The effects of changing redox conditions on the phenolic distributions and carbon cycling at Butterburn Flow

Abstract

Northern peatlands store around 30% of soil carbon and 75% of atmospheric carbon and consist of a complex mixture of humic substances including recalcitrant phenolics such as lignins, tannins and sphagnum acid which make a significant contribution to the organic carbon content. Changes in water table regimes and vegetation inputs will impact the distributions and amounts of these phenols which will in turn significantly influence the carbon storage capacity. This study presents an exploration into the effects of water table fluctuations on redox conditions as well as the accumulation and degradation of lignin and Sphagnum phenols along a bog-fen gradient from Butterburn Flow, Cumbria UK. The insoluble residues of the living vegetation and associated roots as well as the peat were analysed using unlabelled and 13C-labelled tetramethylammonium hydroxide thermochemolysis. All Sphagnum mosses contained significant concentrations of sphagnum acid biomarkers not identified in either the above or below ground sections of any vascular plants. The chemical proxies σ and SR% which utilise these biomarkers proved useful indicators of Sphagnum abundance and were in no way distorted by the presence of other species, unlike the lignin proxy Λ. In some species, concentrations of both lignin-derived phenols and carbohydrates in vascular plant roots exceeded concentrations in above-ground sections of the same species. Therefore, for peats that contain dense and deep-reaching root systems, Λ and carbohydrate concentrations may be distorted. Bryophytes also contained lignin phenols, however, concentrations were minimal. The presence of such phenols is a result of the phenol-rich bog water in which the mosses reside. Total organic carbon increased as a function of depth in all cores, despite concentrations of isosaccharinic acids, metasaccharinic acids and sphagnum acid biomarkers decreasing with depth. The carbon storage capacity of Butterburn Flow was estimated at 5.1 t ha−1 , which agrees with previous estimates for British bog habitats. Overall, sites with lowest water table tended to have lower phenolic content, higher oxidative stress and lower carbohydrate content. Therefore, increased climatic disturbance leading to watertable draw-down will lead to significant oxidative stress and chemical alteration of the peat.