Losses of inorganic carbon and nitrous oxide from a temperate freshwater wetland in relation to nitrate loading

We studied the export of inorganic carbon and nitrous oxide (N2O) from a Danish freshwater wetland. The wetland is situated in an agricultural catchment area and is recharged by groundwater enriched with nitrate (NO3-) (similar to 1000 mu M). NO3- in recharging groundwater was reduced (57.5 mol NO3- m(-2) yr(-1)) within a narrow zone of the wetland. Congruently, the annual efflux of carbon dioxide (CO2) from the sediment was 19.1 mol C m(-2) when estimated from monthly in situ measurements. In comparison the CO2 efflux was 4.8 mol C m(-2) yr(-1) further out in the wetland, where no NO3- reduction occurred. Annual exports of inorganic carbon in groundwater and surface water was 78.4 mol C m(-2) and 6.1 mol C m(-2) at the two sites, respectively. N2O efflux from the sedimenst was detectable on five out of twelve sampling dates and was significantly (P < 0.0001) higher in the NO3- reduction zone (0.35-9.40 mu mol m(-2) h(-1), range of monthly means) than in the zone without NO3- reduction (0.21-0.41 mu mol m(-2) h(-1)). No loss of dissolved N2O could be measured. Total annual export of N2O was not estimated. The reduction of oxygen (O-2) in groundwater was minor throughout the wetland and did not exceed 0.2 mol O-2 m(-2) yr(-1). Sulfate (SO4--) was reduced in groundwater (2.1 mol SO4-- m(-2) yr(-1)) in the zone without NO3- reduction. Although the NO3- in our wetland can be reduced along several pathways our results strongly suggest that NO3- loading of freshwater wetlands disturb the carbon balance of such areas, resulting in an accelerated loss of inorganic carbon in gaseous and dissolved forms.