Tree species identity alters forest litter decomposition through long-term plant and soil interactions in Patagonia, Argentina

1. A major challenge in predicting biodiversity effects on ecosystem functioning is to understand the linkages between above-ground and below-ground components in natural communities. However, incongruities in spatial and temporal scale between plant and soil processes, and confounding ecological factors, have impeded our understanding of biodiversity effects on below-ground processes, particularly in natural ecosystems with long-lived species such as forests. 2. We designed an approach to isolate plant species composition effects from other ecosystem factors, in order to evaluate the effects of individual tree species and tree species mixtures on litter decomposition in a mixed old-growth forest in temperate South America. We identified 'tree triangles' where the intersection of plant canopies directly controlled micro-environmental and biogeochemical conditions on the forest floor. The monospecific treatment included triangles composed of three trees of a single species of Nothofagus dombeyi, N. obliqua or N. nervosa, while the mixed-species triangles consisted in the intersections of the three different Nothofagus species. 3. We placed litterbags with N. dombeyi, N. obliqua or N. nervosa litter and mixed litter of the three species within all these triangles and estimated the decomposition constant (k) after 1 year of incubation. We also used a standard litter type in all triangles to independently evaluate the tree triangle effects on decomposition. 4. We found that plant species affected decomposition through both direct and indirect effects. Direct effects were mediated through leaf litter quality, while indirect effects were related to unique conditions that the plant species created in the surrounding environment. Despite litter decomposition variation among triangles, standard soil biogeochemical conditions such as soil C : N ratios, microbial biomass and pH were similar among microsites. 5. Most interestingly, we explicitly demonstrated that long-term effects of plant species created specific conditions that enhanced decomposition of their own litter, establishing affinity effects between single-species litter and their own microenvironment. 6. Synthesis. Our results highlight that plant species identity and long-term plant-soil feedbacks are important in affecting litter decomposition in this temperate Patagonian forest. Thus, changes or losses in temperate forest above-ground biodiversity can directly impact litter decomposition both through changes in litter quality inputs and, additionally, through the loss of specific plant-soil interactions that affect below-ground processes.