A study of palsa bog organic matter stabilization in response to holocene climate change using ¹³C NMR and FTICR MS

The ¹³C NMR and high-resolution mass spectrometry (HRMS) methods have been employed to elucidate the principal pathways of soil organic matter (SOM) stabilization in the peat deposit of palsa bogs in the European Arctic. Extremes of labile/condensed constituents and structural moieties have been found to be related to climatic and hydrological conditions during different periods of the Holocene. The initial stage of SOM stabilization includes decomposition of plant residues, formation of (pro)humic substances (HSs) molecules from high-molecular-weight precursors, and primary transformation of HS molecules with degradation of the most labile fragments. The more favorable climatic conditions of the Early Holocene (Atlantic Optimum) and Middle Holocene (SB2) determined the botanical composition of the peat, which was dominated by sedge and tree-sedge communities with high contents of lignin components and, as a consequence, a greater proportion of aromatic fragments, characterized by thermo-biodynamic resistance in the humic acids (HAs) of horizons in the lower and central parts of the profile. Older HAs, which underwent intense degradation were enriched in the N,S-containing components as compared to HAs from surface layers. This is in turn accompanied by an overall decrease in the proportion of unoxidised aliphatic and carbohydrate fragments. A substantial proportion of highly condensed fragments were identified in HAs by HRMS. It was suggested that aromatic and condensed HA molecules contain long-chain substituents which contribute to the content of aliphatic carbon. The ratio of pentosans to hexosans in HAs is demonstrated to increase down the peat profile as a consequence of the redistribution of the dominant peat formers. The HA branching indices demonstrate a significant transformation of long-chain structures and an increase in the contribution of substituted aliphatic fragments in the HA structure.