Rapeseed Cover with Cropping System Adjustment Improves Yield and Soil Multifunctionality in Groundwater Overexploited Areas of the North China Plain

Groundwater overexploited areas of the North China Plain (NCP) have long faced the challenge of balancing agricultural water use and food security. Although seasonal fallowing can reduce water consumption, it also leads to underutilization of land and climatic resources, environmental risks such as wind erosion and dust dispersal, and losses in grain production. Integrating cover crops with multiple-cropping systems offers an important approach to improving soil ecological functioning and resource-use efficiency while ensuring food security. In this study, a field experiment was conducted with two one harvest per year systems, in which fallow (FM) or winter rapeseed (RM) was followed by spring maize, and two three harvests over two years systems, in which fallow (FMWM) or winter rapeseed (RMWM) was followed by spring maize and then by winter wheat and summer maize in the following year. We evaluated the effects of winter rapeseed cover and cropping-system adjustment on soil water dynamics, soil multifunctionality, grain yield, and crop water productivity. Winter rapeseed increased water consumption during the fallow period by 12.6–36.2 mm compared with the corresponding fallow treatments. However, abundant rainfall during the subsequent maize season compensated for the soil water deficit caused by winter rapeseed cover. On average, water consumption of spring maize was 86.6 mm higher than that of summer maize. Consequently, the higher water consumption of spring maize and the compensating effects of abundant rainfall offset the extra water use by winter rapeseed, resulting in no significant differences in two-year cumulative water consumption among treatments. Winter rapeseed increased soil available nutrient contents and enzyme activities. Soil alkali-hydrolyzable nitrogen, available potassium, and available phosphorus contents, as well as sucrase, urease, alkaline phosphatase, and catalase activities, were significantly higher under winter rapeseed cover than under the corresponding fallow treatments. Consequently, the soil multifunctionality index was also generally higher under winter rapeseed cover. In addition, RMWM achieved the highest grain yield, with two-year cumulative grain yield increasing by 56.1%, 47.6%, and 8.9% relative to FM, RM, and FMWM, respectively. Crop water productivity under RMWM reached 1.7 kg m−3, representing increases of 56.4%, 47.8%, and 9.4%, respectively. Overall, the system combining winter rapeseed cover with three harvests over two years improved soil functioning and increased grain yield and crop water productivity without significantly increasing two-year cumulative water consumption, providing a feasible cropping-system option for optimizing agricultural water use and safeguarding food security in groundwater-depleted regions of the North China Plain.