Predicting soil water availability to crops in water-repellent sandy soil is complicated as soil water repellency (SWR) responds non-linearly to soil water content. Others have hypothesised that the development of a monolayer of water molecules results in SWR increasing before SWR declines with a further increase in soil water content. In a previous study, we found that SWR increases when above 0.28–0.86% threshold soil water content. Thus, our objective was to determine the underlying mechanisms responsible for why SWR increases when above a certain threshold soil water content in a sandy soil. A water adsorption isotherm was constructed by exposing a water-repellent sandy soil to increasing relative humidity (dynamic vapour sorption technique) to evaluate if the development of a monolayer of water molecules was responsible for the increased SWR response. The increased SWR when above 0.66% threshold soil water content was found to coincide with the capillary condensation of water in the soil. The inverse gas chromatography technique was used for the first time in soil particles' surface energy analysis to investigate why SWR increases when above the threshold soil water content by determining the total, dispersive (non-polar), and specific surface (polar) energy of the soil at two relative humidities (0% and 90%). Wettable sandy soil (98% soil organic carbon removed) was included as a control to further assess if soil organic carbon in the water-repellent soil influences the surface energy of the soil. The mean of total, dispersive, and specific surface energy decreased for both wettable and water-repellent sandy soils when exposed to 90% relative humidity, suggesting that there was limited effect of soil carbon on the increased SWR when above the threshold soil water content since most organic carbon was removed from the wettable soil. We also investigated if there is any difference in the surface energy heterogeneity when exposed to 90% relative humidity to gain insight into surface chemistry heterogeneity of the soil particles' surfaces. Exposing soils to 90% relative humidity decreased the heterogeneity of the total and dispersive surface energy of both wettable and water-repellent sandy soil indicating a more uniform surface chemistry than when exposed to 0% relative humidity.

中文翻译：

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毛细管水的凝结和表面能的降低导致沙土土壤疏水性增加

由于土壤防水性 (SWR) 对土壤含水量呈非线性响应，因此预测防水沙质土壤中作物的土壤水分有效性很复杂。其他人假设单层水分子的发展导致 SWR 在 SWR 随着土壤含水量的进一步增加而下降之前增加。在之前的一项研究中，我们发现当高于 0.28–0.86% 阈值土壤含水量时，SWR 会增加。因此，我们的目标是确定当沙质土壤中土壤含水量超过某个阈值时 SWR 增加的潜在机制。通过将防水沙质土壤暴露于增加的相对湿度（动态蒸汽吸附技术）来构建水吸附等温线，以评估单层水分子的发展是否是增加 SWR 响应的原因。当土壤含水量高于 0.66% 阈值时，SWR 增加与土壤中水的毛细管凝结相吻合。首次将反相气相色谱技术用于土壤颗粒的表面能分析，通过测定土壤颗粒的总能、色散（非极性）和比表面（极性）能来研究当高于阈值土壤含水量时 SWR 增加的原因土壤处于两个相对湿度（0% 和 90%）。将可湿性沙土（去除 98% 的土壤有机碳）作为对照，以进一步评估防水土壤中的土壤有机碳是否影响土壤的表面能。当暴露于 90% 的相对湿度时，可润湿和疏水沙土的总表面能、色散能和比表面能的平均值均有所下降，这表明自 2017 年以来，当土壤含水量高于阈值时，土壤碳对 SWR 增加的影响有限大多数有机碳从可润湿土壤中去除。我们还调查了暴露于 90% 相对湿度时表面能异质性是否存在任何差异，以深入了解土壤颗粒表面的表面化学异质性。