EFFECT OF LONG-TERM FERTILIZATION ON CONTENT AND AVAILABILITY OF FIXED AMMONIUM IN SOIL

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.