栄養塩濃度の高い用水を反復利用する水田群での窒素•リンの動態

摘要

窒素·リン濃度の高い(T-Nで約2～5mg·L~(-1)，T-Pで約0·2～0.7mg·L~(-1)用水を反復利用している滋賀県守山市のA地区（0.84ha)とB地区（l.0ha)の水田群を対象に，窒素，リン等の動態把握調査を行った。その結果，①2地区の水田群でのT-Nの差引排出負荷量は-5611g·ha~(-1)d~(-1)であり，A地区でT-Nは除去されていた。②リンと有機物については，2地区とも水田群への流入量よりも流出量が多くなり，水田は供給源になっていた。2地区の水田群からのT-Pの差引排出負荷量は90～104g·ha~(-1)d~(-1)であり.既往の文献値に比較して高かった。③農業排水を反復利用することは，窒素漉度が高い場合（約2mg·L~(-1))は，下流への排出負荷量削減に結びつくが，リンゃ有機物については，排出負荷量削減効果を期待することは難しいと考えられた。%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.