Plant exudates may stabilize or weaken soil depending on species, origin and time

Abstract

SummaryWe hypothesized that plant exudates could either gel or disperse soil depending on their chemical characteristics. Barley (Hordeum vulgare L. cv. Optic) and maize (Zea mays L. cv. Freya) root exudates were collected using an aerated hydroponic method and compared with chia (Salvia hispanica L.) seed exudate, a commonly used root exudate analogue. Sandy loam soil was passed through a 500‐μm mesh and treated with each exudate at a concentration of 4.6 mg exudate g−1 dry soil. Two sets of soil samples were prepared. One set of treated soil samples was maintained at 4°C to suppress microbial processes. To characterize the effect of decomposition, the second set of samples was incubated at 16°C for 2 weeks at −30 kPa matric potential. Gas chromatography–mass spectrometry (GC–MS) analysis of the exudates showed that barley had the largest organic acid content and chia the largest content of sugars (polysaccharide‐derived or free), and maize was in between barley and chia. Yield stress of amended soil samples was measured by an oscillatory strain sweep test with a cone plate rheometer. When microbial decomposition was suppressed at 4°C, yield stress increased 20‐fold for chia seed exudate and twofold for maize root exudate compared with the control, whereas for barley root exudate decreased to half. The yield stress after 2 weeks of incubation compared with soil with suppressed microbial decomposition increased by 85% for barley root exudate, but for chia and maize it decreased by 87 and 54%, respectively. Barley root exudation might therefore disperse soil and this could facilitate nutrient release. The maize root and chia seed exudates gelled soil, which could create a more stable soil structure around roots or seeds.Highlights Rheological measurements quantified physical behaviour of plant exudates and effect on soil stabilization.Barley root exudates dispersed soil, which could release nutrients and carbon.Maize root and chia seed exudates had a stabilizing effect on soil.Physical engineering of soil in contact with plant roots depends on the nature and origin of exudates.

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

“…Although exudation clearly represents a significant carbon cost to the plant, exudates are involved in engineering the rhizosphere by dispersion and gelling of soil (Naveed et al 2017;Barré and Hallett 2009;Tarchitzky and Chen 2002;Deng et al 2015), modulation of water and nutrient availabilities (Wang et al 2008;Ahmed et al 2014;Deng et al 2015), and attraction of rhizobacteria (Bais et al 2006). To our knowledge this is the first time that plant exudates have been demonstrated to ease soil compression and thus offer the potential for increased root elongation in soil.…”

“…A recent study by Naveed et al (2017) found that chia seed exudate has a greater amount of polysaccharide sugars and less organic acids than barley and maize root exudates, with differing impacts on soil rheology and water retention. Given the scale of samples required for compression experiments, however, harvesting real root exudates in sufficient quantities would be a formidable task.…”

“…It was extracted based on Naveed et al (2017) and Ahmed et al (2014) by mixing 100 g distilled water with 10 g chia seeds using a magnetic stirrer for 2 min at 50°C, followed by cooling to room temperature (20°C) and 4 h standing. The exudate was separated from the seeds by repeatedly pushing the mixture through a 500 μm sieve under pressure using a syringe that was cut at the end.…”

Sticky mucilages and exudates of plants: putative microenvironmental design elements with biotechnological value

“…It is proposed that bioadhesive mucilage components of exudates are important factors, along with root hairs, in the formation of cylinders of soil around roots known as rhizosheaths. It appears that this will involve differing impacts on soil aggregation through varied viscosities and surface properties, on drying/wetting cycles and the potential for hydrophobicity of mucilage after drying and binding properties in general, including the sequestering of heavy metals (Ray et al, 1988;Watt et al, 1994;Dennis et al, 2010;Naveed et al, 2017;Kroener et al, 2018). Rhizosheath bioengineering by some grass species during periods of drought has been observed, where the grasses increased the thickness of their rhizosheaths (Hartnett et al, 2012).…”

Preferential flow systems amended with biogeochemical components: imaging of a two-dimensional study

“…Specifically, in the broad approach used to elucidate the entire root system, Walker et al (2003) found that the root exudates compounded with mucilage under certain circumstances exhibited a stabilizing effect on the rhizosphere (i.e., the stabilization of the soil structure near the roots). Naveed et al (2017) demonstrated that exudates from barley roots may disperse soil while exudates from maize roots and chia seed exudates may increase soil stability around roots or seeds. While the exudates lowered the surface tension of the soil pore water, similar to what the authors determined in this research (Figs.…”