Size compartmentalisation of energy channeling in terrestrial belowground food webs

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Anton Potapov, Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, Russian Federation, Anton Potapov, J.F. Blumenbach Institute of Zoology and Anthropology, University of Göttingen, Göttingen, Germany, Oksana Rosanova, Eugenia Semenina, Vladislav Leonov, Sergey Tsurikov, Andrey Zuev and Alexei Tiunov, A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Anton Esaulov, Penza State University, Anastasiya Korotkevich, Moscow State Pedagogical University, Alexey Kudrin, Institute of Biology of Komi Scientific Centre of the Ural Branch of the Russian Academy of Sciences, Yuri Mazei, Lomonosov Moscow State University

Background/Question/Methods
Trophic interactions in soil are among vital connections, sustaining life below- and above ground. Soil food webs rely on detritus, roots, bacteria and fungi and control decomposition and nutrient mineralization in most terrestrial ecosystems. Traditional soil food-web reconstructions assume that energy is channeled through root, bacterial and fungal energy channels that are coupled by generalistic predators. However, empirical data show little support for the strong differentiation among the energy channels in various primary consumers. Alternative persisting pathways of energy channeling in soil food webs were, however, not suggested. Here, we used stable isotope analysis and metabolic regressions, to explore if energy channeling in soil food webs is organised according to the body size classes of soil fauna. Data on trophic level and community metabolism were collected for consumers spanning 12 orders of magnitude in living body mass, from protists to earthworms, from eight temeperate forests in European Russia.
Results/Conclusions
A remarkable non-linearity in community metabolism and trophic positions was observed across all size classes. Correlation between body mass and trophic level was positive in small-sized, neutral in medium-sized and negative in large-sized consumers, suggesting that these groups form compartments with different trophic organization. Based on this pattern, we propose a concept of belowground food webs being composed of (1) size-structured micro-food web driving fast energy channeling and nutrient release, e.g. in microbial loop, (2) arthropod macro-food web with no clear correlation between body size and trophic level, hosting soil arthropod diversity and subsidizing aboveground predators, and (3) ‘trophic whales’, sequestering energy in their large bodies and restricting its propagation to higher trophic levels in belowground food webs. The three size compartments are based on a similar set of basal resources, but contribute to different ecosystem-level functions and respond differently to variations in climate, soil characteristics and land use. We argue that widely used vision of resource-based energy channeling in belowground food webs can be complemented with size-based energy channeling, where ecosystem multifunctionality, biodiversity and stability is supported by a balance across individual size classes of soil consumers.