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

Plants collected from diverse sites on subantarctic Macquarie Island varied by up to 30‰ in their leaf δ15N values. 15N natural abundance of plants, soils, animal excrement and atmospheric ammonia suggest that the majority of nitrogen utilised by plants growing in the vicinity of animal colonies or burrows is animal-derived. Plants growing near scavengers and animal higher in the food chain had highly enriched δ15N values (mean = 12.9‰), reflecting the highly enriched signature of these animals' excrement, while plants growing near nesting penguins and albatross, which have an intermediate food chain position, had less enriched δ15N values (>6‰). Vegetation in areas affected by rabbits had lower δ15N values (mean = 1.2‰), while the highly depleted δ15N values (below −5‰) of plants at upland plateau sites inland of penguin colonies, suggested that a portion of their nitrogen is derived from ammonia (mean 15N =−10‰) lost during the degradation of penguin guano. Vegetation in a remote area had δ15N values near −2‰. These results contrast with arctic and subarctic studies that attribute large variations in plant 15N values to nitrogen partitioning in nitrogen-limited environments. Here, plant 15N reflects the 15N of the likely nitrogen sources utilised by plants.