Legume cover cropping enhances soil function and yield stability in wheat–maize systems under groundwater overexploitation in the North China Plain

The North China Plain (NCP) is one of China's most important wheat-maize production regions, but the conventional winter wheat-summer maize system relies heavily on groundwater irrigation and high fertilizer inputs, intensifying the conflict between grain production and resource sustainability. Legume cover cropping and intercropping have been proposed as potential strategies to improve resource use and soil health; however, field evidence linking soil water regulation, soil multifunctionality, and yield stability under reduced water and fertilizer inputs remains limited. Here, we conducted a five-year field experiment in a groundwater-overexploited area of the NCP to evaluate four cropping systems: the conventional winter wheat-summer maize system (TWM), a reduced-input winter wheat-summer maize system (WM), wheat-white clover strip intercropping with relay-intercropped maize (CI), and wheat-white clover mixed cropping followed by maize (CM). The results showed that CI improved soil moisture conditions in the wheat strips, whereas CI-C reduced water consumption during the wheat season by 26.2%, 19.3%, and 16.9% compared with TWM, WM, and CM, respectively. CI also enhanced soil nutrient availability and biochemical activity, as indicated by higher contents of alkali-hydrolyzable nitrogen, available phosphorus, available potassium, and soil organic carbon, together with increased soil alkaline phosphatase, urease, and saccharase activities. Consequently, CI-C showed the highest soil multifunctionality index across all sampling periods, while WM consistently showed the lowest values. Yield analysis further revealed that WM reduced wheat and maize yields by 18.6% and 11.3%, respectively, compared with TWM. In contrast, CI increased maize yield by 9.7%, 23.6%, and 16.0% compared with TWM, WM, and CM, respectively, and maintained an annual yield of 18.2 Mg ha⁻¹, comparable to that of TWM. Redundancy analysis indicated that SOC, SAP, SAN, SSC, SUE, SCAT, and ET jointly contributed to yield formation. These findings demonstrate that wheat-white clover strip intercropping with relay-intercropped maize can achieve synergistic improvements in water regulation, soil nutrient activation, soil multifunctionality, and yield stability. This system provides a promising ecological intensification strategy for sustainable wheat-maize production in groundwater-overexploited areas of the NCP.