Long-term plant diversity increases soil extractable organic carbon and nitrogen contents in a subtropical forest

Plant diversity is one of the various factors influencing ecosystem functions such as soil carbon (C) and nitrogen (N) stocks. Soil extractable organic carbon (EOC) and nitrogen (EON) contents are active fractions in soil organic matter, but little is known about the impact of variations in long-term plant diversity on soil EOC and EON contents in forest ecosystems. Utilizing the Biodiversity–Ecosystem Functioning Experiment China platform, we selected long-term plant diversity level treatments, distinguished the functional types of evergreen and deciduous plants, and explored their effects on soil EOC and EON contents. The results showed that soil EOC and EON contents increased significantly with greater plant diversity, which is mainly attributed to proportional increases in complementary effects. After distinguishing plant functional types, we did not find the strong complementary effects at the mixed planting of evergreen and deciduous tree species. Within two-species planting mixtures, evergreen tree species can increase soil EON compared to deciduous tree species. Cyclobalanopsis have a strong carbon and nitrogen storage capacity, suggesting that increasing the plant diversity and the proportion of Cyclobalanopsis planting in forest management will promote carbon and nitrogen accumulation in forest soil. These findings enhance our understanding of long-term forest C and N cycling processes and also provide theoretical support for managing forest soil C sinks.