Rice paddies are a source of food for over half of the world population and also the source of a very potent greenhouse gas, methane. Using Geographic Information Systems (GIS)-linked climate-based and yield-based empirical models, the authors calculated the net primary production (NPP) of rice fields in China. The result confirmed their assumption that rice soils in China, where rice cultivation has occurred for several thousand years, were neither losing or accruing carbon. However, any changes in the hydrology of these soils may transform them into significant carbon sources. Using the Food and Agricultural Organization (FAO) digital map of the soils of the world, they estimated soil carbon content for the rice-growing regions of China and quantified the potential carbon losses that would occur if these soils were drained.