The natural abundance of 15N in mat-forming lichens

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Abstract
Natural abundance of 15N and [N] was studied in thalli of mat-forming lichens collected from tundra and heathland sites in the northern and southern hemispheres. The study includes samples of British Cladonia portentosa from sites in regions of high and low N-loading and in heathland growing both directly on peat and independently of the soil substratum, in a canopy of prostrate gorse (Ulex minor). In the mat-forming lichens examined, a non-random pattern in [N] and δ15N was characterised by a minimum in δ15N, which occurred most frequently at 20–40 mm below the thallus apex. Nitrogen concentration increased above this point, towards the apex, though remained invariably low towards the thallus base. We discuss the significance of the pattern in [N] and δ15N for current theories describing the uptake and recycling of nitrogen by mat-forming lichens in oligotrophic habitats. Our data are incompatible with the suggested uptake of soil organic-N depleted in 15N, though are consistent with possible internal recycling and the development of a structural necromass. The study emphasises the internal fractionation of nitrogen isotopes and provides a caveat against the assumption that values of δ15N provide an unequivocal indicator of source-sink relationships in nitrogen cycling.