Characteristics and behavior of nutrients in a paddy field area equipped with a recycling irrigation system

We explored the characteristics and behavior of nutrients in a paddy field area that had a recycling irrigation system and evaluated the paddy field’s effect on the removal of nutrients during a storm and during the irrigation period from April to September 2002. Because the irrigation water was partly reused, the mean concentrations of total nitrogen (2.31 mg l −1) and total phosphorus (0.14 mg l −1), as well as the total chemical oxygen demand (7.58 mg l −1), were higher in the irrigation water than specified by the Water Quality Standards for Rice enacted by the Ministry of Agriculture, Forestry, and Fisheries, Japan. The hydraulic retention time ranged from 5 to 9 days, except in April, when it ranged from 4 to 5 days. The shorter retention time occurred when large amounts of irrigation water were used to puddle the paddy soil. The net outflow loads ( outflow load − inflow load ) of total nitrogen, total phosphorus and the total chemical oxygen demand during the irrigation period, were −3.98, −0.37, and +0.50 kg ha −1, respectively. During a storm, the runoff loads of nutrients were directly proportional to the water’s flow rate and increased to a lesser extent than in urbanized and other agricultural land-use areas. These results indicate that the paddy field equipped with a recycling irrigation system had the ability to function in removing nitrogen and phosphorus nutrients, because the paddy field had relatively high concentrations of N and P in the irrigation water as well as adequate retention time, both of which facilitated purification.