What shapes leaf litter decomposition in aquatic systems?

Fabienne Santschi, Isabelle Gounand, Eric Harvey & Florian Altermatt

 

One of the most important processes in ecology is how, and how much, biomass is produced and how this biomass eventually decomposes again, making resources available to other organisms. Production of biomass is almost universally confined to plants, while decomposition includes activities of microbes, including bacteria, fungi and protists, as well a wide range of small to large animals. Decomposition of organic biomass is not only important for circulating energy and resources in a food web, but can also link different ecosystems: terrestrial biomass, such as leaf litter, can fall into aquatic ecosystems, where it is decomposed and creates cross-ecosystem flows of nutrients. It is hypothesized that the global loss of species leads to impaired decomposition rates and thus to slower recycling of nutrients. But our understanding of diversity effects on litter decomposition is still incomplete, especially in aquatic systems.

We were interested in how both the type and diversity of the resources (i.e., terrestrial biomass in the form of leaf litter), as well as the composition of the aquatic decomposer community, affects decomposition processes. We firstly tested whether mixing leaf litter from different tree species (oak, beech, alder and poplar) alters litter decomposition rates compared to decomposition of these species in monoculture. Secondly, we tested the effect of the size structure of the aquatic microbial decomposer community (with or without larger microbes) on decomposition rates.

Overall, we found that the identity of the leaf litter strongly affected decomposition rates. The observed decomposition rates matched our independent measures of the activity and abundance of microbial decomposers. We found some evidence that leaf litter mixtures from different tree species were more strongly decomposed than leaf litter from these species in isolation. However, this effect was not coherent across all leaf litter combinations. Importantly, microbial communities that had a reduced structural complexity showed a small but significant overall reduction in decomposition rates compared to communities with the naturally complete microbial complexity. This highlights the importance of a complete natural microbial community for leaf litter decomposition.

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