We determined the natural Cd balance in a cultivated rice paddy field in Japan. The main sources of Cd in the non-polluted paddy field were phosphorus fertilizer and precipitation (annual input of Cd, 2,000 mg ha−1 and 1,020 mg ha−1, respectively). These sources account for 95% of the total input of Cd (3,192 mg ha−1). The actual increase of Cd in the soil was 0.0016 mg kg−1, we thus consider Cd in soil increases only slightly as a result of rice culture. This study indicates that it is difficult to reduce Cd loading by irrigation water treatment in a non-polluted paddy field. This further indicates that once a field is polluted by Cd it is difficult to decrease the accumulated Cd by the ordinary cultivation of rice plants.

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