Explicit microrelief-controlled decoupling of initial aerobic (in hummocks) and anaerobic (in hollows) decay in superficial layers of Sphagnum-dominated peatlands

Abstract Peatlands are important carbon sinks due to their ability to sequester and store large amounts of carbon in the form of organic matter (OM). The microhabitat levels and species distribution within peatlands can significantly influence the mechanism of carbon sequestration, and the quality of OM in moss, specifically its litter, may be a key factor in this process. Sphagnum moss, a common type of moss found in peatlands, has unique physiological, anatomical, and molecular characteristics that can affect the decay of its tissues. To better understand the early stages of OM decomposition and its conversion into peat at the microhabitat level, we conducted a 400-day decomposition experiment on moss and peat core samples. We used Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and thermally assisted hydrolysis and methylation-GC-MS (THM-GC-MS) techniques to analyse the molecular composition of the samples and determine carbon to nitrogen (C/N) ratios and mass loss over time. We identified two distinct degradation processes: long-term decay in anaerobic conditions, characterized by the gradual loss of polysaccharides and the preservation of lignin and aliphatic OM; and the initial stages of aerobic degradation, characterized by the formation of microbial-origin OM, the degradation of labile phenolic compounds (including 4-isopropenylphenol from sphagnum acid, 4IPP) and its mobilization by leaching along with free carbohydrates. These processes were found to be spatially-separated, with anaerobic decay being the initial process in hollows and aerobic degradation through leaching being the dominant process in hummocks. This suggests that the presence of 4IPP in peat cores might not only depend on general peatland water table fluctuations but also on position in the microrelief during peat accretion. Our data suggests the total mass loss of the samples after incubation was lower when the original sample contained a higher amount of structural carbohydrates. Additionally, we observed that decreases in C/N ratios during the decomposition experiment were not significantly correlated with higher mass loss, as is commonly observed in millennial peat records. Our analysis of subsurface moss core samples suggests that in natural conditions, the initial stages of OM degradation may not be consistently characterized by decreasing C/N values. Although the degradation patterns of OM in incubated samples differed between hollows and hummocks compared to naturally decomposed samples, we were able to identify the transport of trace metals from the peatland, which is associated with phenols and driven by leaching from hummocks or surface aerobic layers, through comparison of our THM products with THM products from the peatland outflow.