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

Upland soils are important sinks for atmospheric methane (CH4), a process essentially driven by methanotrophic bacteria. Soil CH4 uptake often depends on land use, with afforestation generally increasing the soil CH4 sink. However, the mechanisms driving these changes are not well understood to date. We measured soil CH4 and N2O fluxes along an afforestation chronosequence with Norway spruce (Picea abies L.) established on an extensively grazed subalpine pasture. Our experimental design included forest stands with ages ranging from 25 to >120 years and included a factorial cattle urine addition treatment to test for the sensitivity of soil CH4 uptake to N application. Mean CH4 uptake significantly increased with stand age on all sampling dates. In contrast, CH4 oxidation by sieved soils incubated in the laboratory did not show a similar age dependency. Soil CH4 uptake was unrelated to soil N status (but cattle urine additions stimulated N2O emission). Our data indicated that soil CH4 uptake in older forest stands was driven by reduced soil water content, which resulted in a facilitated diffusion of atmospheric CH4 into soils. The lower soil moisture likely resulted from increased interception and/or evapotranspiration in the older forest stands. This mechanism contrasts alternative explanations focusing on nitrogen dynamics or the composition of methanotrophic communities, although these factors also might be at play. Our findings further imply that the current dramatic increase in forested area increases CH4 uptake in alpine regions.