In this study, we describe the seasonal variation in N-15 abundance in the litter of two Sphagnum species and four vascular plant species during 3 years of field decomposition in an Italian Alpine bog. Litter bags were periodically retrieved at the end of summer and winter periods, and the delta N-15 in residual litter was related to mass loss, litter chemistry, and climatic conditions. In Sphagnum litter, higher rates of decomposition during summer months were associated with an increase of delta N-15 probably due to the incorporation of microbial organic compounds rich in N-15. The litter of Eriophorum vaginatum and Carex rostrata was characterized by a decrease of delta N-15, so that the final signature was significantly lower than in initial litter. On the other hand, the residual litter of Potentilla erecta and Calluna vulgaris was characterized by a final delta N-15 higher than in initial litter. Our data reported a seasonality of N-15 abundance in the residual litter of Sphagnum species, but not in that of vascular plant species, thus highlighting the role of differences in litter chemistry.