Previous work had shown that rice paddies emit more methane during the wet season than they do in the dry season. But the cause of the decline was unclear. Because the wet season also brings lower crop yields, Nico van Breemen of Wageningen University in the Netherlands and his colleagues posited that methane production is tied to a crop's success. To test that hypothesis, the team experimentally manipulated rice harvests by varying the amount of fertilizer and by removing some of the plants' flowers. They found that as more flowers were removed (and the yield decreased), methane production increased. According to their report, published online this week by the Proceedings of the National Academy of Sciences, plants with fewer flowers exhibit a diminished ability to store carbon produced through photosynthesis. In such cases, the carbon is instead stored in the soil, where bacteria convert it into methane.
Rice production currently accounts for approximately 13 percent of global methane emissions. The scientists suggest that developing varieties of rice capable of storing more carbon, perhaps in additional shoots, could help mitigate methane production. Ronald L. Sass of Rice University and Ralph J. Cicerone of the University of California, Irvine, note in an accompanying commentary that population increases and recent climate change make such efforts necessary. "The food demands of an increasing world population and the disruptive effect of global warming," they write, "both challenge the agricultural science community to pay attention to how these two environmental pressures interact and to accelerate efforts to develop higher yielding, farmer-friendly rice that emits less methane."