Depth-specific effects of tree species and stand density on root exudate carbon inputs and soil organic carbon pools in plantation forests

: Root exudation is tightly coupled to soil organic carbon (SOC) formation and stabilization in forest ecosystems, yet how tree species and stand density jointly regulate root exudate carbon inputs and their associations with SOC pools across soil depths remains unclear. Here, we investigated pure stands of black locust ( Robinia pseudoacacia ) and Chinese pine ( Pinus tabuliformis ) established at five stand-density levels on the Loess Plateau. We quantified root exudation rates (RR, per unit root biomass), root exudate fluxes (RF, integrated over 0–100 cm), and SOC fractions to assess how tree species and stand density affect exudate carbon inputs and SOC pools. Black locust had higher RR in the 0–30 cm topsoil, whereas Chinese pine had greater 0–100 cm RF across density gradients than black locust (6.48–12.24 vs. 4.76–10.58 g C·m −3 ·year −1 ). Increasing stand density reduced RR, RF, and SOC mainly in the topsoil, indicating that density effects on belowground carbon input were concentrated near the surface. Associations between RR and SOC pools were depth- and species-specific: In black locust, RR was positively associated with SOC in the 0–30 and 30–60 cm layers but negatively associated with SOC in the 60–100 cm layer, whereas no comparable pattern occurred in Chinese pine. Overall, root exudate carbon inputs explained SOC variation primarily in the 0–30 cm layer. These findings highlight the importance of species- and depth-specific root exudation processes for predicting SOC dynamics and optimizing stand density in plantation forests.