Experiments were carried out to study the changes of soil microbial biomass C, N and P and the transformation of N after application of organic and inorganic fertilizers by using isotope of ¹⁵N. The results showed that the soil microbial biomass C, N and P increased greatly after application of organic manures at the beginning of the experiment, and thereafter the biomass C decreased, but the biomass N and P remained constant. The immobilization of ¹⁵N rose to the highest about 10 days after application of ¹⁵N-labelled (NH)₂SO₄ and one month latter about 17% of the ¹⁵N was still in the soil microbes; when combined with organic manures the immobilization of ¹⁵N from (NH;), SOS increased. About 19-25% of ¹⁵N from the ¹⁵N-labelled organic manures was also immobilized in the microbes at the 30th day after application. In case (NH)₂SO₄ was applied alone the loss of N was about 18%, however, when combined with organic manure the loss decreased.