Loss of nitrogen fixing capacity in a montane lichen is linked to increased nitrogen deposition

The circumboreal/circumpolar N2-fixing lichen Stereocaulon vesuvianum is among the most widespread and abundant fruticose species in montane Britain but has lost capacity to fix N2 over large areas of country. To investigate whether loss N2-fixation S. linked increased N deposition, we examined thallus morphology, physiology chemistry at twelve locations representing an deposition gradient 3–40 kg ha−1 year−1. Measurements were made parallel on a non-N2-fixing reference (Parmelia saxatilis). presence or absence cephalodia (N2-fixing nodules containing cyanobacterium Stigonema sp) was recorded 500 herbarium specimens dating back 1820. Cephalodium abundance vesuvianum, 15N concentration P. saxatilis, strongly negatively correlated with particularly dry deposited N; do not form total rates ≥8–9 Other morphological oddities N-polluted sites include apothecium (fungal reproductive structure) production green algal biofilm development. Biofilm covered thalli without lacked nitrogenase activity where they rarely develop had activities typical for this species. suggest that present-day N-deposition did exist 19th century largely developed between 1900–1940. Synthesis. We provide clear evidence been regions subjected many decades enhanced atmospheric deposition. This consistent established models diazotrophy, which identify supply combined as inhibitor N2-fixation. Progressive depletion increasing line available data indicating much pollution 15N-depleted. Rates are low compared other symbiotic systems perhaps more likely supplanted by elevated ecosystem compartments fixation (e.g. soils) might also be susceptible merit investigation.