Does stand age affect methane consumption of forest soil? A study in a chronosequence of sessile oak.

Soils play an important role of atmospheric methane sink, consuming about 30 Tg year^-1. Methane consumption is carried out by methanotrophic bacteria whose activity can be affected by different environmental factors. One of the most important factors that impact on methane consumption is the air-filled porosity of soil (AFP), which depends on its total porosity (P) and its water content (SWC). A high AFP enhances gas diffusion in soil, and therefore methane consumption. In forests, P is thought to increase with stand age because of soil decompaction by tree roots and SWC is thought to decrease because of a high evapotranspiration. Another factor that can affect methane consumption and thought to decrease with the aging of forest stands is mineral nitrogen (Nmin) and particularly ammonium that competes with methane for the active site of methane monooxygenase, thus reducing methane oxidation. However only few studies have addressed the effects of stand aging on soil methane consumption.

Our objective was to confirm the hypothesis that methane consumption by forest soil increases with stand age, in relation with an increase AFP and a decrease Nmin. We carried out this study in a chronosequence of 16 stands of sessile oak divided into six age classes (20-30, 40-60, 60-70, 85-90, 125-130 and 140-145). Three sampling campaigns were conducted in late summer 2018 and 2019 (periods of maximum AFP) and in early spring 2018 (period of minimum AFP). Soil methane consumption was measured by incubating the five first centimetres of soil cores at 20°C and by measuring the decrease of CH₄ concentration in the incubation chamber with a laser-based CH₄ analyser.

In contrast to our hypothesis, we did not find any significant effect of stand age on Nmin, P, SWC and AFP, nor on methane consumption. However, methane consumption was higher in stands with high values of AFP and low value of SWC, whatever their age. AFP, through differences in SWC, appeared to be the main driver of soil methane consumption in our study site, explaining both seasonal variations and variations among stands, that could not however be related to their age.