SOIL ORGANISMS AS ECOSYSTEM ENGINEERS OF THE CRITICAL ZONE—THE FORGOTTEN MIDDLE MAN

Soil biota produce burrows, nests, tracks, and trails that are intimately associated with pedogenesis and sediment mixing in the critical zones of fluvial, alluvial, lacustrine, and eolian depositional systems. The significance of these organisms is overshadowed by the emphasis placed on plants and microbes of the critical zone. Field and experimental research on the burrowing activity of soil organisms, however, confirms their tremendous influence on sediment mixing, soil formation, and soil turnover. Organisms are classified as epigeon, geophiles, and geobionts depending on their presence within the soil zone. Epigeon are only involved with the surface, whereas geophiles can have a transient, temporary, or periodic presence in soils depending on the life history of the animal. Geobionts have a permanent presence, as the whole life cycle is underground. Animals of various size and number can displace sediments by diffusive (mm-scale particle movement) and/or advective (cm-to m-scale particle movement) upward or downward mixing via a combination of carrying, pulling, pushing, raking, breaking or cutting, and forcing, resulting in macrochannels and macropores that mediate the activities of plants and microbes. These features have distinct three-dimensional shapes and volumes repeated in space and time by multiple generations of individuals and colonies that act to help build and destroy pedogenic structures and voids, and play a major role as ecosystem engineers in nutrient cycling via their work as herbivores, carnivores, detritivores, and saprovores. The work of soil biota can be explored via neoichnology to provide a basis for understanding structures produced by organism behaviors related to: (1) physicochemical conditions—the effects of climate, topography, parent material—under which they were produced; (2) the size, use, longevity, and ecological significance of the biogenic structure(s); and (3) the groundwater profile and hydrologic system of the landscape.