Understanding how ecosystems sustain plant nitrogen (N) supply under climate change is critical, yet whether increasing plant N demand is met by external inputs or mobilization of soil N reserves remains unresolved. Here we show that climate change increases plant reliance on soil N reserves despite intensive fertilization. Using a two-year 15N-tracing experiment combining elevated CO2, warming, and drought in a montane grassland, we found that plants obtained 82–88% of their N from soil and acquired 4.6–7.3 times more N from soil than from fertilizer despite high N inputs. Elevated CO2 and warming increased plant uptake of soil-derived N but not fertilizer N. Consequently, plant N export exceeded fertilizer inputs, causing ecosystem N deficits and depletion of soil N stocks, with the strongest soil N mining under combined elevated CO2 and warming. Our findings reveal that climate change accelerates biological mining of soil N reserves, potentially constraining the long-term sustainability of intensively managed agroecosystems. A field experiment shows that climate change increases plant reliance on soil nitrogen reserves despite intensive fertilization, accelerating depletion of soil nitrogen stocks and potentially threatening the long-term sustainability of managed ecosystems.