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Johan Emil Kjær

and 8 more

Peatland ecosystems are unsurpassed in their carbon-storing capacity. However, they can be hotspots for emissions of greenhouse gases (GHGs) depending on soil water saturation and oxygen status. Using automated floating chambers, we investigated the spatiotemporal variability of CH4 and CO2 fluxes and their environmental drivers from inundated areas in a temperate, rich fen. We distinguished between two areas: one with continuous inundation, caused by upwelling groundwater and a lower-lying area with periodic inundation by flooding from an adjacent stream. Using hourly measurements, we found mean effluxes of CH4 and CO2 to be 0.16 and 1.23 g C m-2 d-1 between October and May with more than a 10-fold variation between observations. For CO2, efflux were higher in the periodically inundated area compared to the continuously inundated area. In contrast, CH4 fluxes were higher, and dominated by ebullition, at the area with continuous inundation. Both fluxes increased with soil temperature and wind speed. Advective and diffusive fluxes of CH4 and CO2 associated to groundwater upwelling and upwards diffusion of dissolved gases from shallow groundwater (0.5-0.8 meters below ground level) contributed negligibly to the measured fluxes, suggesting that the emitted GHGs were produced close to the terrain. Our data highlight the large spatiotemporal variation of CO2 and CH4 emissions from fens due to variations in hydrology and topography affecting GHG production near the soil surface. Particularly, the temporary dynamics of soil inundation played a major role in controlling the contribution by CO2 and CH4 to wetland GHG release.

Peter Engesgaard

and 5 more

N-loads from subsurface, drains, and groundwater-fed surface (bypass) flows via two riparian zones (crop field and wetland) to a second order stream were investigated by sampling of shallow and deep groundwater on both sides and monthly measurements of flows from springs, drains, and stream including water quality (nitrate). A push-pull test in the crop field gave estimates of first-order denitrification rate (0.23 day–1). Reactive transport modelling evaluated observations of water chemistry and denitrification processes in the groundwater below the crop field showing that nitrate was completely removed by denitrification with pyrite in the aquifer (model rates of 0.6–2.5 mmol NO3 L−1 yr−1). A drain in the crop field routed approximately 10% (bypass) of the regional groundwater inflow to the stream. Buffer efficiency was high at 90%. The wetland on the other side of the stream hosts several locations of focused nitrate-rich groundwater-fed spring discharge, predominantly through a non-maintained drainage system of drainpipes and ditches with bypass accounting for 59% of the regional flow input. Nitrate was completely removed in groundwater by denitrification with dissolved organic matter in shallow groundwater. The regional inflow and N load to the wetland is amongst the highest recorded and data shows that the N load to the stream is very high. The buffer efficiency ranged from 45–83% depending on if all springs contributed to the stream or only the two with visible outflow. A conceptual model for nitrate removal efficiency as a function of Damköhler number and percent bypass flow is proposed.