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. TheKm(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 Km(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 Km(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 +.