LAUSR

This study presents an integrated hydrological‐hydrochemical approach to quantify reactive nitrogen (NR) cycling in temperate mountain catchments. It employs stable isotope analyses (δ2H, δ18O in water, δ15N in NH4+, and NO3− and δ18O in NO3−) to resolve interactions between water flow and N transformations. A two‐component runoff model reveals groundwater as the dominant discharge contribution (75%–90%), with 10%–25% derived from rapidly infiltrating soil water—highlighting a swift hydrological response to precipitation. In addition, this work quantifies all N mineralization in vadose zone and denitrification in groundwater and their seasonal variation within a well‐studied network of N‐saturated temperate forests (i.e., the Czech GEOMON Network). Our results show that increased precipitation infiltration diminishes microbial N production and N2 losses, but maximizes catchment NR exports. Direct input of atmospheric NR to runoff was recorded during a short period of spring snow melting only. Denitrification calculated from 15N fractionation of NO3− in soil and groundwater accounts for 15%–24% of total mineralized N (N loss is 0.6–1.6 kg N ha−1 year−1), with precipitation shifts markedly influencing NR outflows. This framework enhances predictions of climate change impacts on nutrient transport and water quality in mountain catchments, which are critical water sources for ecosystems and human use. Overall, application of this approach can offer key insights for mitigating ecological risks from increased NR mobilization, especially under rising atmospheric N deposition and global warming effects.