Increasing contribution of microbial residues to soil organic carbon in grassland restoration chronosequence

Grassland restoration across the world increases soil organic carbon (SOC) sequestration which is critical for global C cycling and CO2 removal from the atmosphere. However, the relative importance of plant- and microbially-derived C for SOC is still an open question for temperate grasslands. Here, amino sugars and lignin phenols were used as biomarkers to investigate the relative microbial and plant residue contribution to SOC in a 30-year (1-, 5-, 10-, 15-, 25-, 30-year) restoration chronosequence of temperate grassland. The contribution of microbially-derived C (from 4.9 to 13 g kg−1) to SOC was much greater than that of plant-derived C (from 1.3 to 2.3 g kg−1). At the early stage of restoration (<15 years), grassland soils accumulated more C in the form of plant-derived C. In contrast, grassland soils at the late stage of restoration (>15 years) accumulated more microbially-derived C, and less from plant residues. These findings highlight the dominance of microbial contribution to SOC stabilization compared with plant residues. The contribution of bacteria-derived C to SOC gradually increased from 29% to 50% with progress of grassland restoration, while the contribution of fungal C to SOC decreased from 30% to 21%. Consequently, microbial residue contribution to SOC shifts from fungal and bacterial to mainly bacterial residues during grassland restoration. This shift may be due to the faster bacterial growth and a increasing living biomass during grassland restoration, leading to higher accumulation of bacterial residues. Correlation analysis and random forest models showed that belowground plant biomass, soil pH, and living microbial biomass were the main factors regulating plant-derived C. The microbially-derived C in SOC, however, was dependent on living microbial biomass, soil pH and dissolved organic C. Concluding, grassland restoration increased soil C sequestration primarily by microbial necromass (mainly bacterial necromass), and is affected by abiotic and biotic factors, as well as plant C input.