Microbial nitrification inhibitor enhances alpine grassland productivity through narrowing plant niche breadth by utilizing more ammonium

Stimulated microbial nitrification has been reported during grassland degradation when plants and microbes compete for declined nitrogen (N) resources. However, it remains unclear whether inhibiting microbial nitrification would change such competition and alter grassland productivity. Here, we investigated changes induced by the nitrification inhibitor (NI) application in N acquisition strategies, niche breadth and competitiveness of plants and microbes, with different soil N levels in greenhouse. The ¹⁵N labeling technology was employed with ammonium, nitrate and glycine to quantify N uptake. NI significantly (P ≤ 0.02) decreased abundances of AOA and AOB genes for microbial nitrifiers in low-N (1.3 g/kg for total N) soils, and AOB abundance in high-N (1.8 g/kg) soils, validating the efficacy of NI in inhibiting nitrification. NI significantly (P < 0.01) increased the soil ammonium content by 25.50% and 10.43% in low- and high-N soils, respectively. Moreover, NI narrowed the plant niche breadth for N utilization by concentrating more on ammonium uptake in both low- and high-N soils. Consequently, NI significantly (P ≤ 0.04) increased the plant biomass by 10.02% and 10.16% in low- and high-N soils, respectively. In comparison, microbial competitiveness against plants for ammonium decreased by NI in low-N soils, leading to a 23.41% reduction in microbial biomass (P < 0.01); while they remained unchanged in high-N soils. Overall, our study revealed the effectiveness of NI application for enhancing grassland productivity by reducing plant niche breadth through utilizing more ammonium, suggesting a viable strategy to restore degraded grasslands without any external N input.