Complementarity promotes overyielding in grass-legume intercropping

Interspecific complementarity effect and selection effect are crucial for the positive relationship between diversity and productivity in ecosystems, yet their evolution over time in intercropping systems, especially under varying spatial configurations and management practices such as weeding, is not well understood. This study provides the first assessment of the relative importance of climate variability, spatial configuration (planting distance between coexisting species), and weeding practices in shaping the biodiversity effects that drive overyielding in grass–legume intercropping systems. A four-year field experiment (2020-2023) was conducted to investigate net biodiversity effects in grass-legume intercropping (Leymus chinensis and Medicago sativa). Five planting modes—two monocultures and three spatial intercropping configurations (mixed-within-row, alternate-one-row, and alternate-two-rows)—were tested under no-weeding and weeding treatments. Aboveground biomass and biodiversity effects (net biodiversity effect, complementarity effect, selection effect) were assessed. Complementarity effect was the primary contributor to positive biodiversity effects and increased with greater spatial separation. This was most evident in the alternate-two-rows configuration, which reduced interspecific competition and enhanced the complementarity effect by 56.99% (no-weeding) and 136.95% (weeding) relative to the mixed-within-row setup. In contrast, selection effects decreased substantially - by 24.55% without weeding and 166.64% with weeding, indicating a substantial reduction in the influence of dominant species. Climatic factors, especially increased precipitation, boosted the complementarity effect, while uneven rainfall distribution negatively impacted productivity in the final year. Weeding treatments had minimal impact, highlighting spatial configuration and climate as dominant drivers of biodiversity effects. Our findings suggest that strategic intercropping improves yield sustainability and resilience to climatic variations.