Do Plant Roots Modify The Soil Water Retention Properties of Rhizospheric Soil in The Field?

Regulation of water fluxes in soil and plant water uptake are strongly dependent on the hydric properties of the soil. However, these hydric properties might be changed, in turn, by plant roots growing into the soil. Such root-related alteration of soil properties, particularly water retention, can be linked to soil perforation, aggregation of soil particles, release of exudates by roots modifying the soil particles/water properties. This effect could be more pronounced in the rhizosphere, the zone around roots influenced by the activity of root and associated microorganisms, and thus differentiating bulk and rhizospheric soil properties. To test if crops and plant root rhizosphere may have a significant effect on water retention, we conducted experiments from laboratory to field scales, covering various time scales. In the lab, variation in available water content (AWC) between bulk and rhizospheric soil varied from 0 to about 16% increase, depending on plant species and soil type. In the field, the alteration of water retention by root systems was tested in different pedological settings for maize, inoculated or not with the bacteria Azospirillum spp. known to alter root structure and growth, and wheat crops. Again, a range of variation in AWC was evidenced, with a consistent increase of the permanent wilting point in the rhizospheric soil. This rhizospheric effect of soil water retention can be related to exopolysaccharides concentration for plant reaching flowering stage. However, this was not the case for younger plant stages, where microbial activity appears more linked to water retention variations.

Even if alteration of AWC by roots looks modest in the field, this could mitigate drought impacts or modify the water balance of the root zone. How and how much plants modify the hydric properties, however, deserves further research.