Quantitative characterization of soil micropore structure and pore water content using nuclear magnetic resonance: Challenges and calibration methods

Though nuclear magnetic resonance (NMR) has been applied in soil science over several decades, the quantitative relation between NMR signals and soil pore-water distribution is complex and still covered by some cloud. The major debates include: 1) the quantitative relation between transverse relaxation time (T2) and pore radius varies in different studies; 2) Is the relation between NMR signals and soil-water contents unique? To clarify the aforementioned issues over the application of NMR in soil science, a comprehensive study was carried out. The results demonstrate that: 1) a unique linear relationship between peak area of NMR signals and soil volumetric water content (θ) exists, independent of the soil’s initial molding conditions, such as molding dry density (ρd) and molding water content (wini); 2) the ratio between T2and pore radius, defined as the pore structure coefficient (Cr) of NMR, varies with pore water morphologies and soil types; 3) three methods were proposed to determine the value of Crand can help to provide insights for better understanding of the NMR results in soil science.