Fertilizer management practices and greenhouse gas emissions from rice systems: A quantitative review and analysis

Flooded rice systems emit both methane (CH4) and nitrous oxide (N2O). Elevated CH4 emissions in rice systems can lead to a high global warming potential (GWP) relative to other crops, thus strategies to reduce greenhouse (GHG) emissions, particularly CH4, are needed. Altering water, residue (carbon) and fertilizer management practices are commonly suggested as options for mitigating GHG emissions in rice systems. While the effects of water and residue management have been reported on elsewhere, the impact of fertilizer management on GHG emissions has not been reviewed quantitatively. We conducted an exhaustive search of peer-reviewed field studies that compared various side-by-side fertilizer management options. Where sufficient studies were available a meta-analysis was conducted to determine average treatment effects of management practices on both CH4 and N2O emissions. Results show that low inorganic fertilizer N rates (averaging 79 kg N ha−1) increased CH4 emissions by 18% relative to when no N fertilizer was applied, while high N rates (average of 249 kg N ha−1) decreased CH4 emissions by 15%. Replacing urea with ammonium sulfate at the same N rate significantly reduced CH4 emissions by 40%, but may increase N2O emissions. Overall, the fertilizer-induced emission factor for all inorganic N sources was 0.22%. Dicyandiamide (DCD), a nitrification inhibitor, led to lower emissions of both CH4 (−18%) and N2O (−29%). Limited field data suggest that deep placement of N fertilizer reduces CH4 emissions but increases N2O emissions. When compared to inorganic N fertilizers, farmyard manure (FYM) increased CH4 emissions by 26% and the green manure (GrM) Sesbania by 192%. Neither FYM nor GrM had a significant impact on N2O emissions when compared to an inorganic N treatment at the same N rate. Sulfate fertilizers reduced CH4 emissions by 28% and 53% at average rates of 208 and 992 kg S ha−1, respectively. These findings demonstrate that a variety of fertilizer management practices affect GHG emissions from rice systems. To develop effective GHG mitigation strategies future work is needed to (i) quantify the effects on GWP (accounting for both CH4 and N2O emissions), (ii) investigate options for combining mitigation practices (e.g. deep placement of ammonium sulfate), and (iii) determine the economic viability of these practices.

► Low N rates increased CH4 emissions while high rates decreased CH4 emissions.
► Ammonium sulfate reduced CH4 emissions by 39% compared to urea at the same N rate.
► Dicyandiamide led to lower emissions of both CH4 (−18%) and N2O (−29%).
► Farmyard manure increased CH4 emissions by 26% and green manure by 192%.
► Sulfate at rates of 208 and 992 kg S ha−1 reduced CH4 emissions by 28% and 53%, respectively. An equation is provided to determine the effect of sulfate rate CH4 emissions reductions.