Reduction in Mineral‐Associated Organic Carbon Reveal Soil Organic Matter Loss Following Grassland Degradation

It is hugely challenging to predict the influences of grassland degradation on soil organic carbon (SOC) stocks due to the paucity of data, particularly on changes in pools of particulate (POC) and mineral‐associated organic carbon (MAOC). This study investigated the responses of POC, MAOC, and their carbon (C) fractions to grassland degradation in Songnen grassland. Soil samples (0–10 cm depth) from a grassland degradation sequence including undegraded (NDG, Leymus chinensis ), lightly (LIG, Puccinellia tenuiflora and Leymus chinensis ), moderately (MOD, Chloris virgata ), and severely (SEV, Suaeda heteropteran ) degraded grassland were analyzed for contents of total organic C, labile C, and stable C in soil organic matter (SOM), and particulate (POM), and mineral‐associated organic matter (MAOM). Grassland degradation reduced the SOC content by decreasing both POC and MAOC content, primarily from the reduction in plant biomass and substantial soil erosion. The reductions in total C, labile C, and stable C contents resulting from grassland degradation in POM (61%, 54%, and 66%, respectively) were larger than those in MAOM (43%, 38%, and 47%, respectively), indicated that POM was more sensitivity to grassland degradation in comparison to MAOM. Grassland degradation increased the SOC stability because the larger reduction in POC than MAOC content decreased the ratio of POC/MAOC by 50% in degraded grasslands compared with NDG. The loss of SOC under MOD and SEV was mainly due to the higher C contents in MAOM than POM, which resulting in the contributions of MAOM to the loss of total C, labile C, and stable C in SOM were greater than that of POM. Concluding, grassland degradation decreased the SOC content by reducing both the POC and MAOC content because of low C input from plant biomass, and the reduction in MAOM was identified as the primary factor contributing to the SOM loss following grassland degradation due to the high MAOC content.