Disentangling the effects of litter diversity

Abstract

Litter decomposition is a fundamental process about which a great deal is known, but most knowledge comes from studies of single-species decay. Litter-mixing studies have tested whether monoculture data can be applied to mixed-litter systems and have mainly attempted to detect non-additive effects of litter mixing, which addresses consequences of random species loss. Under global change, non-random species loss, characterized by the loss of species more susceptible to changes in environmental factors, is more likely to occur. With this scenario, individual species effects (additivity) as well as species interactions (non-additivity) may alter decomposition processes, potentially showing consequences that differ from those of random loss. To determine the impacts of non-random species loss on decomposition, we looked for both additive and non-additive effects of litter mixing on mass loss, nutrient dynamics, and the decomposer community. To do this, a full-factorial litterbag experiment of four deciduous leaf species was conducted. Data were analyzed using a statistical method that first looks for additive effects based on the presence or absence of species, then significant species interactions occurring beyond that. We found additive species composition (identity) effects on substrate mass loss and most aspects of the decomposer community, suggesting that differences in litter quality override mixing effects for these variables, and the consequences of non-random species loss will be predictable. Additive effects on carbon loss were more evident when the substrate was analyzed separately from microbial biomass colonizing the litter. We found non-additive effects on nutrient dynamics driven by both species richness and composition, with less overall release from multiple-species mixtures than monocultures. This led to great overestimations of ecosystem-level nutrient release when calculated from dynamics of monocultures, as is usually done by other studies, with no net immobilization as was identified by estimations based on the non-additive litter mixtures. Our results suggest a potentially large impact of non-random species loss on this system, which has not been addressed for decomposition, with large repercussions on organic matter and nutrient turnover. Together, these data demonstrate an effect of plant community composition on decomposition and related properties, confirming a link between above- and belowground communities.