Impacts of Plant Functional Group Dominance and Geochemical Factors on Soil Carbon Concentrations and Fractions in Grassland Ecosystems

Climate change and anthropogenic activities are reshaping plant functional group dominance and altering soil physicochemical properties in grassland ecosystems. Although plant carbon inputs, microbial activity, and mineral protection are known to govern soil carbon turnover, how changes in functional group dominance and geochemical factors regulate carbon storage and stability remains unclear. Here, we selected 124 mono-species patches of 12 common grass, forb, and woody species in a temperate grassland nature reserve, measuring plant chemical traits, microbial biomass carbon (MBC), and soil physicochemical properties. We found that across all plant functional groups, root, and microbial contributions outweighed aboveground inputs in soil organic carbon (SOC) formation. Soil mineral properties, especially exchangeable calcium, played predominant roles in influencing soil carbon concentration, surpassing the impact of plant and microbial input. Despite sandier soil and lower plant carbon input in woody patches, bulk soil carbon concentration, and its mineral-associated organic carbon and particulate organic carbon fractions in woody patches did not differ from those in grass and forb patches. Further analysis revealed that woody patches had higher soil moisture, which increased MBC and fostered organo-mineral interactions. These processes could facilitate SOC stabilization, thereby compensating for low root carbon input and the low carbon retention capacity of sandy soils. Overall, our findings reveal how biotic and geochemical factors interact to regulate SOC and its fractions across plant functional groups, highlighting the crucial role of exchangeable calcium and soil moisture in driving organic carbon concentrations in temperate grasslands.