Surface tension, rheology and hydrophobicity of rhizodeposits and seed mucilage influence soil water retention and hysteresis

Abstract

Aims: Rhizodeposits collected from hydroponic solutions with roots of maize and barley, and seed mucilage washed from chia, were added to soil to measure their impact on water retention and hysteresis in a sandy loam soil at a range of concentrations. We test the hypothesis that the effect of plant exudates and mucilages on hydraulic properties of soils depends on their physicochemical characteristics and origin.

Methods: Surface tension and viscosity of the exudate solutions were measured using the Du Noüy ring method and a cone-plate rheometer, respectively. The contact angle of water on exudate treated soil was measured with the sessile drop method. Water retention and hysteresis were measured by equilibrating soil samples, treated with exudates and mucilages at 0.46 and 4.6 mg g^-1 concentration, on dialysis tubing filled with polyethylene glycol (PEG) solution of known osmotic potential.

Results: Surface tension decreased and viscosity increased with increasing concentration of the exudates and mucilage in solutions. Change in surface tension and viscosity was greatest for chia seed exudate and least for barley root exudate. Contact angle increased with increasing maize root and chia seed exudate concentration in soil, but not barley root. Chia seed mucilage and maize root rhizodeposits enhanced soil water retention and increased hysteresis index, whereas barley root rhizodeposits decreased soil water retention and the hysteresis effect. The impact of exudates and mucilages on soil water retention almost ceased when approaching wilting point at -1500 kPa matric potential.

Conclusions: Barley rhizodeposits behaved as surfactants, drying the rhizosphere at smaller suctions. Chia seed mucilage and maize root rhizodeposits behaved as hydrogels that hold more water in the rhizosphere, but with slower rewetting and greater hysteresis.

Keywords: Contact angle; Hysteresis; Root exudate; Seed exudate; Soil water retention; Surface tension; Viscosity.

Fig. 1

The relationship between surface tension (mean ± 1 standard error) and the concentration of the chia seed, chia seed after ball-milling (BM), maize root and barley root exudates and mucilages in water at a range of concentrations. The dashed line is the surface tension of water

Fig. 2

The relationship between viscosity (mean ± 1 standard error) and shear rate for different concentrations of exudates and mucilages; a chia seed, b chia seed after ball-milling (BM), c maize and d barley

Fig. 3

Contact angles (mean ± 1 standard error) on dried soil of a barley root, b maize root, c chia seed exudate after ball milling (BM) and d chia seed exudates and mucilages treated soil at different concentrations in water

Fig. 4

Drying and rewetting curves at a range of matric potentials for unamended soil and soil treated with exudates and mucilages at a concentration of 4.6 mg exudate g⁻¹ dry soil. Grey shows the control soils that were not amended with exudate. The mean ± 1 standard error is shown. The drying curve is fitted with the Fredlund and Xing (1994) model. The wetting curve is fitted with a 3rd order polynomial

Fig. 5

Drying and rewetting curves at a range of matric potentials for unamended soil and soil treated with exudates and mucilages at a concentration of 0.46 mg exudate g⁻¹ dry soil. Grey shows the control soils that were not amended with exudates and mucilages. The mean ± 1 standard error is shown. The drying curve is fitted with the Fredlund and Xing (1994) model. The wetting curve is fitted with a 3rd order polynomial

Fig. 6

Conceptual framework showing the relative significance of surface tension and viscosity of the exudates and mucilages in soil to water retention and hysteresis. Viscosity provides an indirect measurement of long-chain polymers that may act as a hydrogel. The increase or decrease in water retention was observed from the drying limbs in Figs. 4 and 5, where only barley caused a decrease. The Dashed line is a hypothetical cut-off represents the transition between compounds that have a net effect of acting like a surfactant versus a hydrogel