Abstract: Mixed-species plantations potentially alter the stability of soil organic carbon (SOC), playing a crucial role in SOC sequestration. However, how tree species mixtures affect root and rhizosphere soil characteristics and further shape rhizosphere SOC stability are not fully understood. In this study, the effects of mixed-species plantations on rhizosphere SOC stability through root exudation, root morphological traits, rhizosphere properties and microbial biomass carbon were investigated in a Pinus massoniana monoculture and two paired plantations interplanted with Erythrophleum fordii (a nitrogen-fixing species) and Castanopsis hystrix. Our findings show that when interplanting with C. hystrix, root exudation of P. massoniana increased significantly, which was positively correlated with increases in mass proportion (+ 38% and + 11%) and carbon contents (+ 77% and + 12%) of large and small macro-aggregates in the P. massoniana rhizosphere. This suggests that root morphological traits and exudation inputs largely affected P. massoniana rhizosphere SOC stability. When interplanting with E. fordii, there was no significant increase in root exudation and no correlations between rhizosphere aggregate mass proportion, carbon content and root exudation rates, while available nitrogen attributed to P. massoniana rhizosphere SOC stability. Our results suggest divergent mechanisms underlying P. massoniana rhizosphere SOC stability in mixed-species plantations with different companion species, and highlight the critical role of selecting appropriate companion species in improving SOC stabilization of mixed-species plantations.

Key words: Mixed-species, Rhizosphere SOC stability, Root exudation inputs, Root morphological traits, Available nitrogen