes) of the surface SOC density versus MAT increased from dry subhumid to semi-arid regions and then gradually shifted to decrease from semi-arid to hyper-arid regions, indicating that the surface SOC stock is more sensitive to temperature in semi-arid regions. This study provides a comprehensive survey of ecosystem carbon stocks in China’s drylands, fills a gap in our knowledge about carbon stocks in deserts, complements estimates of ecosystem carbon stocks throughout China, and provides insights into our understanding of the carbon cycle in drylands.

地球干旱区储存着大量有机碳，因此在调节大气二氧化碳（CO₂）浓度、减缓气候变化方面发挥着关键作用。然而，由于实测数据匮乏，尤其是荒漠生态系统相关数据不足，学界对有机碳储量的规模及其驱动机制仍知之甚少。为填补这一研究空白，我们针对中国干旱区涵盖草原与荒漠生态系统的170个样点开展野外调查，共布设763个样方，采集土壤样品5091份。研究结果表明，上述生态系统中共储存有机碳10.96 Pg，其中草原生态系统储碳7.07 Pg，荒漠生态系统储碳3.89 Pg。研究发现，环境变量对碳密度的影响会因生态系统类型与土壤深度的不同而呈现异质性。年均降水量对两类生态系统的植被碳密度空间分布均具有显著正向调控作用。草原生态系统的表层（0~40 cm）与亚表层（40~100 cm）土壤有机碳（soil organic carbon, SOC）密度的空间变异分别主要与植物生产力和年均温（mean annual temperature, MAT）相关；而荒漠生态系统的该类参数则主要与土壤盐度（电导率）呈显著相关。表层SOC密度相对于MAT的空间敏感性（线性回归斜率）从半湿润偏旱区到半干旱区呈上升趋势，随后从半干旱区到超干旱区则逐渐下降，这表明半干旱区的表层SOC储量对温度变化更为敏感。本研究对中国干旱区生态系统碳储量开展了系统性全面调查，填补了荒漠生态系统碳储量研究的空白，完善了全国范围内生态系统碳储量的估算结果，并为深化人们对干旱区碳循环的理解提供了重要理论参考。