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

The long-term monoculture management of Cunninghamia lanceolata pure forests tends to lead to the soil degradation. The understory intercropping with precious tree species is an important measure to enhance their ecological functions. However, the 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 reference for the soil fertility maintenance and close-to-nature transformation of C. lanceolata plantations. In the C. lanceolata plantation in Qingyuan County of Zhejiang Province, three treatments were set up: pure C. lanceolata (CT), C. lanceolata + O. hosiei (COT), and C. lanceolata + P. chekiangensis (CPT). Four replicate plots were set up in each treatment. The soil samples were collected from the 0–10 cm and 10–30 cm soil layers. Then, the soil bulk density, porosity, water-holding capacity, pH, organic carbon, total and available nutrient contents were determined, and the stoichiometric ratios of carbon, nitrogen, phosphorus, and potassium were calculated. The results showed that compared with CT, interplanting precious 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 the total porosity and field capacity were significantly increased. The non-capillary porosity of COT soil was significantly increased. Interplanting precious tree species also enhanced the contents of nutrient and organic carbon in the surface soil. Specifically, the content of alkali-hydrolyzable nitrogen 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, the understory intercropping with P. chekiangensis or O. hosiei could effectively improve the surface soil structure, enhance the nutrient availability, and promote the organic carbon accumulation in C. lanceolata plantations, but has limited the effects on soil acidity and alkalinity and deeper soil layers.