Physical long-term regeneration dynamics of soil organic matter as followed by 1H solid-state NMR methods

Abstract

Environmental context The mobility of soil organic matter and water molecules has a strong influence on the availability of fertilisers as well as on the fate of pollutants in soil. Magnetic resonance techniques identified two regimes of mobility change on the molecular level occurring on a timescale of 1 year after initially heating the sample. The results can help to understand the effect of soil type and water content for agricultural use and soil protection. Abstract 1H wide-line solid-state NMR methods have been applied to monitor long-term mobility changes in the supramolecular network of soil organic matter and water induced by short thermal treatment. NMR line widths are a direct measure of the mobility of water molecules and organic matter components. For the first time, we obtained an insight into the long-term physical mechanisms in terms of molecular mobility governing soil organic matter–water interactions. All time series reveal a systematic, attenuated proton demobilisation on time scales with a maximum of 1 year that depends on water content and type of soil. Results are discussed in the context of water molecule bridges and are compared with the results of structural transition temperatures obtained from differential scanning calorimetry measurements. The analysis is based on a porous system with random field characteristics. Two major features, a logarithmic time dependence in the first hours and a linear time dependence at longer times after the heating event, are observed in all investigated samples. In peat samples, a temporary increase of mobility was observed, the point in time depending on water content. The soil organic matter physicochemical matrix aging mechanism could also be relevant for the aging of organic chemicals in soil samples, suggesting a long-term reduction in molecular mobility.