1 H NMR relaxometry is used in earth science as a non-destructive and time-saving method to determine pore size distributions (PSD) in porous media with pore sizes ranging from nm to mm. This is a broader range than generally reported for results from X-ray computed tomography (X-ray CT) scanning, which is a slower method. For successful application of 1 H NMR relaxometry in soil science, it is necessary to compare PSD results with those determined from conventional methods. The PSD of six disturbed soil samples with various textures and soil organic matter (SOM) content were determined by conventional soil water retention at matric potentials between −3 and −390 kPa (pF 1.5-3.6). These PSD were compared with those estimated from transverse relaxation time (T 2 ) distributions of water in soil samples at pF 1.5 using two different approaches. In the first, pore sizes were estimated using a mean surface relaxivity of each soil sample determined from the specific surface area. In the second and new approach, two surface relaxivities for each soil sample, determined from the T 2 distributions of the soil samples at different matric potentials, were used. The T 2 distributions of water in the samples changed with increasing soil matric potential and consisted of two peaks at pF 1.5 and one at pF 3.6. The shape of the T 2 distributions at pF 1.5 was strongly affected by soil texture and SOM content (R 2 = 0.51 − 0.95). The second approach (R 2 = 0.98) resulted in good consistency between PSD, determined by soil water retention, and 1 H NMR relaxometry, whereas the first approach resulted in poor consistency. Pore sizes calculated from the NMR data ranged from 100 μm to 10 nm. Therefore, the new approach allows 1 H NMR relaxometry to be applied for the determination of PSD in soil samples and for studying swelling of SOM and clay and its effects on pore size in a fast and non-destructive way. This is not, or only partly, possible by conventional soil water retention or X-ray CT.

1 H NMR relaxometry is applied for the investigation of pore size distributions in geological substrates. The transfer to humous soil samples requires the knowledge of the interplay between soil organic matter, microorganisms and proton relaxation. The goal of this contribution is to give first insights in microbial effects in the 1 H NMR relaxation time distribution in the course of hydration of humous soil samples. We observed the development of the transverse relaxation time distribution of the water protons after addition of water to air dried soil samples. Selected samples were treated with cellobiose to enhance microbial activity. Besides the relaxation time distribution, the respiratory activity and the total cell counts were determined as function of hydration time. Microbial respiratory activities were 2-15 times higher in the treated samples and total cell counts increased in all samples from 1Â10 9 to 5Â10 9 cells g )1 during hydration. The results of 1 H NMR relaxometry showed tri-, bi-and mono-modal relaxation time distributions and shifts of peak relaxation times towards lower relaxation times of all investigated soil samples during hydration. Furthermore, we found lower relaxation times and merging of peaks in soil samples with higher microbial activity. Dissolution and hydration of cellobiose had no detectable effect on the relaxation time distributions during hydration. We attribute the observed shifts in relaxation time distributions to changes in pore size distribution and changes in spin relaxation mechanisms due to dissolution of organic and inorganic substances (e.g. Fe 3+ , Mn 2+ ), swelling of soil organic matter (SOM), production and release of extracellular polymeric substances (EPS) and bacterial association within biofilms.Abbreviations: 1 H NMR -Proton Nuclear Magnetic Resonance; q 0 -initial mean surface relaxivity; d pore ¼ 4V 0 S 0 -pore diameter of a cylindrical pore (Hinedi et al., 1997); T 2 -initial mean relaxation time; q i ¼ k T iS -surface relaxivity; d 0 -initial mean pore diameter; k-layer thickness, in which T iS takes place; DAPI--4¢,6-diamidino-2-phenylindol; d BET -mean pore diameter estimated from BET data; DOCDissolved Organic Carbon; EPS -extracellular polymeric substances; PGA -polygalacturonic acid; S 0 -water covered pore surface; S BET -specific surface area estimated from BET data; SOM -soil organic matter; t -hydration time [days], s-time constant of the first order process [days]; T i -relaxation time of the longitudinal (i=1) or transverse (i=2) relaxation of proton magnetization; T iB -bulk relaxation time;T iS -surface relaxation time; V 0 -water filled pore volume; Y -replacement character for the peak relaxation time [ms]; Y ¥ -peak relaxation time for infinite hydration time; Y 0 -initial value for t=0.

Proton nuclear magnetic resonance (1H NMR) relaxometry has been used to analyze pore size distributions of wet porous samples. To make this method applicable to soil samples, knowledge about contribution from the soil solution to the total proton relaxation is needed. We extracted soil solutions from nine soil samples and determined transverse proton relaxation rates, the concentration of Fe, Mn, and total organic C (TOC), and the pH of the solutions. The effects of Fe, Mn, and TOC on the proton relaxation in the soil solution were compared with those of dissolved Fe2+, Fe3+, and Mn2+ and of glucose, d‐cellobiose, potassium hydrogen phthalate, sodium alginate, and agar in model solutions. Proton relaxation rates in the soil solutions were up to 20 times larger than in pure water, which was mainly due to dissolved Fe(III) and Mn(II) species. The relaxivities of Fe and Mn in soil solution were reduced to 40 and 70% compared with Fe(III) and Mn(II) in a model solution, respectively. Smaller relaxivities were primarily due to the formation of metal–organic complexes. We conclude that the proton relaxation in soil samples is generally accelerated by the soil solution, and its contribution must be considered to estimate pore sizes from relaxation times. By using the calculated relaxivities of Fe and Mn in soil solution, the contribution of the soil solution to the total proton relaxation can be estimated from the Fe and Mn concentration in the soil solution.

Proton NMR relaxometry is a very powerful tool for investigating porous media and their interaction with water or other liquids and the mobility and interaction of organic molecules in solution. It is commonly used in material science or earth science. However, it is only scarcely applied in soil science although it shows great potential for helping to understand water uptake into the soil matrix and processes occurring at the solid-liquid interface at soil particle surfaces. This review introduces proton NMR relaxometry in the context of soil science and discusses the most important applications of the method in this field. Relevant results from different applications of NMR relaxometry in soils are described and research gaps identified. Some original data is presented concerning biofilm formation in soils, which was investigated using proton NMR relaxometry. NMR relaxometry is a sensitive, informative and promising method to study pore size distribution in soils as well as many kinds of soil physicochemical processes, among which are wetting, swelling or changes in macromolecular status. It is further a very helpful method to study interactions between molecules in soil organic matter and can serve to study the state of binding of water or organic chemicals to soil organic matter. Relaxation times determined by NMR relaxometry are sensitive to various factors that play a role in soil-water interaction which is both an advantage and shortcoming of the method: NMR relaxometry can be applied to numerous investigations in soil science, but at the same time interpretation of the results may be very difficult in such complex and heterogeneous systems like soils.

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Dramatic physical and physico-chemical changes in soil properties may arise due to temperature and moisture variations as well as swelling of soil organic matter (SOM) under constant conditions. Soil property variations may influence sorption/desorption and transport processes of environmental contaminants and nutrients in natural-organic-matterrich soils. Notwithstanding the studies reported in literature, a mechanistic model for SOM swelling is unavailable yet. The objective of the present study was the evaluation of the swelling of peat soils, considered as SOM models, by 1 H NMR relaxometry and differential scanning calorimetry (DSC). Namely, information on the processes governing physical and physicochemical changes of peat during re-hydration were collected. The basic hypothesis of the present study was that the changes are slow and may affect water state as well as amounts of different water types into the peats. For this reason, such changes can be evidenced through the variations of mobility and thermal behaviour of the involved H 2 O molecules by using 1 H NMR relaxometry and DSC. According to the experimental results, a mechanistic model, describing the fundamental processes of peat swelling, was obtained. Two different peats re-wetted at three temperatures were used. The swelling process was monitored by measuring spin-spin relaxation time (T2) over a hydration time of several months. Moreover, DSC, T1 -T2 and T2 -D correlation measurements were done at the beginning and at the end of the hydration. Supplementary investigations were also done in order to discriminate between the swelling effects and the contributions from soil solution, internal magnetic field gradients and/or soil microorganisms to proton relaxation. All the results revealed peat swelling. It was evidenced by pore size distribution changes, volumetric expansion and redistribution of water, increasing amounts of nonfreezable and loosely bound water, as well as formation of gel phases and reduction of the translational and rotational mobility of H 2 O molecules. All the findings implied that changes of the physical and physicochemical properties of peats were obtained. In particular, three different processes having activation energies comprised in the interval 5 -50 kJ mol -1 were revealed. The mechanistic model which was, then, developed included water reorientation in bound water phases, water diffusion into the peat matrix and reorientation of SOM chains as fundamental processes governing SOM swelling. This study is of environmental significance in terms of re-naturation and re-watering of commercially applied peatlands and of sorption/desorption and transport processes of pollutants and nutrients in natural organic matter rich soils.

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