Abstract
Nutrient and carbon contents of the sediment in a drainage canal may increase under cyclic irrigation although the effluent loadings from a paddy-field district are reduced by the reuse of drainage water. In this study, a paddy-field district adjacent to Lake Biwa implementing cyclic irrigation was investigated and the nitrogen, phosphorus and carbon contents of the sediment in the drainage canal are determined. The results show no clear relationship between irrigation period and the temporal variation in nitrogen, phosphorus or carbon content of the sediment. The amounts of nitrogen, phosphorus and carbon in the upper layer of the sediment are larger than those in the lower layer. The mean nitrogen, phosphorus and carbon contents of the sediment were respectively 1.96 g·kg-1, 2.44 g·kg-1, and 20.0 g·kg-1 in the upper layer and 1.36 g·kg-1, 0.75 g·kg-1, and 12.1 g·kg-1 in the lower layer. In particular, the phosphorus content of the sediment in the drainage canal was higher than mean phosphorus contents of the sediments in rivers and lakes. Nitrogen in the sediment existed mainly in the organic state. In contrast, phosphorus was mainly in the inorganic state.
