Nitrogen-fixing tree species enhance the positive effects of tree species richness on soil organic carbon sequestration by increasing fine root phosphorus loss

Abstract

Mixed planting of multiple tree species and introducing nitrogen (N)-fixing tree species have been regarded as the most effective methods to enhance the ecological function of planted forests. Although there is increasing concern about the effects of tree richness on litter decomposition, knowledge gap still exists about whether N-fixing tree species regulate the effect of tree richness on soil organic carbon (SOC) sequestration by changing fine root decomposition. A 9-year tree species richness experiment with four species gradients (1, 2, 4, and 6 species) interacted with N-fixing tree species was conducted in subtropical forests. Fine root decomposition rate and changes in SOC based on an in situ microcosm incubation were measured to determine the effects of tree species richness and N-fixing tree species on fine root decomposition and SOC storage. We found that tree species richness positively affected fine root C, N, and phosphorus (P) loss. Regardless of the effects of tree richness, the presence of N-fixing tree species enhanced P loss by 30.6%. The increased SOC induced by fine root decomposition showed a positive non-linear response to tree species richness. The presence of N-fixing trees amplified the promoting effects of tree species richness on the net accumulation of SOC by 54.6% probably attributed to the alleviated soil P limitation calculated by soil extracellular enzyme stoichiometry, due to the stimulated root P release and soil available P content. This study demonstrates that the presence of N-fixing tree species significantly enhances the positive effects of tree species richness on SOC storage in subtropical forests. This process is driven by the accelerated release of root P, which alleviates soil P limitation and promotes microbial activity. These findings underscore the critical role of N-fixing tree species in enhancing the ecological functionality of mixed plantations through improved nutrient cycling and C sequestration.