Quantitative evaluation of environmental risk associated with nitrogen flow in agricultural production and mitigation plan for 2 typical prefectures in Japan

Nitrogen (N) flow and environmental risk in Japanese agricultural production were estimated for each prefecture in 1997 using a modified nutrient balance calculation system. N flows associated with agricultural production varied widely among the prefectures. The amount of excess N that remained on farmland (residual N), which is a source of environmental risk, also varied widely and was strongly affected by the level of inputs of chemical fertilizer and manure. The prefectures were grouped into 3 categories in relation to environmental risk. 1) Prefectures which had a lower environmental risk than the Japanese average. 2) Prefectures which had a higher environmental risk than the average due to high application of chemical fertilizer, and 3) due to the high production of livestock excreta. Two typical prefectures were selected to implement mitigation plans. Prefecture A belonged to category 2) and had the highest chemical fertilizer input in Japan. Therefore, it was considered that by decreasing the input of chemical fertilizer could be effectively reduced environmental risk. The mitigation plan was drafted to reduce the environmental risk to 69% of the present level. Prefecture B belonged to category 3), and was characterized by a high livestock excreta production of which 47% was not used as manure in the prefecture. By substitution of chemical fertilizer with manure for forage crops and more effective use of manure in paddy fields, it was estimated that the environmental risk in Prefecture B could be reduced to 77% of the present level.