Oxidation of methane in peat: kinetics of CH4 and O2 removal and the role of plant roots

Vertical profiles of oxygen uptake potential were measured in peat. When both O2 and CH4 were in excess, methanotrophy accounted for 85% of the O2 uptake potential, showing a high capacity for CH4 oxidation in the peat. In the absence of CH4, maximum O2 uptake potential was near the peat surface where available labile organic matter was present and decreased with depth as organic matter became more refractory. The oxidation of CH4 followed saturation kinetics with respect to both CH4 and O2 when they were at limiting concentrations. For CH4 oxidation in the peat, the ko2 was 32 micromolar O2 and kCH4 was 57.9 micromolar CH4. The V max for O2 was 209 nmol O2 ml-1 peat h-1, which was approximately double that for CH4, correctly reflecting the stoichiometry of aerobic CH4 oxidation. The CH4 oxidation kinetics were used in a mathematical model to examine the effect of plant roots on increasing the vertical transport rate of CH4 out of and O2 into the peat, by gas phase transport through the roots. In the absence of roots, CH4 oxidation was confined to a narrow layer near the peat surface where vertical gradients of O2 and CH4 overlapped. Little CH4 diffused through this surface layer. With roots present, the model confirmed the possibility of sub-surface peaks of aerobic CH4 oxidation potential below the water table in an apparently anoxic peat. These were due to active CH4 oxidation in the oxic rhizosphere maintained by the plant roots in the otherwise anoxic peat. The extrusion of O2 from the root tips also tended to diminish in situ CH4 formation by inhibition of the anaerobic methanogenic bacteria. The presence of plant roots increased the flux of CH4 out of the peat to the atmosphere, by-passing the surface oxic layer in which active CH4 oxidation mopped up vertically diffusing CH4.