Kinetics of nonexchangeable ammonium release from soils

The rate of nonexchangeable NH4+ release from soils can have a significant effect on N dynamics and environmental quality. Nonexchangeable NH4+ is that N fraction which is fixed in the interlayers of clay minerals. The objective of this study was to determine the kinetics of nonexchangeable NH4+ release from two topsoils from Germany [Giessen Ap (Aqualf) and Hungen Ap (Alfisol)] and from two subsoils, one from Germany [Hungen C (Alfisol)] and one from Kentucky [Shrouts Bt (Hapludalf)]. Calcium-saturated, freeze-dried soils were extracted with H-resin for 0.25 to 384 h. Sodiumethylmercurythio-salisylate (Thimerosal) was added to the soil/H-resin suspension to inhibit microbiological ammonification. The kinetics of nonexchangeable NH4+ release from the soils were biphasic and were best described by Elovich and heterogeneous diffusion models. The release of nonexchangeable NH4+ as a percentage of total nonexchangeable NH4+ in the soils ranged from 4 to 25% and was lower in subsoils than in topsoils. This was ascribed to the higher levels of indigenous nonexchangeable NH4+ in the subsoils, which is more tightly held than recently fixed nonexchangeable NH4+.