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Abstract
Fallow flooding at converted paddy fields on the middle stream of the Shirakawa river basin, Kumamoto Japan is encouraged by the local government since the practice is effective for artificial groundwater recharge. In this study, we investigated the effects of fallow flooding on the water quality of percolating water, soil fertility, and succeeding carrot (Daucus carota L.) production. The flooding was conducted at a converted paddy field in 2007. Nitrate concentration of percolating water surged immediately after the start of flooding, due to the leaching of residual nitrate in the soil layer; then dropped to less than the concentration of irrigation water, supposedly due to denitrification thereafter. Denitrification rate was calculated as 0.22 g-NO3 −-N m−2d−1; such a rate may alleviate the nitrate contamination caused at the very beginning of flooding. Regardless of severe leaching conditions, soil nutrient contents were maintained or rather increased in the drained soil. Carrot yield 158 days after sowing in the test field was 67 Mg ha−1, far exceeding the yield of 41 Mg ha−1 in the untreated adjacent field. Flood fallowing, known as an efficient practice for suppressing pathogenic nematodes and/or continuous cropping injury, appears to be a favorable practice for improvement of soil fertility.