Nitrogen supplying capacity of three types of paddy soils in Tai-lake region were studied. Results obtained showed that (1) the mean value of the iricretnents of clay mineral-fixed arnynonium after anaerobic incubation of 9 air-dried soil samples was 17 ppm, which was equivalent to 28% of the increment of the exchangeable ammonium. It seems that the sum of the increments of exchangeable and clay mineral-fixed ammonium show-ld be taken as the nitrogen mineralization capacity-of the soils capable of fixing ammonimn; (2) results oatained in the field microplot experiments revealed that the contributions of subsoil to the nitrogen supplying capacity of the paddy fields ranged ffom 16% to 49%, with a mean value of 30%;and (3) difficulties in the prediction of nitrogen supplying capacity, of a paddy field with incubation technique are discussed.