Exploring northern peatland biogeochemistry as a function of climate change

Abstract

Soil organic matter (SOM) is a major store of terrestrial carbon (C), and peat soils make up a large proportion of this terrestrial C sink. Northern peatlands themselves cover an area of around 4 x 106 km2 , and currently store around 547 Gt of organic carbon as waterlogged peat. Some of the major peat forming plants are the Sphagnum mosses, which are known to synthesize phenolic compounds such as trans-sphagnum acid as structural support components. The use of molecular marker compounds to identify specific plant species or SOM inputs is well established, as is the use of molecular proxies to assess existing or past climatic conditions and/or SOM dynamics and turnover. Samples from a global range of peatlands were analysed using thermally assisted hydrolysis and methylation (THM) in the presence of 13C-labelled tetramethylammonium hydroxide (13C-TMAH) in order to confirm the presence of a suite of Sphagnum marker compounds. The corresponding Sphagnum yield parameter (σ), along with the parameter for total lignin-derived phenols (Λ) was assessed in relation to branched glycerol dialkyl glycerol tetraether (brGDGT) derived proxies under simulated climatic warming conditions in the Frasne Peatland, France. In addition to this, analytical pyrolysis gas chromatography mass spectrometry (Py-GCMS) was carried out on peat samples in order to obtain new plant-specific biomarkers, and the viability of these markers was assessed with 13C-TMAH thermochemolysis. The proposed Sphagnum marker compounds I, IIa, IIb and III were observed across all Sphagnum capitula sampled and all peat deposits with Sphagnum input, while being absent from all vascular plants or non-Sphagnum bryophytes studied. Ferulic acid methyl ester was proposed as a graminoid-specific marker after being found exclusively in graminoid pyrolysates, and 3-methoxy-5-methylphenol was proposed as a marker for lichens after being observed in six of the seventeen lichen species sampled. 13C-TMAH thermochemolysis of these proposed markers confirmed a primarily monomethoxy source, confirming their viability as biomarkers. The observed decrease in σ and Λ in surficial peats under simulated warming conditions suggests an effect due to drying and aeration of surficial peat layers as a result of increasing temperatures, which corresponds with brGDGT-based proxies such as the methylation index of branched tetraethers (MBT), which is seen as an indicator of air temperature.