Effect of Varying Dissolved Oxygen Concentration on Elution of Phosphate from Lotus Paddy Field Soil in Paddy with Abundant Green Algae

Ibaraki Prefecture produces the largest volume of lotus root in Japan. In a previous study, we reported that the runoff load of total phosphorus (TP) in the Teno area, Tsuchiura city, Ibaraki Prefecture, became maximum in summer. Therefore, in this study, we investigated lotus paddy field with abundant green algae in the Teno area, focusing on the concentrations of dissolved oxygen (DO), TP, and phosphate (PO4-P). As a result, DO in the green-algae-abundant lotus paddy field became supersaturated with increasing amount of green algae. On the other hand, DO in the greenalgae-abundant lotus paddy field showed a concentration decrease to less than 4 mg L with increasing amount of lotus leaves in summer. Furthermore, the concentration of TP with DO of less than 4 mg L was about twice that after the lotus was harvested. The green-algae-abundant lotus paddy field had a significantly higher percentage of PO4-P in TP than the Sakai River agricultural water that was used. These results indicated that the green-algae-abundant lotus paddy field exhibited a reduced DO concentration and increased elution of phosphate from the paddy soil in summer. As a consequence, the concentration of TP in the green-algae-abundant lotus paddy field was increased in summer.