Study of dissolved organic matter in peatlands: molecular characterisation of a dynamic carbon reservoir

Abstract

Northern peatlands represent a significant carbon reservoir, containing approximately a third of the terrestrial carbon pool. The stability of these carbon stores is poorly understood, and processes of accumulation and degradation appear to be finely balanced. Over the last decade, it has become increasingly clear that losses of dissolved organic carbon (DOC) from peatlands can be of considerable size and this flux appears to have increased substantially over the last 20 years. Despite its significance, the chemical composition of peatland-derived DOC remains poorly understood. This study aimed to characterise dissolved organic matter (DOM) at the molecular level using a novel combination of techniques. The study site (Cors Fochno, Wales, UK) is an ombrotrophic bog on which a number of studies into carbon cycling and hydrology have been carried out, providing a useful context for this project. The size and compositions of the DOC pool was monitored over 18 months, from three banks of piezometers, sampling from depths of 15 cm to 6 m. DOM which is representative of bog runoff was also monitored. DOC concentrations varied considerably between locations, spanning an order of magnitude (11.4 to 114 mgC l-1). Several relationships between DOC concentration and environmental and physical factors were established: DOC levels near the surface of the peatland varied with temperature, those in the runoff were most affected by recent rainfall events and the apparent DOC concentration at depth was related to the hydraulic conductivity of peat at that depth. The annual flux of DOC from the site was estimated at 113 tonnes, or 17.4 gC m-2. Only a small portion of the DOC pool could be characterised by analysis of dissolved combined amino acids (DCAA) and dissolved carbohydrates (as neutral sugars). Non-protein amino acids were most abundant in runoff samples, suggesting microbial reworking of DOM on entering drainage systems. DCAA yields decreased with depth, and the DCAA pool in deeper peat layers was characterised by more hydrophobic compounds. Interpretation of semi-quantitative results from TMAH thermochemolysis GC-MS analysis suggested oxidative degradation of organic matter near the surface of the peatland and photochemical degradation where DOM entered drainage networks, and this was supported by novel interpretation of results from ultrahigh resolution mass spectrometry analysis. The deepest porewaters were dominated by nalkanes, with notable contributions from fatty acids, suggesting a plant wax source for this DOM. The highest DOC concentrations were found at intermediate depth from a site midway between the centre of the bog and the southern boundary where hydraulic conductivities were low, and DOM from these piezometers were characterised by high contributions from a suite of phenolic compounds (with mainly para-hydroxyphenyl structures). These compounds have been linked to Sphagnum species, and are known to be functionally important to the development and maintenance of the unusual chemical environment in peatlands which slows decay rates, reduces microbial activity, and allows the sequestration of the large carbon reservoir. The findings of this study highlight the dynamic nature of peatland derived DOM, both in the size of the carbon pool and its composition which change dramatically with both season and depth.