Microbial ethylene production and inhibition of methanotrophic activity in a deciduous forest soil

Production of C2H4, but not of CH4, was observed in anoxically incubated soil samples (cambisol on loamy sand) from a deciduous forest. Ethylene production was prevented by autoclaving, indicating its microbial origin. Ethylene production gradually decreased from 4 to 12 cm soil depth and was not affected by moisture or addition of methionine, a possible precursor of C2H4. Oxidation of atmospheric CH4 in soil samples was inhibited by C2H4. Ethylene concentrations of 3, 6 and 10 μl l−1 decreased CH4 uptake by 21, 63 and 98%, respectively. Methionine and methanethiol, a possible product of methionine degradation, also inhibited CH4 oxidation. Under oxic conditions, C2H4 was consumed in the soil samples. Ethylene oxidation kinetics exhibited two apparent Km values of 40 μl l−1 and 12,600 μl l−1 suggesting the presence of two different types of C2H4-oxidizing microorganisms. Methanotrophic bacteria were most probably not responsible for C2H4 oxidation, since the maximum of C2H4 oxidation activity was localized in soil layers (2–8 cm depth) above those (8–10 cm depth) of CH4 oxidation activity. Our observations suggest that C2H4 production in the upper soil layers inhibits CH4 oxidation, thus being one reason for the localization of methanotrophic activity in deeper soil layers.