Distinct mechanisms of soil organic carbon formation in natural and legume-based grasslands on the Loess Plateau, China

The conversion of degraded land into grassland, through either abandonment or the introduction of legumes, is recognized as an effective strategy for increasing soil organic carbon (SOC). However, the impacts of this land use change on plant-derived lignin phenol and microbial residue carbon (MRC) remain insufficiently understood. In this study, we investigated the effects of grassland development following cropland abandonment and legume establishment on these SOC sources. We analysed 30 abandoned-cropland grasslands (CGs) and 30 alfalfa-established grasslands (AGs) with revegetation ages ranging from 1 to 30 years located in the semiarid region of the Loess Plateau, China. Our findings revealed that CGs exhibited a linear increase in lignin phenol content and its contribution to SOC (from 7.2 to 12.5 mg g⁻¹ SOC); however, there was no corresponding increase in MRC. In contrast, AGs demonstrated increases in both lignin phenol and MRC contents, along with their contributions to SOC (from 4.8 to 11.8 mg g⁻¹ SOC and from 32.8 % to 44.4 % SOC, respectively). Enzymatic stoichiometry analysis indicated that both grassland establishment alleviated microbial carbon limitation; nevertheless, microbial nitrogen limitation persisted, with its intensity remaining unrelated to the duration of grassland establishment. Soil exchangeable magnesium and root biomass primarily influenced the lignin phenol contribution to SOC, whereas soil dissolved organic carbon and microbial biomass carbon mainly affected the MRC contribution across both grassland types. These results underscore the distinct mechanisms of SOC formation and regulation in natural and legume-based grasslands. To enhance soil carbon sequestration in this and comparable dryland regions, priority should be given to the introduction of leguminous plants and the alleviation of microbial nitrogen limitation, thereby increasing plant carbon inputs and improving microbial conversion efficiency.