1992 Volume 63 Issue 6 Pages 639-645
We studied ogranic material flow in each of upper (Ooho District and Toyosato District in Tsukuba City), middle (Yatabe District in Tsukuba City) and lower (Ushiku City, Kukizaki Town and Ina Town) reaches of the Ushiku lake basin, Ibaraki, Japan. The rate of human wastes that loaded the environment were highest in the lower reaches which were highly urbanized. Most of animal wastes were recycled to the agricultural land as manure in the middle and lower reaches. Only half of the animal wastes were recycled in the upper reach, where less animal wastes were recycled because a large number of pigs were bred there. About 90% of the crop residual, most of which was rice straw, was recycled to the agricultural land in the middle and lower reaches. Less amount (75%) was recycled in the upper reach, because farmers raised large areas of turf and burn mown lawns. We estimated the amount of nitrogen accumulated in soil organic matter and that of nitrogen mineralized in soil of the agricultural land in each reach of the basin. Amount of accumulated and of mineralized nitrogen were respectively about 1200 kg ha^<-1> in the middle reach, and were about 900 kg ha^<-1> and 50 kg ha^<-1> in the lower reach. In the upper reach, amount of accumulated and of mineralized nitrogen were about 1100 kg ha^<-1> and 80 kg ha^<-1> at 1980, and decreased to about 800 kg ha^<-1> and 50 kg ha^<-1> at 1985. We estimated reasonable and maximum rates of organic nitrogen application on the basis of nitrogen budget in the agricultural land of each reach. The reasonable rates under the actual nitrogen application (70-120 kg ha^<-1>) were 30-70 kg ha^<-1>. The maximum rates which assumed the complete replacement of the inorganic nitrogen by the organic source were 140-160 kg ha^<-1>. The amount of organic nitrogen generated in each reach exceeded the reasonable rates but was within the maximum rates. This fact shows that the organic nitrogen generated in the area can be used as an alternative to fertilizer nitrogen.
