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We used PLFA analysis to monitor the soil microbial community structure in a gramineous–legume forage field under different inorganic nitrogen (N) fertilization regimes in Southwest China. The gramineous–legume forage system included one grass (Paspalum wetsfeteini) and one legume (Medicago sativa) in three planting systems: P. wetsfeteini monoculture, M. sativa monoculture, and P. wetsfeteini and M. sativa mixed culture. The fertilization treatments included three N levels: 338 (low), 450 (moderate), and 675 (high) kg N ha⁻¹ yr⁻¹. The results showed that biomasses of total microbes, bacteria, fungi, and green alga were significantly greater and protozoan biomass tended to be greater under legume monoculture than that under grass monoculture; and fungal biomass was significantly greater under grass-legume mixed culture than under grass monoculture in wet season. However, principal component …