The purpose of this study was to clarify the nitrogen (N) purification capacity of a paddy field in a recycling irrigation system. Irrigation water was sampled at 12-h intervals during the irrigation period from April to September 2003. In addition, ponded water in a paddy field was collected at three points (inlet, centre and outlet). Total amounts of N were 30.7 kg ha−1 in inflow and 27.8 kg ha−1 in outflow. Thus, the net outflow load was −2.9 kg ha−1. The N removal rate constant when N removal is expressed as a 1st-order kinetic was 0.017–0.024 m d−1. This value is close to values of wetlands and paddy fields in the literature. We found a good correlation between recycling ratio and N removal effect. These results indicate that the recycling irrigation system accumulates N in the irrigation/drainage system, and thus the paddy field does a good job of water purification by removing N.

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