Chemical Composition of Soil Organic Matter in a Subarctic Peatland: Influence of Shifting Vegetation Communities

Climate change in the subarctic region has increased the rate of inundation of peatlands due to increased temperatures, precipitation, and permafrost thaw. Increased inundation may result in vegetation community shifts, as documented in a subarctic mire near Abisko, Sweden. The wet fen communities have established in former sphagnum areas, and sphagnum colonized in degraded palsa hummocks. At Stordalen mire, we studied the influence of vegetation community on chemical composition of peat soil organic matter (SOM). Vegetation and soil samples were obtained along a hydrologic gradient with representative communities: palsa, sphagnum, and fen. Soil organic matter chemical composition indicated shifts in vegetative communities. Total N and N isotope signatures in fen soils showed characteristics of sphagnum and palsa communities at >6-cm depth, and sphagnum soil profile signatures shifted from sphagnum to palsa properties at a 20-cm depth. Soil chemical composition measured by Fourier Transform Infrared (FTIR) spectroscopy and C-13 Nuclear Magnetic Resonance (NMR) spectroscopy showed increasing recalcitrant C (alkyl and aromatic) in palsa soil. Sphagnum soil profiles sustained labile organic C (O-alkyl) until 15 cm then shifted to humified soil, and fen soil profiles showed areas of sphagnum and palsa signatures. Furthermore, the strong relationship between functional group C (O-alkyl and alkyl) and total N demonstrated that soil N is an effective indicator of peat decomposition. Our results identified change points in soil chemical composition in regards to N content and C functional group which highlights the importance of historic vegetation community on chemical composition of peat soils.