Optimizing root distribution and water use efficiency in maize/soybean intercropping under different irrigation levels: The role of underground interactions

In maize/soybean intercropping system, achieving optimal yields depends on a thorough understanding of the complex interactions occurring in the belowground processes. Therefore, a two-year field experiment was conducted to assess crop productivity, root distribution, and soil water distribution and utilization in response to underground interaction (UI) and underground separation (US) under three irrigation levels (60 %, 80 %, and 100 % ETc, represented by W1, W2, W3, respectively) in maize/soybean intercropping system. The results indicated that the UI not only significantly increased the maize yield but also enhanced its root length density and root investment, thereby improving its root competitive ability compared to the US; but the UI had negative effects on these indicators for soybean. US resulted in soil water differences between the maize and soybean sides, with higher soil water levels observed on the soybean side compared to the maize side. In contrast, under UI, the soil water differences were minimal, soil water transport from the soybean side to the maize side facilitated complementary water uptake, thereby enhancing the water use efficiency of maize. Furthermore, as the irrigation level increased, the yields, root length densities, soil water content, and soil water transport of both maize and soybean increased. However, increased irrigation level reduced the advantages of interspecific underground interactions and the water use efficiency. Under UI, correlation analysis revealed significant positive relationships among most measured parameters, with the exception of water use efficiency, which exhibited a negative correlation with evapotranspiration. Radar chart analysis demonstrated distinct performance patterns across irrigation levels under UI: W3 showed superior results in soil water content and evapotranspiration, W2 showed balanced performance across all indicators, while W1 excelled in water use efficiency. These findings highlight the importance of underground interactions and irrigation management in optimizing crop productivity and water use efficiency in maize/soybean intercropping system.