How water connectivity and substrate supply shape the turnover of organic matter – Insights from simulations at the scale of microaggregates

• Published in: Geoderma Vol. 405; p. 115394
• Main Authors: Zech, Simon, Ritschel, Thomas, Ray, Nadja, Totsche, Kai Uwe, Prechtel, Alexander
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2022
• Subjects: Cellular automaton; Dissolved organic matter; Michaelis–Menten kinetics; Microbial habitats; X-ray microtomography; Michaelis–Menten kinetics; Cellular automaton; Dissolved organic matter; Microbial habitats; X-ray microtomography
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2021.115394

•We model soil microbial dynamics by a cellular automaton and differential equations.•Simulations are based on CT-scans of microaggregates and realistic parameter values.•Scenarios in silico allowed investigations at scales still inaccessibe ex-situ.•Connectivity, not supply controls microbial degradation at the microaggregate scale.•We contribute to the understanding of metabolic factors driving the Birch effect. Microaggregates are hot spots of microbial activity at a scale that frequently poses a severe experimental challenge or defies a direct observation. Mathematical models that combine the mechanisms of spatially resolved organic matter transport with the processes of organic matter turnover can facilitate the understanding of soil microbial dynamics and the function of soils at these scales. In this study, we investigate microbial population dynamics and the turnover of particulate organic matter (POM) in soil microaggregates. CT images of microaggregates obtained from samples of natural soils serve as basis for selecting the simulation domain. For different levels of water saturation, the fluid (liquid and gas) distribution within the pore space is calculated according to a morphological model. We consider bacteria and POM, which are heterogeneously distributed within the liquid phase. Dissolved organic carbon (DOC) is released by hydrolyzing POM, assuming a reaction following first-order kinetics. DOC spreads by diffusion and can subsequently be consumed by bacteria and turned into CO2. The growth of bacteria is realized by a cellular automaton framework (CAM) and based on Michaelis–Menten kinetics due to the uptake of DOC. Our simulations show that the heterogeneous distribution of substrate and bacteria results in an overall biodegradation kinetics and CO2 output that strongly depends on the microaggregate scale (<250μm). Only very specific cases can be distinguished globally, e.g., when the substrates are isolated from bacteria due to a disconnected liquid phase. Locally, however, heterogeneities in substrate distribution impact the development of bacteria populations, e.g., a smaller geodesic distance of bacteria to the substrate promotes bacterial growth locally.