The present work is a part of the cooperative project between Northwesters Agricultural University Shaanxi,China and Justus Liebig University,Gissen,W.Germany.The content and variation of nonexchangeable ammonium in manured loessial soil and its availability for winter wheat were studied through a common plot experiment and a micro plot experiment using 15N-labelled ammonium under field condition during wheat growing period.The experimental results showed that manured loessial soil possessed a greater reserve of nonexch-angeable ammonium;in the cultivated layer of 0-20 cm of the soil tested,the nonexchangeable ammonium amounted to 236 ppm N,constituting about 23% of the total N,and in the soil layer of 0-75 cm,it was 252 ppm N on average,constituting a quarter of the total N.Both the newly fixed ammonium and the ammonium oringinally fixed by soil played an imortant role in wheat growth;the former was predominated by its higher availability,while the latter was cheristerized by its greater quantity.The maximume net release (MNR) rate of newly fixed amonium ranged from 38.0 to 70.5% in the soil layer of 0-75 cm,being much higher than that of the ammonium oringinally fixed by soil that ranged from 10.4 to 22.2% in the same soil layer.Howerver,the MNR amount of the ammonium oringinally fixed by soil was 12-72 ppm in the soil layer of 0-75 cm,it is much more than that of the newly fixed ammonium that was only 7.4-10.9 ppm in the same soil layer.It was also proved that the fixation of ammonium by clay minerals in manured loessial soil could act as a pool of N in soil and applying N tended to increase the nonexchangeable ammonium in soil.