Effect of the organic horizon on methane oxidation and uptake in soil of a boreal Scots pine forest

We studied in situ and in the laboratory whether the organic horizon (O horizon) could act as a physical ‘buffer system’ against soil dryness and water stress for the CH4 oxidizing microbes in boreal coniferous forest soil. The laboratory experiments with samples from a Scots pine forest soil showed that the CH4 oxidation took place in the uppermost mineral soil and was not affected by removal of the O horizon (peeling) and irrigation. The removal of a thin O horizon (2–3 cm) in early June increased the mean summertime uptake of CH4 by 50% and decreased the release of CO2 by 50%. Thus, the O horizon acts as a diffusion barrier, lowering the uptake rate of atmospheric CH4. The experimental 50% increase in precipitation by irrigation (from 58 to 84 mm month−1) had only a marginal effect on the gas fluxes in both control plots and peeled plots. During the study summer two dry periods with low matric potentials (<−1 MPa) in mineral soil occurred without there being any reduction in CH4 uptake. In laboratory experiments, the gravimetric water content of 12% (−9 kPa) led to the maximum CH4 oxidation. The CH4 oxidation was still 60% of the maximum at a water content of 4% (<−1.5 MPa) and totally inhibited at a water content of 1%. The results reveal that a low soil water content is not an important factor in restricting CH4 oxidation in mineral soil of boreal forests.