Pore‐Scale Distribution of Mucilage Affecting Water Repellency in the Rhizosphere

Abstract

The physical properties of the rhizosphere are strongly influenced by rootexuded mucilage, and there is increasing evidence that mucilage affects the wettability of soils on drying. We introduce a conceptual model of mucilage deposition during soil drying and its impact on soil wettability. We hypothesized that as soil dries, water menisci recede and draw mucilage toward the contact region between particles. At low mucilage contents (milligrams per gram of soil), mucilage deposits have the shape of thin filaments that are bypassed by infiltrating water. At higher contents, mucilage deposits occupy a large fraction of the pore space and make the rhizosphere hydrophobic. This hypothesis was confirmed by microscope images and contact angle measurements. We measured the initial contact angle of quartz sand (0.5-0.63-and 0.125-0.2-mm diameter), silt (36-63-mm diameter), and glass beads (0.1-0.2-mm diameter) mixed with varying amounts of chia (Salvia hispanica L.) seed mucilage (dry content range 0.2-19 mg g −1 ) using the sessile drop method. We observed a threshold-like occurrence of water repellency. At low mucilage contents, the water drop infiltrated within 300 ms. Above a critical mucilage content, the soil particle-mucilage mixture turned water repellent. The critical mucilage content decreased with increasing soil particle size. Above this critical content, mucilage deposits have the shape of hollow cylinders that occupy a large fraction of the pore space. Below the critical mucilage content, mucilage deposits have the shape of thin filaments. This study shows how the microscopic heterogeneity of mucilage distribution impacts the macroscopic wettability of mucilageembedded soil particles.With an extent of millimeters to a few centimeters, the rhizosphere is the part of soil actively modified by root growth and exudation (Gregory, 2006;Hinsinger et al., 2009;York et al., 2016;Roose et al., 2016). Its impact on soil hydrology might be profound, as about 40% of all terrestrial precipitation flows through the rhizosphere-plant-atmosphere continuum (Bengough, 2012). In view of this immense flow of water, Dakora and Phillips (2002) and Sposito (2013) proposed rhizosphere research as key for the sustainable management of water resources.One of the substances released by root tips is mucilage, a gel consisting mainly of polysaccharides and <1% lipids (Oades, 1978;Read et al., 2003). In combination with other sources of organic matter and root hairs, plant mucilage contributes to the formation of the rhizosheath, a region of interconnected soil particles bound to the root surface (Watt et al., 1993). The enhanced connection between roots and soil is supposed to have a major effect on microbial growth and plant nutrient uptake (Dakora and Phillips, 2002). Furthermore, mucilage is known to alter the hydraulic properties of the rhizosphere (Young, 1995;Hallett et al., 2003;Carminati et al., 2010;Moradi et al., 2012;Carminati, 2013;Zarebanadkouki et al., 2016). After a drying cycle, Carminati et al. (2010) found the rewett...

Microhydrological Niches in Soils: How Mucilage and EPS Alter the Biophysical Properties of the Rhizosphere and Other Biological Hotspots

“…For low mucilage and EPS contents and large pores, the final shape of arising structures is thin filaments. At intermediate content or at the contact between soil particles, the pendular bridges are cylindrical (Albalasmeh and Ghezzehei, 2014; Benard et al, 2018). They form during soil drying as the gas–liquid interface retreats and the polymers adhering to the soil particle surface are stretched.…”

Physics and hydraulics of the rhizosphere network

“…(a) Illustration of the water distribution around roots, showing increasing water content toward the root surface. (b) Filament of chia mucilage between sand particles (from Benard et al, ). (c) Hollow cylinder between glass beads mixed with chia mucilage (experimental details are found in Benard et al, ).…”

“…(b) Filament of chia mucilage between sand particles (from Benard et al, ). (c) Hollow cylinder between glass beads mixed with chia mucilage (experimental details are found in Benard et al, ). (d) Conceptual model of mucilage drying between soil particles.…”

“…40% of terrestrial precipitation; Bengough, 2012) and hosts a tremendous number of microorganisms (Philippot et al, 2013), which together with the roots engineer the soil physical properties (Hallett et al, 2013). Prominent among such engineering activity is the secretion of mucilage from the root tips, which changes key physical properties of the soil solution: it adsorbs water (McCully and Boyer, 1997), it increases its viscosity and decreases its surface tension (Read and Gregory, 1997;Naveed et al, 2017), affecting also the contact angle (Moradi et al, 2012;Zickenrott et al, 2016;Benard et al, 2017). By changing these properties, mucilage brings into question well accepted paradigms, such as the fact that the soil water content should decrease towards the root surface (but it does not, as illustrated in Fig.…”

Spatial Distribution of Mucilage in the Rhizosphere Measured With Infrared Spectroscopy

“…While the polysaccharides contained in mucilage have a very high water holding capacity (McCully and Boyer, 1997), the lipids which are also contained (Read et al, 2003), can have a reverse effect when mucilage dries. It is therefore likely that mucilage is responsible for the observed rhizosphere hydrophobicity (Carminati, 2013;Ahmed et al, 2015;Benard et al, 2017). Fatty acids are composed of a hydrophilic and a hydrophobic tail of nonpolar fatty acid chains (C-H groups).…”