NITROGEN MINERALIZATION, PLANT-GROWTH AND GOOSE HERBIVORY IN AN ARCTIC COASTAL ECOSYSTEM

1 Lesser snow geese feed intensively on graminoid vegetation in intert idal salt marshes at La Pi rouse Bay, Manitoba. Relatively little graz ing occurs in more inland sites, where the same vegetation persists bu t tidal inundations are infrequent. At some sites geese have grubbed v egetation exposing sediments. 2 Feeding preferences of geese may be li nked to the availability of nitrogen for plant growth in the different areas. Total soil nitrogen, exchangeable inorganic nitrogen, net mine ralization of nitrogen, together with soil properties, were measured i n intertidal and inland sites (both grazed and ungrazed), and in inter tidal grubbed sites. Where applicable, rates of net mineralization of nitrogen were compared with aboveground biomass and with the nitrogen content of shoots of graminoids in both grazed and ungrazed (exclosed) swards. 3 All soils were regosolic static cryosols with a thin Ah hum us 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 t he surface was 0.87 g cm(-3) and 0.53 g cm(-3), respectively, in the i ntertidal and inland marsh, and water content and salinity of soil wer e higher in the inland marsh. 4 Total nitrogen in soils (0-2 cm below the surface) in the inland marsh was 118 g +/- 6 g N m(-2) compared wi th 80 +/- 2 g N m(-2) in the intertidal marsh. Total soil nitrogen, ex changeable inorganic nitrogen and net mineralization of soil nitrogen were significantly greater beneath intact swards compared with grubbed swards in the intertidal marsh. 5 Seasonal net cumulative amount of n itrogen 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 inl and marsh was 0.06 g N m(-2). In 1992 (54 days) the net amount of nitr ogen 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). 6 Cumulative above-gr ound biomass of graminoid species in exclosed plots was greater in the intertidal marsh compared with that in the inland marsh. The amount o f nitrogen (as percentage) in above-ground biomass was also higher in plants from the intertidal marsh. 7 Differences in the quantity and qu ality of vegetation are associated with variation in biogeochemical cy cling in soils. Geese exploit patches of vegetation as their primary s ource of forage where net above-ground primary production is high and plant tissues are rich in nitrogen.