Long-term drainage reduces CO₂ uptake and increases CO₂ emission on a Siberian floodplain due to shifts in vegetation community and soil thermal characteristics

With increasing air temperatures and shifts in precipitation patterns forecasted in the Arctic over the coming decades, thawing of ice-rich permafrost is expected to change the hydrological conditions in large parts 15 of the region by creating mosaics of wetter and drier areas. The objective of this study is to investigate how lowered water table depths of formerly wet floodplain ecosystems affect CO2 fluxes measured with a closed chamber system, focusing on the roles of changes in vegetation community structure and soil thermal characteristics. We found that a decade-long drainage significantly increased the abundance of shrubs but decreased that of Eriophorum angustifolium, which subsequently made Carex species dominant. These two 20 changes had opposing influences on photosynthetic uptake during the growing season: increased abundance of shrubs slightly increased gross primary production while replacement of Eriophorum by Carex significantly decreased it. Drainage also diminishes the heat capacity and thermal conductivity of soil, leading to increased soil temperatures in shallow layers during the daytime and decreased soil temperatures in deeper layers, and therefore reduced thaw depths. This soil temperature regime can intensify growing-season ecosystem respiration by up to 93 % theoretically. Overall, drainage increased net CO2 uptake (net ecosystem 25 exchange) by 16 % over 20 days in 2013 but decreased it by 37 % over 66 days in 2014. During the frozen season, the drained transect emitted four times more CO2 than the undrained transect. In summary, the net effect of these complex changes recently weakened net CO2 uptake in the drained areas