Size-structured food webs form integrated trophic systems where energy is being channelled from small to large consumers. Empirical evidence suggests that size structure prevails in aquatic ecosystems while in terrestrial food webs trophic level is largely independent of body size.Compartmentalisation 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 community-wide decline in trophic level with body mass in invertebrates and a remarkable non-linearity in community metabolism and trophic positions across all size classes. Specifically, we found that correlation between body mass and trophic level is positive in small-sized (protists, nematodes, arthropods below 1 µg in body mass), neutral in medium-sized (arthropods of 1 µg to 1 mg) and negative in 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, 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 suggest 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 compartments.

Climate and human activity affected significantly the Eurasian on the forest vegetation zone through the Holocene. This paper presents new multi-proxy records of environmental changes at the southern boundary of the mixed coniferous-broadleaf forest zone in the eastcentral part of the East European Plain during the middle and late Holocene. Palaeoecological analyses of a peat core for pollen, charcoal, peat humification, plant macrofossils and testate amoebae with dating using radiocarbon have shown that climate appears to have been a dominant control on vegetation. There is strong evidence for a reduced precipitationevapotranspiration ratio and high fire frequency during the Holocene thermal maximum (6.9-5.3 ka BP), leading to dominance of Betula-Pinus forests. By contrast subsequent climatic cooling led to the expansion of broadleaf forests and establishment of Picea. Human activities influenced vegetation from the Neolithic onwards but played a role which was secondary to climate until the recent past. Over the last century human impacts considerably increased due to harvesting of broadleaf trees and contributed to the formation of the current mixed coniferous-broadleaf forests.

Global climate warming disproportionately affects high-latitude and mountainous terrestrial ecosystems. Warming is accompanied by permafrost thaw, shorter winters, earlier snowmelt, more intense soil freeze-thaw cycles, drier summers, and longer fire seasons. These environmental changes in turn impact surface water and groundwater flow regimes, water quality, greenhouse gas emissions, soil stability, vegetation cover, and soil (micro)biological communities. Warming also facilitates agricultural expansion, urban growth, and natural resource development, adding growing anthropogenic pressures to cold regions’ landscapes, soil health, and biodiversity. Further advances in the predictive understanding of how cold regions’ critical zone processes, functions, and ecosystem services will continue to respond to climate warming and land use changes require multiscale monitoring technologies coupled with integrated observational and modeling tools. We highlight some of the major challenges, knowledge gaps, and opportunities in cold region critical zone research, with an emphasis on subsurface processes and responses in both natural and agricultural ecosystems. Expected final online publication date for the Annual Review of Environment and Resources, Volume 46 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.