Nitrogen mineralization, plant growth and goose herbivory in an Arctic coastal ecosystem

Lesser snow geese feed intensively on graminoid vegetation in intertidal salt marshes at La Perouse Bay, Manitoba. Relatively little grazing occurs in more inland sites, where the same vegetation persists but tidal inundations are infrequent. At some sites geese have grubbed vegetation exposing sediments. Feeding preferences of geese may be linked to the availability of nitrogen for plant growth in the different areas. Total soil nitrogen, exchangeable inorganic nitrogen, net mineralization of nitrogen, together with soil properties, were measured in intertidal and inland sites (both grazed and ungrazed), and in intertidal grubbed sites. Where applicable, rates of net mineralization of nitrogen were compared with above-ground biomass and with the nitrogen content of shoots of graminoids in both grazed and ungrazed (exclosed) swards. All soils were regosolic static cryosols with a thin Ah humus horizon. Most graminoid roots were confined to the top 2 cm of soil, few penetrated deeper than 5 cm. Bulk density of soil 1-2 cm below the surface was 0.87 g cm-3 and 0.53 g cm-3, respectively, in the intertidal and inland marsh, and water content and salinity of soil were higher in the inland marsh. Total nitrogen in soils (0.2 cm below the surface) in the inland marsh was 118 g +/- 6 g N m-2 compared with 80 +/- 2 g N m-2 in the intertidal marsh. Total soil nitrogen, exchangeable inorganic nitrogen and net mineralization of soil nitrogen were significantly greater beneath intact swards compared with grubbed swards in the intertidal marsh. Seasonal net cumulative amount of nitrogen mineralized in the intertidal marsh (86 days) in 1991 was 0.53 g N m-2 at a soil depth of 1-2 cm. The comparable value for the inland marsh was 0.06 g N m-2. In 1992 (54 days) the net amount of nitrogen mineralized in vegetated intertidal sites was 0.18 g N m-2 (1-2 cm) and in grubbed sites it was 0.06 g N m-2. Cumulative above-ground biomass of graminoid species in exclosed plots was greater in the intertidal marsh compared with that in the inland marsh. The amount of nitrogen (as percentage) in above-ground biomass was also higher in plants from the intertidal marsh. Differences in the quantity and quality of vegetation are associated with variation in biogeochemical cycling in soils. Geese exploit patches of vegetation as their primary source of forage where net above-ground primary production is high and plant tissues are rich in nitrogen.