Does structure really matter? Exploring implications of microbe-structure interactions to carbon dynamics at the field scale

Soil structure determines crucial soil physical processes, such as water distribution, gas flow, and cycling of carbon and nutrients. This way, it builds and constrains biological habitats. The resulting environmental conditions at the micro- or mesoscale builds and constrains habitats for plants, soil fauna, and microorganisms. However, the direct impact of soil structure on these biological actors remains poorly understood. So far, research primarily focused on mechanisms at the aggregate or pore scale. Although process knowledge on this scale is still needed, it is also essential to understand the implications of such interactions for soil functions at the field scale.

Within this study, we tested different concepts of microbe-structure interactions using the systemic soil model BODIUM (König et al., 2023; bonares.de/bodium), and analysed the consequences for carbon dynamics at the field scale.

Our model integrates a dynamic soil structure with distinct pore size classes and explicit representation of microorganisms, recently extended to distinguish between bacteria and fungi. This framework allows us to explore microbe-structure interactions, by adjusting fungi and bacteria mobility, growth strategies as well as microbe and carbon distributions within the pore size classes.  The evaluation of the soil structure is supported by the Soil Structure Library (Weller et al., 2022; https://structurelib.ufz.de/lit/), which provides a collection of analysed soil CT images with pore size distributions down to 10 µm. For some of the images additional information was obtained on the distribution of particulate organic matter and its correlation with the pore system. This allows further process analysis on aerobic and anaerobic matter turnover.

We performed simulations spanning temporal and spatial scales relevant to agriculture, and analysed the implications for soil total soil carbon and C to N ratio, the proportions of fungi and bacteria, as well as emission rates. Additionally, we simulated scenarios involving tillage and bioturbation, which alter soil structure, to account for soil structure dynamics and resulting spatial distribution of organic matter.

Our simulation results suggest that soil structure indeed exerts a significant influence on field-scale soil functions, but rather by shaping environmental conditions for microbes and not due to direct interactions. However, the extent of this influence critically depends on our assumptions for the mobility and growth behaviour of microorganisms. This dependency also suggests that in our scenarios soil structure is not a limiting factor, and we should extend our simulations to more extreme scenarios such as a high compaction.

Consequently, further modelling and experimental research is needed to unravel the underlying mechanisms and develop robust upscaling approaches.

König, S., Weller, U., Betancur-Corredor, B., Lang, B., Reitz, T., Wiesmeier, M., Wollschläger, U., Vogel, H.-J. (2023): BODIUM - a systemic approach to model the dynamics of soil functions. Eur. J. Soil Sci. 74 (5), e13411, 10.1111/ejss.13411

Weller, U., Albrecht, L., Schlüter, S., Vogel, H.-J. (2022): An open Soil Structure Library based on X-ray CT data. Soil 8 (2), 507 – 515, 10.5194/soil-8-507-2022