Low-Concentration Kinetics of Atmospheric CH₄ Oxidation in Soil and Mechanism of NH₄ ⁺ Inhibition

ABSTRACT

NH₄ ⁺ inhibition kinetics for CH₄ oxidation were examined at near-atmospheric CH₄ concentrations in three upland forest soils. Whether NH₄ ⁺-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 NH₄ ⁺ were very low in the three soils, desorption of endogenous NH₄ ⁺was not a significant factor in this study. TheK_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 CH₄ 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 NH₄ ⁺ salts, whereas the V _max(app) values decreased substantially, indicating that there was a mixture of competitive and noncompetitive inhibition mechanisms for whole NH₄ ⁺ salts. Compared to the corresponding K⁺ salt controls, the K_m(app) values for NH₄ ⁺ salts increased substantially, whereas the V _max(app) values remained virtually unchanged, indicating that NH₄ ⁺ acted by competitive inhibition. Nonammoniacal salts caused inhibition to increase with increasing CH₄ concentrations in all three soils. In the birch taiga soil, this trend occurred with both NH₄ ⁺ and K⁺ salts, and the slope of the increase was not affected by the addition of NH₄ ⁺. Hence, the increase in inhibition resulted from an NH₄ ⁺-independent mechanism. These results show that NH₄ ⁺inhibition of atmospheric CH₄ oxidation resulted from enzymatic substrate competition and that additional inhibition that was not competitive resulted from a general salt effect that was independent of NH₄ ⁺.