The organic N forms in soil are determined by identification and quantification of organic compounds released by acid hydrolysis. There are no available data on the chemical nature of organic N in the Amazon soils. The objective of this study was to identify and quantify the transformation of 15N-labeled nitrogen fertilizers to different organic nitrogen fractions in three soils of Central Amazonia: an Oxisol, an Ultisol and a Low-Humic Gley (LHG). The soils were sampled after cultivation in greenhouse experiment. The chemical fractionation of soil organic nitrogen, as acid-hydrolysable and distillable N (NSAD), acid-hydrolysable non-distillable N (NSAnD) and non-hydrolysable N (NnH), was performed. In the Oxisol and Ultisol, 63 to 66% and 69 to 73%, respectively, of fertilizer N applied was incorporated in the NSAnD fraction. This suggests that N immobilization of microbial origin is important in these soils. In the LHG, there was equal immobilization in both NSAnD and NSAD fractions: 46 to 53% of N applied was immobilized in NSAnD, while 42 to 53% was immobilized in NSAD. In this soil, the presence of 2:1 type clays probably contributed to the high stock of 15N incorporated in the NSAD fraction, due to fixation of NH4+.
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