Physical properties of a sandy soil as affected by incubation with a synthetic root exudate: Strength, thermal and hydraulic conductivity, and evaporation

Abstract

Plant roots release various organic materials that may modify soil structure and affect heat and mass transfer processes. The objective of this study was to determine the effects of a synthetic root exudate (SRE) on penetrometer resistance (PR), thermal conductivity (λ), hydraulic conductivity (k) and evaporation of water in a sandy soil. Soil samples, mixed with either distilled water or the SRE, were packed into columns at a designated bulk density and water content, and incubated for 7 days at 18°C. Soil PR, λ, k and evaporation rate were monitored during drying processes. Compared with those incubated with water, samples incubated with SRE had visible hyphae, greater PR (0.7–5.5 MPa in the water content range of 0.11 to 0.22 m3 m−3) and λ (0.2–0.7 W m−1 K−1 from 0.05 to 0.22 m3 m−3), and increased k in the wet region but decreased k in the dry region. SRE treatment also reduced the overall soil water evaporation rate and cumulative water loss. Analysis of X‐ray computed tomography (CT) scanning showed that the SRE‐treated samples had a greater proportion of small pores (<60 μm). These changes were attributed mainly to SRE‐stimulated microbial activities. Highlights The effects of incubating a sandy soil with a synthetic root exudate (SRE) on soil physical properties and evaporation are examined. SRE incubation increased the fraction of small pores. SRE incubation increased soil penetrometer resistance and thermal conductivity. Soil hydraulic conductivity was increased in the wet region but was reduced in the dry region. SRE incubation reduced the overall evaporation rate and cumulative water loss.

Effects of mucilage concentration at different water contents on mechanical stability and elasticity in a loamy and a sandy soil

“…Higher exudate concentrations resulting in lower PR were reported by Oleghe et al (2017) for a clay loam and-to a smaller extent-for a sandy loam at À50 kPa Ψ m, which is consistent with our findings in loam. In contrast to this, Zhang et al (2020) detected an increase in PR within defined water content groups when adding a synthetic root exudate, specifically at lower water contents in a sand with a similar texture to ours but a higher bulk density of 1.61 g cm À3 . The decisive difference in approach was the incubation of the soil samples at 18 C in Zhang et al (2020) allowing a microbial community to develop, whereas we stored them at 4 C, largely suppressing microbial activity.…”

“…In contrast to this, Zhang et al (2020) detected an increase in PR within defined water content groups when adding a synthetic root exudate, specifically at lower water contents in a sand with a similar texture to ours but a higher bulk density of 1.61 g cm À3 . The decisive difference in approach was the incubation of the soil samples at 18 C in Zhang et al (2020) allowing a microbial community to develop, whereas we stored them at 4 C, largely suppressing microbial activity. Gao et al (2017) recognised that PR depends on incubation temperature affecting the microbial community.…”

“…After incubation at 4 C, he could not detect an effect of exudate treatment on PR. Comparing the technical details in Zhang et al (2020) with Oleghe et al (2017) and our study suggests that results also depend on boundary conditions during tests and that direct comparison of absolute values should be interpreted with caution. Cone geometries and penetration rates used might alter resulting forces to different extents depending on substrate and water contents (Ruiz, 2018).…”

Root and root‐derived biopore interactions in soils: A review

“…Moreover, Zhang et al. (2021) found that microbial activity resulting from root exudates increased the proportion of small pores with diameters <60 μm, which improved soil hydraulic conductivity. Similarly, Feeney et al.…”

Rhizosheath–root system changes exopolysaccharide content but stabilizes bacterial community across contrasting seasons in a desert environment

“…In addition, the rhizosheath provides a favourable niche that enriches bacteria and fungi from the surrounding sand [13,22]. Rhizosheath-root system microbial communities are dominated by desert-adapted Actinobacteria and Alphaproteobacteria, and saprophytic Ascomycota fungi, which form stable interactions and carry a broad portfolio of plant growth promoting (PGP) traits and ecological services to the holobiont, including nitrogen fixation and EPS-production [6,13,[23][24][25][26][27].…”