Two-Decade Nitrogen and Water Additions Stimulate Temperate Grassland Carbon Fluxes through Plant Trait Shifts and Soil Moisture Enhancement

Atmospheric nitrogen deposition and increased precipitation are key drivers of grassland carbon cycling; however, their long-term interactive effects and mechanisms remain poorly understood. Based on a 20 year field experiment with sustained nitrogen (10 g N m–2 yr–1) and precipitation (+∼50% precipitation) addition in a temperate grassland in northern China, we found that both nitrogen deposition and precipitation addition significantly enhanced net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP), with additive effects of nitrogen and water addition on NEE and GEP. Structural equation modeling revealed that nitrogen deposition increased carbon fluxes by enhancing community-weighted mean leaf area (CWMLA) and chlorophyll content of dominant species, whereas precipitation addition stimulated carbon fluxes mainly through improving soil moisture (SM) and CWMLA. Notably, nitrogen deposition and precipitation addition enhanced GEP to a greater extent than ER, which led to a net ecosystem carbon sink and exhibited a significant interaction effect exclusively on ER. These findings underscore previously underexplored mechanisms linking plant trait and SM dynamics to carbon flux responses. Our results indicate that long-term increases in nitrogen deposition and precipitation may accelerate carbon cycling in temperate grasslands and enhance the carbon sequestration function of grasslands. Our study provides unique long-term evidence to demonstrate that nitrogen and precipitation additions exert progressively stronger positive effects on grassland carbon fluxes over time, providing critical insights for predicting ecosystem responses to sustained climate change and informing adaptive nitrogen and SM management strategies.