Abstract/Summary

Abstract: Roots are near‐ubiquitous components of soils globally but have often been regarded as separate from the soil rather than a substantial factor in determining what soil is and how it functions. The start of rapid soil formation commenced about 400 million years ago with the emergence of vascular plants and the evolution of roots and associated microbes. Roots and associated microorganisms contribute significantly to soil formation by altering rocks and soil minerals through a variety of biogeochemical processes and supply carbon to a depth that can have long residence times. Living root inputs of carbon via rhizodeposits are more efficient than shoot and root litter inputs in forming slow‐cycling, mineral‐associated soil organic carbon pools. The current functionality of soils in providing food and fuel and fibres, supplying plant nutrients, filtering water and flood regulation, and disease suppression are all dependent on the activities of plant roots. Roots are actively communicating and collaborating with other organisms for mutual benefit, and the signals underlying this modulation of the rhizosphere microbiome are being identified. In this review I examine how plant roots (an organ not an organism) affect soil formation and function and conclude that, from several perspectives, roots are not just “in” soil but “of” it and that definitions of soil should recognise this. A possible definition is: “Soils are altered surficial rock or sediment, composed of organic matter, minerals, fluids, and organisms whose formation and functionality are influenced by biogeochemical weathering and interactions of these components with plant roots.” Highlights: Paleoclimatic and paleosoil research shows the key role of roots and mycorrhiza in soil formation. Deep roots and living root inputs are substantial contributors to long‐term C storage. Root/microbe signalling facilitates mutualistic symbioses, nutrient uptake and disease suppression. Definitions of soil should explicitly include roots as an important component of the soil system.