Studies On Fixed Ammonium Of Chief Upland Cultivated Soils In Hunan Province

The content of fixed ammonium, capacity of fixing-ammonium, available fixed ammonium, kinetics of ammonium fixation and release of fixed ammonium of chief upland cultivated soils in Hunan Province were studied by using the method of field investigation and sampling, laboratory incubation and analyses. The seasonal change of fixed ammonium content in soil and_ its bioavailability in different growth stage of Huanghuacai by use of field plot experiment. The main results were as follows:1. The content of fixed ammonium of cultivated horizon in soils tested ranged from 59.4 to 435.7 mg-kg-1, the percentage of fixed ammonium to total N ranged from 4.6 to 60.1%. In the profile of 0~40 centimeter depth, the content of fixed ammoniumhad no distinct change, and it increased or had no distinct change in the profile of 40~75 centimeter depth. The percentage of fixed ammonium to total N in soils steadily increased with the increasing of depth. The soil factors affecting fixed ammonium content in soils tested were mainly parent material, soil clay composition, pH, CEC and the content of clay (<0.01mni) .2. The capacity and maximum capacity of fixing-ammonium in the cultivated horizon ranged from 3.3mg-kg-1 to 203.3 mg-kg-1 and 80.9mg-kg-1 to 533.9 mg-kg-1, respectively. And that in soil profiles tested ranged from 3.6mg-kg-1 to 242.2mg-kg-1 and 60.6mg-kg-1 to 574.7 mg-kg-1, respectively. The soil factors affecting the capacity of fixing-ammonium in tested soils were mainly parent material, soil clay composition, pH, CEC, organic matter, available N and concentration of NH4+ in soil.3.The fixing quantity of ammonium in soils tested increased with the increasing of the fixation time, and the fixation rate was the largest at the beginning several hours. The releasing amount of fixed NH/ in soils tested increased with the incubation time, and the velocity of fixed-NH4+-release was very slow. The release of fixed ammonium still continued at the reaction time of 42 days. pH was an affecting factor of NH4+-fixation in soil and the category of cation was an affecting factor of fixed-NH4+-release from soil. Both first-order equation and parabolic diffusion law equation were the most adaptable to describe the NH4+-fixation and fixed-NH4+-release kinetics. 4.0.0 ~ 17.4% of inherently fixed ammonium and 54.1 ~ 100.0% of recently fixedammonium in soils tested released during the incubation time. The availability ofrecently fixed ammonium was far higher than inherently fixed ammonium. Change of the fixed ammonium content in soil in different growth stage of Huanghuacai was similar with available N content. The total N uptake and the biomass of Huanghuacai was positively correlated with the net release of soil N at the significant level of P0.05.