Differential Responses of Community Biomass Stability to Short-Term Nitrogen and Phosphorus Inputs in Two Dominant Plant Communities of the Songnen Grassland, Northeast China

The Songnen region in northeastern China contains one of the world’s three largest area of soda saline–alkali soils. Its grasslands are experiencing severe salinization and nutrient impoverishment, which threaten regional ecological security. While nitrogen (N) and phosphorus (P) additions are widely applied to restore degraded grasslands, how the biomass stability of different dominant plant communities responds to such inputs remains unclear. Here, we compared two representative communities—one dominated by Leymus chinensis and the other by Puccinellia distans—under four treatments: control, N addition (15 g N m−2), P addition (15 g P2O5 m−2), and combined N + P addition. N and P additions increased species dominance in both communities, yet diversity and evenness declined in L. chinensis and increased in P. distans. Aboveground biomass peaked under N + P (increasing by 146.82% and 82.16%, respectively). Critically, biomass stability showed opposing trajectories: it rose by 27.69% in L. chinensis under N + P, but fell by 50.90% in P. distans under N alone. Mantel tests and random forest analyses linked biomass stability in L. chinensis to multiple soil enzymes, nutrients, and plant diversity, whereas P. distans biomass stability was primarily regulated by microbial nitrogen and phosphorus. Structural equation modeling further indicated that nutrient inputs in L. chinensis triggered a complex soil microbial–enzymatic cascade that positively coupled plant biomass and soil nutrients with biomass stability, while diversity exerted a negative effect. In P. distans, biomass stability was negatively driven by soil nutrients and biomass, with enzyme activity playing a mediating role. These findings reveal contrasting biomass stability-regulation mechanisms and provide a mechanistic basis for targeted restoration of saline–alkali grasslands.