Abstract: This paper studies the effect of microbial population and soil enzyme on nitrogen transformation in forest soil under different forest types. The rates of nitrogen fixation, denitrification, ammonification and nitrification are quite different under different forest types: broadleaved forest>mixed broadleaved-coniferous forest>coniferous forest.Nitrogen fixation mainly occurs in soil horizon A₁, and denitrification in horizon A₀. There are two kinds of nitrification in forest soil, one is autotrophie and the other is heterotrophic. The former is brought about by chemoautotrophie nitrifying bacteria and the latter is by a number of heterotrophic microbes. Their occurence is correlated with forest type. In some forest soils under mixed broadleaved-coniferous forest, both autotrophie and heterotrophic nitrifications are existed, but under coniferous forest, the nitrification is mainly heterotrophic.

Key words: Nitrogen transformation in forest soil, Soil enzyme, Nitrogen fixation, Denitrification, Ammonif ication, Nitrification