Peer review

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. We used the FAO soil map of the world to produce a high-resolution rice location map. Using published GIS-linked climate-based and yield-based empirical models, we calculated the net primary production (NPP) of rice fields in China. Values varied between 136·1012 g C using climate drivers from digital maps to 222·1012 g C using published grain production figures for 1988. We assumed that either 5% of NPP or 30% of the organic matter added to the soil during rice cultivation was transformed into methane, adding up to a total emission of 7 to 16·1012 g C. We also gathered published data on fertilizer inputs and management practices and, using linear regression techniques, calculated the correlation between methane emission and carbon inputs to obtain a total emission value of 10·1012 g C. Using the results for NPP (135–222·1012 g C), methane emission (7–16·1012 g C) and published grain production figures, we balanced the carbon budget of rice paddies estimating soil respiration at 51·1012 g C for all Chinese rice fields or 159 g C m−2 y−1 for an average Chinese rice field, a number which agrees with published values for similar systems. This result confirmed our assumption that rice soils in China, where rice cultivation has occurred for several thousand years, were neither losing nor 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, we 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.

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