Is the Rhizosphere Temporarily Water Repellent?

Abstract

The rhizosphere has a controlling role in the flow of water and nutrients from soil to plant roots; however, its hydraulic properties are not well understood. As roots grow, they change the pore size distribution of the surrounding soil. Roots release polymeric substances such as mucilage into their rhizosphere. Microorganisms living in the rhizosphere feed on these organic materials and release other polymeric substances into the rhizosphere. The presence of these organic materials might affect the water retention properties and the hydraulic conductivity of the rhizosphere soil during drying and rewetting. We used neutron radiography to monitor the dynamics of water distribution in the rhizosphere of lupin (Lupinus albus L.) plants during a period of drying and rewetting. The rhizosphere was shown to have a higher water content than the bulk soil during the drying period but a lower one during the subsequent rewetting. We evaluated the wettability of the bulk soil and the rhizosphere soil by measuring the contact angle of water in the soil. We found significantly higher contact angles for the rhizosphere soil than the bulk soil after drying, which indicates slight water repellency in the rhizosphere. This explains the lower soil water content in the rhizosphere than the bulk soil after rewetting. Our results suggest that the water holding capacity of the rhizosphere is dynamic and might shift toward higher or lower values than those of the surrounding bulk soil, not affected by roots, depending on the history of drying and rewetting cycles.

Interweaving Monitoring Activities and Model Development towards Enhancing Knowledge of the Soil-Plant-Atmosphere Continuum

“…New methods such as magnetic resonance imaging (MRI) and tomography are applied to study subterranean parts of plants (see also Skaggs and Shouse, 2008). In their contribution, Moradi et al (2012) explained the unexpected behavior of the rhizosphere observed by using neutron tomography, that is, lower soil water content in the rhizosphere compared to the bulk soil aft er rewetting. Measuring rhizosphere and soil wettability properties, they found signifi cantly higher contact angles for rhizosphere soil compared to the bulk soil aft er drying, which indicates slight water repellency in the rooting zone.…”

Plant exudates improve the mechanical conditions for root penetration through compacted soils

“…They are so effective at improving soil physical conditions that biological tillage through the action of plant roots is a growing practice that is advocated in sustainable crop rotations. At the root-soil interface, the release of exudates by plant roots into the rhizosphere provides a major food source for microorganisms (Jones et al 2004), induces a physico-chemical release of nutrients for plant uptake (Malamy 2005;Marco et al 2015), and alters soil water retention and flow (Moradi et al 2012;Zarebanadkouki and Carminati 2014). Whereas a large number of studies have explored biological and chemical properties of the rhizosphere, most physical investigations are limited to measures of soil stability or pore structure visualisation, as it is difficult to perform measurements at such a small scale (Peng et al 2011;Czarnes et al 2000;Morel et al 1991).…”

Management options for water-repellent soils in Australian dryland agriculture

“…Because these compounds are common components of soil organic matter, water repellency is generally confined to the topsoils where organic matter accumulates. It is in this zone, too, that plant roots proliferate and produce root exudates that may also contribute to repellency after drying cycles as described above (Hallett et al 2003Moradi et al 2012).…”