Effects of Biological Nitrogen Fixation and Nitrogen Deposition on Soil Microbial Communities in Karst Grassland Ecosystems

Diverse exogenous nitrogen (N) sources have a considerable impact on microbial community structure in terrestrial ecosystems. Legume plants and N deposition can relieve N limitations and increase net primary productivity. However, the differences in their effects on soil microbial communities remain unclear. Here, the responses of the soil microbial community to a legume-planting system and simulated N deposition were examined in karst grasslands in Southwest China over five years by analyzing soil microbial phospholipid fatty acids (PLFAs). The experiment included three treatments-legume plant introduction (N_L, <i>Indigofera atropurpurea</i>), N deposition (N_D, NH₄NO₃:10 g N m^-2 yr^-1), and a control with no treatment. The effects of N_L and N_D on soil microbial community composition differed significantly. N_D significantly reduced the biomass of bacteria, actinobacteria, and arbuscular mycorrhizal fungi. N_L insignificantly increased the biomass of all microbial groups. However, the total amounts of PLFAs and fungal biomass were significantly higher in N_L than in N_D. The effect of legume plant introduction on soil microbial community composition was more powerful than that of N_D. Overall, the introduction of legume plants is beneficial in terms of increasing the biomass of the soil microbial community and stabilizing the soil microbial community structure in karst grassland ecosystems.