HONG KONG -- Agriculture is known as a major contributor to greenhouse gas emissions, but a new study finds that organic farming can reverse the agriculture ecosystem from a carbon source to a carbon sink.

In a paper published in Science Bulletin, a group of scientists estimated that more than 1 billion tons of excess carbon dioxide can be stored in China's farmlands annually through regenerative organic farming, ranching and land use. Meanwhile, crop yields can also increase as the soil fertility is improved by the use of organic manure.

"To mitigate greenhouse gas emissions and retain soil fertility, organic agriculture might be a wise choice for decreasing the intensive use of synthetic fertilizers, protecting environments, and further improving crop yields," the scientists said.

They demonstrated how to do so by integrating organic farming with cattle breeding in a rural area of eastern China's Shandong province. The majority of nutrient inputs in farmlands there traditionally came from chemical fertilizer. During the experimental run, the scientists fed cattle with crop residues, collected and composted cattle manure, and used it to replace chemical fertilizer for crop production.

The study's finding shows that although cattle breeding causes higher emissions of methane and other type of greenhouse gases, the new practice still sequesters more carbon, thanks to crop residue recycling and chemical reduction.

Putting that into numbers, the study notes, farmlands using cattle manure absorb greenhouse gas emissions equivalent to 8.8 tons of carbon dioxide per hectare every year. By contrast, the farmland using chemical fertilizer releases greenhouse gas emissions equivalent to 2.7 tons of carbon dioxide.

Besides that, turning crop residues into animal feed helps make full use of agricultural waste. According to the scientists' estimation, China produces about 630 million tons of crop residues annually, with more than two-thirds of them being abandoned or burned -- causing air pollution and driving up greenhouse gas emissions.

Unlike ranchers in Brazil who have cleared forests to build facilities for cattle, most farmers in eastern China enclose part of their existing agricultural land as cattle pasture. As a result, emissions from land-use change are barely a concern there.

A 2011 report from the Food and Agriculture Organization of the United Nations says that agriculture causes about one-third of global greenhouse gas emissions when direct energy use; emissions from livestock; the production of fertilizers, pesticides, machinery and equipment; as well as soil degradation and land-use change for feed production are taken into account. An analysis by the Washington, D.C.-based think tank World Resources Institute shows that in 2011, China generated more agriculture-related emissions than any other nation.

Jiang Gaoming, one of the study's authors and a professor at the Chinese Academy of Sciences' Institute of Botany, said that using organic manure can significantly reduce greenhouse gas emissions from crop production. If the country completely replaces chemical fertilizer with organic manure, "the [mitigation] potential should be 1.38 billion tons of carbon dioxide for the whole China's farmland per year," Jiang said.

However, there are barriers to making such a switch. For one, according to Jiang, farms in China are reluctant to use organic fertilizer as this requires more labor, and labor costs have increased greatly in recent years. In addition, the country may not be able to find enough organic fertilizer to use if all the farmlands are replaced with organic fertilizer, Jiang said.

Jiang and his team suggested that the Chinese government could encourage farmers to use half the amount of chemical fertilizer while the rest is replaced with organic ones. "A combination of organic manure and chemical fertilizer demonstrated the best result in improving soil quality and crop yields, while decreasing greenhouse gas emissions," the scientists said.