Drought and warming enhance the risk of nitrogen losses in intensively managed grasslands

Background and aims: Climate change is expected to intensify drought and heatwaves, with major consequences for nutrient cycling in grasslands. Plant-soil-microbe interactions regulate nitrogen (N) dynamics, yet their responses to simultaneous warming and drought remain unclear, especially across plant species and land management histories. Methods: We conducted a factorial warming and drought manipulation on six temperate grassland species belonging to three functional groups (grasses, forbs, legumes) grown in soils from intensively (Int) and extensively (Ext) managed grasslands. We quantified N content and biomass production across soil, microbes, roots, and shoots. We inferred fungi:bacteria by microbial C:N, not by direct soil community determination. Results: Under control and warming conditions, Int soils supported up to 50% higher plant biomass than Ext soils, but this advantage disappeared under drought, reducing plant biomass by ~ 40%. Warming consistently reduced microbial biomass by up to 30% in both soil types. In contrast, drought decreased microbial biomass in Int soils but increased it by ~ 20% in Ext soils, likely reflecting their more drought-resistant fungal communities. These changes altered N flows: extractable soil N was up to three times higher in Int soils under warming and drought, indicating greater vulnerability to N losses, whereas Ext soils maintained low extractable soil N except under combined stress. Conclusions: Intensive management boosts plant biomass under mild and warm conditions but increases susceptibility to N loss during climate extremes. Extensive management supports more stable biomass and N cycling. Integrating microbial dynamics and land-use history is essential for predicting grassland resilience and improving N cycling models under global change.