Diversification of wheat-maize double cropping with legume intercrops improves nitrogen-use efficiency: Evidence at crop and cropping system levels

The sustainability of traditional maize-wheat (M-W) double cropping in the North China Plain (NCP) is threatened by excessive nitrogen (N) input and surplus. Meanwhile, there is strong market demand of more protein and oil crops, such as soybean or peanut. Incorporation of legumes into M-W via intercropping with maize is emerging in the NCP to foster China's self-sufficiency for edible oils and proteins. It is unknown how such a change in cropping system affects the required annual fertilizer N input, and the resulting N-use efficiency (NUE) and N surplus (Ns). We conducted a four-year field experiment involving four N fertilizer rates and rotations of winter wheat with six different summer crops: maize (conventional and density-increased), peanut (P), soybean (S), and intercrops of density-increased maize and peanut or soybean (MP, MS). The "three-quadrant diagram" and NUE proposed by the EU Nitrogen Expert Panel (EUNEP) were used to assess NUE at crop and cropping system level, respectively. Land equivalent ratios of N uptake in intercrops averaged 1.02–1.07 while N fertilizer equivalent ratios averaged 1.15–1.20, indicating more efficient N uptake and more yield per unit fertilizer than sole crops. Intercropped maize exhibited greater N acquisition efficiency than sole maize. Inclusion of intercrops lowered required annual N inputs and Ns and increased the apparent recovery efficiency (RE) of applied N and EUNEP-NUE. Soybean was a more productive and N-use efficient companion species for maize than peanut. Increasing N decreased RE and EUNEP-NUE while elevated Ns of all rotation systems. N productivity responses of each rotation system to increasing N followed a "linear-plateau" model. Compared to M-W with an optimal 240–360 kg N/ha, MS-W with 210–320 kg N/ha saved 11.1–12.5% fertilizer, increased N uptake by 12.4–16.0%, augmented RE from 36.0–37.0% to 47.8%, increased EUNEP-NUE from 0.50 to 0.67 kg/kg (within the target of 0.50–0.90 kg/kg), lowered Ns by 38.8–39.2%, and reduced N emission by 48.6–49.3%. Therefore, here we show for the first time, using multiple N performance indicators, that MS-W with moderate N provides diversified products, higher N productivity, NUE and lower N loss than M-W, thus being a suitable option for sustainable intensification of agricultural production. Such a diversified rotation approach with legume intercropping aligns with the principles of agricultural green development and has a global relevance for countries with sequential double cropping or rotation systems.