Root‐ and microbial‐derived mucilages affect soil structure and water transport

Abstract

The production of exudates by plant roots and microbes in the rhizosphere, together with intense wetting and drying cycles due to evapotranspiration, stimulate changes in soil structure. We have attempted to separate these two processes using an experimental model with bacterial exopolysaccharides (dextran and xanthan) and root mucilage analogues (polygalacturonic acid, PGA), and up to 10 cycles of wetting and drying. To characterize the soil structure, tensile strength, water sorptivity and ethanol sorptivity of the amended soils were measured, and thin sections were made. Xanthan and PGA induced greater tensile strength of the amended soil, suggesting that they increased the bond energy between particles. Porosity increased with each cycle of wetting and drying, and this increase was less pronounced for the PGA 2 g l ±1 than for the xanthan and dextran. This suggests that PGA stabilized the soil against the disruptive effect caused by the wetting and drying. The PGA was the only polysaccharide that in¯uenced water sorptivity and repellency, resulting in slower wetting of the treated soil. Wetting and drying led to an increase of the sorptivity and a decrease of the repellency for all treatments with the exception of the PGA-amended soils. The PGA may therefore stabilize the soil structure in the rhizosphere by increasing the strength of bonds between particles and decreasing the wetting rate.

Does the presence of glomalin relate to reduced water infiltration through hydrophobicity?

“…Other authors have found strong links between microbial activity and water relations, either through pore blockage mechanisms under high substrate loads (Baveye et al 1998), or though increasing water repellency as shown in our previous work (Czarnes et al 2000). Plant root mucilages have also been well documented to increase aggregate stability (Watt et al 1993).…”

“…Plant root mucilages have also been well documented to increase aggregate stability (Watt et al 1993). In a previous paper we (Czarnes et al 2000) reported increases in soil water repellency after amendment of soil with synthetic root mucilage. The presence of the plant in our experimental system could also have caused increased microbial activity, through the increased quantities of carbon input.…”

Mapping soil deformation around plant roots using in vivo 4D X-ray Computed Tomography and Digital Volume Correlation

“…There are important factors of root/soil interaction that were not directly observable in these data, including mucilage exudation (Czarnes et al, 2000;McCully, 1999), and root hair traits (Moreno-Espíndola et al, 2007). Applying the techniques developed in this paper to investigate genotypes that contrast in mucilage exudation (Vollsnes et al, 2010) and root hair traits (Gahoonia et al, 2001) will offer compelling opportunities to probe the fundamentals of rhizosphere development.…”

A brief overview of the causes, impacts and amelioration of soil water repellency - a review

“…The area of soil around plant roots, generally referred to as the rhizosphere, has also been shown to have greater levels of water repellency than bulk soil (Hallett et al 2003). Specific compounds produced by plant roots have been shown to induce water repellency (Czarnes et al 2000), but the effects could also be due to secondary microbial metabolites from root exudate decomposition. Links between hydrophobic compounds in soil and the development of water repellency have not been convincing.…”