Invasion of a Sphagnum-peatland by Betula spp and Molinia caerulea impacts organic matter biochemistry. Implications for carbon and nutrient cycling

Peatlands act as a sink of carbon (C) through the accumulation of dead remains of plants. Under global changes triggered by human activities, it is not only the sink capacity of peatland that is in danger, but also the C already stored. Invasion of Sphagnum peatlands, mainly by Molinia caerulea and Betula spp, is a growing preoccupation. This study aims to assess the extent of the influence of this invasion on the biochemical characteristics of the peat. Elemental analysis, sugar and Rock-Eval pyrolysis parameters were measured in 50 cm profiles collected in invaded and intact plots. The results show that oxygen index ratios (OICO2/OICO) can be used to detect new C substrate injection as invading plants have a lower ratio than Sphagnum spp and Sphagnum peat. Total hemicellulosic sugar contents and organic matter (OM) degradation indices (R400, PPI) suggest that the invading plants promote a faster OM decomposition probably through a faster degradability and a relatively higher nutrient content of their litter. Differences in terms of nutrient status between areas of the peatland are suggested to be of great importance in determining the extent of OM transformation likely due to stoichiometric constraints.