River-floodplain restoration and hydrological effects on GHG emissions: Biogeochemical dynamics in the parafluvial zone.

ABSTRACT

The parafluvial zone is frequently exposed to drying-rewetting cycles with critical consequences for the biogeochemistry of soil and sediment in river-floodplain landscapes. Upon restoration of the hydrological connectivity, substantial changes in biogeochemical processes are expected. The effects of water fluctuation on the magnitude of GHG emissions were investigated in the parafluvial zone of a restored river floodplain in Austria. Sediment composition, DOM quality and N2O, CO2, CH4 fluxes were quantified during distinct hydrological periods (intermittent, desiccation and post flood) and along a hydrological gradient. The hydrological gradient ranged from non-flooded plots in the floodplain soil (used as reference plots after restoration), to rarely-flooded and frequently flooded sediment plots in the parafluvial zone. Enhanced biogeochemical turnover rates were identified during the intermittent period, when N2O and CO2 emissions peaked. In particular, the frequently flooded plots showed significantly higher CO2 and CH4 emissions compared to non-flooded and rarely-flooded plots. This indicates a strong effect of water level fluctuation on GHG emissions, with higher emissions occurring during transitional stages of drying and rewetting. Strong positive relationships were found between individual GHG fluxes, suggesting a tight link between C and N cycles. Both the C and N cycles are dependent on similar substrate characteristics that are governed by the quality of the DOM pool. Interestingly, drier sediments in the rarely-flooded plots were also active areas for emissions. This highlights the importance to include dry phases and sites in the overall C and N emission estimates of riverine landscapes. From the restoration point of view, N2O emissions in the parafluvial zone did not differ significantly from the emissions in the reference plots, whereas CO2 and CH4 fluxes did. When making management decisions to restore connectivity, one needs to carefully consider the interplay between nutrient removal from water versus GHG emissions, to reach maximum environmental benefits.