Optimizing nitrogen application rates to maximize productivity while reducing environmental risk by regulating nitrogen and water utilization in mixed cropping systems

Optimizing the nitrogen (N) fertilization level and cropping system is critically important for achieving good production performance with low environmental pollution. However, there is a knowledge gap on the relationship between crop production sustainability, water and nitrogen consumption, greenhouse gas (GHG) emission under the change in N application rate under cereal and legume mixed systems. A 3-year field experiment with two cropping systems [sole forage sorghum (SS) and forage sorghum/lablab mixed cropping (SL) with four N fertilizer application rates (N0, 0 kg N ha−1; N90, 90 kg N ha−1; N180, 180 kg N ha−1; and N270, 270 kg N ha−1)] was conducted. Results obtained showed that mixed cropping combined with N fertilization enhanced forage biomass and crude protein yield by 34.0 % and 51.1 %, respectively, particularly in mixed cropping combined with the N180 treatment compared with the N0 level in the sole cropping system. Similar improvements were observed for yield stability and sustainability (2.97 % and 0.95, respectively), which reached maximum values at N180 in the mixed cropping system. In addition, mixed cropping increased the water productivity of dry matter yield (WPDM) and water productivity of crude protein yield (WPCPY) by 17.2 % and 27.6 %, respectively, at 180 kg N ha−1 compared with the corresponding treatments of the sole planting system. Furthermore, mixed cropping combined with appropriate N application significantly improved N physiological efficiency (PEN) and reduced GHG intensity (GHGI), where N application of 180 kg ha−1 N increased PEN by 21.3 % and reduced GHGI by 17.6 % compared with the corresponding monoculture in the mixed cropping system. This study demonstrated that reducing N fertilization in cereal–legume cropping systems can promote forage productivity by optimizing water and N management while decreasing environmental pollution. Therefore, forage sorghum mixed with lablab and fertilization at 180 kg N ha–1 is a preferable approach for sustainable agricultural production in the Northwest arid region of China.