Abstract
Mass balances of nitrogen, phosphorus and organic carbon during irrigation period in a rice paddy in Shiga Prefecture, Japan, were determined in detail for three years. Size of the rice paddy is 100m×30m. Irrigation water is supplied from Lake Biwa through a pipeline irrigation system. Observation results are as follows :
1) Nitrogen and phosphorus outflows from a rice paddy were considerably influenced by the amount of precipitation.
2) Nitrogen and phosphorus outflows in May were the largest during irrigation period, i.e., those outflows in May occupied 45-55% of the total outflows during irrigation period. Net outflows of nitrogen and phosphorus in May occasionally exceeded the total net outflows during irrigation period because net outflows in some other months had negative values. Therefore, prevention of excessive nutrient outflows in May is important for achieving significant reduction in nutrient loading from rice paddies.
3) Saving the amount of intake water is important, especially in May, for reducing nutrient loading from rice paddies. Controlling the amount of applied fertilizer corresponding to the growth of rice is also important.
