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Kinetics of Nonexchangeable Ammonium Release from Soils

Inst. for Plant Nutrition, Justus-Liebig-University, Suedanlage 6 G-35390 Giessen, Germany

D. L. Sparks

College of Agricultural Sciences, Univ. of Delaware, Dep. of Plant and Soil Sciences, 147 Townsend Hall, Newark, DE 19717-1303

*Corresponding author (diedrich.steffens{at}ernaehrung.uni-giessen.de).

ABSTRACT

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⁺.

Received for publication September 25, 1995.