Grassland Degradation With Saline-Alkaline Reduces More Soil Inorganic Carbon Than Soil Organic Carbon Storage

Grassland is an important part of terrestrial ecosystems and contain substantial amounts of soil carbon. However, 90% of grasslands suffer from degradation in northern China, where soil inorganic carbon (SIC) is a major reservoir. Previous studies have focused primarily on soil organic carbon (SOC) stock, while the effects of grassland degradation in SIC stock remains largely unexplored. Therefore, accurate assessments of both SOC and SIC stocks and their profile distributions are necessary to fully understand the role of grassland degradation in China's carbon budget. Here, we conducted an experiment to estimate the stocks, profile distributions, and environmental controls of both SOC and SIC in patchy saline-alkaline grasslands under three degraded degrees (non-degraded patches, ND; moderately degraded patches, MD, and heavily degraded patches, HD). Our results showed that grassland degradation destroys not only SOC but also SIC stocks in saline-alkaline grasslands. SIC is the main component of soil carbon in saline-alkaline grasslands, and their distributions changed dramatically. Compared with ND, SIC losses caused by degradation accounted for 84% and 86% of total carbon loss under MD and HD patches, respectively. SIC loss primarily occurred at the intermediate soil layers (30-70 cm), while the loss of SOC was mainly in the topsoil (0-40 cm). Moreover, the distributions of SIC from 20 to 60 cm were closely related to soil pH, while it is mainly regulated by EC from 60 to 100 cm. In addition, soil pH, EC, and above-ground biomass were important variables driving the profile distributions of SOC in the upper soil. Our work provides evidence that grassland degradation mainly damages the SIC stocks. Our findings, therefore, highlight the non-negligible role of SIC dynamics in the carbon budget of degraded grassland ecosystems and the need to consider these dynamics in terrestrial carbon cycle research.