Microbial distribution in soils: physics and scaling
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In a handful of fertile soil there are billions of microorganisms and yet, even with a conservative estimate, the surface area covered by these organisms is considerably less than 1%. What does this tell us about the function of the physical structure in which soil organisms reside and function, collecting, and separating micropopulations from each other and from resources? It would seem that most of the soil is akin to desert regions with little life been supported on its terrains, yet with vast communities of individuals, from an amazing array of species, supported in small-scale habitats, connected or disconnected by saturated or unsaturated pore space over relatively short time-scales. The biodiversity of these communities remains impressive yet overall functionally illusive, bar some considerations of inbuilt redundancy. What is far more impressive is the range of habitats on offer to populations with short-term evolutionary time frames. The availability of spatially and temporally diverse habitats probably gives rise to the biodiversity that we see in soil. It is not too far fetched to state that the majority of habitats on Earth (and indeed extraterrestrial) are revealed in that handful of soil. The key question is what is the functional consequence of such habitat heterogeneity? To answer this it is clear that we need to bring together a new discipline that combines the biology and physics of the soil ecosystem. This biophysical approach, com- bined, where required, with important mineral-microbe knowledge is needed to help us understand the mechanisms by which soils remain productive, and to identify the tipping-points at which there may be no return to sustainability. This review aims to highlight the importance of addressing the soil ecosys- tem as a dynamic heterogeneous system focusing on microbiota–habitat interactions.