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Size‐structured food webs form integrated trophic systems where energy is channeled from small to large consumers. Empirical evidence suggests that size structure prevails in aquatic ecosystems, whereas in terrestrial food webs trophic position is largely independent of body size. Compartmentalization of energy channeling according to size classes of consumers was suggested as a mechanism that underpins functioning and stability of terrestrial food webs including those belowground, but their structure has not been empirically assessed across the whole size spectrum. Here we used stable isotope analysis and metabolic regressions to describe size structure and energy use in eight belowground communities with consumers spanning 12 orders of magnitude in living body mass, from protists to earthworms. We showed a negative correlation between trophic position and body mass in invertebrate communities and a remarkable nonlinearity in community metabolism and trophic positions across all size classes. Specifically, we found that the correlation between body mass and trophic level is positive in the small‐sized (protists, nematodes, arthropods below 1 μg in body mass), neutral in the medium‐sized (arthropods of 1 μg to 1 mg), and negative in the large‐sized consumers (large arthropods, earthworms), 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, for example 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 suggest that the 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 are supported by a balance across individual size compartments. [ABSTRACT FROM AUTHOR]

[Display omitted] • We studied cormorant-related processes of marine matter uptake by soil food webs. • Cormorants are important in bringing marine nitrogen but not carbon deeper inland. • Wave transfer secures marine carbon uptake by soil animals only close to the coast. Marine organic matter can potentially enter terrestrial ecosystems via abiotic or biotic vectors. The former is facilitated by e.g. waves, tides, winds, etc.; the latter – by the activity of organisms, e.g. seabird migration. However, differences in its assimilation rate in detrital food webs at different distance from water remain generally unexplored. To fill this gap, we compared the consumption of marine organics by terrestrial invertebrates within coastal ecosystems of the Black sea using stable isotope analyses of carbon and nitrogen. We focused on relatively inland (100 m from the coast) forested areas with the recently established overwintering colonies of the great cormorant (Phalacrocorax carbo) and control ecosystems isolated from any marine organic matter input. Guano and cormorant food leftover input led to the significant increase of δ15N values of soil, litter and all trophic groups of soil invertebrates. However, δ13C values changed to a smaller extent. The importance of marine resources (in a form of guano) as a carbon source for soil arthropods was relatively low: approximately 15% of their diet. In contrast, coastal soil invertebrates were significantly enriched with 13C compared to control and cormorant-influenced ecosystems as they were greatly dependent on the seaweed and other marine resources (on average 45% of coastal invertebrate diet). We conclude that marine resources delivered by an abiotic vector may form the basis of the soil food web diets in a 15 m zone from the water edge. Alternatively, the biotic transfer further inland (forest on a cliff 100 m away from the water edge) with e.g. nitrogen-rich seabird guano or food remains may geographically extend proliferation of marine nutrients into terrestrial ecosystems. As far as we know, our results are among the first to demonstrate the spatial zoning of mechanisms securing bottom-up subsidy of biogenic elements to soil ecosystems having originated from the sea. [ABSTRACT FROM AUTHOR]