Role of CH4 oxidation, production and transport in forest soil CH4 flux

Abstract

Forest soils are an important sink for atmospheric CH4 but the contribution of CH4 oxidation, production and transport to the overall CH4 flux is difficult to quantify. It is important to understand the role these processes play in CH4 dynamics of forest soils, to enable prediction of how the size of this sink will respond to future environmental change. Methane oxidation, production and transport were investigated for a temperate forest soil, previously shown to be a net CH4 consumer, to determine the extent to which physical and biological processes contributed to the net flux. The sum of oxidation rates for soil layers were significantly greater (P < 0.05) than for the intact soil cores from which the layers were taken. Combined with the immediate inhibition of CH4 uptake on waterlogging soils, the findings suggested that soil CH4 diffusion was an important regulator of CH4 uptake. In support of this, a subsurface maximum for CH4 oxidation was observed, but the exact depth of the maximum differed when rates were calculated on a mass or on an areal basis. Markedly varying potential CH4 uptake activities between soil cores were masked in intact core rates. Potential CH4 oxidation conformed well to Michaelis-Menten kinetics but Vmax, Kt and aAO values varied with depth, suggesting different functional methanotrophic communities were active in the profile. The presence of monophasic kinetics in fresh soil could not be used to infer that the soil was exposed only to CH4 mixing ratios ≤ atmospheric, as challenging soils with 20% CH4 in air did not induce low-affinity oxidation kinetics. Atmospheric CH4 oxidation potentials exceeded production potentials by 10-220 times. The results show that the forest soil CH4 flux was dominated by CH4 oxidation and transport, methanogenesis played only a minor role.