Effect of nitrogen reduction and soybean intercropping on nitrogen balance in sweet maize fields in South China

The increasing demand for fresh sweet maize (Zea mays L. saccharata) in South China has led to the prioritization of finding solutions to environmental pollution caused by continuous production of the crop and high inputs of chemical nitrogen fertilizer. A promising method for improving crop production and environmental conditions is to intercrop sweet maize with legumes. Here, a field experiment was conducted at Experimental Center of South China Agriculture University for two years (2015-2016) to investigate nitrogen input, output and balance in sweet maize farmlands in South China under two nitrogen levels reduced nitrogen dose of 300 kg·hm^-2 (N1) and conventional nitrogen dose of 360 kg·hm^-2 (N2) and three cropping patterns sole sweet maize (SS), sweet maize//soybean intercropping with sweet maize to soybean line ratios of 2:3 (S2B3) and 2:4 (S2B4). The purpose of the study was to provide scientific basis for reducing chemical nitrogen fertilizer input, improving nitrogen use efficiency and developing a sustainable sweet corn production model in South China. Results showed that:1) reduced nitrogen application and sweet maize//soybean intercropping decreased total nitrogen input and the proportion of chemical nitrogen input through soybean nitrogen fixation and straw return. Nitrogen fertilizer input under SS, S2B3 and S2B4 accounted respectively for 84.29%, 55.42% and 59.06% of total annual nitrogen input under N1 and for 87.53%, 49.93% and 53.70% under N2. 2) Accumulated nitrogen amount of grain under intercropping system was significantly higher than that under sole sweet maize. Average annual grain nitrogen accumulation of sweet maize//soybean intercropping was 2.18-2.43 times of that of SS. Sweet maize//soybean intercropping reduced ammonia volatilization significantly and thereby reduced the risk of nitrogen leaching. Compared with SS, S2B3 and S2B4 reduced annual ammonia volatilization and nitrogen leaching under N1 by 35.97% and 14.74%, 42.07% and 11.54%, respectively. Treatment S2B4-N1 had the lowest ammonia volatilization, which was 38.72 kg·hm^-2. Meanwhile, annual ammonia volatilization and average annual nitrogen leaching in S2B3 and S2B4 under N2 reduced respectively by 24.55% and 12.89%, 29.98% and 16.23% than that under N1 treatment. 3) Annual nitrogen surpluses under S2B3-N1, S2B3-N2, S2B4-N1 and S2B4-N2 were respectively 71.03 kg·hm^-2, 133.7 kg·hm^-2, 42.87 kg·hm^-2 and 100.64 kg·hm^-2, which were 81.27%, 64.75%, 88.69% and 73.47% lower than the average of SS under N1 and N2. Overall, the study demonstrated that intercropping combined with reduced-nitrogen rate maintained sweet maize production, reduced production cost, while also reducing environmental impact. Intercropping with soybean and reduced nitrogen application may be a more sustainable and environmentally friendly way for production of sweet maize in South China.