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The role of microorganisms in the temporal dynamics of aggregation is examined. Bacteria, fungi, actinomycetes and unicellular algae have minute dimensions, yet they can affect soil structure not only at their own scale but also at much larger scales. The regulation of the temporal and spatial dynamics of aggregate formation, stabilization and destruction by microorganisms involves various feedbacks, because microorganisms may directly or indirectly benefit or may have a reduced activity, because of the structures they contributed to create. The various mechanisms on how microbes control soil structure are described in terms of: structural form; aggregate stabilization; adhesion of microbial cells to solid particles; formation of hyphal networks by fungi and actinomycetes; and decreasing hydrophobicity of particles and aggregates. Also discussed are the different factors controlling microbially mediated aggregation, the dynamics and factors of microorganism-soil structure interaction, and predicting and managing microorganism-soil structure interactions.

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The mechanisms controlling the genesis of rhizosheaths are not well understood, despite their importance in controlling the flux of nutrients and water from soil to root. Here, we examine the development of rhizosheaths from drought-tolerant and drought-sensitive chickpea varieties; focusing on the three-dimensional characterization of the pore volume (> 16 μm voxel spatial resolution) obtained from X-ray microtomography, along with the characterization of mucilage and root hairs, and water sorption. We observe that drought-tolerant plants generate a larger diameter root, and a greater and more porous mass of rhizosheath, which also has a significantly increased water sorptivity, as compared with bulk soil. Using lattice Boltzmann simulations of soil permeability, we find that the root activity of both cultivars creates an anisotropic structure in the rhizosphere, in that its ability to conduct water in the radial direction is significantly higher than in the axial direction, especially in the drought-tolerant cultivar. We suggest that significant differences in rhizosheath architectures are sourced not only by changes in structure of the volumes, but also from root mucilage, and further suggest that breeding for rhizosheath architectures and function may be a potential future avenue for better designing crops in a changing environment.

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