The effect of ammonium fixation on the transformation of N labelled fertilizer N in three soils was studied in a pot experiment. Little fertilizer N was fixed by clay minerals in a paddy soil derived from red earth, while 56-77% of fertilizer N applied was fixed in bleacked paddy soil and calcareous permeable paddy soil. Availability of these "newly" fixed ammonium N was so high that over 90% of it was recovered by rice plant or microorganisms within 30-50 day after the transplantation of rice seedlings. The availability of the biologically immobilized N to the current crop was much less than that of the "newly" fixed ammonium N. In a field microplot experimant conducted on bleached paddy soil, it was found that 20-86% of the residual fertilizer N taken up by the succesive crops (2nd and 3rd crops) was derived from fixed ammonium nitrogen. The authors consider that far soils with fairly strong NH₄⁺-N-fixing capability, only the NH₄⁺-N-fixation by clay minerals and its release are known, can the N immobilization, remineralization be evaluated.