Water dynamics of the root zone: rhizosphere biophysics and its control on soil hydrology

Approximately 40% of total terrestrial precipitation transits the tiny volume of rhizosphere soil around plant roots before being transpired, making it one of the most hydrologically active regions of the biosphere. This study considers several findings at the root-soil interface that affect our understanding of water retention and flow in the root zone and hence the water relations of all vegetated soil profiles. Imaging methods, including neutron radiography and light transmission, are illuminating the dynamics of water content around plant root systems. These methods, together with studies on samples of mucilage and mucilage-compound-amended soils, have provided increasing evidence that rhizosphere hydraulic properties differ from those of bulk soil. Changes in soil structure due to root growth, rhizodeposition, and repeated drying cycles change the pore size distribution and coat soil particles with organic compounds. Some of these compounds exhibit hydrophobic or hydrophilic behavior, depending on the soil water content, giving rise to the hysteretic-like behavior in the rhizosphere that has been observed in dynamic image sequences. Data from studies that consider the water retention properties of maize (Zea mays L.) mucilage from primary and nodal roots together with polymer gels are compared to consider the likely impact of mucilage on soil water release. Roots often generate and are intimately associated with flow paths for water and solutes in the soil, and vegetation is well known to exert a major influence on catchment hydrology. The potential use of vegetation to manage hydrologic processes at field scale is considered briefly as a way of influencing water outflow rates and engineering soils for particular purposes.