Grazing exclusion enhanced the capability of soil microorganisms to access photosynthetic carbon in Loess Plateau grassland

Photosynthetic carbon (C) has a pivotal role in the C cycle of the plant-soil system, contributing significantly to soil organic C (SOC) accrual. Grassland soils have a large capacity to store organic C and grazing is an important factor influencing the C cycle, but few studies have quantitatively how grazing exclusion affects the transfer of photosynthetic C in a plant-soil-microbial system. We used in situ isotope pulse-chase methodology to study photosynthetic C allocation patterns in the grazed and grazing-excluded grassland soil of the Loess Plateau, China. Grazing exclusion increased the total assimilated 13C by 46% compared with the grazed grassland, but did not significantly change the 13C allocated to the aboveground (75%) and belowground (25%) plant biomass. The 13C transferred faster to soil via root exudates in the grazed soil with lower aboveground biomass, suggesting that removal of aboveground biomass by grazing animals influences the rate of C transfer. Most (79%) the SOC gained from grazing exclusion accumulated in the mineral associated organic C (MAOC) pool, which is a stronger predictor of SOC accrual than particulate organic C (POC). Grazing exclusion increased the transformation of POC to MAOC, mainly through the accumulation of microbial necromass. Grazing exclusion significantly reduced the G+/G- ratio and the fungal/bacteria ratio, indicating a shift in soil microbial community composition in favor of bacteria over fungi under grazing exclusion. Grazing exclusion increased the microbial biomass by 48% and significantly enhanced the capability of soil fungi and G- bacteria to access photosynthetic C. In summary, grazing exclusion increases the magnitude of C transfer from the atmosphere to soil microbial biomass, and the gradual conversion of POC to MAOC.