Plant species richness and legume presence increase microbial necromass carbon accumulation

Biodiversity control experiments have demonstrated that higher plant species richness increases plant production and soil organic carbon (SOC) stocks. However, although microbial necromass C constitutes a substantial and stable component of SOC, the effect of species richness on microbial necromass C accumulation remains unclear, especially concerning the presence or absence of legumes. We conducted an 8-year grassland biodiversity experiment to investigate how changes in plant diversity, with or without legumes, affect microbial necromass C and its proportion in SOC. Aboveground and root biomass increased by 254 % and 195 %, respectively, from monoculture plots to those with twelve species. SOC, total nitrogen (N), and microbial biomass C (MBC) all increased with plant species richness, independent of legume presence. The presence of legumes further increased the SOC, total N, MBC, and inorganic N compared to plots without legumes. Microbial necromass C increased by 48.3 % in twelve-species mixtures compared to monocultures; however, the ratio of microbial necromass C to SOC remained stable with increasing species diversity. Notably, the presence of legumes increased both the accumulation of microbial necromass C and its proportion in SOC compared to the absence of legumes at both high and low species diversity levels. This greater accumulation was primarily driven by increased plant productivity and microbial biomass, induced by legume-associated nitrogen fixation. Our findings highlight significant implications for ecosystem management: maintaining high levels of plant species diversity, particularly in conjunction with legumes, can increase microbial necromass C and enhance soil C sequestration.