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

Abstract

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.