Spatial micro-distribution of methanotrophic activity along a 120-year afforestation chronosequence

Aims: Methanotrophic bacteria drive upland soil methane (CH₄) uptake. Land-use change often affects their activity, but the mechanisms involved are not well understood. We studied soil-atmosphere CH₄ fluxes along a 120-year Norway spruce afforestation chronosequence on subalpine pasture, testing whether effects were related to shifts in the spatial niche of methanotrophs. Previous field data had shown that soil ¹⁴CH₄ uptake increased with forest age, and that this effect was driven by decreased water filled pore space due to higher rainfall interception in the more developed canopies of older forest stands. Methods: The spatial distribution of methanotrophic activity was determined by ¹⁴CH₄-labelling followed by soil section preparation, aggregate size fractionation, aggregate erosion, and micro-autoradiographic imaging. Results: Uptake rates of CH₄ measured in laboratory incubations of soil cores as well as their water contents largely followed the in situ measurements previously made in the field. ¹⁴CH₄ assimilation was heterogeneously distributed, and occurred further down the soil profile in older forest that had a more developed organic layer that did not contribute to CH₄ uptake. Assimilation was largest in 2—8 mm aggregates, and higher at the exterior than in the interior of aggregates. Conclusions: Our data indicates that differences in soil aggregation and related methanotrophic activities did not contribute substantially to higher CH₄ uptake in older forest, mostly because aggregation did not change much with age. On a per mass basis, however, large aggregates contributed less to CH₄ uptake due to their unfavorable surface to volume ratio. More generally, we argue that the (sub-)aggregate heterogeneity of soil microbial activity and diversity is underexplored, although it critically determines ecological interactions that drive ecosystem-level processes.