Résumé
1. Introduction Sugarcane-legume intercropping systems have attracted much attention worldwide to improve soil fertility (Tang et al., 2021) and to reduce herbicide application (Soulé et al., 2024). Nonetheless, a recent meta-analysis highlighted how introducing companion plants with sugarcane leads to highly variable yield response (Viaud et al., 2023), and the processes and resources involved in competition between sugarcane and companion plants are still poorly understood. The aim of this study, based on Viaud's (2023) thesis work, was to evaluate the competition mechanisms between a legume (Jack bean, Canavalia ensiformis) and sugarcane depending on environmental conditions (water and nitrogen availability) in the tropical island of La Réunion. 2. Materials and Methods A three-year experiment has been set up to test the interactions between irrigation, nitrogen fertilization, and sugarcane-legume intercropping through a factorial trial with three crossed treatments and four repetitions in the experimental station of La Mare (Reunion Island). Above and belowground sugarcane traits were measured overgrowth to assess the impact of legume presence on sugarcane traits, and their temporal relation was assessed through structural equation modeling. The competition for soil N and N from fertilizer was distinguished through 15N-labelled fertilizer, and the competition for water was assessed through soil water content measurements. 3. Results The presence of a legume reduced sugarcane yield by 9% on average through an overall decrease in both above- and below-ground biomass. At the beginning of growth, the decrease in biomass was linked to a reduction in the number of tillers (-8%), adding the effect of lodging amplified by the presence of legumes at the end of growth (+25% in the irrigated conditions, Figure 1). While legume intercropping did not impact nitrogen fertilization uptake by sugarcane, soil N competition occurred under low water availability conditions and impacted the sugarcane N nutrition index (-23%). On the contrary, no evidence of water competition was found. The sugarcane root response was determinant in the sugarcane-legume interaction. The presence of legume decreased sugarcane root colonization around six months after the beginning of growth, which was correlated with the higher lodging at the end of the growth (twelve months). The lower root colonization could also explain the lower nitrogen nutrition index during the first six months of growth. Despite the absence of Nitrogen fertilization uptake by legumes, the fertilization did not give sugarcane a competitive advantage. However, irrigation ensured better sugarcane growth at the beginning of the cycle, making it more competitive with the legume, although increasing the risk of lodging at the end of the growing year. 4. Discussion Our results illustrate the complexity of sugarcane response to intercropping as a function of resource availability. Despite a significant competitive advantage of high-biomass sugarcane over the legume, sugarcane's root-avoidance strategy highlighted in previous studies (Christina et al., 2023) makes it sensitive to inter-specific competition. Adequate irrigation management at the beginning of sugarcane growth is essential to limit sugarcane yield loss. Finally, the interest of this legume in sugarcane intercropping systems in terms of symbiotic fixation, N return to the soil, and sugarcane nutrition was explored in Viaud's (2023) subsequent thesis work.