Life Underground: Who, Where, Why?

Without life, there would be no soil; however, soil taxonomy does not explicitly include soil biology in its classification system(s). Microbial processes are, without question, intertwined with and drivers of many of the criteria and distinguishing characteristics—pH, organic matter, structure, among others—used to help group soils. Beijerinck’s (1913) hypothesis that “everything is everywhere, but, the environment selects” was commonly accepted, though rigorous testing of this idea was not possible until major advances were made in molecular biology, analytical chemistry, and visualization tools. Biogeography of soil microorganisms—the study of the distribution of organisms in space and time—has expanded rapidly over the last two decades. Investigations of how biodiversity and abundance vary in space have tackled different physical scales: aggregate, column, field, landscape, continental, global. Different environmental factors are important in explaining variations in biodiversity at different scales. Among soil organisms, bacteria, in particular, are small and easy to disperse, survive harsh conditions for long periods, and their populations and communities respond rapidly to change; these biological traits also shape patterns in overall diversity. The human factor-- land use and disturbance (e.g., agriculture, contamination, deforestation)—also plays a major role in alteration and loss of soil biodiversity and overrides underlying patterns. Despite the sensitivity of soil biodiversity to so many factors, there appears to be a “core” biodiversity or portion of the community that remains relatively constant in a given soil. Specific examples are presented and how, if at all, soil biodiversity can be included in a soil taxonomic framework is discussed. Given that a healthy soil is a living soil, understanding who lives where and why will help us find ways to protect and regenerate our soils.