Nitrogen management to reduce yield-scaled global warming potential in rice

Fertilizer N is usually required to achieve optimal yields but when applied in excess there is increased risk of pollution, including higher greenhouse gas (GHG) emissions. Thus, optimal N management must consider both yields and environmental effects. Yield-scaled GWP (Global Warming Potential), which is the GWP (in CO2 equivalents) per Mg of grain yield, is a useful metric for evaluating management options where the goal is to achieve both high yields with minimal environmental burden. A 6-year field study was conducted to test the hypothesis that the lowest yield-scaled GHG emissions for rice occur when N is applied at optimal N rates for maximum yields, independent of the source of N applied. We tested this hypothesis for organic (manure) and inorganic (urea) N sources. The N rates and sources in each growing season were: 0, 90, 180 and 270 kg N ha−1 applied as either urea alone or pig manure combined with urea (where N was added as manure and supplied 60% of the total N rate). The N rates to achieve maximum yields (90 to 180 kg N ha−1 depending on year) were similar for both N sources. Seasonal CH4 and N2O emissions varied significantly between years but the magnitude of emissions was determined largely by N source. Across N rates, application of manure increased GWP by almost 60% relative to the urea treatments due to higher CH4 and N2O emissions. When urea was used as the sole N source, yield-scaled GWP (87 kg CO2eq. Mg−1 grain) was lowest at optimal N rates for maximum yields. In contrast, when manure was used, yield-scaled GWP was higher than for urea and increased with increasing manure-N rates (from 104 to 171 kg CO2eq. Mg−1 grain). The lowest yield-scaled GWP for manure was when no manure was applied - despite the low yields. Thus, when manure is used as an N source in flooded rice systems, over application should be avoided.