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

Abstract

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 (ρ_d) and molding water content (w_ini); 2) the ratio between T₂ and pore radius, defined as the pore structure coefficient (C_r) of NMR, varies with pore water morphologies and soil types; 3) three methods were proposed to determine the value of C_r and can help to provide insights for better understanding of the NMR results in soil science.

Graphical abstract

Keywords

nuclear magnetic resonance / unsaturated soil / pore water distribution / soil–water characteristic curve / calibration method

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Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11709-025-1137-z and is accessible for authorized users.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant Nos. 51979002 and 52025094).

Competing interests

The authors declare that they have no competing interests.

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