Surface soils and their microbiology have been studied for decades. However, subsurface soil, more broadly referred to as the vadose zone, is of increasing interest to microbiologists. The vadose zone, extending from the terrestrial surface to the groundwater table, is rich in microbes of many types. This review summarizes what is known about the abundance and diversity of microbes in the vadose zone and the environmental factors that influence vadose zone microbes and microbial processes. We discuss the roles of vadose zone microbes in nutrient cycling as well as their importance in pollutant remediation. We address a number of fundamental questions in vadose zone microbial ecology, including: What do we need to learn about vadose zone microbes to improve our ability to predict the fates of pollutants? How different are microbial communities and microbial activities in the terrestrial subsurface compared with surface soil? Numerous questions and arguments justify “deepening” soil microbiology's spatial context to include the whole unsaturated subsurface.

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