Wetland hotspots on the edge – large N2O emissions dominate the soil greenhouse gas budget in a recipient fen

Abstract

Rewetting fens in agricultural landscapes serves as a method to counteract net emissions of greenhouse gases (GHGs) into the atmosphere. The prevalent assumption is that the rewetted area exhibits uniform behavior; however, peripheral zones of a wetland may experience elevated nutrient levels from the surrounding landscape's drainage, leading to internal gradients of biogeochemical processes within the wetland. This aspect is frequently overlooked in GHG budgets for rewetted fens. In this investigation, we employed an automated GHG flux system (SkyLine) to quantify the annual soil GHG budget at the transition from mineral upland to organic soils in a wet fen, including a partially obstructed drainage ditch. From February 2022 to January 2023, CO2, CH4, and N2O fluxes were automatically measured at 27 plots along a 30-meter transect resulting in over 40.000 fluxes per gas for the site. Spatiotemporal patterns of GHG fluxes were studied alongside measurements of groundwater level, soil moisture, and temperature. Due to the chamber configuration, vegetation was excluded from the measurement plots, allowing for the assessment of net soil GHG exchange. Overall, CO2 and N2O fluxes exhibited similar seasonal trends, indicating comparable climatic and hydrological drivers. CO2 fluxes displayed a distinct seasonal pattern, peaking during the warmest periods. Similarly, N2O fluxes reached maximum values in the summer, however, responding rapidly to fluctuating groundwater caused by precipitation. During these hot moment N2O fluxes increased from close-to-zero to maximum values and reaching minimum again within hours to days. CH4 fluxes were overall below zero with minimal seasonal variability, resulting in a net uptake, though occasional emission spikes occurred. Temporal stability of GHG fluxes across the transect was observed, but flux magnitudes varied significantly between individual plot. Annual soil CO2 effluxes varied sixfold, and annual N2O emissions varied tenfold across the transect. Converted to CO2-equivalents, it became evident that, in the absence of plants, that the GHG budget in the border zone of the fen was dominated by N2O emissions, likely due to the net import of nitrogen with groundwater from upland fields fueling high rates of denitrification in the subsoil. CH4 did not significantly contribute to the GHG budget for the plots on peat but dominated for the ditch due to ebullitions. Our findings show the dynamic nature of GHG fluxes in response to environmental variations in peat soils, emphasizing the impact of fluctuating groundwater. While rewetting may enhance complete denitrification and reduce net N2O, border zones of rewetted wetlands may still experience dynamic hydrology and nutrient inputs. Factors, that collectively promote N2O emissions, particularly during critical, short-lived hot moments. Episodic N2O emissions from this zone can disproportionately influence the magnitude of GHG emission reduction following rewetting. Preliminary results of our net soil GHG budget analysis for this location will be presented, highlighting the necessity for high-frequency flux measurements to elucidate underlying causes of temporal patterns in GHG fluxes and their relationship to biogeochemical, hydrological, and climatic drivers.