The role of tree species and microbes for the development of net greenhouse gas fluxes from soils after afforestation of agricultural lands

Greenhouse gas (GHG) emissions, in the form of CO₂, CH₄ and N₂O, from land use change and agriculture are responsible for up to 20% of anthropogenic emissions, mainly due to deforestation, livestock production and crop fertilization. Afforestation is proposed as an effective means to sequester atmospheric carbon in biomass and soils. However, there is a lack of knowledge about the resultant soil GHG fluxes from temperate afforested ecosystems, how they develop in the field and over many years. Furthermore, tree species choice (deciduous/conifers) may impact the soil biogeochemistry differently through the soil physicochemical properties and the soil microbiome with a currently uncertain outcome in relation to GHG fluxes and the climate mitigation potential.

In this study, we investigate the development of soil GHG fluxes, soil physicochemistry, and the soil microbiome on arable land using a well-established forest chronosequence (Vestskoven, Denmark), which is a former cropland area afforested over the last 50 years with Norway spruce (Picea abies), oak (Quercus robur) and beech (Fagus sylvatica). The total of 19 selected sites in Vestskoven includes 6 to 7 stand ages per tree species. We measured CH₄ and CO₂ fluxes in situ, and sampled soil for physicochemical and microbial analyses.

We present data on how net soil CH₄ uptake and soil CO₂ efflux develop with time since planting and how the net soil CH₄ uptake correlates with the relative abundance of methanotrophic and methanogenic soil communities. We expect these relationships to be dependent of tree species due to differences in how soil physicochemical properties impact the microbial communities responsible for soil CH₄ cycling.

Preliminary results show that afforestation increases net soil CH₄ uptake, since all tree species had higher net soil CH₄ uptake rates compared to cropland, but the effect of plantation age was only visible in oak stands after 50 years. This indicates tree species-specific regulation of the net CH₄ flux and its development over time. There was no clear trend for a development of the soil CO₂ efflux after planting for either tree species. We will further present analyses of structural equation modelling elucidating the interactions between gas fluxes, soil physicochemical environment and microbial communities.