Hydraulic lift promotes selective root foraging in nutrient-rich soil patches

Hydraulic lift (HL) - the passive movement of water through plant roots from deep wet to shallow drier soil layers - can improve root survival in dry soils by providing a source of moisture to shallow roots. It may also enhance plant nutrient capture, though empirical evidence for this is scarce and whether HL promotes the selective placement of roots in nutrient-rich soil enhancing nutrient capture in dry soils remains unknown. We tested this with a split-pot design in which we separated the root system of Retama sphaerocarpa (L.) Boiss shrubs into two pot compartments: a lower, well-watered one; and an upper, drier one. Half the shrubs grew under natural light conditions hence allowed to perform HL, whereas the other half had impaired HL by maintaining continuous illumination at night. Resource-rich (organic matter enriched in N-15 and P) and resource-poor soil patches were inserted in the upper compartment after a drought treatment was imposed. Artificial illumination did impair HL at night. Soil moisture in both the whole upper compartment and in soil patches was lower in plants illuminated at night and reduced the allocation of roots to nutrient-rich soil patches at the expense of root growth in nutrient-poor patches (i.e. root foraging precision). Plant nitrogen capture was also lower in shrubs with impaired HL. Overall, these results demonstrate that HL favoured the selective placement of roots in nutrient-rich patches as well as nutrient capture under drought, a process that may secure nutrient capture and maintain plant performance during drought periods.