Diversified crop rotations improve crop water use and subsequent cereal crop yield through soil moisture compensation

The water-intensive conventional winter wheat–summer maize (WM) double cropping system in the North China Plain (NCP) has significantly decreased the groundwater table. To address this issue, we undertook a two-year field experiment to explore the potential and mechanisms of water-saving and yield increase of five newly designed diversified crop rotations incorporating spring crops (sweet potato, soybean, peanut, spring maize, and millet) into cereal crops compared with the conventional WM (as control). The results revealed that the five diversified crop rotations significantly decreased annual actual crop evapotranspiration by 7–12% and net groundwater use by 21–31% compared to the conventional WM. Sweet potato and peanut-based rotations significantly enhanced annual average equivalent yields up to 32% and economic benefit (+50%, +7%) while improving water productivity by 24–68% compared to WM. Shallow-rooted crops (sweet potato, soybean, peanut, and millet), when used as the preceding crop, improved soil water storage in the 0–180 cm soil layer at the start of the succeeding wheat planting season by 3–9% compared to the conventional WM. These shallow-rooted crops mainly concentrated their root systems in the 0–120 cm soil water, particularly the top 80 cm, complementing the deeper root systems of wheat, which extended down to 180 cm. Consequently, this optimal soil water use regime in diversified crop rotations increased the leaf area index and aboveground biomass of the succeeding wheat and maize crops, increasing total grain yields by 4–11%. Thus, introducing shallowed-root annual crops as preceding crops to the current WM rotation is beneficial for decreasing irrigation inputs, enhancing overall crop productivity, and mitigating groundwater table decline in the NCP.