Suppression of rice methane emission by sulfate deposition in simulated acid rain

Abstract

Sulfate in acid rain is known to suppress methane (CH 4 ) emissions from natural freshwater wetlands. Here we examine the possibility that CH 4 emissions from rice agriculture may be similarly affected by acid rain, a major and increasing pollution problem in Asia. Our findings suggest that acid rain rates of SO 4 2− deposition may help to reduce CH 4 emissions from rice agriculture. Emissions from rice plants treated with simulated acid rain at levels of SO 4 2− consistent with the range of deposition in Asia were reduced by 24% during the grain filling and ripening stage of the rice season which accounts for 50% of the overall CH 4 that is normally emitted in a rice season. A single application of SO 4 2− at a comparable level reduced CH 4 emission by 43%. We hypothesize that the reduction in CH 4 emission may be due to a combination of effects. The first mechanism is that the low rates of SO 4 2− may be sufficient to boost yields of rice and, in so doing, may cause a reduction in root exudates to the rhizosphere, a key substrate source for methanogenesis. Decreasing a major substrate source for methanogens is also likely to intensify competition with sulfate‐reducing microorganisms for whom prior SO 4 2− limitation had been lifted by the simulated acid rain S deposition.