Nitrogen application increases the productivity of perennial alpine cultivated grassland by improving soil physicochemical properties and microbial community characteristics

Nitrogen fertilizer application has significantly contributed to the overall improvement of grassland productivity. However, the effects of different nitrogen forms in cultivated grasslands and the mechanisms by which nitrogen addition improves plant productivity remain unexplored.

In this study, four nitrogen application rates were assessed using urea as the nitrogen source (CK: 0 kg N hm⁻² y⁻¹, U1: 22.5 kg N hm⁻² y⁻¹, U2: 45 kg N hm⁻² y⁻¹, and U3: 67.5 kg N hm⁻² y⁻¹), as well as three nitrogen forms (amide nitrogen: urea, U3; ammonium nitrogen: ammonium sulfate, A; nitrate nitrogen: calcium nitrate, N) at 67.5 kg N hm⁻² y⁻¹.

Aboveground biomass increased gradually with increasing N application, being 12.02% higher in the U3 treatment compared to the A treatment and 7.59% higher compared to the N treatment. Nitrogen addition significantly increased soil nutrient content, such as soil total nitrogen. The application of 67.5 kg N hm⁻² y⁻¹ significantly increased the species' evenness index. Microbial diversity was not significantly affected by different N application rates and nitrogen forms, but the microbial community structure and its functions were significantly altered. Both heat map and redundancy analyses suggested that soil physicochemical properties and bacterial communities significantly regulated plant communities. Structural equation modeling analyses indicated that plant productivity was similarly regulated by soil physicochemical properties and bacterial functions.

This study implies that short-term N application indirectly increased plant productivity in perennial cultivated grasslands by improving soil physicochemical properties and soil bacterial function.