Fates of Added Nitrogen in Freshwater Arctic Wetlands Grazed by Snow Geese: The Role of Mosses

Abstract

Previous studies have shown that the growth of freshwater grasses and sedges eaten by breeding colonies of Snow Geese responds weakly to nitrogen additions, and also is poorly able to compensate within the same season for tissues lost to geese. These results contrast with the rapid responses to grazing and fertilization that have been observed in salt-marsh species. A possible explanation is that the mosses prominent in freshwater wetlands rapidly sequester added nitrogen, preventing access by forage species to the fecal inputs provided by foraging geese. To investigate this hypothesis, I added ecologically realistic amounts of ammonium and nitrate labelled with ¹⁵N to the surface and rooting zone of experimental plots in freshwater wetland vegetation at two Snow Goose colonies. Results indicate that the presence of mosses did not prevent forage species from rapidly taking up ammonium and nitrate added either at or below the moss surface. Nonetheless, most of the added ¹⁵N was absorbed by the moss layer; consequently, mosses tend to divert nitrogen away from forage species and into long-lasting peat. In the long term, this may reduce the ability of freshwater forage plants to recover from damage by increasing populations of Snow Geese.