Water and nutrient dynamics in surface roots and soils are not modified by short-term floodng of phreatophytic plants in a hyperarid desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water andnutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the watertable, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viableshallow roots may be equally necessary for nutrient uptake. We sought to better understand the interactionbetween water and nutrient uptake from soil horizons differing in the relative abundance of these resources.To this end, we monitored plant water and nutrient status before and after applying flood irrigation to fourphreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sapflow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica(Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using theheat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitratereductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in theleaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on waterand nutrient status. Rates of sap flow measured during the day and at night in the absence of transpirationdid not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h)1)detected during the day in soils that had a near zero water content at the surface indicated that all specieshad contact to groundwater. There was no evidence from sap flow data that plants had utilised flood waterto increase maximum rates of transpiration under similar climatic conditions, and there was no evidence ofa process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. thepassive movement of water from moist soil to very dry soil via roots). Measurements of plant water status,xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were alsounaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of waterand nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newlyavailable nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.