Abstract: Nuclear magnetic resonance (NMR) technology has been applied in soil science due to the characte-ristics of high efficiency, rapidity, no damage to soil structure, and harmlessness to the human body. However, the effect of the presence of paramagnetic materials in soils on the characteristics of NMR signals was still unclear. In this study, we investigated the effects of paramagnetic material on the low field nuclear magnetic (LF-NMR) signals and soil water content measurement in soils with different texture. The results showed that the LF-NMR signal of soil water could reach about 150, while that of all the solid materials including soil minerals, organic matter and microbes was less than 0.3, which was relatively negligible. Compared with the NMR signals produced by solid materials in soils, soil texture and paramagnetic material had stronger impact on the measured LF-NMR signals of soil water. LF-NMR equipment had a relaxation time monitoring blind area, and the loss of NMR signal was mainly due to the acceleration of the relaxation process of hydrogen protons in water by magnetic materials, resulting in extremely fast LF-NMR signals feed back by water in small pores that could not be captured by monitoring equipment. For loamy fluvo-aquic soil (1.2%) and clay loamy black soil (1.3%) with low paramagnetic material contents, the loss of LF-NMR signals was not large, which was linearly related to soil water content. For clayey red soil with high content of clay (45.3%) and paramagnetic materials (4.0%), a part of the LF-NMR signals would be lost in the measurement, and the monitored LF-NMR signal was not linearly related to the soil water content. In addition, external addition of paramagnetic materials (3.0 g·L^-1 MnCl₂ solution) would further reduce the LF-NMR signals that could be monitored in black and red soils. The maximum signal loss rates of black soil and red soil were 41.0% and 46.7%, respectively, which greatly changed the quantitative relationship between it and soil water content. Therefore, the influence of paramagnetic materials on the LF-NMR signals should be reduced first through correction when using LF-NMR to measure the water content of clay soil with rich internal paramagnetic materials (>1.3%) or external addition of paramagnetic materials. Our results would provide valuable insights into the study of soil water content measurement and soil pore structure analysis using low field nuclear magnetic resonance technology.

Key words: low-field nuclear magnetic resonance, paramagnetic material, relaxation time, soil water content, clay content