Intercropping with Peanut Promoted the Root Growth of Young Coconut and Improved the Soil Quality

Intercropping with peanuts in coconut plantation is a traditional agroforestry model in tropical regions. However, current agricultural practices lack a solid scientific basis. In this study, a four-year field experiment with three treatments (coconut–peanut intercropping, coconut monoculture and peanut monoculture) was conducted in a young coconut plantation in Hainan, China. The regulatory mechanism of intercropping was examined by analyzing the variability in coconut root characteristics (n = 3), soil physicochemical properties (n = 5) and microbial community structure (n = 5) across different treatments. The results indicated that more than 80% of the intercropped coconut roots were concentrated within a horizontal distance of 100 cm from the trunk and at a soil depth of 0–40 cm. Compared to monocropped coconuts, intercropping with peanut significantly promoted the root growth of young coconut palms, increasing their RLD and RAD by 93.70–442.86% and 82.72–358.54%, respectively, in the 0–40 cm soil layers at horizontal 50 cm from the coconut. Intercropping with peanuts also improved soil quality by raising pH, total phosphorus and available phosphorus contents. The improved nutrient levels might result from the long-term fertilization accumulation in soil of peanut intercropping. Compared to the coconut monoculture, the soil bacterial community in the intercropping system showed increases in the Chao1 and Shannon indices by 19.00% and 7.39%, but fungal diversity did not differ significantly between the two treatments. Furthermore, intercropping with peanuts enriched beneficial taxa such as Pseudomonadota, Gemmatimonadota, and Ascomycota. Soil pH, available phosphorus, and available potassium were identified as the primary drivers of microbial community variations. This study highlights that coconut–peanut intercropping improves coconut root growth, soil properties and microbial composition, providing an effective management strategy for young coconut plantations.