Leaf Hydraulic Conductance Mediates the Effect of Precipitation on Leaf Nutrients in Subtropical Forests

Precipitation will increase in some regions and decrease in others. These changes can alter plant nutrient status, and consequently, impact the terrestrial carbon cycle. However, the ecological and physiological mechanisms underlying plant nutrient responses to changes in precipitation remain poorly understood. Here, we propose that both soil nutrients and plant hydraulic traits mediate precipitation effects on leaf nutrients. To test this hypothesis, we measured leaf nutrient and hydraulic traits alongside soil properties across 12 Eucalyptus urophylla plantations along a mean annual precipitation gradient (MAP, from 1401 to 1898 mm yr^-1), with relatively constant mean annual temperature (22.1 ± 0.1°C, mean ± SE) in subtropical China. Leaf nitrogen (N) to phosphorus (P) ratio increased significantly from 12.5 to 18.5 with increasing MAP. The shift was driven primarily by an elevation in leaf N content (from 1.24 to 2.08 g m^-2), while leaf P content remained relatively constant (0.10 ± 0.01 g m^-2). Leaf N and P contents correlated positively with leaf hydraulic conductance (p < 0.05) but not with soil available (or total) N and P contents (p > 0.05) across the MAP gradient. Together, these results suggest that leaf hydraulic conductance, rather than soil nutrients, mediates the response of leaf nutrients to the MAP gradient. A theory-based structural equation modelling analysis further corroborated this hydraulic-mediation pathway. Our findings suggest that high leaf hydraulic conductance under high MAP enhances N and P transport from soil to leaf, especially for N due to its solubility and mobility. These results highlight the need to incorporate nutrient-water interactions into ecosystem models to better predict carbon and nutrient cycles under climate change.