Abstract
Using administrative data and a pollutant load factor method, we estimated the amount of nitrogen load generated in the basins of the Chikuma and the Sai in Nagano during the past 30 years. Total amount of nitrogen load generated in the basins was 22,000 - 25,000 ton N yr-1, in which the waste from livestock was greatest, followed by residential wastes and nitrogen leached out from farmland. Nitrogen derived from livestock and farmland gradually decreased due to the decline of those activities. The amount of chemical fertilizer consumption drastically decreased from 1980, and fell short of agricultural nitrogen demand calculated by using standard application rates for crop plants. It was assumed that the nitrogen shortage was compensated by livestock waste. Due to the reduction in chemical fertilizer use coupled with recycling of the livestock waste and the construction of sewage system, the nitrogen load entered into the rivers has decreased by 40% in the middle of 1990s. In order to make further reduction of nitrogen load into the rivers, it might be effective to increase the treatment efficiency of sewage treatment plants and to reduce the nitrogen leached out from agricultural area.
