Low-Concentration Kinetics of Atmospheric CH(4) Oxidation in Soil and Mechanism of NH(4)(+) Inhibition

NH(4)(+) inhibition kinetics for CH(4) oxidation were examined at near-atmospheric CH(4) concentrations in three upland forest soils. Whether NH(4)(+)-independent salt effects could be neutralized by adding nonammoniacal salts to control samples in lieu of deionized water was also investigated. Because the levels of exchangeable endogenous NH(4)(+) were very low in the three soils, desorption of endogenous NH(4)(+) was not a significant factor in this study. The K(m(app)) values for water-treated controls were 9.8, 22, and 57 nM for temperate pine, temperate hardwood, and birch taiga soils, respectively. At CH(4) concentrations of ≤15 μl liter(−1), oxidation followed first-order kinetics in the fine-textured taiga soil, whereas the coarse-textured temperate soils exhibited Michaelis-Menten kinetics. Compared to water controls, the K(m(app)) values in the temperate soils increased in the presence of NH(4)(+) salts, whereas the V(max(app)) values decreased substantially, indicating that there was a mixture of competitive and noncompetitive inhibition mechanisms for whole NH(4)(+) salts. Compared to the corresponding K(+) salt controls, the K(m(app)) values for NH(4)(+) salts increased substantially, whereas the V(max(app)) values remained virtually unchanged, indicating that NH(4)(+) acted by competitive inhibition. Nonammoniacal salts caused inhibition to increase with increasing CH(4) concentrations in all three soils. In the birch taiga soil, this trend occurred with both NH(4)(+) and K(+) salts, and the slope of the increase was not affected by the addition of NH(4)(+). Hence, the increase in inhibition resulted from an NH(4)(+)-independent mechanism. These results show that NH(4)(+) inhibition of atmospheric CH(4) oxidation resulted from enzymatic substrate competition and that additional inhibition that was not competitive resulted from a general salt effect that was independent of NH(4)(+).