Objective Differences in tree species composition may alter the stoichiometry of soil and microbial elements, thereby influencing soil organic carbon (SOC) accumulation, but the relationship between them remains unclear. This study explored the changes in soil nutrient and enzyme stoichiometry across different mixed Pinus massoniana plantations and their relationship with SOC content, to deepen the understanding of the potential mechanisms of species composition affecting SOC accumulation.

Method Three stand types in Longquan, Zhejiang Province, were examined: pure P. massoniana plantation, mixed P. massoniana + Cunninghamia lanceolata plantation, and mixed P. massoniana + C. lanceolata + Schima superba + Liquidambar formosana plantation. The changes in soil nutrients and the activities of enzymes related to carbon (C), nitrogen (N), and phosphorus (P) cycling in the topsoil (0 - 10 cm) of these three stands were analyzed, as well as the relationship between the stoichiometric ratios of soil nutrient and enzyme C∶N∶P stoichiometric ratios and SOC content.

Results The contents of SOC, total N, total P, dissolved organic C, total dissolved N, and available P tended to increase with increasing tree species numbers in the mixed plantations. Compared to the pure P. massoniana plantation, the soil C/P ratios in the P. massoniana + C. lanceolata and P. massoniana + C. lanceolata + S. superba + L. formosana mixed plantations decreased by 17.58% and 22.94%, and the ratios of soil dissolved organic C/available P decreased by 24.90% and 55.82% in the two plantations. The activities of N-acetyl-β-glucosaminidase and β-glucosidase in the soil of the three stands increased with the increase in mixed tree species. The activities of leucine aminopeptidase in the P. massoniana + C. lanceolata and P. massoniana + C. lanceolata + S. superba + L. formosana mixed plantations increased by 61.52% and 48.61%, compared to the pure P. massoniana plantation. There were no significant differences in the soil enzyme C/N ratio and enzyme N/P ratio among the three stands. The enzyme C/P ratio in the mixed plantations was significantly higher than that in the pure plantation. Vector analysis revealed that, as tree species richness increased in mixed plantations, microbial C limitation gradually intensified, whereas microbial N limitation remained unchanged. SOC content was significantly and positively correlated with the availability of N and P, β-glucosidase activity, and the soil enzyme C/P, but was not significantly correlated with soil C∶N∶P stoichiometric ratio. The variations in SOC contents were mainly regulated by the availability of soil N and P, as well as the activities of C metabolism enzymes.

Conclusion Establishing mixed P. massoniana plantations enhanced SOC content and altered the C∶N∶P stoichiometric ratios of both soil nutrient and enzymes. However, the increase in SOC was mainly driven by improved nutrient availability and enhanced activities of carbon-metabolizing microorganisms, resulting from changes in the tree species composition in mixed plantations. These findings deepen understanding of SOC accumulation mechanisms in mixed plantations and provide valuable insights for optimizing stand structure to enhance the soil C sequestration potential of P. massoniana plantations.