The North China Plain (NCP) supports more than 20% of China’s grain output with only 7% of the country’s water resources. The long-established winter wheat–summer maize double cropping system in this region relies heavily on groundwater irrigation, with annual agricultural groundwater withdrawals reaching 2.1×10¹⁰ m³. Wheat irrigation alone accounts for more than 40% of total regional irrigation, and groundwater overexploitation has become a core bottleneck constraining sustainable agricultural development. In pursuit of high yields, farmers commonly apply excessive nitrogen (N) fertilizer. However, excessive N application not only reduces crop water use efficiency (WUE) but also exacerbates soil N surplus and non-point source pollution, further deteriorating the water environment.
Against this background, cropping systems that incorporate legumes show unique advantages—legumes can supply N to subsequent crops through biological nitrogen fixation, and previous studies have shown that legume-based rotations can improve system WUE. The spring mung bean–summer sweet maize double cropping system, as a water-saving alternative, has both economic and environmental potential. However, the regulatory effects of N application rate on yield, WUE, and economic benefit in this system remain unclear, and systematic research is urgently needed to determine the optimal N application strategy that balances grain productivity and sustainable water use.
The team led by Yadong YANG at China Agricultural University published a research paper titled “Nitrogen application increases water use efficiency and economic benefit of spring mung bean–summer sweet maize cropping system on the North China Plain” in ENGINEERING Agriculture, an Engineering cluster journal of the Chinese Academy of Engineering (DOI: 10.15302/J-FASE-2026680). The study confirmed that moderate N application in the spring mung bean–summer sweet maize system on the NCP can increase yield, improve efficiency, and save water, and recommended a low-N application strategy to support regional water-saving and low-carbon agricultural development.
The experiment was conducted at the Wuqiao Experimental Station in Cangzhou, Hebei Province, with four N application regimes: N0 (no N for both seasons), N1 (60 kg·ha⁻¹ N for mung bean + 60 kg·ha⁻¹ N for sweet maize), N2 (60 kg·ha⁻¹ N for mung bean + 120 kg·ha⁻¹ N for sweet maize), and N3 (60 kg·ha⁻¹ N for mung bean + 180 kg·ha⁻¹ N for sweet maize). Soil moisture, crop evapotranspiration (ET), maize equivalent yield (MEY), WUE, and production income were monitored for two consecutive years. Water consumption over the entire growth period was calculated using the water balance method, and the yields of the two crops were converted into a unified equivalent for annual productivity comparison. Two-way analysis of variance was used to analyze the independent and interactive effects of experimental year precipitation and N application level, systematically revealing the seasonal complementary mechanism of N application on water consumption of the two crops, quantifying the economic output and water productivity of different N strategies, and ultimately identifying the optimal N application threshold that balances productivity, water saving, and income.
Based on the two-year experimental results, the study drew clear conclusions:
N application stably increased annual system productivity, WUE, and income. Compared with the no-N control, annual MEY increased by 7.3%–10.7%, annual WUE increased by 4.8%–9.5%, and annual total output value increased by 7.3%–10.7%.
N application had seasonal regulatory and complementary effects: in 2021, N1 and N2 significantly reduced mung bean season ET by 15.1% and 18.9%, respectively, while significantly increasing sweet maize season ET by 14.4% and 18.8%, respectively. Water consumption in the two seasons was complementary, with no significant difference in total annual water consumption.
N application significantly improved annual WUE: N3 increased annual WUE by 10.2% and 11.4% in 2020 and 2021, respectively. In the mung bean season, all N treatments had higher WUE than the control, but in the sweet maize season, only the high-N treatment maintained high efficiency.
Considering resource input, output benefit, and water sustainability, the low-N strategy was optimal. Stable yield and water saving could be achieved without pursuing high N input.
This study directly addresses the agricultural pain points of groundwater overexploitation and excessive N fertilizer in the NCP and aligns with national strategies such as fertilizer reduction and agricultural carbon neutrality. The low-N mung bean–sweet maize rotation relies on legume nitrogen fixation to reduce chemical fertilizer input, balancing stable yield and income growth with water saving and carbon reduction. It fills the technical gap in water and fertilizer management for water-saving rotations and provides practical guidance and data support for cropping structure adjustment and the implementation of green, sustainable agriculture in the NCP.
DOI: 10.15302/J-FASE-2026680