Nitrogen enrichment offsets the stimulation of soil respiration by increased precipitation in temperate grasslands

Long-term interactive effects of increased precipitation and nitrogen deposition on grassland carbon cycling remain poorly understood, despite their individual impacts being well documented through short-term experiments. Based on a three-year investigation within a 19-year precipitation (+ 180 mm per year) and nitrogen addition (+ 10 g N m⁻² yr⁻¹) experiment in temperate grasslands of northern China, we demonstrated that while long-term increased precipitation significantly stimulated soil respiration (Rs) and its components (heterotrophic [Rh] and autotrophic respiration [Ra]), long-term N enrichment exerted suppressive effects. Crucially, N enrichment offset 86.7 % of the precipitation-induced stimulation of Rs, leaving only 13.3 % of the stimulation under combined N and precipitation addition. Furthermore, N addition also weakened the promotive effect of increased precipitation on both Rh and Ra. Mechanistic analyses revealed that Rh was mainly regulated by soil moisture, β-glucosidase (BG) activity, and the abundance of the key carbon-degrading functional gene (apu). Whereas, Ra was primarily driven by soil moisture interacting with vegetation characteristics, including species richness, leaf area, and leaf C:N ratio. Also, by providing the first experimental evidence that long-term N deposition can counteract enhanced soil C emissions under increased precipitation in grasslands through biochemical pathways and biotic restructuring, our findings advance the predictive understanding of grassland C cycling under concurrent global change scenarios.