Interactive effect of tree species and soil organic layer thickness on methane consumption.

Soils play an important role of atmospheric methane sink, consuming about 30 Tg year^-1, with forest soils the main sink. Methane consumption is carried out by methanotrophic bacteria whose activity can be affected by different factors like tree species. Some species can affect methane consumption through organic molecules released by the roots (exudates) or decomposing litters, including monoterpenes. These compounds mainly secreted by coniferous trees, are thought to inhibit methane consumption. The composition of the soil organic layer and its thickness can also differ between stands of different tree species, with stands of coniferous species known to have thicker organic layer than stands of broad-leaved species. Methane consumption is thought to be lower in organic layer compared to mineral because of a slower diffusion of methane and dioxygen. However only few studies have addressed the effects of tree species on soil methane consumption without finding consensual results.

Our objective was to confirm the hypothesis that tree species can affect soil methane consumption. We carried out this study in a triplicate of four nearby stands, one of sessile oak, one of European beech, one of Scots pine and one of Norway spruce. The sampling was conducted during summer 2020. Soil methane consumption was measured by incubating the five, ten or fifteen first centimetres of soil cores at 20°C and by measuring the variation of CH₄ concentration in the incubation chamber with a laser-based CH₄ analyser. Monoterpenes were identified by GC-MS.

In contrast to our hypothesis, we did not systematically find a significantly higher methane consumption for stands of broad-leaved species compared to stands of coniferous species. However, in first five centimetres of the soil, methane consumption was significantly higher in beech stands compared to the three other species. This result may be related to lower monoterpenes concentration in soils of beech stands and to a thinner soil organic layer at the top of the soil.