In order to reduce environmental burdens from grassland, nitrification inhibitors were added to cow's urine before application onto grassland. Levels of nitrous oxide generation, yields of pasture grass, levels of nitrogen absorption, levels of nitrogen in soil solution and levels of inorganic nitrogen in soil were determined A highly humic thick-layer andosol field was sowed with a mixture of Italian ryegrass, orchard grass, tall fescue and red clover, and divided into the following zones : chemical fertilizer, nitrogenfree, cow's urine and cow's urine+Dicyandiamide (Dd). 1. Among three nitrification inhibitors added to solutions of ammonium sulfate to investigate their effects on nitrification inhibition, Dd exhibited the greatest effect. 2. Nitrous oxide generation, nitrogen absorption by pasture grass, yield of pasture grass and levels of nitrogen as NO_3 in underground water and soil were determined. The nitrogen application level was 50 kg ha^<-1> at one time, except for the cow's urine+Dd Zone, which had a nitrogen application level of 55 kg ha^<-1>because of the replacement by Dd for 10% of nitrogen in cow's urine. 3. Nitrous oxide generation was inhibited by 66% and 40%, respectively, in 1998 and 1999 in the cow's urine+Dd zone, compared to that in the cow's urine Zone. Ratios of nitrous oxide generation to nitrogen application were O.22% in the cow's urine+Dd zone and O.36% in the cow's urine zone in 1998 ; and O.23% in the former and 0.08% in the latter in 1999. 4. The nitrogen absorption level and dry matter yield in the cow's urine+Dd zone were higher than those in the cow's urine zone. The increments were 24% for both, which were even greater than those of nitrogen supply through Dd. 5. The determination of nitrogen levels in underground water and soil revealed no downward transfer of nitrogen.
