The role of lesser snow geese as nitrogen processors in a sub-arctic salt marsh.

Abstract

Ammonia volatilization losses from faeces of Lesser Snow Geese were measured during the summer of 1987 on the salt-marsh flats at La Pérouse Bay. Amounts of ammonia volatilized increased with increasing ambient temperature, and ranged from 1.0 to 15.1 mg N per 100 mg of nitrogen present as soluble ammonium ions at the start of the 8-h experiment. Using estimates of faecal deposition reported previously, the annual loss via volatilization was estimated at 0.08 g N m-2, or 7.9% of the nitrogen present in goose faeces. Percent change in soluble ammonium ions in fresh faeces after 8 h ranged from -51.1% to +41.1%, indicating that net mineralization of organic nitrogen occurred in some of the faeces. Microbial respiration of fresh goose faeces increased exponentially with temperature. However, variable rates of net mineralization per unit rate of respiration indicated that the substrate quality affected microbial immobilization and thus net nitrogen mineralization. In feeding experiments, captive goslings grazed different types of vegetation, each with distinctive nutritional qualities. Forage quality had significant effects on goose feeding behavior and subsequent rates of nitrogen mineralization in fresh faeces. Net nitrogen mineralization rates in faeces from geese which grazed the three vegetation types ranged from 1.31 to 4.97 mg NH 4+ -N gDW-1 24 h-1. Because plant growth in this salt marsh is nitrogen-limited, where swards are grazed, mineralization of organic faecal nitrogen represents an essential link in the maintenance of the flow of nitrogen into the sediments and the sustained growth of vegetation at a time when most required by the geese.