Recent advances and challenges on the use of magnetic and electrical properties to explore biological signatures in soils

Abstract:

In the past few decades, there has been an increasing recognition of the pivotal role of biological activity in subsurface processes. Microorganisms play a crucial part in organic matter decomposition, precipitation/dissolution of minerals, and a multitude of other biogeochemical cycles. Additionally, plant root systems can physically and chemically transform geological formations through cracks, water and nutrient uptake and storage. An improved understanding of the influence of the biosphere on subsurface processes is crucial in the coming years because it is linked with major economical, societal, and environmental challenges. Although the importance of the biosphere for physical and chemical transformations of geological media is widely acknowledged, the complexity of the induced changes remains hard to capture and explain. Compared to soil or plant analyses, geophysical methods offer the advantage of being non-destructive, and have good spatial and temporal resolution. Among the existing geophysical methods, electrical and magnetic methods are particularly well-suited to study biological processes in the subsurface. Indeed, regarding electrical methods, the measured complex electrical conductivity is sensitive not only to changes in the electrolyte chemistry through electrical conduction (charge transport), but also to changes at the surface of the mineral grains through electrical polarization (charge storage). A lot of the biological activity in the soil can be monitored following by-products precipitation/dissolution, interestingly including metallic particles (e.g., pyrite, magnetite). If electrical methods can detect areas of enhanced activity, magnetic methods can add information on the type of metallic minerals forming in the soil, which are controlled by the redox conditions of the environment and thus by the biological activity.

Based on a few case studies, this presentation will emphasize recent advances in the understanding of the electrical and magnetic signals stemming from biological materials and their activity; and identify existing knowledge gaps such as up- and down-scaling between laboratory and field studies, as well as joint interpretation of geophysical data and other environmental data.