Seasonal water uptake for halophytic shrubs in northwest China salinity-affected area: an isotopic investigation

摘要

Shallow groundwater table is the main cause of salinization in the northern area of Ningxia Plain.The uptake of groundwater by halophytes can control groundwater table.Potential water sources of four halophytic shrubs,i.e.twenty-year-old Tamarix ramosissima Ledeb.,three-year-old T.ramosissima.,Lycium barbarum L.and Atriplex canescens(Pursh)Nutt.were studied using hydrogen and oxygen isotopic compositions during a whole growth period in salinity-affected area of northwest China.Irrigation was applied for comparison of contrasting water conditions on water use of the above shrubs.The result showed that the adult T.ramosissima had the phreatophytic nature.The three year old T.ramosissima and Lycium barbarum L.exhibited higher dependence on water from unsaturated soil in upper profile during early period.The the three year old Atriplex canescens was found to have phreatophytic tendency with higher deep water uptake.Apparent depletion in xylem δD values rather than δ18O,compared with water sources,was found.There was discrepancy when using δ18O or to determine water use uptake by the shrubs.This discrepancy was more obvious after irrigation.Hydrogen isotopic fractionation would occur during water uptake by halophytic shrubs.δ18O values of xylem water should be adopted to study halophytic shrub water use in salinity-affected regions.