Divergent pathways of rhizosphere SOC stabilization in mixed-species plantations: The role of root exudates versus nitrogen availability

Transitioning from monocultures to mixed-species plantations is a key strategy for enhancing soil organic carbon (SOC) sequestration. However, the specific mechanisms by which tree species interactions, particularly through root exudation and morphological traits, shape rhizosphere SOC stability remain poorly understood. This study investigated the effects of introducing broad-leaf species into coniferous plantations on rhizosphere SOC dynamics.

We examined a near-mature Pinus massoniana monoculture and two paired plantations interplanted with Erythrophleum fordii (a nitrogen-fixing species) and Castanopsis hystrix. We quantified root exudation rates, root morphological traits, microbial biomass carbon, and rhizosphere physicochemical properties to identify the controlling factors of SOC stability.

Our results revealed divergent stabilization pathways depending on the companion species. Interplanting with C. hystrix significantly stimulated the root exudation of P. massoniana. This increase in exudates was positively correlated with the mass proportion and carbon content of both large and small macro-aggregates, suggesting that exudate-mediated physical protection is the primary driver of SOC stability in this mixture. Conversely, interplanting with the N-fixing E. fordii did not significantly alter root exudation rates or their relationship with aggregation. Instead, SOC stability in the P. massoniana rhizosphere was primarily attributed to increased nitrogen availability.

Our findings highlight that root exudates play a conditional role in SOC stabilization, heavily dependent on the identity of neighbor species. We conclude that selecting appropriate companion species is critical for managing specific SOC sequestration pathways in mixed-species plantations.