Recently nitrate outflow from the agricultural area became one of big problems in Japan. We continue the long term experiment on nitrate removal in the flooded paddy field from 1991. Experimental plots consist of four plots ; weed plant, rice, non-plant and covered plot. In the non-plant plot any plant does not grow, but some of algae grow. In the covered plot, no algae grows due to the shield of the light. Natural water with high nitrate concentration about 20 mg L_1 are supplied to each plot continuously. Nitrate is removed through the flow process in the plots by denitrification and the uptake by the plant. Average values of nitrate removal rate for 8years are 0.30gm 2d_1 in a weed plot and 0.27gm 2d-1 in a rice and a non-plant plots. Nitrate removal rates gradually decreased in the first few years, but now they keep the ability of nitrate removal after 9 years. In order to clarify the influence of plant growth and algae on nitrate removal these data are examined by the nitrate removal equation as shown in eq. (2), which indicates the relation between the nitrate removal by denitrification (Ro) and nitrate concentration of water (X) and temperature (T).
