Movements of Nitrogen and Phosphorus in Paddy Fields Utilizing Irrigation Water with High Nutrient Concentrations

Abstract

We investigated the inflow and outflow of nitrogen and phosphorus in two paddy field areas (0.84 ha and 1.0 ha) where irrigation water has relatively high concentrations of nitrogen (app. 2-5 mg•L^-1) and phosphorus (app. 0.2-0.7 mg•L^-1) in Moriyama City in Shiga Prefecture, Japan. The results are as follows: 1) The net loads of nitrogen of the two paddies were -56 and 11 g•ha^-1•d^-1. One paddy showed nitrogen removal during the irrigation period. 2) the net loads of phosphorus were 90 and 104 g•ha^-1•d^-1. These values were higher than those reported by other researchers in other researches performed in Shiga Prefecture. The high load of phosphorus may be due to the accumulation of bioavailable phosphorus in paddy soil. 3) The net loads of dissolved organic carbon were 213 and 326 g•ha^-1•d^-1. These values were the same as previously reported values. 4) From the results of this study, we found that the reuse of wastewater with high nutrient concentrations is effective for the removal of nitrogen, but not for the removal of phosphorus or organic carbon. The removal of phosphorus may be suppressed by the accumulation of phosphorus in paddy soil. The removal of organic carbon is difficult because of the production of organic carbon by phytoplankton in surface water of paddies.