The Biomantle-Critical Zone Model

It is a fact that established fields, like geomorphology, soil science, and pedology, which treat near surface and surface processes, are undergoing conceptual changes. Disciplinary self examinations are rife. New practitioners are joining these fields, bringing novel and interdisciplinary ideas. Such new names as "Earth's critical zone," "near surface geophysics," and "weathering engine" are being coined for research groups. Their agendas reflect an effort to integrate and reenergize established fields and break new ground. The new discipline "hydropedology" integrates soil science with hydrologic principles, and recent biodynamic investigations have spawned "biomantle" concepts and principles. One force behind these sea shifts may be retrospectives whereby disciplines periodically re-invent themselves to meet new challenges. Such retrospectives may be manifest in the recent Science issue on "Soils, The Final Frontier" (11 June, 2004), and in recent National Research Council reports that have set challenges to science for the next three decades (Basic Research Opportunities in Earth Science, and Grand Challenges for the Environmental Sciences, both published in 2001). In keeping with such changes, we advocate the integration of biomantle and critical zone concepts into a general model of Earth's soil. (The scope of the model automatically includes the domain of hydropedology.) Our justification is that the integration makes for a more appealing holistic, and realistic, model for the domain of Earth's soil at any scale. The focus is on the biodynamics of the biomantle and water flow within the critical zone. In this general model the biomantle is the epidermis of the critical zone, which extends to the base of the aquifer. We define soil as the outer layer of landforms on planets and similar bodies altered by biological, chemical, and/or physical agents. Because Earth is the only planet with biological agents, as far as we know, it is the only one that has all three agents. Earth's biomantle is where most biota live, reproduce, metabolize, use and expend energy, generate heat and waste, and/or where their remains accumulate upon death. The biomantle is the epidermis of soil and dominantly a product of bioturbation and organic accumulations, impacted by subordinate processes (leaching, eluviation, illuviation, rainwash, throughflow, mass transfer, weathering, biochemical transformations, etc.). Biomantles are either one layered, those without basal stonelayers, or two layered, those with basal stonelayers, depending on presence or absence of gravels and the style of bioturbation. Earth's critical zone integrates the studies of water with soil, biota, air, and rock in the terrestrial surface and near surface environment. Its upper limit is the tops of trees and its lower is the base of the aquifer. Water is the plasma that drives Earth's environmental systems and circulates through the critical zone. The critical zone encompasses the soil, which acts as a geomembrane through which water and solutes, energy, gases, solids, and living organisms biodynamically exchange with the atmosphere, biosphere, hydrosphere, and lithosphere, thus creating a life sustaining environment. Soil also functions as a central link in hydrologic and biogeochemical cycles. The biomantle-critical zone approach provides an appealing way forward in pedology, soil geomorphology, soil ecology, soil science, and near surface geophysical studies. It fosters fresh process and results understandings, and offers Earth science practitioners an array of new interpretive options.