Tree species richness and N-fixing tree species enhance the chemical stability of soil organic carbon in subtropical plantations

Tree species diversity enhances productivity and soil carbon storage in subtropical forests. However, the effects of tree species diversity and N-fixing tree species on the chemical stability of soil organic carbon (SOC) have been poorly understood. In this study, a manipulative experiment involving varying tree species richness (i.e., one, two, four and six species) and the inclusion of N-fixing trees was conducted to investigate the effects of tree species richness and the presence of N-fixing tree species on the chemical stability of SOC. Biological traceability was used to separate SOC into plant- vs. microbial-derived components, and then assessed by four measures of the chemical stability, including the cyclic/acyclic lipid ratio, average carbon chain length (ACL), Pielou's evenness of acyclic lipids and Pielou's evenness of cyclic lipids. We found that tree species richness was positively correlated with the cyclic/acyclic lipid ratio, ACL and evenness of acyclic lipids. In the presence of N-fixing tree species, tree species richness significantly increased both plant- and microbial-derived lipids, resulting in a higher cyclic/acyclic lipid ratio through stimulated root-microorganism interactions. Plant-derived lipids were positively correlated with the cyclic/acyclic lipid ratio, ACL and evenness of acyclic lipids, suggesting the important role of plant-derived components in shaping the chemical stability of SOC. This study reveals that the chemical stability of SOC is not only a function of tree species richness, but also manifested by the presence of N-fixing tree species. These results demonstrate that mixed plantations with N-fixing trees as an appropriate silvicultural option will be beneficial to the enhancement of the chemical stability of SOC through increased soil recalcitrant C components.