中海及び宍道湖の富栄養化と輸入食糧

Japan has been imported 27 million tons of foodstuff annually in 1980s, which contained 0.82 million tons of Nitrogen and 0.16 million tons of phosphorous.
Of the 2300 tons of annual total Nitrogen discharge to lake Nakanoumi and Shinji, domestic wastes and livestock wastes contributed 50%, 1155 tons. Of the 230 tons of the annual total phosphorous discharge to the two lakes, the domestic wastes and livestock waste contributied 68%, 156 tons. In 1980s, most of the nitrogen and phosphorous in the wastes were originated from the imported foodstuffs, while the contribution from imported foodstuffs were less than 30% in 1960s.
Although the contribution from surrounding natural environmental components including agricultural land, forests land and rain fall are reached to 42% of total discharge in nitrogen and 25% in phosphrous discharge, these contribution can be considered as an essential flows to supply nutrients in order to maintain the ecosystems of the two lakes
Suface soils of crop and foest land have great function to control nitrogen and phosphorous in the two lake ecosystems. Thus, the key point to improve the quality of the two lake water depend on the efficient use of the soils ability to control nitrogen and phosphorous.