Polymer-coated urea effects on maize yield and nitrogen losses for hilly land of southern China

The rational nitrogen (N) application rate for crops depends not only on yield but also on the environmental cost of N loss to the surrounding ecological environment. To explore the environmental and economic optimal N rate (EEONR) for dryland maize in the hilly areas of southern China, a field experiment was conducted with spring maize from 2015 to 2017. Common urea (CU, control, 240 kg N ha−1) and four polymer-coated urea (PCU) fertilization levels (100PCU, 90PCU, 80PCU, 70PCU, which respectively denotes 0, 10%, 20% and 30% reduction of PCU–N application based on the control) were employed in this trial. Maize yield, N loss and economic return were investigated over 3 years. The results were as follows: (1) relative to CU, a 0–20% reduction in the PCU-N application rate did not significantly affect the maize yield during the three consecutive years, however, a 30% reduction in the PCU-N application rate led to significant yield loss in the third year despite the consistent yields achieved in the first 2 years. (2) PCU application significantly abated N loss to the environment, and as N application rate decreased, N loss significantly reduced. Our results showed that a 0–30% reduction in the PCU-N application rate mitigated N loss by 12.8–46.1% via N2O, NH3, runoff and leaching, and the two main N-loss pathways were leaching (55.2–62.2%) and NH3 volatilization (29.8–36.0%). (3) A 0–30% reduction in the PCU-N application rate resulted in a deficit, ranging from 784 to 2438 CNY ha−1 for increased earnings over CU because of the high purchase price of PCU. The net economic benefit was calculated from the N input cost, yield economic income and environment cost of N lost in the treatments with 0, 10%, 20% and 30% reduction of PCU-N and were − 1820 to 14,479, 402 to 14,612, 2306 to 14,613, and 3941 to 14,205 CNY ha−1, respectively, and the net economic benefit of the CU plot was − 4003 to 14,671 CNY ha−1. In summary, our findings suggest that reducing the PCU-N application rate by 20% is a reasonable EEONR for spring maize cultivation in hilly areas of southern China.