Phosphorus (P) is a limiting factor of eutrophication in many lakes. Paddy-field districts are major non-point sources of P; water pollution prevention measures are to be implemented in these districts. However, sediment in drainage canals has a large P buffering capacity and can release P to the water column when input of P from paddy fields is reduced. Therefore, it is necessary to evaluate the amounts of P in the sediment in drainage canals. In this study, the sediments in drainage canals in paddy-field districts around Lake Biwa were investigated. The amounts and fractions of P in the sediments during both the irrigation period and the non-irrigation period were analyzed. Results showed that the mean value of total P in the sediment was 0.6 g/kg in both periods, but total P in non-irrigation periods tends to be greater than that during irrigation periods for each site in the study. Furthermore, over 70% of total P was inorganic on the average. Moreover, the fractions of organic and inorganic P exhibited seasonal variability. The potential contribution of water-extractable P in the sediment to total P in drainage water increased from June to November.

Japanese paddy rice systems commonly adopt the rotation of vegetables, wheat and soybeans with paddy rice. Crop rotation may, however, increase the nutrient load in effluent discharged from the district because more fertilizer is applied to the rotation crops than is applied to paddy crops. We investigated a paddy-field district subject to collective crop rotation and quantified the annual nutrient load of effluent from the district in three consecutive years. The total annual exports of nitrogen and phosphorus over the investigation period ranged from 30.3 to 40.6 kg N ha(-1) and 2.62 to 3.13 kg P ha(-1). The results suggest that rotation cropping increases the effluent nutrient load because applied fertilizer is converted to nitrate, and surface runoff is increased due to the absence of shuttering boards at the field outlets.

Many water quality conservation measures have been implemented in agricultural areas to reduce pollutant loading. However, controlling discharged drainage water enhances sedimentation of suspended solids in drainage canals in those areas. The sediment, which has high concentrations of nitrogen (N) and carbon (C), may release N and C and lower the effect of the conservation measures. In this study, we clarified the characteristics of N and C in the sediment in drainage canals. We have investigated paddy field districts around Lake Biwa and measured N and C contents in the sediment. We also conducted incubation experiment of the sediments sampled in two different paddy field districts. Changes in N and C fractions in the sediments and in the overlying water during the incubation period were analyzed. As a result, concentrations of total N and total organic C in the overlying water of the incubated sediment were respectively about two and three times those of drainage water measured at the study points. Total N flux was 33.0 -41.1 mgN/m 2 /day and total organic C flux was 25.3 -48.4 mgC/m 2 /day. It was indicated that the sediments had enough N and C for determining the water quality of drainage water.

Many water quality conservation measures have been implemented in agricultural areas to reduce pollutant loading. We investigated agricultural drainage canals in three paddy-field districts around Lake Biwa. In this paper, we focus on nitrogen (N) and organic carbon (C) released from the sediments in the drainage canals. We collected sediment cores and conducted an incubation experiment. We measured temporal change in concentrations of N and organic C in the overlying water of the sediments. As a result, we found that the nitrogen is released mainly as inorganic nitrogen (ammonium N). Ammonium N flux for each sediment sampled from St. 1, St. 2 and St. 3 was 24, 13, and 41 mg N/m 2 /d, respectively. Nitrogen was released from the sediments mainly in an inorganic state. The C/N ratio of the sediments was about 10 and almost constant during the incubation period. It is suggested that the fraction of organic decomposed in the sediments was small during the incubation experiment.

X phosphorus total phosphorus phosphate phosphorus cyclic irrigation lake-water irrigation non-irrigation phosphorus concentration, mg • L-1 volumetric flow rate, mm • d-1 phosphorus concentration of influent water, mg • L-1 phosphorus concentration of effluent water, mg • L-1 volumetric flow rate of influent water, mm • d-1 volumetric flow rate of effluent water, mm • d-1 net phosphorus export, kg • ha-1 • d-1 cyclic irrigation ratio surplus irrigation water ratio concentration ratio concentration ratio of total phosphorus neutral effect curve for the net phosphorus export 2 ABSTRACT Phosphorus (P) effluent load discharged from paddy fields has a substantial impact on water quality in downstream areas. We evaluated the effectiveness of cyclic irrigation, in which drainage water is reused for irrigation purposes, in reducing the P load from a paddy-field district neighboring Lake Biwa, Japan. We measured temporal variations in the P concentration in drainage water and estimated the P mass balance for the study district for three consecutive years (2007-2009). A simple model was developed to characterize the hydrological structure and effects of cyclic irrigation using three parameters: the ratio of the P concentration in drainage water to that in irrigation water, the degree to which drainage water is reused, and the ratio of surplus irrigation water to the total amount of irrigation water. The total annual exports ofP were 2.62-3.13 kg•ha-1. Phosphorus exports during the cyclic irrigation periods (April-June) were 0.27-0.63 kg•ha-1 (period average rate= 3.7-8.8 g•ha-1 •d-1), lower than those during the lake water irrigation periods (July-August) of0.82-1.66 kg•ha-1 (8.9-18.0 g•ha-1 •d-1). Phosphorus export was lowest during the cyclic irrigation period, consistent with the small amounts of discharged drainage water. Our results suggest that cyclic irrigation is an effective district-scale method for reducing effluent P loads. Reuse of drainage water and efficient use of irrigation water are important in reducing the P load from the district.