A thawing boreal peat landscape along the southern limit of permafrost presently is carbon neutral

Along the southern limit of permafrost in northwestern Canada rising air temperatures have caused widespread land cover changes at unprecedented rates. A prominent change includes thermokarst wetland expansion at the expense of black spruce-dominated boreal forest stands due to the permafrost thaw-induced collapse of peat plateaus. We present a multi-year (2013 – 2017) net ecosystem carbon (C) balance (NECB, g C m^-2year^-1) at Scotty Creek near Fort Simpson, NT. The highly fragmented study site is dominated by permafrost-free wetlands and forested permafrost peat plateaus. Eddy covariance  measurements of net ecosystem carbon dioxide (CO₂) and methane (CH₄) exchanges (2013 – 2017) are complemented by discharge (2014 – 2016) and water chemistry monitoring (2015 and 2016) at the outlets of three small headwater catchments (<0.5 km²) draining the eddy covariance footprint area. In addition to net ecosystem CO₂and CH₄exchanges, the NECB includes the export of dissolved C (DC) as the sum of inorganic and organic C (DIC and DOC), free CO₂and CH₄through runoff, and the estimated import of DOC through precipitation. We use absorbance spectroscopy for dissolved organic matter (DOM) characterization to distinguish different DOM sources among catchments and characteristic land cover types. Between 2013 and 2017, the NECB varied between a weak net C source (~16 ±5 g C m^-2year^-1) and sink (~-22 ±5 g C m^-2year^-1) in 2015 and 2013, respectively, with a mean value of -1 ±7 g C m^-2year^-1. The net C sink-source strength was largely controlled by variations in net CO₂exchange, ranging between a weak net CO₂ sink (~-29 ±3 g C m^-2year^-1) and source (~8 ±4 g C m^-2year^-1) in 2015 and 2013, respectively. In contrast, our study site was a persistent annual net CH₄source (~8 ±1 g C m^-2year^-1). Compensated by the import of DOC through precipitation, DC exported from the three catchments was a negligible component of the NECB. There were no significant differences in DOC concentrations and absorbance indices among catchments, and thawed and frozen land cover types, overall illustrating high DOM aromaticity (SUVA₂₅₄= 3.3 ± 0.6 L mg^-1m^-1) and high molecular weight (a254:a365 = 4.3 ± 0.3) characteristic for peatlands and peat-dominated landscapes outside the circumpolar permafrost region. We conclude that a rapidly thawing boreal peat landscape along the southern limit of permafrost presently appears to be C neutral.