Headwater Mires Constitute a Major Source of Nitrogen (N) to Surface Waters in the Boreal Landscape

Nutrient exports from soils have important implications for long-term patterns of nutrient limitation on land and resource delivery to aquatic environments. While plant-soil systems are notably efficient at retaining limiting nutrients, spatial and temporal mismatches in resource supply and demand may create opportunities for hydrologic losses to occur. Spatial mismatches may be particularly important in peat-forming landscapes, where the development of a two-layer vertical structure can isolate plant communities on the surface from resource pools that accumulate at depth. Our objectives were to test this idea in northern Sweden, where nitrogen (N) limitation of terrestrial plants is widespread, and where peat-forming, mire ecosystems are dominant features of the landscape. We quantified vertical patterns of N chemistry in a minerogenic mire, estimated the seasonal and annual hydrologic export of organic and inorganic N from this system, and evaluated the broader influence of mire cover on N chemistry across a stream network. Relatively high concentrations of ammonium (up to 2 mg l(-1)) were observed in groundwater several meters below the peat surface, and N was routed to the outlet stream along deep, preferential flowpaths. Areal estimates of inorganic N export from the mire were several times greater than from an adjacent, forested catchment, with markedly higher loss rates during the growing season, when plant N demand is ostensibly greatest. At broader scales, mire cover was positively correlated with long-term concentrations of inorganic and organic N in streams across the drainage network. This study provides an example of how mire formation and peat accumulation can create broad-scale heterogeneity in nutrient supply and demand across boreal landscapes. This mismatch allows for hydrologic losses of reactive N that are independent of annual plant demand and potentially important to receiving lakes and streams