Excess Carbon Dioxide Affects Rice Yields and Amount of Methane

The new study focuses on how rising temperature and extra carbon dioxide in the atmosphere affect rice yields and the amount of methane that is being released by rice paddies.

Methane in rice paddies is produced by microscopic organisms that respire CO2 similar to how humans respire oxygen. Greater the amount of Co2 in the atmosphere, faster the rice plants grow. The extra plant growth supplies soil microorganisms with extra energy, pumping up their metabolism.

Greater levels of CO2 boost the rice yields, but to a smaller extent then CH4 emissions. This leads to increase in the CH4 emitted per kilogram of rice yield.

"Our results show that rice agriculture becomes less climate friendly as our atmosphere continues to change. This is important, because rice paddies are one of the largest human sources of methane, and rice is the world's second-most produced staple crop," said Dr Kees Jan van Groenigen, Research Fellow at the Botany Department at the School of Natural Sciences, Trinity College Dublin, and lead author of the study.

This study was conducted by Van Groenigen, along with colleagues from Northern Arizona University and the University of California in Davis.

For this study they analyzed the published research to date from 63 different experiments on rice paddies that were mainly from Asia and North America. 

The researchers worked using a technique known as meta-analysis, a statistical tool for finding general patterns in a large body of experimental data.

 "Two strong patterns emerged when we analysed all the data: first, more CO₂ boosted emissions of methane from rice paddies, and second, higher temperatures caused a decline in rice yields," explained Professor Bruce Hungate of Northern Arizona University and co-author of the study.

Apart from this, rising temperature were found to have a minor effect on CH4 emission.  But because they decrease rice yield, they also increase the amount of CH₄ emitted per kilogram of rice.

 "Together, higher CO₂ concentrations and warmer temperatures predicted for the end of this century will about double the amount of CH₄ emitted per kilogram of rice produced." explained Professor Chris van Kessel of the University of California in Davis and co-author of the study.

"Because global demand for rice will increase further with a growing world population, our results suggest that without additional measures, the total CH₄ emissions from rice agriculture will strongly increase."

But the authors suggest that there are several options available to reduce CH₄ emissions from rice agriculture.

 "These findings, together with our own results really stress the need for mitigation and adaptation measures to secure global food supply while at the same time keeping greenhouse gas emissions in check." van Groenigen concluded.

This study was published in the online edition of Nature Climate Change.