Nitrogen application increases water use efficiency and economic benefit of spring mung bean-summer sweet maize cropping system on the North China Plain

The spring mung bean-summer sweet maize cropping system can alleviate groundwater scarcity problems on the North China Plain. However, the effects of nitrogen application on water use efficiency (WUE) and economic benefit of spring mung bean-summer sweet maize cropping system remain unclear. This study investigated the crop yield, economic benefit and WUE of spring mung bean-summer sweet maize cropping system under four N application regimes: N0, no N for spring mung bean and summer sweet maize; N1, 60 and 60 kg·ka−1 N for spring mung bean and summer sweet maize, respectively; N2, 60 and 120 kg·ka−1 N for spring mung bean and summer sweet maize, respectively; and N3, 60 and 180 kg·ka−1 N for spring mung bean and summer sweet maize, respectively. The results indicated that N application significantly increased annual maize equivalent yield (MEY), and N1 and N3 increased annual MEY by 8.6% and 10.7% (p < 0.05), respectively, compared to N0 in 2021. N application significantly influenced evapotranspiration (ET) in spring mung bean and summer sweet maize seasons (p < 0.05). N1 and N2 decreased ET by 15.1% and 18.9%, respectively, in spring mung bean season, and increased ET by 14.4% and 18.8%, respectively, in summer sweet maize season compared to N0 in 2021, respectively (p < 0.05). N3 improved annual WUE by 10.2% and 11.4% compared to N0 in 2020 and 2021, respectively (p < 0.05). Also, experimental year significantly affected MEY, ET and WUE in spring mung bean and summer sweet maize seasons, and annually (p < 0.001), which can be attributed to the variations in seasonal rainfall between the two experimental years. In summary, N application increased grain yield and WUE of spring mung bean-summer sweet maize cropping system, however, a lower N application rate was recommended to achieve a balance between economic benefit and WUE on the North China Plain.