Аннотация:We carried out long-term incubations to study the contribution of endogenous and exogenous nitrogen to decomposition of various fractions of forest litter, sampled in linden, pine and aspen mixed forest in Prioksko-Terrasny Nature Reserve. Based on C: N ratio signatures (including endogenous nitrogen) the following sequence was found: mortmass of cyanobacteria Nostoсcommunae (C : N ratio 9), deciduous litter (C : N ratio 32), pine needles litter (C : N ratio 66), pine bark litter (C : N ratio 84), coarse woody debris of linden (C : N ratio 206), coarse woody debris of pine (C : N ratio 510). To find the effect of exogenous nitrogen we applied NH4NO3 to the litters until the prescribed C : N ratio in the range of 5 to 204 has been reached. Mineralization was assessed by CO 2 emission intensity. Humification was measured by changes in the share of structural fragments in organic matter of litter from solid-state 13 C NMR. We found logarithmic relationship between the rate of carbon mineralization and initial C:N ratio in litter, having maximum at C : N ratio of 22. Mineral nitrogen treatment increased the intensity of mineralization of the litter fractions poor in nitrogen (C : N ratio exceeding 66) and inhibited CO 2 emission from decomposition of litter with high nitrogen content (C : N ratio from 9 to 32). During the litter decomposition the Alkyl/O-Alkyl Ratio increased. It corresponds to the level of humification of plant matter in soils. Additional nitrogen treatment has stimulated humification, especially during pine needles decomposition. Thus we found the effect of endogenous and mineral nitrogen on both mineralization and humification of forest litter.