Intercropping effects of agro-crops under walnut forests in Wuxi, Chongqing, China: soil environment, walnut growth and intercropping benefits

Abstract Walnut is an important woody oil-bearing tree crops, and the compound planting with agro-crops can provide a solution to food shortages. This study focuses on two monoculture planting patterns (bare soil and retaining grass) and four common local intercropping planting patterns in Wuxi County, Chongqing City, China, investigating soil physical and chemical properties, microbial diversity, walnut growth and mineral nutrition, as well as the yield and benefit of walnuts and agro-crops. The results showed that: (1) Intercropping treatments and retaining grass can reduce soil bulk density, increase soil moisture content, and significantly enhance both soil total porosity and capillary porosity. (2) Intercropping treatments and retaining grass can significantly lower soil pH and increase the content of soil organic matter, total nitrogen, total phosphorus, available phosphorus, available potassium, available zinc, and available manganese. Intercropping with soybeans can significantly decrease the available copper content in soil, while intercropping with chili peppers can significantly reduce the alkaline hydrolyzable nitrogen content in the soil. (3) Except for the intercropping with tobacco, which slightly reduces soil fungal community diversity, retaining grass and other intercropping treatments can significantly increase both the richness and diversity of soil bacterial and fungal communities. (4) Retaining grass and intercropping treatments increased the annual increment in walnut tree meter diameter, spring shoot length, and the nitrogen and manganese content of leaves. They also significantly increased leaf length, width, thickness, and magnesium content. Intercropping with soybeans significantly increased the length of summer shoots and significantly reduced leaf moisture content as well as the content of potassium, calcium, and copper. Intercropping with chili peppers significantly increased the length of summer shoots and significantly reduced leaf moisture content and potassium, copper, and zinc content. (5) Intercropping treatments significantly increased the yields and benefits of walnuts. Among these treatments, intercropping with chili peppers provides the highest benefits, while intercropping offers the lowest. However, retaining grass resulted in no output or benefits from agro-crops. Intercropping planting patterns not only significantly improve the soil environment and promote walnut growth but also increase yield and benefits. Additionally, retaining grass contributes to the improvement of soil environment to a certain extent, further promoting walnut growth.