Explicit microrelief-controlled decoupling of initial aerobic decay and leaching (in hummocks) and anaerobic decay (in hollows) in surface layers of a Sphagnum-dominated peatland

Understanding decay processes in peat deposits is fundamental for predicting their role as sources or sinks of atmospheric carbon in a changing environment. It is known that the distribution of microhabitats –hummock, lawn and hollow– within peatlands affects organic matter quality and degradation, but microtopography-dependent carbon dynamics are poorly understood on the molecular level. We studied early decomposition across microtopography levels through analyses of superficial moss cores from a Sphagnum -dominated ombrotrophic peatland in Central Germany, and a 400-day incubation experiment, using analytical pyrolysis. Interpretations were aided by analysis of living vegetation and a deep peat core as reference. Stable and labile pools of polysaccharides dominated the pyrolyzates and played a crucial role in decay dynamics. Two distinct degradation processes emerged: 1) anaerobic decay, characterized by loss of polysaccharides and selective preservation of lignin and aliphatic OM; and 2) leaching of labile phenolic compounds (including sphagnum acid) and free carbohydrates with concomitant initial aerobic degradation and selective preservation of structural polysaccharides. The relative importance of these initial decay processes is spatially dependent; anaerobic decay was detectable in only some of the more evolved hollow layers, while aerobic degradation and leaching dominated in hummocks. Sphagnum acid’s molecular markers appeared useful tracers of early decay as it probably has a leaching-sensitive component in hyaline cells (corroborated by SEM micrographs) that is lost rapidly from hummocks, but not from hollows. Hence, the occurrence of sphagnum acid in peat cores is influenced by microrelief position during peat accretion. This study highlights how microhabitat variations within peatlands influence decay mechanisms on the molecular level. • Molecular characterization was used on superficial cores from Sphagnum -dominated peatland. • Pyrolyzates were dominated by products of polysaccharides (holocellulose, sphagnan). • Early decay causes loss of sphagnum acid, other phenolics and mobile carbohydrates. • Leaching and aerobic decay characterized hummocks; anaerobic decay occurred in hollows. • Leaching from hyaline cells might drive early decomposition of peat moss.