Sulfate-containing amendments to reduce methane emissions from rice fields: mechanisms, effectiveness and costs

Application of sulfate-containing amendments is oftensuggested as a mitigation option to reduce methane (CH4) emissionsfrom rice (Oryza) fields. This paper discusses the mechanism andpotential of this mitigation option, reviews the relevant experimental data,and presents first, indicative costs of application. CH4 emission datafor rice fields with sulfate-containing amendments are compiled toreinterpret the resulting reduction in CH4 emission and find a generalrelationship between emission reduction and amount of sulfate applied. Thereduction in CH4 emission depends on the amount of sulfate applied.However, absolute emission reduction is location specific and cannot bederived from the amount of sulfate (SO2-4) applied only. We established alogarithmic relationship, across locations, between SO2-4 application andfractional emission reduction relative to the emission of the non-amendedcontrol field. Recycling of SO2-4 in the rhizosphere was essential to explainthe observed reductions in CH4 emission for a number of theexperiments. The cost of applying SO2-_4-containing fertilizers varies acrosscountries and depends on local fertilizer prices. Since a fractional reductionis obtained, the cost-efficiency in terms of CH4 mitigation per unitof SO2-4 applied will be highest in high-emitting rice production systems.Provided the proper target areas are selected, the cost of SO2-4-containingfertilizer as a mitigation option to reduce CH4 emissions in rice fieldsis estimated at 5–10 US dollar per Mg CO2-equivalent.