Impact of Introduced Spatholobus suberectus and Dalbergia balansae on Soil N Accumulation and P Depletion in Chinese Fir Plantations

The introduction of understory vegetation can increase species diversity and potential productivity in forest ecosystems, which is considered a viable solution to the global problem of declining soil quality caused by deteriorating climatic conditions and human activities. The forest management model that achieves economic and ecological benefits by introducing legumes is widely used. However, there have been rare studies on the effects of introducing legumes under Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations on soil nutrient content and microbial diversity. In this study, we investigated the soil chemical properties, microbial diversity, and enzymatic activities in Chinese fir plantations introduced with Spatholobus suberectus (SRS), Dalbergia balansae (DRS), both species (BS), and in a monoculture plantation (CK). As indicated by the results, soil pH, total phosphorus (TP), available phosphorus (AP), available potassium (AK), urease activities, and the ratios of C:P and N:P decreased in SRS, DRS, and BS treatments, whereas soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), phosphatase, and sucrase activities increased. The decrease in soil pH and the effects of Chytridiomycota and Glomeromycota elevated phosphatase activity. Accordingly, the mineralization rate of soil phosphorus increased. This increase enhanced phosphorus availability and the risk of loss, resulting in the depletion of soil phosphorus and the inhibition of urease activity. The findings of this study reveal that the introduction of legumes effectively improves the soil microbial community and nitrogen accumulation in Chinese fir plantations while resulting in phosphorus depletion, highlighting the need for balanced nutrient management.