Songbo Tang, Jianyang Xia, and Liming Yan, East China Normal University, discusses their article: Long-term drought triggers contrasting responses of foliar stable nitrogen isotopes and soil available nitrogen in a subtropical forest
Introduction: The Hidden Link Between Drought and Nitrogen Dynamics
As climate change intensifies, drought events increasingly threaten the functionality of global forest ecosystems. Subtropical evergreen broad-leaved forests, critical carbon sinks in terrestrial ecosystems, rely heavily on nitrogen (N) cycling to sustain productivity. Yet, how drought disrupts coupled plant-soil nitrogen dynamics—potentially limiting forest growth—remains a pressing scientific question. The Zhejiang Tiantong Forest Ecosystem National Observation and Research Station (Tiantong Station), a long-term monitoring platform nestled in China’s subtropical evergreen broad-leaved forests, offers unique insights into this complex interplay.
Research Context: The Scientific Significance of Tiantong Station
Located in East China’s monsoon region (29°48’N, 121°47’E, 160 m elevation), Tiantong Station preserves a pristine subtropical evergreen broad-leaved forest, serving as a natural laboratory for studying ecosystem responses to climate stressors. Dominant tree species like Lithocarpus glaber and Schima superba not only underpin ecosystem productivity but also reflect adaptive strategies under environmental pressures. Through a long-term experimental drought platform, researchers systematically evaluated how water limitation alters soil N stocks, plant N uptake, and growth trajectories.
Key Findings: Drought-Induced Imbalances in Nitrogen Cycling
1. Sharp Decline in Soil Available Nitrogen
Prolonged drought significantly reduced soil ammonia (14.6%) and available N (14.3%), directly diminishing soil N accessibility under water stress. Declines in total soil N stocks further suggest accelerated N loss, heightening risks of ecosystem-scale “N starvation.”
2. Adaptive Downregulation of Plant Nitrogen Demand
Despite minor reductions in foliar N concentration (1.5%-2.9%), canopy leaf area index dropped by 10.5%, and relative growth rates of dominant species declined by 19.0–32.1%. This “conservation strategy” reveals plants prioritizing survival over growth under N scarcity—a trade-off that may weaken long-term carbon sequestration capacity.
3. Foliar δ15N Enrichment: A Signal of Open Nitrogen Cycling
Foliar δ15N in L. glaber and S. superba increased from -3.0‰ to -2.1‰ and -4.5‰ to -3.5‰, respectively, indicating drought-driven shifts toward more open N cycling. Isotopic signatures suggest plants may increasingly rely on heavier N forms or altered organic N mineralization pathways since the high N losses under water stress.
Ecological Implications: Rising Nitrogen Limitation Risks in Subtropical Forests
The study highlights a dual threat to subtropical forests: drought simultaneously depletes soil N availability and forces plants to decrease growth, potentially triggering a negative feedback loop of “low N–low productivity.” Notably, enriched foliar δ15N signals reduced ecosystem N retention, implying higher risks of N loss to the atmosphere or waterways. As droughts intensify, such open N cycling could exacerbate N limitation, undermining forest resilience and carbon storage potential.
Tiantong Station’s Role in Long-Term Monitoring
As a flagship site for studying East Asia’s evergreen broad-leaved forests, Tiantong Station provides critical evidence for predicting carbon-nitrogen interactions amid declining global N availability. Its long-term experimental data link physiological responses to ecosystem-scale processes, offering a foundation for adaptive forest management. Future work integrating multi-scale models and cross-regional comparisons will further unravel how subtropical forests navigate climate stressors.
Conclusion
Drought is not merely a water crisis—it is a catalyst for nutrient cycle disruption. Research at Tiantong Station underscores that protecting subtropical evergreen broad-leaved forests is vital, not only for biodiversity, but also for maintaining N balance and carbon sink functions. In an era of intersecting climate and anthropogenic pressures, mitigating N limitation risks may emerge as a cornerstone for safeguarding these ecological lifelines.
