ISOTOPIC STUDY OF THE WATER EXCHANGEBETWEEN ATMOSPHERE AND BIOSPHERE ATSELECTED SITES IN PAKISTAN

摘要

Study of water exchange between atmosphere and biosphere was initiated tounderstand the ties between these two spheres. This paper presents the isotope ( 18 O,2 H) results of moisture in the soil and plants (root, stem and leaf samples). Non woodyplants (wheat, grass) and soil samples from wheat and grass fields (from the surface and7 cm below the surface) were collected during January 2005 to April 2005. Woody plantsof many species were sampled from two sites near Islamabad and Lahore. Air moisturewas also collected in the field. Moisture contents from these samples were extractedusing the vacuum distillation method and analysed for 18 O, 2 H isotope contents. Datadepicts that the 18 O and 2 H of moisture in the leaves of non-woody and woody plantsare much more enriched than those of stems and follow the evapourative enrichmenttrend originating from the soil moisture in the active root zone. Degree of enrichmentdepends on the size of leaves. Significant evaporation effect in the moisture of grassstems indicates alteration during water trasport through the stem of tiny/thin plants.Reflection of typical isotopic values of individual rain events in the soil and stem and thecorresponding values in the leaves shows that these isotopes can identify the source ofmoisture uptake, which can be used for studying water-use efficiency. Leaves of woodyplants have relatively depleted 18 O values during wet (monsoon) period mainly due toretardation of fractionation resulting from higher humidity.