Soils and sediments act as open bio-physical-chemical reactors where chemical changes are driven by the interactions between moving fluids, solid- and aqueous-phase constituents, and microorganisms. Soils are typically heterogeneous, composed of individual aggregates that form a network of interconnected microenvironments. Their structural heterogeneity determines the spatial and temporal distributions of reactive sites and of flow paths for solutes, substrates and microorganisms and the consequences of such spatial organization on chemical fate are potentially important. As a matter of fact the fate of chemicals in soils or sediments, is the result of interrelated processes including their transport by convection, diffusion, dispersion, their binding by sorption, their transformation by (bio)chemical processes, occurring over a wide range of length and time scales. An additional constraint on chemical fate is the rate at which bacteria produce or consume chemical constituents, however, environmentally relevant kinetic expressions and parameters for most microbial reactions are still lacking.
Our current research projects involve redox-sensitive metals (iron, Fe), contaminants (selenium, Se) and nutrients (nitrate, NO3-, and sulphate, SO42-). For many elements dissolved in natural water, redox transformations results in adsorption or precipitation and thus immobilization, often over distances of only few millimetres. For that reason, part of our research focuses on redox-stratified environments, such as soil aggregates and the interface between surface waters and underlying sediments. A major gap still exists in translating basic microbiological, geochemical and physical knowledge obtained in simple model systems to complex, real-world systems characterized by variably saturated conditions, multiple mineral surfaces, and naturally occurring organic matter and microorganisms. To fill that gap it is crucial to examine biogeochemical dynamics in systems of intermediate complexity, which include the critical characteristics and coupling of relevant microenvironments and which permit examination of interdependent effects of transport and microbial and geochemical reactions under environmentally realistic conditions and well known geometry. Our current projects focus on using flow-through systems, progressing from artificial soil aggregates inoculated with known bacterial strains, to intact soil or sediment columns and covering the range between residual and saturation water content. Current topics under investigation include (1) understanding how localized biogeochemical environments are controlled by the intrinsic microbial activity and dynamics versus mass transfer limitations, (2) identifying the key environmental determinants that control biogeochemical reaction kinetics and microbial community structure, and (3) establishing how microbial community structure and biogeochemical (micro-)heterogeneity affect the transformation rates of nutrients and contaminants in the subsurface.
Overall, our research will lead to a better understanding of coupled transport and biogeochemical processes in natural environments and improved mathematical representation and parameterisation of biogeochemical reaction kinetics, ultimately providing the predictive framework needed to assess chemical fate and transport in soils and sediments. This capability is critical for understanding the cycling of elements and chemical compounds, with important implications for the preservation of diversity, maintenance of groundwater and soil quality, evaluation of pollution risks, and management of microbial communities for bioremediation.