Soil-water interactions unveiled by fast field cycling NMR relaxometry

Soil and water are two paramount environmental compartments. Their interactions are fundamental for food production and, hence, the sustainability of life. In this article, we revise the literature concerning applications of fast field cycling (FFC) NMR relaxometry in soil science in order to demonstrate how this technique provides a molecular understanding of soil-water interactions. On the basis of the reviewed literature, a model for water and nutrient dynamics in soil is proposed. The model is based on the idea that the degrees of freedom of water associated with a solid surface and/or constraint in pores are reduced as compared to bulk water. While low-density ice-like water layer (LDW) surrounds solid soil particles, high-density water (HDW) domain predominates in bulk. Water moves from high-density domains to the low-density ones, thus allowing not only the exchange of water molecules adsorbed on the solid surface but also the movement of the dissolved organic and inorganic nutrients. Any solute adsorbed on the soil surface, and especially to highly porous clay or other porous materials (e.g., pyrogenic carbon also referred to as biochar), can be exchanged with the solute dissolved in the bulk by the HDW-LDW mechanisms. The mechanism suggested here, based on kinetics considerations, explains the dynamics of nutrients from soil, clay, and biochar surfaces toward microbes and/or plant roots, which are essential to plant nutrition. © 2017 John Wiley & Sons, Ltd.