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

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 (δ15N) in two dominant tree species, i.e., 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 δ15N was significantly enriched in both dominant species, with the foliar δ15N 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 δ15N 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.