Analyzing the degree of organic matter transformation of rewetted European peatlands in the context of their greenhouse gas emission potential

Peatlands are a vast and vulnerable carbon stock. Although they cover only 3% of the land area, globally the organic matter stored in peat accounts for 20% of soil carbon. In peat-rich countries, peat soils contribute much to the carbon dioxide (CO₂) emissions from croplands and grasslands in the national greenhouse gas (GHG) budgets. This may seriously diminish any possible net carbon sink of the land use sector. Owing to their high emission reduction potential per area, cultivated peat soils are often considered as a very effective target for GHG mitigation measures in the agriculture and land use sector. This goal can be achieved by rewetting. Rewetting often comes with synergies such as water protection, as peat soils are also high sources of dissolved organic matter and nutrient efflux. In wet soil transformation is driven by anaerobic processes and the resulting methane emissions can be considered a tradeoff of CO₂ savings by wet management. How groundwater table raise influences GHG emissions depends e.g. on peat properties, environmental conditions and site management. The goal of the EU project EJP Soil INSURE, to which our research project belongs, is to evaluate the significance of different factors regulating the GHG balance of wet cultivated peat soils at different European sites and to seek for indicators of successful GHG mitigation. We contribute to this goal by analyzing the organic matter of peat profiles. We study the molecular composition of the different peats using analytical pyrolysis GC-MS, in conjunction with organic matter stoichiometry. Our aim is to better understand the role of peat properties for the biogeochemical cycles in rewetted peatlands and their GHG balance. By using different classes of compounds, we were able to distinguish between transformed organic matter and peat that is relatively rich in undecomposed plant material. We identified marker compounds that were highly specific for fresh plants, transformed plant material or microbial abundance. For example, the abundance of undecomposed plants can be linked to levosugars, decomposed plant material is pictured by low weight polysaccharides such as 5-methyl-2-furalaldehyde and microbial matter is displayed by specific nitrogen compounds, as pyridines, pyrroles and indene’s, such as 1H-indene, 3-methyl. In addition, we studied the correlation between peat stoichiometry, compound abundance and the degree of transformation. We find that for example a high C/N ratio is negatively correlated with a high amount of low weight polysaccharides and compounds indicative for microbial abundance. We conclude that our method gives a detailed insight of peat composition. Further, it enhances our knowledge of peat quality and could therefore help to gain insights into the dependency of GHG emissions on peat quality.