Effects of Intercropping Precious Tree Species on Soil Physicochemical Properties in Cunninghamia lanceolata Plantations

Long-term monoculture management of Cunninghamia lanceolata plantations tends to lead to soil degradation. Understory intercropping with precious tree species is an important measure to enhance their ecological functions. However, systematic studies on the effects of intercropping with Phoebe chekiangensis and Ormosia hosiei on the comprehensive soil properties are still lacking. This study aimed to investigate the effects of intercropping these two tree species on the soil physicochemical properties in C. lanceolata plantations, with the goal of providing a scientific basis for soil fertility maintenance and close-to-nature transformation of C. lanceolata plantations. In a C. lanceolata plantation in Qingyuan County, Zhejiang Province, three treatments were established: pure C. lanceolata（CT）, C. lanceolata + O. hosiei（COT）, and C. lanceolata + P. chekiangensis（CPT）. Each treatment included four replicate plots. Soil samples were collected from the 0–10 cm and 10–30 cm soil layers to determine soil bulk density, porosity, water-holding capacity, pH, organic carbon, total and available nutrient contents, and to calculate the stoichiometric ratios of carbon, nitrogen, phosphorus, and potassium. The results showed that compared with CT, interplanting valuable tree species improved the physical structure of the 0–10 cm soil layer to some extent. The soil bulk density of CPT was significantly reduced, while total porosity and field capacity were significantly increased. The non-capillary porosity of COT soil was significantly increased. Interplanting also enhanced the nutrient and organic carbon contents in the surface soil. Specifically, the alkaline hydrolysis nitrogen content in COT soil was significantly higher than that in CT, and the soil organic carbon contents in both CPT and COT were significantly higher than that in CT. After interplanting, the soil C/N, C/P, and C/K ratios showed an increasing trend, but the differences among different stands were not significant. In conclusion, understory intercropping with P. chekiangensis or O. hosiei can effectively improve the surface soil structure, enhance nutrient availability, and promote organic carbon accumulation in C. lanceolata plantations, but has limited effects on soil acidity and deeper soil layers. Future studies should focus on the long-term effects of intercropping and the dynamics of deeper soil layers, as well as conduct in-depth research on biological mechanisms and the optimization of multi-species configurations.