Abstract In the hydrological cycle, the infiltration process is a critical component in the distribution of water into the soil and in the groundwater system. The nonlinear dynamics of the soil infiltration process yield preferential flow which affects the water distribution in soil. Preferential flow is influenced by the interactions between water, soil, plants, and microorganisms. Although the relationship among the plant roots, their rhizodeposits and water transport in soil has been the subject of extensive study, the effect of microbial exudates has been studied in only a few cases. Here the authors investigated the influence of two artificial microbial exudates–catechol and riboflavin–on the infiltration process, particularly unstable fingered flow, one form of preferential flow. Flow experiments investigating the effects of types and concentrations of microbial exudates on unstable fingered flow were conducted in a two-dimensional tank that was filled with ASTM C778 graded silica sand. The light transmission method (LTM) which is based on capturing the light intensity transmitted through a sand-water system and then converting it into degree of water saturation was used to visualize and characterize the flow of water in porous media as well as to image and measure the spatial and temporal distribution of water in porous media. Flow patterns, vertical and horizontal profiles of the degree of water saturation of the fingers, as well as measurements of the fingers dimension (width), number, and velocity were determined using the light transmission method. Interfacial experiments exploring the influence of microbial exudates on the wettability behavior of water were performed by measuring the contact angle and the interfacial tension of the (solid)-gas-microbial exudate solution systems. Unstable wetting front generating fingered flow was observed in all infiltration experiments. The experimental results showed that the microbial exudate addition affected the infiltration process, as the measurements of the degree of saturation profiles and widths of the fingers differed from those of the control NaCl solution. These differences may be due to an improved water holding capacity in the presence of the microbial exudates. The lowest catechol solution concentration (10 μM) produced the largest finger width (9.69 cm) among the tested catechol solution concentrations and all the other solutions including the control solution (7.24 cm). Moreover, the wettability of the medium for the catechol solution increased with an increase in concentration. The highest riboflavin solution concentration (1000 μM) generated the highest finger width (7.75 cm) among the tested riboflavin solution concentrations. However, the wettability of the medium for the riboflavin solution decreased with an increase in concentration. Our study demonstrated that the microbial exudates which are biochemical compounds produced and released by microbes in the environment are capable of influencing the soil infiltration process. The results of this study also demonstrated that the influence of the contact angle expressed as ( cos θ ) 1 / 2 should be integrated in the scaling of the finger dimension, i.e., finger width, when the Miller and Miller (1956) scaling theory is applied for the hydrodynamic scaling in porous media.

中文翻译：

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渗流区的优先流动和界面动力学：微生物渗出液的影响

摘要 在水文循环中，入渗过程是水向土壤和地下水系统分配的关键组成部分。土壤入渗过程的非线性动力学产生影响土壤中水分分布的优先流。优先流量受水、土壤、植物和微生物之间相互作用的影响。尽管植物根系、根系沉积物和土壤中水分输送之间的关系已成为广泛研究的主题，但仅在少数情况下研究了微生物渗出物的影响。在这里，作者研究了两种人工微生物渗出物 - 儿茶酚和核黄素 - 对渗透过程的影响，特别是不稳定的指状流，一种优先流形式。研究微生物渗出物的类型和浓度对不稳定指状流的影响的流动实验是在一个装有 ASTM C778 分级硅砂的二维罐中进行的。光透射法 (LTM) 基于捕获通过沙水系统传输的光强度，然后将其转换为含水饱和度，用于可视化和表征多孔介质中的水流以及成像和表征。测量多孔介质中水的时空分布。流动模式、手指含水饱和度的垂直和水平剖面，以及手指尺寸（宽度）、数量和速度的测量值都是使用光传输方法确定的。通过测量（固体）-气体-微生物渗出液溶液系统的接触角和界面张力，进行了探索微生物渗出液对水润湿性行为影响的界面实验。在所有渗透实验中都观察到不稳定的润湿前沿产生指状流。实验结果表明，微生物渗出物的添加影响了渗透过程，因为饱和度分布和手指宽度的测量与对照 NaCl 溶液的测量不同。这些差异可能是由于存在微生物渗出物时保水能力的提高。最低的儿茶酚溶液浓度 (10 μM) 产生最大的手指宽度 (9. 69 cm) 在测试的儿茶酚溶液浓度和包括对照溶液 (7.24 cm) 在内的所有其他溶液中。此外，培养基对儿茶酚溶液的润湿性随着浓度的增加而增加。在测试的核黄素溶液浓度中，最高的核黄素溶液浓度 (1000 μM) 产生最高的手指宽度 (7.75 cm)。然而，培养基对核黄素溶液的润湿性随着浓度的增加而降低。我们的研究表明，微生物分泌物是环境中微生物产生和释放的生化化合物，能够影响土壤入渗过程。