We studied mass loss and nitrogen dynamics during fall and spring initiated decomposition of an N2-fixing epiphytic lichen, Lobaria oregana (Tuck.) Müll. Arg., using 15N. We developed a method of labeling lichens with 15N for use in a decomposition study that involved spraying lichen material with a nutrient solution containing 15N-enriched ammonium. Through the first 180 days of sampling, lichens placed in the field during the spring had a smaller decay constant (k = 1.24 year1) than the lichens placed in the field during the fall (k = 3.1 year1). However, both spring and fall lichen samples were decomposed beyond recognition after 1 year. Patterns in exogenous N uptake and N concentration did not differ by season. Both spring and fall lichens took up N from the surrounding environment during decay while simultaneously losing N to the environment. The N concentration in both sets of lichen additions increased during decay to a peak of around 2.8% N, equal to a C to N ratio of about 16, and then began to decrease. This indicates that early in decay, net N immobilization occurred in the remaining lichen, but this was followed by net N mineralization in later stages of decay.Key words: decomposition, nitrogen, Lobaria oregana, lichen, mineralization, immobilization.