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

Abstract

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.