Stable isotope evidence for marine-derived avian inputs of nitrogen into soil, vegetation, and earthworms on the isle of Rum, Scotland, UK

The largest breeding colony of Manx shearwaters (Puffinus puffinus) in the world is found on the Isle of Rum in the inner Hebrides of Scotland. We collected a feather, guano, soil, vegetation, and earthworms near shearwater burrows to determine whether inputs of nitrogen (N) from guano were incorporated into belowground foodwebs. For comparison, similar samples were collected from a nearby plot that was experimentally fertilised 40 years prior. The shearwater feather had the highest level of 15N enrichment (+18‰), followed by guano (+12‰). Soil (+7.5‰) and vegetation (+5.7‰) collected at the burrow entrance were enriched with 15N relative to those collected at 2 m or more away (+0.5 to +3.1‰ for soil, and −2.5 to −4.6‰ for vegetation). In contrast, soil inside the fertilised plot had δ15N ≈ 0, but was enriched with 15N away from the plot edge. Earthworms collected from shearwater greens had enriched 15N signatures relative to earthworms from the fertilised plot (+3.8 and −0.9‰, respectively). Our data suggest that available N is tightly cycled in vegetation and soil for decades, and that shearwater derived N is substantially assimilated by earthworms. Therefore, because earthworms do not occur outside areas of shearwater influence, the birds should be viewed as ecosystem engineers of soil invertebrate foodwebs on Rum.

► Manx shearwater guano is a source of nutrients for infertile soils on Isle of Rum. ► Earthworms inhabit shearwater greens, and assimilate guano-derived 15N. ► Seabird N is assimilated into belowground food webs. ► Manx shearwaters are ecosystem engineers of soil food webs on Rum.