Field-Scale Inhibition and Recovery of Atmospheric-Methane Oxidation in Soil

Abstract:

Aerobic methane (CH₄) oxidation in upland soils is the only known terrestrial sink for atmospheric CH₄. It is mediated by methane-oxidizing bacteria (MOB) that possess a high-affinity form of the enzyme methane monooxygenase (MMO), allowing utilization of CH₄ at near-atmospheric, low concentrations (≤ 1.9 µL/L). As cultivation attempts for high-affinity MOB have shown little success to date, there remains much speculation regarding their functioning in different environmental systems.

For quantification of microbial functions at the field scale, inhibition experiments are often used as a control and to verify that observed substrate turnover is microbially mediated. Targeting MMO, several compounds have been proposed as inhibitors of CH₄ oxidation. However, previous inhibition experiments were mostly conducted in systems dominated by low-affinity MOB, which mediate CH₄ oxidation at elevated CH₄ concentrations. On the contrary, inhibition experiments targeting high-affinity MOB are scare, particularly at the field scale.

We present results of field-scale experiments to investigate effectiveness of and recovery from inhibition of atmospheric CH₄ oxidation using the competitive inhibitors CH₃F and CH₂F₂, as well as the non-competitive inhibitor C₂H₂. The latter is of particular interest, because C₂H₂ irreversibly binds to MMO, requiring de-novo synthesis of MMO for recovery of CH₄ oxidation activity. Experiments were conducted during both winter and summer seasons in a sandy soil. Atmospheric CH₄ oxidation was quantified in regular intervals at reference and treatment locations using the soil-profile method with concurrent measurements of soil-water contents and -temperature. Whereas C₂H₂ inhibition was highly effective in both seasons, the time required for recovery to the level of the reference location was much shorter during the summer experiment (~1 mo compared with 4 mo during winter). Our data provide new insights into the physiology of high-affinity MOB.