Root morphological traits and dissolved organic carbon regulate root exudation during orchard development

Root exudation is a key interface in plant-soil interactions and strongly influences nutrient cycling within orchard ecosystems. However, how root exudation varies with orchard age and what ecological factors drive these shifts remain insufficiently understood. Therefore, we quantified the relationships between root exudation and root traits, rhizosphere soil properties, and microbial biomass across a six-age orchard chronosequence of Citrus reticulata 'Chachi'. We found that the specific root length (SRL), root tissue density (RTD), specific root tip number (RTN), and root nitrogen concentration (RN) increased with orchard age, whereas the root diameter (RD), root carbon concentration (RC), and the root C:N ratio decreased. Principal component analysis (PCA) revealed that root traits varied significantly along the orchard-age gradient on PC1. Root exudation rates also differed significantly among orchard ages (p < 0.05) and were most strongly associated with soil dissolved organic carbon (DOC) and the DOC:total dissolved nitrogen (TDN) ratio. Specifically, along the orchard-age gradient, the exudation rates of carbon (CEr), nitrogen (NEr), sugars (SEr), and organic acids (OAEr) initially increased and then decreased, with peaks at 3 or 5 years. However, root exudation was not significantly related to root chemical traits. Partial least squares path modeling (PLS–PM) further revealed that orchard age was associated with variation in root exudation both directly and indirectly through its influences on root traits, rhizosphere soil properties, and microbial biomass. With increasing orchard age, root traits became more strongly associated with variation in CEr, whereas NEr shifted from being primarily linked to soil nutrient availability to being more closely influenced by microbial biomass. These results suggest that soil nutrient availability and root traits jointly shape how strongly root exudation responds along the orchard-age gradient. These findings enhance our understanding of belowground C processes associated with root exudation along the orchard-age gradient and offer practical insights for fertilization strategies in long-term perennial orchard systems.