Nitrogen deposition and biodiversity change are important drivers of global change with unclear consequences on C and N turnover in terrestrial ecosystems. N deposition effects have been evaluated in the Northern Hemisphere where N levels have already increased, making it difficult to predict future impacts on undisturbed ecosystems. In addition, plant diversity effects on decomposition have been studied in artificial herbaceous communities on short time scales. On the contrary, we evaluated the simultaneous effects of diversity and N deposition on decomposition within an unpolluted mixed old-growth forest in Patagonia, Argentina. Our diversity treatment was defined by ‘triangles’ created by the intersection of three tree-canopies that directly controlled micro-environmental conditions on the forest floor; and combined it with a nitrogen fertilization treatment (0 and 80 kgN/ha*yr). One- or three-species triangles were composed of single or multiple Nothofagus species (N.dombeyi, N.obliqua, or N.nervosa), where we placed litterbags with different litter types. Litter mass loss and N dynamics after one year were significantly and interactively affected by N addition, triangle composition, and litter type. Litter mass loss consistently and significantly increased with N additions but triangles differed in their response, with a 24% and 11% increase in decomposition in the pluriespecific and in the N.obliqua triangles. Also litter types showed different sensibility to fertilization, with 6% and15% of increase in decomposition. At the same time, litter N immobilization decreased (35% on average) with N additions, and the magnitude of the effect also differed among triangles. These results suggest that plant diversity, in addition to its litter type, creates particular conditions in the soil in the long-term that promote varying ecosystem responses to nitrogen deposition.