Root exudation as a major competitive fine-root functional trait of 18 coexisting species in a subtropical forest.

Root exudation stimulates microbial decomposition and enhances nutrient availability to plants. It remains difficult to measure and predict this carbon flux in natural conditions, especially for mature woody plants. Based on a known conceptual framework of root functional traits coordination, we proposed that root functional traits may predict root exudation. We measured root exudation and other seven root morphological/chemical/physiological traits for 18 coexisting woody species in a deciduous-evergreen mixed forest in subtropical China. The root exudation, respiration, diameter and nitrogen (N) concentration all exhibited significant phylogenetic signals. We found that root exudation positively correlated with competitive traits (root respiration, N concentration) and negatively with conservative trait (root tissue density). Furthermore, these relations were independent of phylogenetic signals. A principal components analysis showed that root exudation and morphological traits loaded on two perpendicular axes. Root exudation is a competitive trait in a multi-dimensional fine-root functional coordination. The metabolic dimension which root exudation loaded on was relatively independent of the morphological dimension, indicating that increasing nutrient availability by root exudation might be a complementary strategy for plant nutrient acquisition. The positive relation between root exudation with root respiration and N concentration is a promising approach for future prediction of root exudation.