About this Research Topic
Over the last 15 years researchers have increasingly recognized that in order to understand many soil processes, and be able to predict their future evolution, the traditional macroscopic approach would not do, and a microscopic perspective needed to be adopted instead.
This conclusion has been particularly clear for soil processes involving microorganisms. Several key studies have demonstrated that to better understand what controls the growth and activity of microorganisms, one needed to make observations at their scale, i.e., with a resolution of at most a few microns. The shift in emphasis from the macroscale to the microscale has been rendered possible by tremendous technological advances, such as the advent of dedicated table-top X-ray computed tomography scanners, and the development of various synchrotron-based techniques (such µXRF or NEXAFS) as well as NanoSIMS. Experimental research in this field has been very active and is currently undergoing a major expansion around the world.
In parallel with the experimental work that has been carried out, a significant amount of research has also been devoted to the mathematical modelling of soil processes at the microscale. Various “mathematical network” models of soils have been developed, which in theory were based on a microscale description of the pore space, but these models assumed pores to have a regular geometry, most often that of cylindrical tubes. In the last 15 years, modelling efforts have started instead from the actual geometry of pores, as seen in 3D X-ray CT images of soils. Significant progress has been achieved in the description of water retention and movement in those pores. The approach of choice in that context has been the Lattice-Boltzmann method, whose application to soils has achieved substantial progress. Other methods have also been implemented, involving the use of geometrical primitives (spheres or ellipsoids) or including finite difference and finite element schemes, after discretization of the pore space. Relatively limited work has been devoted to the description of (bio)chemical processes at the microscale in soils but, by contrast, a very sizeable body of literature has focused on modelling the growth and activity of microorganisms, especially bacteria and fungi, in the pore space.
A previous Research Topic, spearheaded by the Soil Processes section, has focussed on elucidating microbial processes in soils and sediments. The 22 articles published in that Research Topic have presented in detail some of the experimental research being carried out in this context, with comparatively slightly less attention spent on the modelling side. In the present Research Topic, we would like to focus specifically on the work being done on the mathematical modelling, at the microscale, of all soil processes, not just those involving microorganisms.
In this general context, the objective of this Research Topic is to serve as an outlet for articles that demonstrate how the use of novel modelling tools at the microscale, starting from actual 3D images of soils, allows us to better understand and predict a variety of soil processes. Of special interest are:
• Articles presenting an interdisciplinary perspective on the modelling effort, for example by combining physical and microbiological or chemical aspects;
• Comparisons of the application of different modelling approaches on sets of CT images;
• Articles dealing with situations where the pore space varies over time, for example in soils that get progressively compacted, or in swelling/shrinking soils; and
• Articles that bridge the gap between the micro- and macroscales, i.e., allowing the much needed and still largely elusive step of upscaling, which will need to be resolved for the research on the microscale to lead to outcomes that are relevant to the everyday practice of soil management.
Manuscripts submitted to this research topic can be of the following types: Original Research, Review, Mini Review on specific topics or Opinion articles.
Keywords: Microbial ecology, carbon sequestration, soil organic matter, greenhouse gas production, dynamics, mathematical modelling, simulation