Molecular characterization of terrestrial organic carbon in some organic-rich soils in the northern latitudes

Abstract

Northern peatlands store around one third of the global soil carbon, however over the past 100 years significant areas of these peats and peaty gley soils have been drained and planted with coniferous forest. Afforestation could accelerate peat decay due to land disturbance causing the peatland to become a net carbon source, alternatively carbon may accumulate due to increased carbon fixation, causing the peatland to remain a net carbon sink. Despite the global importance of these mechanisms, our understanding of the fate of soil carbon stores in afforested carbon-rich soils (i.e. peaty gley and peat soils) remains unclear. Peat and litter were analysed using thermally assisted hydrolysis and methylation (THM) in the presence of 13C-labelled and unlabelled tetramethylammonium hydroxide (TMAH) which revealed the distribution, degradation transformations and turnover rates of vascular plant- and Sphagnum-derived phenols in carbon-rich soil profiles. The studied sites included afforested peaty gley soils under a first- and second-rotation Sitka spruce plantation, unplanted moorland, and self seeded Sitka spruce on unprepared moorland, all of which are located in Kielder Forest, northern England. A pristine peatland in central Sweden was also extensively sampled to assess the carbon related processes occurring in carbon-rich soils prior to afforestation. The effects of afforestation on total carbon stocks were also investigated. The establishment of coniferous forests on peaty gley soils led to a net accumulation of soil carbon during the second rotation, surpassing the moorland carbon capacity. Whilst the encroachment of Sitka spruce on to open moorland via self seeding has resulted in a decreased carbon stock. The phenol composition of soil horizons displayed a maximum lignin content at deep soil across the afforested sites caused primarily by the horizon inversion that occurred prior to planting. Sphagnum acid THM products were identified across the peatland and serve as putative biomarkers for the contribution of Sphagnum-derived organic matter in peats and afforested peatlands, as well as showing potential to provide information on peatland oxic conditions. Sphagnum phenols accumulate preferentially in the anoxic saturated peat, suggesting changes introduced via land-use change or climate change could affect the water table, and thus increase the potential peat decomposition, and the subsequent loss of carbon in peatlands.