Origin and transformation of soil organic matter in permafrost soils

Soil organic matter (SOM) is a highly heterogeneous component of soils and its composition differs strongly between sites, depending on the specific environmental conditions. Thawing of permafrost leads to the exposure of large amounts of SOM to decomposition, resulting in the release of greenhouse gases. Moreover, SOM might be mobilized as dissolved organic matter (DOM), possibly contributing to losses of carbon from soils. Detailed knowledge of SOM composition is key for understanding mineralisation processes and for the quantification of greenhouse gas emissions from thawing permafrost soils of different moisture, thaw depth, parent material and slope position.

We sampled permafrost soils along two transects on Disko Island, West Greenland, to characterize SOM from soils with different characteristics. Installation of suction cups allowed pore water sampling to determine the amount and composition of DOM. We measured emissions of CO₂ and CH₄ with a manual chamber system to quantify greenhouse gas fluxes at the different sites. To determine the degree of SOM decomposition and the potential impact of site characteristics on greenhouse gas emissions and SOM leaching, we fractionated SOM and subsequently analysed lignin components, amino sugars, and stable isotopes (δ¹³C and δ¹⁵N). Molecular microbial analyses were carried out to understand the underlying biological processes that control SOM cycling and greenhouse gas production.

Lignin components and derived molecular ratios matched with the recent vegetation. Sites are characterized by woody angiosperms in the well aerated and drained soils at the top of the slopes and by herbaceous plants in the wetland area at the lower end of the transects. The data indicated weak decomposition at the wet sites and stronger decomposition at the dry sites, which correlated with the proportion of particulate OM within the total SOM. Stable isotopes showed according patterns, becoming more positive with depth within the soil profile but becoming more negative along the transects. Leaching of DOM showed a more complex pattern with the lowest C contents in the wettest areas and the highest C contents in the intermediate slope positions but increasing C contents within the soil profiles. Only the wettest sites emitted CH₄, while the drier locations were neutral in terms of CH₄ or acted as CH₄ sinks. We observed decreasing CO₂ emissions along the transects during the day, with the driest sites being sources of CO₂. The observations of CH₄ fluxes were supported by higher abundances of methanogenic microorganisms in the wetter areas.

The results underline the susceptibility of SOM to decomposition in thawing permafrost. While topsoils and litter layers contain larger amounts of SOM than subsoils, results suggest that C is transported downwards along the soil profile with infiltrating water, possibly buffering decomposition. DOM appears to be transported down the hillslopes until it is either drained into waterbodies or emitted as CH₄. Concerning scenarios of soil moisture changes, and daily and seasonal variations in CO₂ uptake, the observed soils might therefore turn from C sinks to sources. However, the extend of this C-relocation by lateral DOM transport has not yet been quantified and needs further observation.