The carbon balance in terrestrial ecosystems is determined by the difference between inputs from primary production and the return of carbon to the atmosphere through decomposition of organic matter. Our understanding of the factors that control carbon turnover in water-limited ecosystems is limited, however, as studies of litter decomposition have shown contradictory results and only a modest correlation with precipitation. Here we evaluate the influence of solar radiation, soil biotic activity and soil resource availability on litter decomposition in the semi-arid Patagonian steppe using the results of manipulative experiments carried out under ambient conditions of rainfall and temperature. We show that intercepted solar radiation was the only factor that had a significant effect on the decomposition of organic matter, with attenuation of ultraviolet-B and total radiation causing a 33 and 60 per cent reduction in decomposition, respectively. We conclude that photodegradation is a dominant control on above-ground litter decomposition in this semi-arid ecosystem. Losses through photochemical mineralization may represent a short-circuit in the carbon cycle, with a substantial fraction of carbon fixed in plant biomass being lost directly to the atmosphere without cycling through soil organic matter pools. Furthermore, future changes in radiation interception due to decreased cloudiness, increased stratospheric ozone depletion, or reduced vegetative cover may have a more significant effect on the carbon balance in these water-limited ecosystems than changes in temperature or precipitation.

Summary 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 plantsoil interactions that affect below-ground processes.

Summary Litter decomposition in terrestrial ecosystems is an important first step for carbon and nutrient cycling, as senescent plant material is degraded and consequently incorporated, along with microbial products, into soil organic matter. The identification of litter affinity effects, whereby decomposition is accelerated in its home environment (home‐field advantage, HFA), highlights the importance of plant–soil interactions that have consequences for biogeochemical cycling. While not universal, these affinity effects have been identified in a range of ecosystems, particularly in forests without disturbance. The optimization of the local decomposer community to degrade a particular combination of litter traits is the most oft‐cited explanation for HFA effects, but the ways in which this specialized community can develop are only beginning to be understood. We explore ways in which HFA, or more broadly litter affinity effects, could arise in terrestrial ecosystems. Plant–herbivore interactions, microbial symbiosis, legacies from phyllosphere communities and attractors of specific soil fauna could contribute to spatially defined affinity effects for litter decomposition. Pyrosequencing soil communities and functional linkages of soil fauna provide great promise in advancing our mechanistic understanding of these interactions, and could lead to a greater appreciation of the role of litter–decomposer affinity in the maintenance of soil functional diversity.