Increasing soil methane uptake along 120-year afforestation chronosequence is driven by soil moisture

Background/Question/Methods

Methanotrophic bacteria in upland soils are the only biological sink for atmospheric methane (CH₄). Forest soils generally are larger sinks for atmospheric methane than pastures, but the mechanisms involved are not well understood to date. Soil gas diffusivity often is higher in forest soils than in pasture, which may favor CH₄ oxidation because this process often is diffusion-limited. Also, nitrogen inputs to forest soils may be lower than in pasture. Afforestation can therefore increase soil methane uptake, but the few studies available indicate that this process is slow, for reasons not well understood to date. Here we report soil CH₄ uptake along an afforestaion chornosequence with Norway spruce (Picea abies L.) established in extensively grazed subalpine pasture. We measured soil CH₄ uptake in situ throughout a growing season, and found a progressive increase of the soil methane sink with forest stand age.  We further labeled intact soil cores with ¹⁴CH₄ in the laboratory and mapped the spatial distribution of methane uptake using an auto-radiographic technique.

Results/Conclusions

Soil moisture and soil porosity data indicated that the increase in soil CH₄ uptake in the older forests was primarily driving by reduced soil water content. Likely reasons for the drier soils were increased interception and increased evapotranspiration in old forest stands. As a consequence, water-filled pore space decreased and diffusion of atmospheric CH₄ down the soil profile to the methanotrophically active soil layer was facilitated. This mechanism contrasts alternative explanations including altered soil N status, altered soil structure or shifts in the methanotrophic community structure, although these factors also might be at play. Our findings further imply that the currently dramatic increase in forested area in alpine regions increases CH₄ uptake in Alps.