Biogeochemical change of the nitrogen in the turbid water compulsorily discharged after ploughing and irrigatin the field was clarified by the incubation experiment. The sample was collected from paddy field in Lake Biwa basin. 95% of total nitrogen in the turbid water was the organic nitrogen. The ammonium nitrogen increased in the culture with the nitrification inhibitor, while nitrate nitrogen inceased in the culture with inhibitor free. About 30% of total nitrogen was decomposed, and was nitrified to nitrate nitrogen within 1 month. About 70% of Total nitrogen was undecomposed. It was considered that this fractionation was a recalcitrant form like the rice straw, and were accumulated in the bottom of Lake Biwa for long term.