Hydraulic conductivity of the root-soil interface of lupin in sandy soil after drying and rewetting

Aims: The putative role of the rhizosphere in controlling root water uptake is receiving increasing attention. Recent experiments showed that the rhizosphere turned temporarily hydrophobic after drying and subsequent rewetting. Our objective was to investigate whether the rhizosphere hydrophobicity influences the hydraulic conductivity of the rhizosphere-root continuum. Methods: Lupins were grown in aluminium containers filled with a sandy soil. When the plants were 30 days-old, the soil was let dry to a water content of 2–4 % and then it was irrigated. The soil water content during irrigation was imaged using a time-series neutron radiography. By image processing, we quantified the increase in the volume of water in the roots upon irrigation. Using this information and additional measurements of the root pressure after irrigation, we calculated the water flow into the roots and the total hydraulic conductance of the rhizosphere-root continuum, K_tot. Results: The radiographs showed that: 1) the rhizosphere stayed temporarily dry after irrigation; 2) as the rhizosphere slowly rewetted, the roots rehydrated of 63 %. During 2 to 3 h subsequent to irrigation, K_tot increased from 1.36 ± 1.09 × 10⁻¹¹ m² s⁻¹ MPa⁻¹ to 5.02 ± 2.13 × 10⁻¹⁰ m² s⁻¹ MPa⁻¹, approaching the conductance of lupin roots in wet soils measured in previous experiments. Based on our calculations, these values of K_tot correspond to a rhizosphere conductivity of 3.87 ± 1.91 × 10⁻¹⁴ m s⁻¹ (shortly after irrigation) and 1.09 ± 1.64 × 10⁻¹² m s⁻¹ (2–3 h after irrigation). Conclusions: We conclude that during a drying/wetting cycle, the conductivity of the root-soil interface is a temporary limit to root water uptake. We postulate that the temporary reduced hydraulic conductivity is primarily caused by the rhizosphere water repellency.