Application of a Cellular Automaton Method to Model the Structure Formation in Soils Under Saturated Conditions: A Mechanistic Approach

Andreas Rupp,Andreas Meier,Thomas Ritschel,Nadja Ray,Tom Guhra,Kai Uwe Totsche,Alexander Prechtel

doi:10.3389/fenvs.2019.00170

Abstract

Soil functions are closely related to the structure of soil microaggregates. Yet, the mechanisms controlling the establishment of soil structure are diverse and partly unknown. Hence, the understanding of soil processes and functions requires the connection of the concepts on the formation and consolidation of soil structural elements across scales that are hard to observe experimentally. At the bottom level, the dynamics of microaggregate development and restructuring build the basis for transport phenomena at the continuum scale. By modeling the interactions of specific minerals and/or organic matter, we aim to identify the mechanisms that control the evolution of structure and establishment of stationary aggregate properties. We present a mechanistic framework based on a cellular automaton model to simulate the interplay between the prototypic building units of soil microaggregates quartz, goethite, and illite subject to attractive and repulsive electrostatic interaction forces. The resulting structures are quantified by morphological measures. We investigated shielding effects due to charge neutralisation and the aggregate growth rate in response to the net system charge. We found that the fraction as well as the size of the interacting oppositely charged constituents control the size, shape, and amount of occurring aggregates. Furthermore, the concentration in terms of the liquid solid ratio has been shown to increase the aggregation rate. We further adopt the model for an assessment of the temporal evolution of aggregate formation due to successive formation of particle dimers at early stages in comparison to higher order aggregates at later stages. With that we show the effect of composition, charge, size ratio, time and concentration on microaggregate formation by the application of a mechanistic model which also provides predictions for soil aggregation behaviour in case an observation is inhibited by experimental limitations.

Highlights

Soil structure is organized in a hierarchy of patterns and properties, as e.g., found in pores and aggregates at different scales
We conducted the following numerical experiments to illustrate the impact of composition, surface charge, particle size ratio, and concentration on microaggregate formation
We started with a solid phase consisting of Qz only (i.e., 0% of the solid is Gt and Il, 100% of the solid is Qz) and increase the portions of Il and Gt stepwise until no Qz remains in the solid (i.e., 50% of the solid is each Il and Gt, while 0% is Qz)

Summary

Introduction

Soil structure is organized in a hierarchy of patterns and properties, as e.g., found in pores and aggregates at different scales. Portell et al (2018), e.g., established an individual based model approach to account for growth of microbial species on explicit pore geometries, and combined it with solute transport realized by a Lattice Boltzmann method. Cellular automaton model (CAM) methods have already successfully been used to describe the structural development of biofilms at the pore scale (Tang and Valocchi, 2013; Tang et al, 2013) or self-organization of soil-microbe systems (Crawford et al, 2012). On a smaller interaction scale, DLVO theory and the fractal growth of diffusion-limited aggregation has been used recently to illustrate the detailed interplay of molecular forces between particles in the formation of microaggregates (Ritschel and Totsche, 2019)

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