| The method of Silva-Bremner was used to study the contents of native fixed ammonium and available fixed ammonium, maximum ammonium fixation capacities, and kinetics of ammonium fixation in main type paddy soils derived from different parent materials in Hunan Province with investigation and sampling in situ, and laboratory incubation. The newly laky clayey soil and the reddish yellow clayey soil were used to study the seasonal change of fixed ammonium contents in the soils, the bioavailability and the relationship between the releasing rate of fixed ammonium and the N uptake during the rice growth with pot experiment. The results are as follows:For cultivated horizon of the tested soils, the contents of fixed ammonium were significantly different, and ranged from 88.3 mg-kg"1 to 386.5 mg-kg"1, averaged 268.6?3.2 mg-kg"1, and the percentage of fixed ammonium to total N varied from 4.2% to 15.3%, averaged 9.9%?.5%. The contents of fixed ammonium were higher than those in soil of North Hunan region and in soil of red earth region , and were a main N resource in the province. The content of fixed ammonium was found in order of newly laky clayey soil>alluvial sandy soil>yellow clayey soil>purple clayey soil>reddish yellow clayey soil>granitic coarse sandy soil.There were four distribution patterns of fixed ammonium in the profiles of 1m depths. Firstly, fixed ammonium contents increased with increasing depth in the profiles. Secondly, they decreased with increasing depth in the profiles. Thirdly, there were no distinct change in the profiles. Lastly, they abruptly increased or decreased in some horizons of the profiles. % of fixed ammonium to total N in profile soils mostly increased with increasing depth, they were different in different horizons, with 14,4% in the cultivated horizon soils, with 21.7% in the plow pan soils and with 31.3% in other layers.The factors affecting fixed ammonium contents in the soils included mineral composition in soil clays, soil clay content, total N content and alkali-hydrolyzable N, organic matter content in the soils, and so on. The contents of fixed ammonium in newly laky clayey soil, alluvial sandy soil, purple clayey soil and yellow clayey soil in which the 2:1 type clays were predominant were higher than those in granitic coarse sandy soil and reddish yellow clayey soil in which the 1:1 type clays were predominant. The contents of fixed ammonium were significantly correlated with the contents of soil clay (0.01mm), total N, alkali-hydrolyzable N and organic matter in the soils at the significant level of PO.OSor P0.01, the correlation coefficients were 0.485*(n=16), 0.590**(n=23), 0.468*(n=17) and 0.596**(n=23), respectively.The maximum ammonium fixation capacities ranged from 0 to 272.2 mg-kg"1, averaged 63.1 mg-kg" in the cultivated horizon soils. The maximum ammonium fixation capacities of newly laky clayey soil was the highest , while that of the purple clayey soil derived from the weathering product of purple plate was the lowest. There were also different maximum ammonium fixation capacities in the soil profiles, the yellow clayey soil profile had the lowest maximum ammonium fixation capacities, and the newly laky clayey soil profiles had the highest maximum ammonium fixation capacities. This was related to the mineral composition and the CEC in soils.The kinetics results of ammonium fixation indicated that the fixing velocity for added NH4+ was very quick, and the reaction took place at the initial several hours. Both first order and Elovich equations were the most adaptable to describing the NlV-fixation kinetics as supported by their higher determination coefficient (r2) and lower value of the standard error of estimate (SE). The parabolic diffusion equation can also describe the experimental data satisfactorily, so the NH4+-fixation may be a process controlled by diffusion.The contents of available fixed ammonium in cultivated horizon soils varied greatly. The change in available fixed ammonium contents of different layers in the soil profiles ha...
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