Subantarctic Macquarie Island - a model ecosystem for studying animal-derived nitrogen sources using N-15 natural abundance

Plants collected from diverse sites on subantarctic Macquarie Island varied by up to 30 parts per thousand in their leaf delta(15)N values. N-15 natur al abundance of plants, soils, animal excrement and atmospheric ammonia sug gest that the majority of nitrogen utilised by plants growing in the vicini ty of animal colonies or burrows is animal-derived. Plants growing near sca vengers and animal higher in the food chain had highly enriched delta(15)N values (mean = 12.9 parts per thousand), reflecting the highly enriched sig nature of these animals' excrement, while plants growing near nesting pengu ins and albatross, which have an intermediate food chain position, had less enriched delta(15)N values (> 6 parts per thousand). Vegetation in areas a ffected by rabbits had lower delta(15)N values (mean = 1.2 parts per thousa nd), while the highly depleted delta(15)N values (below -5 parts per thousa nd) of plants at upland plateau sites inland of penguin colonies, suggested that a portion of their nitrogen is derived from ammonia (mean N-15 = -10 parts per thousand) lost during the degradation of penguin guano. Vegetatio n in a remote area had delta(15)N values near -2 parts per thousand. These results contrast with arctic and subarctic studies that attribute large var iations in plant N-15 values to nitrogen partitioning in nitrogen-limited e nvironments. Here, plant N-15 reflects the N-15 Of the likely nitrogen sour ces utilised by plants.