Optimizing Belowground Interactions in Young Rubber Tree−Banana Intercropping via Partial Organic Fertilizer Substitution: Root Competition, Soil Nutrients, and Microbial Communities

Aims Optimal fertilization strategies for rubber tree-banana intercropping systems and their comprehensive impacts on belowground interactions remain inadequately characterized. This study systematically evaluated the effects of different fertilization regimes on root distribution, interspecific competition, and rhizosphere properties. Methods The experimental design included conventional chemical fertilization (M1), 30% organic nitrogen substitution (M2), 45% controlled-release nitrogen combined with chemical nitrogen (M3), and rubber tree monoculture as control (CK). Results Results demonstrated that the M2 treatment produced the most significant improvements, enhancing rubber tree height by 16.26% and stem girth by 8.66% compared to CK, while simultaneously supporting optimal banana growth within the intercropping system. Detailed analysis of root spatial-temporal distribution revealed that M2 treatment improved root niche complementarity and enhanced system adaptability to subsurface competition. Furthermore, M2 significantly modified rhizosphere microbial community structure by increasing microbial diversity and enriching beneficial bacterial and fungal phyla. These microbial community shifts showed strong positive correlations with improved soil fertility indicators and plant growth performance. Importantly, this fertilization strategy effectively reduced root competition intensity across different soil layers during critical banana developmental stages, including the vigorous growth and bud development phases. Conclusions Our integrated analysis demonstrates that substituting 30% of chemical nitrogen with organic nitrogen represents an optimal fertilization approach, as it simultaneously enhances plant productivity, alleviates belowground competition, and improves soil ecological conditions. Therefore, this study provides a scientifically-grounded and sustainable management strategy for enhancing the productivity and ecological sustainability of rubber tree-banana intercropping systems.