Our results highlight the importance that soil physical properties be considered in greater detail in soil microbiological studies than is currently the case. Bacillus did spread faster than Pseudomonas and both did spread faster at a lower bulk-density. Depending on the physical conditions, bacteria in half the samples took between 1.62 and 9.22 days to spread 1.5 cm. At a bulk-density of 1.6 g cm −3 the average number of Pseudomonas was 63% lower than at a bulk-density of 1.3 g cm −3. We use X-ray CT and show how pore geometrical properties at microbial scales such as connectivity and solid-pore interface area, are affected by the way we prepare microcosms. We study the growth of populations introduced in replicated microcosms packed at densities ranging from 1.2 to 1.6 g cm −3, as well as packed with different aggregate sizes at identical bulk-density. In the current paper we therefore ask the following questions: (i) To what extent can we control the pore geometry at microscopic scales in microcosm studies through manipulation of common variables such as density and aggregate size? (ii) What is the effect of pore geometry on the growth and spread dynamics of bacteria following introduction into soil? To answer these questions, we focus on Pseudomonas sp. These bulk-measures are too crude and do not describe the heterogeneity at microscopic scales where microorganisms operate. We then use surrogate measures of soil architecture such as aggregate size distribution and bulk-density, in an attempt to recreate conditions encountered in the field. The difficulty to capture the architectural heterogeneity in microcosms means that we typically disrupt physical architecture when collecting soils. It is widely recognized that soil architecture is the key driver of biological and physical processes underpinning ecosystem services, and the role of soil architecture and soil physical conditions is receiving growing interest. Simplified experimental systems, often referred to as microcosms, have played a central role in the development of modern ecological thinking on issues ranging from competitive exclusion to examination of spatial resources and competition mechanisms, with important model-driven insights to the field. 4School of Water, Energy and Environment, Cranfield University, Cranfield, United Kingdom.3UMR ECOSYS, AgroParisTech, Université Paris-Saclay, Thiverval-Grignon, France.2Soil Microbial Ecology, FB 2 (Biology/Chemistry), University of Bremen, Bremen, Germany.1School of Science Engineering and Technology, Abertay University, Dundee, United Kingdom.Baveye 3 Andrew Spiers 1 Wilfred Otten 1,4 Archana Juyal 1,2 Thilo Eickhorst 2 * Ruth Falconer 1 † Philippe C.
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