The natural abundance of (15)N in mat-forming lichens.

Natural abundance of (15)N 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 delta(15)N was characterised by a minimum in delta(15)N, 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 delta(15)N 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 (15)N, 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 delta(15)N provide an unequivocal indicator of source-sink relationships in nitrogen cycling.