Changes in Aquatic Carbon Following Rewetting of a Nutrient‐Poor Northern Peatland

Abstract Rewetting drained peatlands by raising the groundwater table is currently suggested, and widely implemented, as an efficient measure to reduce peat soil degradation and decrease CO 2 emissions. However, limited information exists regarding effects of peatland rewetting on lateral carbon export (LCE) via the aquatic pathway. Any changes in LCE are critical to consider, as they affect the overall peatland C balance, and may offset any climatic benefits from rewetting. Additionally, altered LCE could have consequences for downstream water quality and biota. Here, we monitored aquatic C content (DOC, DIC and CH 4 ) in runoff and pore water, as well as radiocarbon content of DOC in runoff from a drained, nutrient‐poor boreal peatland that was rewetted during autumn 2020. By comparing pre‐ (2019–2020) and post‐ (2021–2022) rewetting periods, we detected changes in the aquatic C export. The results showed that the rewetting effect was site‐, season‐ and C form‐specific. Overall, one catchment showed elevated (DOC, DIC) or highly elevated (CH 4 ) concentrations and exports post‐rewetting, whereas the other site showed only elevated DOC. Changes in runoff C concentrations after rewetting were likely driven by site‐specific factors such as expansion of open‐water areas, altered hydrological flow paths and proportion of filled in ditches of total ditch length. Finally, radiocarbon measurements indicated enhanced export of contemporary DOC via runoff following rewetting. These initial (short‐term) findings highlight the need for site‐specific before‐after assessments to better evaluate the C sequestration capacity of peatlands while undergoing rewetting operations.