Rooted in reciprocity: interactions and feedbacks in the soil-plant hydraulic continuum

Soils play a critical role in regulating plant water availability, with characteristics like bulk density, porosity, and texture determining soil hydraulic properties, that is, properties that affect the soil water retention and water transport. Together with mycorrhizal activity, which influences the conductance of water between the soil and the roots, soil hydraulic properties affect the ease with which plants can access soil water. In turn, root growth also modifies soil structures and mycorrhizal communities, influencing soil water retention and soil hydraulics. Despite a good theoretical understanding of the dynamic interactions between soils and plants, limited information is available on: (i) how much soil texture affects plant hydraulic properties across plant species; and (ii) how much plant roots affect soil hydraulic properties across soil textures. To assess the extent of the feedback loop between soil and plant hydraulics, we transplanted 4-year-old Quercus robur (N=12) and Quercus cerris (N=12) saplings into either a loam or a clay loam, in equal numbers for each species. Following an acclimation period of three to five months, a total of 28 soil water retention curves were measured from soil cores collected at depths of 7-12 cm, 25-30 cm, and 55-60 cm in the vicinity of the trees (i.e., likely to contain root fragments, mycorrhiza, etc.). We measured a further eight water retention curves in the absence of trees, allowing the determination of a baseline of soil hydraulic characteristics. Finally, after the soil sample collection, we established two hydraulic vulnerability curves per tree. Preliminary results show no indication of plant hydraulic acclimation to soil under well-watered conditions. The presence of tree roots affected soil bulk density at depth in the loam, as well as hydraulic properties like the field capacity at -33 kPa and the permanent wilting point, but not in the clay loam. Whether these effects are the same after longer acclimation periods or under water-stress conditions remains to be determined.