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首页> 外文期刊>Applied Magnetic Resonance >NMR Fast Field Cycling Relaxometry of Unsaturated Soils
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NMR Fast Field Cycling Relaxometry of Unsaturated Soils

机译：非饱和土壤的NMR快速场循环弛豫法

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摘要

The bioavailability of water for plant nutrition in natural soils is controlled by the pore system structure and the interaction of water with the pore walls at variable degrees of saturation. For the characterization of these processes T1 relaxometry is particularly suitable because it is not influenced by internal gradients and the frequency dependence of T_1 includes detailed information about the local dynamics at the pore walls. Using Fast Field Cycling Relaxometry, we have determined T_1 relaxation dispersion curves of unsaturated soil materials which cover a broad range of textures between pure sand and silt-loam. The mean relaxation rates scale inversely with the water content, as expected according to the Brownstein-Tarr model, which means that the effective pore volume is the only water-contributing fraction. By further analysis of the relaxation dispersion curves we find a bi-logarithmic behavior which is describable by a model of two-dimensional diffusion at the liquid-solid interface in the neighborhood of paramagnetic impurities at the surface. The microscopic wettability, as expressed by the ratio of surface residence time and correlation time, is identical for the soil material but decreases by a factor of two for the sand. This relaxation mechanism is unique for all textures and water contents and proves that the water mobility at the surface does not decrease even at the lowest water contents.
机译：天然土壤中水作为植物营养的生物利用度受孔隙系统结构以及水与孔隙壁在不同饱和度下的相互作用控制。为了表征这些过程，T1弛豫法特别适用，因为它不受内部梯度的影响，并且T_1的频率依赖性包括有关孔壁局部动力学的详细信息。使用快速场循环弛豫法，我们确定了非饱和土材料的T_1弛豫色散曲线，该曲线涵盖了纯砂和粉质壤土之间的各种纹理。正如Brownstein-Tarr模型所预期的那样，平均松弛速率与含水量成反比，这意味着有效孔隙体积是唯一的水贡献分数。通过对弛豫色散曲线的进一步分析，我们发现了一个双对数行为，该行为可以通过表面顺磁性杂质附近的液-固界面处的二维扩散模型来描述。用表面停留时间和相关时间之比表示的微观可湿性对于土壤材料是相同的，但对于沙子来说降低了两倍。这种松弛机制对于所有纹理和含水量都是唯一的，并证明即使在最低含水量下，表面的水迁移率也不会降低。

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