Fundamental drivers of nutrient diffusion in soils - a comprehensive data synthesis based on microdialysis studies

The bioavailability and delivery of nutrients (solutes) to plant roots and soil microbes is governed by diffusive processes. According to Fick’s first law solute diffusion is driven by the diffusivity of the solute, the concentration gradient and the path length. We first shortly review the main factors potentially affecting these drivers i.e. the diffusivity (solute-soil interactions including ion exchange and other sorption reactions, microbial metabolism), the concentration gradient (source-sink relations via mobilization through biotic and abiotic reactions and solute uptake by organisms) and the path length (soil water filled pore space, pore size distribution, texture). Then, based on data synthesis of available microdialysis studies we provide evidence for the ranking of these factors in soils, depending on soil horizon (organic versus mineral soil), fertilization (agriculture versus forests), nutrient form (ammonium versus nitrate versus free amino acids; nitrate versus phosphate) and soil moisture, including drying-rewetting. Microdialysis represents a relatively recent minimal invasive technique for in situ and lab use to measure rates of diffusive solute (nutrient) transport on a microscale level. Since its first application in soil in 2005, this technique has provided unique, non-isotopic and non-destructive insights into major drivers of solute transport in soils, with ~45 studies published thus far.