Variations in pore size and contact angle distributions control rhizosphere rewetting

Abstract

Rhizosphere wettability decreases upon severe drying leading to periods of prolonged low water content around roots after precipitation or irrigation. These observations were explained by the temporal hydrophobic character of mucilage, while structural alterations of the pore space caused by mucilage, such as pore clogging, remained mostly unexplored. In this study, time-series neutron radiography and a pore network model were used to assess the impact of pore geometry and wettability on water flow following the addition of mucilage in a sand substrate. To do so, we monitored the capillary rise of ethanol and water separately for mucilage contents, ranging from 0.0 to 2.0 mg g−1. A pore network model was developed to analyze the impact of alterations in pore geometry and wettability. Results are compared with analytical solutions of the Lucas-Washburn equation. Rewetting dynamics were explained by a combination of a decrease in effective pore throat size and a global decrease in wettability. The local distribution of wettability, however, appeared of minor importance as dynamics of water imbibition could be matched by a uniform effective contact angle. For 0.1 mg g−1 mucilage content increased wettability was predicted for both approaches: the analytical solution and the pore network model fit. At larger contents, a decrease in wettability occurred which was accompanied by a decrease in derived effective pore and pore throat size. On a minute scale, rewetting appeared to steadily progress at all mucilage contents with accelerated rewetting observed at 0.6 mg g−1 likely related to an increased wetting front length. This study highlights the importance of mucilage on pore geometry in combination with wettability modifications in the rhizosphere. Aside from rhizosphere rewetting, the presented approach provides an opportunity to investigate further wettability-related processes in other soil environments on various spatial scales.