Long‐term drought triggers contrasting responses of foliar stable nitrogen isotopes and soil available nitrogen in a subtropical forest

Abstract Drought disrupts nitrogen (N) cycling by simultaneously reshaping N dynamics in leaves and soils, potentially limiting plant growth and ecosystem productivity. Understanding how drought affects these coupled dynamics in forests is crucial for predicting future carbon–nitrogen interactions amid declining global N availability. Based on a long‐term experimental drought platform in a subtropical evergreen broad‐leaved forest, foliar N concentration, stable N isotopes (δ 15 N) in two dominant tree species, that is Lithocarpus glaber and Schima superba , and four soil N stocks (i.e. total, ammonia, nitrate and available N) were determined to assess drought‐induced shifts in N cycles in subtropical forests. Experimental droughts significantly decreased soil N stocks, with typical declines in soil ammonia (14.6%) and available N (14.3%) contents, indicating less soil available N under water limitation. While foliar N concentration decreased slightly (1.5%–2.9%), the canopy‐level leaf area index (10.5%) and relative growth rates of the dominant species (19.0%–32.1%) significantly declined, indicating a reduction in plant N demand under droughts. Moreover, foliar δ 15 N was significantly enriched in both dominant species, with the foliar δ 15 N of L. glaber and S. superba enriching from −3.0 ± 0.2‰ to −2.1 ± 0.2‰ and from −4.5 ± 0.2‰ to −3.5 ± 0.2‰, respectively. Synthesis . Our results show that long‐term drought accelerates losses of soil available N due to reduced moisture, leading to a more open N cycle and enriched foliar δ 15 N in dominant species. This shift suggests altered plant N uptake and reduced ecosystem N retention, increasing the risk of N limitation under climate change in subtropical forests.