LOW-CONCENTRATION KINETICS OF ATMOSPHERIC CH4 OXIDATION IN SOIL AND MECHANISM OF NH4+ INHIBITION

NH4+ inhibition kinetics for CH4 oxidation were examined at near-atmos pheric CH4 concentrations in three upland forest soils. Whether NH4+-i ndependent salt effects could be neutralized by adding nonammoniacal s alts to control samples in lieu of deionized water was also investigat ed. Because the levels of exchangeable endogenous NH4+ were very low i n the three soils, desorption of endogenous NH4+ 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 bi rch taiga soils, respectively. At CH4 concentrations of less than or e qual to 15 mu l liter(-1), oxidation followed first-order kinetics in the fine-textured taiga soil, whereas the coarse-textured temperate so ils exhibited Michaelis-Menten kinetics. Compared to water controls, t he K-m(app) values in the temperate soils increased in the presence of NH4+ salts, whereas the V-max(app) values decreased substantially, in dicating that there was a mixture of competitive and noncompetitive in hibition mechanisms for whole NH4+ salts. Compared to the correspondin g K+ salt controls, the K-m(app) values for NH4+ salts increased subst antially, whereas the V-max(app) values remained virtually unchanged, indicating that NH4+ acted by competitive inhibition. Nonammoniacal sa lts caused inhibition to increase,vith increasing CH4 concentrations i n all three soils. In the birch taiga soil, this trend occurred with b oth NH4+ and K+ salts, and the slope of the increase was not affected by the addition of NH4+. Hence, the increase in inhibition resulted fr om an NH4+-independent mechanism. These results show that NH4+ inhibit ion of atmospheric CH4 oxidation resulted from enzymatic substrate com petition and that additional inhibition that was not competitive resul ted from a general salt effect that was independent of NH4+.