H-NMR relaxometry to monitor wetting and swelling kinetics in high-organic matter soils

Variations of soil moisture conditions affect sorption properties of soil organic matter and the pore size distribution of the soils and thus are expected to have an impact on the availability of pollutants and nutrients in soils. At least two principal processes that occur when a soil-water contact is established, are involved. Wetting, which is the very first step, is governed by the interactions of water with the surface of soil organic matter (SOM). The wettability of the pore walls determines the pore accessibility for water. Only in wettable soils, water will occupy the smallest pores first. In the course of wetting, the wettability of the pore walls increases, leading to water redistribution. Swelling of SOM is accompanied by an increase of volume due to the water uptake of the solid SOM phase and will change the SOM polarity. Swelling will thus affect sorption processes in the bulk SOM phase and is expected to change the pore sizes. In this contribution, we investigated swelling and wetting kinetics of soil samples by H-NMR-Relaxometry. We found different effects of wetting and swelling on the development of relaxation time distribution and thus of the pore size distribution. Both swelling and wetting can be slow processes, lasting for up to some weeks. During this time, we found changes in the pore size distribution. For swelling phenomena, we observed a continuous change of the effective pore size, and for wetting phenomena, we found a change in water distribution in a probably rigid pore system. Thus, during swelling and wetting, neither pore size distribution nor sorbent properties of SOM nor hydraulic properties remain constant. Due to the slow kinetics, both processes play an important role in sorption, transport and accessibility for water in hydrophobic areas within a time scale of weeks after e.g. a rainfall event. This will affect the environmental availability and the transport of pollutants and nutrients in the field.