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.

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.

Nutrient effluent load from paddy fields has a large impact on water quality in the downstream areas. The sediment in drainage canals can act as nutrient source by releasing nutrients to the overlying drainage water. In this study, we evaluated the effect of bioturbation caused by tubificid worms, which live commonly in paddy fields, on nutrient release from the sediment in drainage canals. We investigated a low-lying paddy field district adjacent to Lake Biwa. We also sampled drainage water and the sediment in the study site and conducted incubation experiment of the sediment for four weeks. In the sediment incubation experiment, we added different densities of tubificid worms to the sediment. The effect of tubificid worms on nitrogen release was much larger than that on phosphorus. Nitrogen was mainly released as ammonium nitrogen by the excretion of tubificid worms.

Paddy-field areas are among the major non-point sources of phosphorus (P). Drainage canals can influence P concentration in drainage water because the sediments can absorb P and have large P buffering capacity. Assessment of the P buffering capacity and management of drainage canal sediments are important to reduce P effluent loadings from paddy-field areas. In this study, we investigated three paddy-field districts around Lake Biwa. Using adsorption and incubation experiments, we clarified the P adsorption and release characteristics of the sediments. The concentrations of total sedimentary P were highest in the surface layer of the sediments. The zero-equilibrium P concentrations of the sediments, estimated from the P adsorption experiment, varied widely among sites, and were similar to the peak concentrations of P released from the incubated sediments and the mean P concentrations in the drainage water (0.021 to 0.068 mgP/L). These results suggest that the sediments determine the concentrations of P in the overlying drainage water. Adsorption isotherms for dry sediments were described by a linear model with higher slope than wet sediments, indicating that drying drainage canals might enhance the P buffering capacity of sediments.

In paddy fields, the drainage canal sediments can act as a pollutant source without proper management. The objective of this study is to evaluate the role of the sediments on the nitrogen (N) and organic carbon (C) concentrations in drainage water in paddy fields. Drainage canal sediments were sampled at three paddy-field districts around Lake Biwa and incubated for 28 days. Results of the experiments confirmed that the drainage canal sediments had enough potential as N and C sources. The sedimentary total N and C concentrations were 1.95 -3.38 and 21.1 -32.8 mg/kg, respectively. Nitrogen in the sediments existed mainly in the organic state. During the incubation experiment, large amounts of NH 4 -N were released from the sediments. Fluorescence peaks of humic-like substance had strong correlation with dissolved organic N and C in the overlying water on the incubated sediments. It can be concluded that large amounts of organic N in the sediments can be a long-term NH 4 -N source for the overlying drainage water through decomposition and release.