Mixed grass species differ in rhizosphere microbial community structure and function response to drought compared to monocultures

Abstract

Natural grassland succession is the interactions among different grass species and various environmental factors. The dynamics of soil microbial communities along succession stages has received tremendous attention. The effects of the coexistence of different grass species on the structure and function of rhizosphere soil microbial communities, however, are poorly understood, especially under drought stress conditions. Here, we investigated how different grass planting patterns and soil water content affect rhizosphere microbial community structure and function. Two typical grass species (Bothriochloa ischaemum and Stipa bungeana) on the Loess Plateau were planted in three planting systems: B. ischaemum monoculture, S. bungeana monoculture, and a mixed culture of the two species under two water regimes (optimal water: 80% field capacity [FC]; drought stress: 60% FC). Rhizosphere soil samples were collected to evaluate the rhizosphere microbial community structure (via phospholipid fatty acid [PLFA] biomarkers) and function (via six enzyme activities). The mixed culture of grass species noticeably shifted the rhizosphere microbial community structure. It caused the rhizosphere gram-positive:gram-negative PLFA ratio and fungal:bacterial PLFA ratio to increase under optimum water conditions. In comparison, drought stress caused total PLFA, bacterial PLFA, and gram-positive PLFA biomass to increase. Drought stress caused rhizosphere-specific enzyme activity to increase under monoculture treatment and rhizosphere-specific enzyme activity to decrease under mixed culture treatment. Soil water-soluble organic carbon, pH, and water-soluble organic phosphorus were the most important factors driving changes to rhizosphere microbial community composition. In comparison, pH was the predominant factor driving variation in rhizosphere specific enzyme activities. Thus, the coexistence of grass species altered the structure and function of the rhizosphere microbial community through changes in rhizosphere pH and water-soluble nutrients. In conclusion, changes to rhizodeposits under a mixed culture of grass species likely affect the structure and function of the rhizosphere microbial community. The results of the present study improve our understanding of the impacts of grass species interaction on soil biogeochemical processes under future global drought stress scenarios.