KINETICS OF INHIBITION OF METHANE OXIDATION BY NITRATE, NITRITE, AND AMMONIUM IN A HUMISOL

The kinetics of inhibition of CH4 oxidation by NH4+, NO2-, and NO3- in a humisol was investigated. Soil slurries exhibited nearly standard Michaelis-Menten kinetics, with half-saturation constant [K-m(app)] values for CH4 of 50 to 200 parts per million of volume (ppmv) and V-max values of 1.1 to 2.5 nmol of CH4 g of dry soil(-1) h(-1). With one soil sample, NH4+ acted as a simple competitive inhibitor, with an estimated K-i of 8 mu M NH4+ (18 nM NH3). With another soil sample, the response to NH4+ addition was more complex and the inhibitory effect of NH4+ was greater than predicted by a simple competitive model at low CH4 concentrations (<50 ppmv). This was probably due to NO2- produced through NH4+ oxidation. Added NO2- was inherently more inhibitory of CH4 oxidation at low CH4 concentrations, and more NO2- was produced as the CH4-to-NH4+ ratio decreased and the competitive balance shifted. NaNO3 was a noncompetitive inhibitor of CH4 oxidation, but inhibition was evident only at >10 mM concentrations, which also altered soil pHs. Similar concentrations of NaCl were also inhibitory of CH4 oxidation, so there may be no special inhibitory mechanism of nitrate per se.