Discontinuity of the concentration and composition of dissolved organic matter at the peat-pool interface in a boreal peatland

Abstract. Pools are common features of peatlands and can represent from 5 to 50 % of the peatland’s surface area. They play an important role in the peatland carbon cycle by emitting carbon from their surfaces to the atmosphere. However, the origin of this carbon is not well known. A hypothesis is that carbon emitted from pools is the product of mineralised peat-derived dissolved organic matter (DOM). To test this hypothesis, this study examined the origins, compositions, and degradability of DOM in peat porewaters and peat pools within an ombrotrophic boreal peatland in northeastern Québec (Canada) for two years during the growing season. The temporal evolutions of dissolved organic carbon (DOC) concentrations, the optical properties, molecular compositions (TMH-GC-MS), stable isotopic signatures (δ13C-DOC), and degradability of DOM were determined. This study demonstrates that DOM is a complex and highly dynamic component of peatland ecosystems. If the molecular analyses reveal that DOM in porewaters and pools share a common vegetation origin, the compositions of the DOM in the two environments are markedly different. Peat porewater DOM is more aromatic, with a higher molecular weight DOC : DON ratio. The temporal dynamics of DOC concentrations and DOM compositions also differ. In peat porewaters, the DOC concentrations followed a strong seasonal increase from 9 mg L-1, reaching a plateau above 20 mg L-1 in summer and autumn. This is best explained by seasonal vegetation productivity, which is greater than DOM degradation through microbial activity. In pools, DOC concentrations also increased but remained two times lower than in the peat porewaters at the end of the growing season with an average concentration of 10 mg L-1. Those differences might be explained by a combination of physical, chemical, and biological factors. The limited hydraulic conductivity in deeper peat increased the DOM residence time in peat. This might favour both DOM microbial transformation within the peat and the interaction of DOM aromatic compounds with the peat matrix, explaining part of the shift of DOM compositions between peat porewaters and pools. The DOM might be even further transformed at the interface between peat and pools with the production of low molecular weight compounds. This study could not highlight any photolability of DOM and only limited microbial degradability. We estimate that the carbon emissions related to DOM transformation in peatland pools could represent from 4.2 to 8.7 % of the long-term apparent rate of carbon accumulation.