Background <p>Change-driven alterations in precipitation regimes and increasing nitrogen inputs profoundly influence grassland productivity, yet the mechanisms linking soil resources to coordinated aboveground plant traits remain poorly understood.</p> Methods <p>We conducted an in situ soil-column microcosm experiment in a Leymus chinensis-dominated temperate grassland in northeast China to examine how precipitation amount (150, 300, and 450 mm during the growing season) and nitrogen addition (0 and 10 g N m-2 yr-1) jointly regulate soil conditions, plant functional traits, and aboveground biomass.</p> Results <p>Both the increased precipitation and N addition significantly enhanced aboveground biomass. Variations in soil water content, nitrate nitrogen, and pH explained the majority of the variation in the examined root morphological, anatomical, and chemical traits, as well as leaf functional traits. Under the unfertilized treatment, the cascade was: soil environment → root uptake (cortex) → leaf transport and photosynthesis → aboveground biomass. Under the fertilized treatment, the cascade was: soil environment → root uptake and transport (cortex, stele diameter, and vessel number) → leaf transport and photosynthesis → aboveground biomass. The measured root anatomical traits were strongly coupled to leaf photosynthetic performance, and these trait–physiology associations were substantially restructured by nitrogen addition.</p> Conclusions <p>Our findings demonstrate that grassland productivity is governed by coordinated, multi-level plant functional strategies driven by interactive water and nitrogen availability, highlighting the central role of trait coordination in linking soil resource acquisition to aboveground carbon gain. These findings deepen understanding of grassland adaptive responses, thereby strengthening predictive models and guiding conservation efforts in a changing global environment.</p>