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

• Published in: Applied and Environmental Microbiology
• Main Authors: Gulledge, Jay, Schimel, Joshua P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Society for Microbiology 1998
• Subjects: taiga
• Online Access: http://dx.doi.org/10.1128/aem.64.11.4291-4298.1998; https://journals.asm.org/doi/pdf/10.1128/AEM.64.11.4291-4298.1998

ABSTRACT 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 + .