Increasing phosphorus availability reduces grassland soil N₂O emission: Plants and microbes move from mutualism to self-reliance

Phosphorus (P) availability directly affects grassland soil physicochemical properties and plant growth, which in turn alters microbially mediated nitrous oxide (N₂O) emission. Linking plant, soil, and microbial processes is helpful to reveal processes that affect the effects of soil P on N₂O emission. Here, we established five P-application treatments (control, with no P addition, and 1–12.5 g P m⁻² yr⁻¹ in treatments P1 to P12.5) to vary soil P availability. We investigated how the nutrient-acquisition strategies of Leymus chinensis, soil physicochemical properties, and microbial metabolic activity responded to P availability and assess effects on N₂O emission. The N₂O flux in the fertilization treatments was significantly lower than in the control but differed among the treatments. Plant biomass and root nonstructural carbohydrates increased significantly in P1 and P2.5, and plants increased root carbon allocation and recruited more microbes and greatly increased the nitrogen mineralization rate. This symbiotic plant–microbe association promoted plant water uptake, and soil drying increases the abundance of amoA functional gene, thereby promoting nitrification and reducing N₂O emission. Plants obtained more nutrients associated with an increase in the number of root tips and carboxylate exudation in P5 and P12.5. This self-reliance strategy increased nutrient competition, and the resulting substantial reduction of microbial biomass decreased the N₂O flux. However, the abundance of the narG gene and N₂O emission increased slightly in P12.5, whereas the microbial biomass was low but maintained a high carbon-use efficiency, reflecting a self-reliant microbial strategy to acclimate to their environment. Overall, P availability in grassland soils was inversely proportional to N₂O emission, and strongly determined plant–microbe interactions. Our results provide support for managing grass growth and N₂O emission in P-deficient grassland.