Mechanistic, Integrative Multiscale Modeling of the Turnover of Soil Microaggregates
Mechanistische, integrative Multiskalenmodellierung des Umsatzes von Mikroaggregaten im Boden
Creators: name: Zech, Simon
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: DDC Classification::5 Naturwissenschaften::51 Mathematik::510 Mathematik

description: This work is concerned with the mechanistic mathematical modeling of the turnover of soil microaggregates in interaction with the microbial turnover of soil organic matter at different scales. Microaggregates - composite soil structures smaller than 250µm - are considered to be the fundamental structural building blocks of soils and are therefore intrinsically linked to soil functions such as water retention, microbial habitat, and carbon storage and turnover. Soil microaggregates are composed of various mineral, organic and biotic components and are formed by an interplay of physical, biological and chemical mechanisms. Mechanistic models operating at the microscale can help to unravel the complex interplay of factors affecting aggregate formation, complement experimental evidence and are a prerequisite for deriving macroscopic understanding. However, an explicit description of the dynamically evolving soil structure has been neglected in most modeling approaches of microbial turnover of soil organic matter (SOM). In the beginning of this thesis, we present a versatile discrete continuum microscale model that operates on a spatially explicit domain. The model combines partial and ordinary differential equations for the reaction and diffusive transport of different species with a cellular automaton method (CAM) to describe the evolution of the microstructure in terms of solid phase reorganization and biomass evolution. A key component of the model is the ability to account for the effect of soil structure on SOM degradation and vice versa. In addition, data from real soil samples in combination with different imaging techniques (CT, dynamic image analysis) and a morphological fluid distribution model are used to initialize the model in a realistic way. A major focus of this thesis is the application of the microscale model in realistic settings based on experimental data to gain insights that are difficult to obtain from experiments. In the first study, we investigated microbial population dynamics and particulate organic matter (POM) turnover in response to carbon supply, pore space heterogeneity, and water connectivity in soil microaggregates using CT scans. The simulations revealed that heterogeneities in substrate and bacterial distribution have a strong local effect, but are averaged out over the scale of a microaggregate, with water connectivity identified as the key factor at this scale. Second, we studied the formation of microaggregate building units influenced by the size, shape and surface charge of the model minerals goethite and illite under different driving forces. Numerical simulations were in good agreement with laboratory experiments and facilitated the understanding of the formation and stability of microaggregate units with quantitative data that are difficult to obtain through experimental approaches, such as the contact area between particles. In the third model application, we disentangled the interplay between changing soil structure, POM turnover, and the simultaneous modulation of soil surface properties that may promote aggregation, as affected by soil texture, POM input, and POM decomposition rate. Our results showed that POM storage increases with clay content, demonstrating that surface interactions can delay the turnover of OC to CO2. In addition, we observed a priming effect, in which increased POM input stimulated the mineralization of old POM and revealed its origin through an increased rearrangement of the soil structure. As a next major objective, we addressed the challenging task of upscaling microscale models to larger scales. For this purpose, we introduced a macroscale model concept describing the diffusive transport of CO2 over a soil profile, where the CO2 production is calculated by representative microscale models taking into account macroscopic environmental conditions varying with soil depth. Model applications based on the previously investigated microscale scenarios underlined the importance of explicitly taking into account the microscale structure at the macroscale. Finally, we deal with the numerical investigation of a macroscopic flow and transport problem that can be challenging in many porous media applications when structural changes of the pore space lead to the degeneration of hydrodynamic parameters such as porosity, and permeability and diffusion tensors. Applying scalings of the unknowns in terms of the porosity as a remedy, we discretize a combined degenerating flow and transport problem with a mixed finite element method and prove the stability of the scheme. Numerical simulations confirm that optimal convergence is obtained for the transformed variables, while the non-transformed variables may not converge.