Resource omnivory in soil food webs and meta-ecosystem connections

Soil food webs were long considered an ecosystem sink for primary production and a black box of reticulated interactions. Quantification of multiple and changing interactions among consumers and resources within and beyond soil food webs stands up as a major challenge. In this mini-review/opinion paper, I present development of ideas on soil food-web structure focusing on resource omnivory – a central characteristic that is linked to food-web structure and stability. There is plenty of empirical evidence for trophic differentiation among soil invertebrates along different food-web dimensions (food resources, trophic levels, microhabitats, time). This comes along with the pervasive idea of widespread omnivory in soil food webs. I argue that we need to quantitatively assess multiple-resource feeding by soil consumers and related drivers across various taxa and different ecosystem types to come closer to predictions of soil food-web structure and dynamics. At the meta-ecosystem level, cross-ecosystem omnivory (i.e. feeding across energy fluxes from different ecosystems) plays an important role in connecting soil with aboveground and aquatic food webs. Aboveground-belowground studies have been focusing on the interfaces such as the rhizosphere and litter surface. Broader cascading impacts of the energy and organismic fluxes across these interfaces within the recipient ecosystem are, however, less understood. Of particular interest here are connections of vertebrate communities to soil food webs and the central role of soil-borne insects in cross-ecosystem energy exchange. Interactions between soil and aquatic food webs span for dozens to hundreds of meters from the terrestrial-aquatic interface, transferring significant amount of energy and matter between these ecosystems. Consequent changes of the structure and functioning in the recipient ecosystem requires more attention, especially how biodiversity-ecosystem functioning relationships manifest across ecosystems. Continuously developing methods, such as compound-specific isotopic analyses, can facilitate quantification of cross-ecosystem omnivory, helping to understand effects of food-web changes across the ecosystem borders. Overall, I present soil food webs as an open and dynamic pool with multiple cross-ecosystem connections and call for quantitative studies in this direction.