Effects of controlled release N fertilizers and reduced application rate on nitrous oxide emissions from soybean fields converted from rice paddies

This study aimed to investigate the effects of using controlled release fertilizers with reduced application rates on nitrous oxide (N2O) gas emissions. For this, we monitored the N2O emissions released from soybean fields converted from rice paddies in Japan for three years, i.e., from 2017 to 2020, under six nitrogen fertilizer treatments: conventional (AC: ammonium chloride, 20 kg N ha−1), controlled release coated urea (CRCU: ammonium chloride, 5 kg N ha−1, coated urea, 15 kg N ha−1, total 20 kg N ha−1), controlled release coated nitrate (CRCN: coated calcium nitrate, 20 kg N ha−1), CRCU at a reduced rate (CRCU-R: 10 kg N ha−1), CRCN at a reduced rate (CRCN-R: 10 kg N ha−1), and no nitrogen fertilizer (NF: 0 kg N ha−1). The soil of the field was classified as Gleyic Fluvisol.The annual N2O emissions of the CRCN treatment were significantly lower than those of the AC in the first and second years (p < 0.05) and were not significantly different in the third year, with reductions of 17–32%, while the same yield was maintained. The annual N2O emissions of the CRCU treatment tended to be lower than those of the AC for the three years, with reductions of 14–19%, although not statistically significant. This suggests that coated nitrate fertilizers were more effective in reducing N2O emissions, since nitrate produces this gas via denitrification only. In addition, the N2O emissions of the CRCU-R and CRCN-R treatments were 22–37% and 24–41% lower than that of the AC treatment, respectively. Although not statistically significant, these reductions were slightly greater than those obtained for the N2O emissions of CRCU and CRCN, suggesting the effect of further mitigation of N2O emissions through the reduction of application rate. Furthermore, the N2O emissions per yield tended to decrease due to the use of controlled release fertilizers for the three years, and further decrease due to the reduction of application rate. In conclusion, the use of controlled release fertilizers with reduced application rates can be regarded as an improved climate-smart soil management in soybean fields converted from rice paddies.