Dissolved organic carbon concentrations vary with season and land use – investigations from two fens in Northeastern Germany over two years

Abstract. The rising export of dissolved organic carbon (DOC) from peatlands during the last 20 years is of great environmental concern, as DOC harms drinking water quality and diminishes the carbon storage of peatlands. Lack of knowledge particularly exists for fens. The aim of our study was to determine DOC concentrations at an agriculturally used fen and a rewetted fen throughout the year. We measured DOC concentrations in ditch water of these fens in 2011 and 2012. Furthermore, discharge measurements were condcucted to detect DOC export. Overall DOC concentrations at our agriculturally used site and at our rewetted site were 35 mg L⁻¹ and 26 mg L⁻¹ (median), respectively. The maximum DOC concentration at our agriculturally used site was twice as high as at the rewetted site (134 mg L⁻¹ vs. 61 mg L⁻¹). Annual DOC export was calculated for the rewetted site, amounting to 200 kg C ha⁻¹ on average. Our results suggest that rewetting of degraded fens reduces DOC export in the long-term, while agricultural use of fens leads to enhanced decomposition and thus, elevates DOC export.