The results of 13 Years long-term field experiment on Shenyang brown earth showed that application of chemical fertilizers
alone or low amount organic manure had little effect on the content of fixed NH4 in soil. However, the continuous application of high amount organic manure increased the content of fixed NH4 in soil significantly. In all high amount organic manure plots the content of flxed NH4 increased by 30. 2% in average after 13 years. The freshly fixed NH4 came mainly from the mineralization of organic nitrogen in soil. The net increase of fixed NH4 after fertilization was more than the application rates of nitrogen
fertilizer. It was the resul of priming effect of soil nitrogen. The content of fixed NH4 in soil varied greatly in a growing season due to uptake by crops and mineralization of soil organic nitrogen. During the growth period of soybean the total amount of fixed NH4 released from 0～20cm layer in CK,
chemical fertilizer and chemical fertilizer combined with organic manure plots reached up to N 43. 110 and 165kg/hm2, respectively. This showed that the release of fixed NH4 offered a main available nitrogen source for crops.