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

Sulfate in acid rain is known to suppress methane (CH₄) emissions from natural freshwater wetlands. Here we examine the possibility that CH₄ 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^2-₄ deposition may help to reduce CH₄ emissions from rice agriculture. Emissions from rice plants treated with simulated acid rain at levels of SO^2-₄ 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 CH₄ that is normally emitted in a rice season. A single application of SO^2-₄ 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^2-₄ 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 ^2-₄ limitation had been lifted by the simulated acid rain S deposition.