Acid rain reduces methane emissions from rice paddies

This is potentially a beneficial side effect of the high pollution levels China – the world’s largest producer of rice – is often associated with. Methane is 21 times more powerful as a greenhouse gas than CO2.

“The reduction in pollution happens during a stage of the lifecycle when the rice plant is producing grain. This period is normally associated with around half of all methane emissions from rice and we found that simulated acid rain pollution reduced this emission by 24 per cent,” said Dr Gauci.

The project – funded by the Natural Environment Research Council – used rice soils and grain from Portuguese paddies. Soils from these paddies have been exposed to very little acid rain and are similar to Asian rice soils before they became polluted. To test the effects of acid rain, the researchers added frequent small doses of sulphate, which simulate acid rain experienced in polluted areas of China.

“We had similar results when exposing natural wetlands to simulated acid rain but this could be more important since natural wetlands are mostly located far from major pollution sources, whereas for rice agriculture, the methane source and the largest source of acid rain are both in the same region – Asia,” added Dr Gauci.

“We need to do further research but it looks like there could be a combination of processes at work. One line of investigation we’d like to confirm is that the sulfate component of acid rain may actually boost rice yields. This might, paradoxically, have the effect of reducing a source of food for the methane producing micro-organisms that live in the soil.”

This is because some sugars produced by rice plants are lost in the soil and micro-organisms feed on these sugars. But when the rice plant is producing grain, the carbohydrates are directed into grain production and away from soil so limiting the amount of food available for micro-organisms.

“There is also likely to be competition between these micro-organisms and sulphate-reducing bacteria. Normally in these conditions sulphate-reducers win which results in less methane.”

Dr Gauci added a note of caution to the results. “Acid rain is one of several pollution problems in Asia that need solving in the coming decades but we need to appreciate the potential consequences of that clean up, one of which could be an increase in methane emissions as the effect of the acid rain wears off.”