Optimizing belowground interactions in young rubber-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 four treatments: (1) rubber monoculture with conventional fertilization (CK); and three rubber-banana intercropping systems with different fertilization regimes: (2) conventional chemical fertilization (M1), (3) 30% organic nitrogen substitution (M2), and (4) 45% controlled-release nitrogen combined with chemical nitrogen (M3). Thus, the effect of intercropping per se can be assessed by comparing CK with M1, while the effect of fertilization strategies within intercropping is evaluated by comparing M1, M2, and M3. Results: Results demonstrated that the M2 treatment produced the most significant improvements, enhancing rubber tree height by 16.26% and stem girth by 8.17% compared to CK, while simultaneously supporting optimal banana growth. During the banana bud development stage, M2 increased rubber root biomass in the 0-20 cm and 20-40 cm layers by 66.91%-115.84% and 140.08%-625.47%, respectively, compared to other treatments, and promoted banana root length density mainly in the surface layer (0-20 cm) by 79.98%-88.50%. Soil available N under M2 was elevated by 14.43%-26.49% across different soil layers at harvest, and available P increased by 12.72% in the 0-20 cm layer, while soil organic matter increased by 7.77%. Microbial community shifts showed strong positive correlations with increased soil nutrients; for example, Firmicutes was enriched by 131.00%, and key taxa such as Proteobacteria and Bacteroidota were also significantly increased and positively correlated with root development. Importantly, M2 reduced root competition intensity (UICII) by 22.69%-31.65% during vigorous growth and bud development stages. Conclusions: Our integrated analysis demonstrates that substituting 30% of chemical nitrogen with organic nitrogen appears promising under the experimental conditions of this study, as it simultaneously enhances plant productivity, alleviates belowground competition, and improves soil ecological conditions. Therefore, this study provides a scientifically-grounded and potentially sustainable management strategy for enhancing the productivity and ecological sustainability of rubber tree-banana intercropping systems.