Effects of drought on carbon–nitrogen dynamics in root exudates of Qinghai spruce

Root exudates play a critical role in plant adaptation and the regulation of soil carbon and nutrient cycling, especially under climate change conditions. Despite their importance, the dynamics of root exudation under drought, particularly during drought-induced tree mortality, remain poorly understood. Here, we investigate how drought affects root exudation dynamics, root morphology traits and non-structural carbohydrate (NSC) concentrations in Qinghai spruce (Picea crassifolia Kom.) seedlings across the progression of drought-induced mortality. We conducted a two-stage drought manipulation experiment, beginning with moderate drought (50% irrigation reduction) followed by complete water cessation to induce lethal drought conditions. Our results reveal that drought significantly decreased the exudation rate of total organic carbon (C), while nitrogen (N) exudation rate remained unaffected, resulting in a lower C:N ratio in root exudates, indicative of a higher proportion of N-rich compounds. In addition, drought induced a shift in root morphological traits toward a more competitive strategy, marked by increased specific root length (SRL), specific root area (SRA) and branch intensity, and a reduction in root diameter (RD), root tissue density and root NSC concentrations. Notably, root exudation rates were negatively correlated with competitive root traits (higher SRL, SRA and branch intensity) and positively correlated with conservative traits (larger RD and root tissue density), suggesting that root exudation serves a more conservative function under drought conditions, prioritizing carbon storage and morphological adaptations over exudation. These findings provide valuable insights into the adaptive mechanisms of trees under drought stress, with implications for nutrient cycling, forest resilience and ecosystem productivity in water-limited environments.