Impacts of Nitrogen Fertilizer Substitution on Greenhouse Gas Emission in a Paddy Field of South China Under Ridge Irrigation

Ridge irrigation (RI) is a water-saving irrigation mode for paddy fields, and the effects of part of urea substituted by organic manure and controlled-release urea on greenhouse gas emission under RI mode are not clear and need to be investigated. A field experiment with four nitrogen treatments (F0: only urea; F1: Chinese milk vetch replacing 30% urea; F2: pig manure replacing 30% urea; and F3: controlled-release urea replacing 30% urea) and two irrigation modes (conventional irrigation CI and RI) in south China was conducted to determine rice yield, methane (CH4), and nitrous oxide (N2O) emissions, global warming potential (GWP), and greenhouse gas emission intensity (GHGI). Compared to F0, three urea substitution treatments increased the yield by 14.6–23.2% and reduced N2O emission by 26.4–57.9%; F1 decreased CH4 emission by 22.3–30.8%; and F2 increased CH4 emission by 59.9–120.9%. Compared to F0, F1 decreased GWP and GHGI by 23.1–31.3 and 39.0–41.2% while F2 increased them by 53.9–111.6 and 55.7–57.2%, respectively. Compared with CI mode, RI mode decreased GWP and GHGI by 7.8–34.0 and 16.7–28.4%, respectively. Chinese milk vetch replacing 30% urea was a suitable urea substitution method under ridge irrigation, which can increase rice yield and decrease GWP and GHGI.