Mycorrhizal type shifts the controls on tree root exudation from soil-driven to carbohydrate-driven mechanisms

Exudation is crucial for carbon and nutrient cycling in forests. However, the underlying mechanism controlling exudation in mature trees, especially its dependence on mycorrhizal type, remains unknown. Based on the control of carbon acquisition by roots, we propose an updated 'push-trade-off-pull' framework for exudation. We investigated three controlling categories, that is, nonstructural carbohydrates (NSCs) in branches and roots, root functional traits, and soil nutrients, as proxies for 'push', 'trade-off', and 'pull', respectively, over exudation for trees colonized by arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi in subtropical forests of China. The NSCs, root traits, and soil nutrients together controlled exudation of trees, particularly distinguishing AM from ECM species. Soil nutrients dominantly impacted the exudation of AM species (47%), that is, increased exudation linked with decreased soil nutrients, supporting the 'pull' effect. However, the NSCs mainly mediated that of ECM species (56%), that is, enhanced exudation associated with declined NSCs, which rejects the 'push' effect. For the 'trade-off', greater exudation was correlated with greater root branching for AM and with lower root tissue density for ECM species. Our findings highlight the mycorrhizal symbiosis-dependent mechanism of exuded carbon that provides a new perspective for understanding exudate-mediated belowground carbon cycling in forests.