Effects of monoculture and mixed culture of grass and legume forage species on soil microbial community structure under different levels of nitrogen fertilization

• Plant functional groups (grass vs legume) have different effects on soil microbes.
• Presence of legume is conducive to soil microbial biomass.
• Legume increases the bacterial and fungal biomasses rather than F:B ratio.

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 analysis (PCA) only revealed a tendency that the microbial PLFA composition under legume monoculture differed from that under grass monoculture in the wet season. In addition, the soil microbial community structures were not significantly different among the three planting systems in the dry season. The PCA results showed that the microbial PLFA composition under low N fertilization was apparently different from that under moderate and high N fertilization in the wet season. Particularly, the biomasses of total microbes, bacteria, and green algae were significantly greater under moderate N fertilization than under low N fertilization and the green algal biomass was significantly greater under high N fertilization than under low N fertilization in the wet season. Additionally, PCA also revealed that the microbial PLFA compositions were different under the low and moderate N fertilizations in the dry season. However, the biomass and diversity of microbial community had no significant difference among the three levels of N fertilization. The results suggest that legume cultivation increased the biomasses of soil microbial community.