The Determination Of Soil Water Stable Isotopes And Its Utilization In Soil Evaporation

Water stable isotopes(?²H,?¹⁸O)as the fingerprint of water are widely used in eco-hydrological studies.In order to obtain water stable isotopic compositions,we can use direct liquid-vapour equilibration method to analyze soil and plant samples directly or extract water from soils and plants and subsequently analyze the isotopic compositions of the extracted waters.There are many methods that we can use to extract waters from soils and plants,and cryogenic vacuum distillation(CVD)is the gold-method.The water extracted by CVD is assumed to be the bulk water.However,recent researches showed that there are unanswered questions for both of the isotope measurement methods.For example,different liquid-vapour equilibration times and calibration methods for direct liquid-vapour equilibration method are used in the literature;due to uncompleted extraction,the isotopic composition of CVD extracted water is smaller than the true value.Further,the isotopic bias affects the result of former researches on ecohydrological processes.Therefore,evaluating the optimum equilibration time and calibration method for direct liquid-vapour equilibration method and the effect of the isotopic bias caused by CVD are the urgent scientific questions that are needed to be answered in stable isotope ecohydrological studies.Thus,lab experiments of oven-dried and rehydrated soils and field experiments were conducted,by comparing the isotopic compositions in the mixture water extracted by high-speed centrifuge and centrifugation residual water extracted by CVD from that in water extracted by CVD alone to validate the conjunction of CVD and high-speed centrifuge to obtain large and small soil pores water isotopic composition;by comparing soil water isotopic values measured by direct liquid-vapour equilibration method using six equilibration times and three calibration methods to elucidate the effect of equilibration time and calibration method to the result of direct liquid-vapour equilibration;by comparing evaporative water losses derived from pre-and post-corrected water isotope values obtained by CVD to evaluate the effect of isotopic bias caused by CVD to calculate the evaporative water loss;by comparing evaporative water losses derived from isotopic composition in evaporating water and CVD extracted bulk soil water to explore the effect of water stable isotope variation in large and small pores to calculate the evaporative water loss.The main results are followed:(1)There was no isotopic difference of water extracted by high-speed centrifuge from reference water(P>0.05 or|Z-score|<2);however,water extracted by CVD was significantly depleted in heavy isotopes than reference water(P<0.05 and|Z-score|>2).Furthermore,the depletion increased with the increase of clay content and the decrease of water content.Moreover,there was no difference of the mixture(large pores water extracted by high-speed centrifuge and residual small pores water in post-centrifugation soils extracted by CVD)isotopic composition from the bulk soil water extracted by CVD(P>0.05 or|Z-score|<2).The results indicated that the combination of CVD and high-speed centrifuge can be used to determine water isotopic compositions in small and large pores.There is no isotopic fractionation for large pores water,but the isotopic composition biased in small pores water extracted by CVD.Therefore,calibration is needed to correct the isotopic bias caused by CVD.When there was residual water in oven-dried soils,the isotopic bias in CVD extracted water was affected by the isotopic values in reference water.Thus,it is needed to omit the effect of oven-drying residual water to the calibration equation.(2)Direct liquid-vapour equilibration method demonstrated higher?²H and?¹⁸O as equilibration time increased,but there was no significant difference of isotopic value measured at 12 h from that measured at 24 h.Further,the isotopic composition measured at 12 and 24 h was closest to the reference value.Therefore,12 to 24 h could be used as the optimal equilibration time at 20?.For field soil samples,the isotopic values measured by direct liquid-vapour equilibration with liquid waters as standards>the values using calibration with soil texture>the values using calibration with both soil texture and water content?the values derived from CVD.The results indicated that the water measured by direct liquid-vapour equilibration was large pores water.Considering soil clay content and water content in the calibration equation made the measured soil water isotopic composition close to the value obtained by CVD.(3)There was significant difference of the evaporative water loss derived from post-corrected isotopic composition from that derived from pre-corrected isotopic value(P<0.05)on several time points.For a long time period(>23 days),there was no difference(P>0.05).The results indicated that,under most cases,the spatial variability of soil water isotope is larger than the isotopic bias caused by CVD.Further,the effect of the isotopic bias to the calculation of evaporative water loss can be diminished by long-term observation.(4)When newly added water was“heavier”than pre-event soil water,soil water depleted with the increase of evaporation time;when newly added water was“lighter”than pre-event soil water,soil water enriched with increasing evaporation time.This suggested that evaporating water was newly added water,both of which were different from bulk soil water.However,soil evaporative water losses derived from soil water and evaporating water did not differ significantly(P>0.05).Overall,12?24 h was the optimum equilibration time for direct liquid-vapour equilibration method to measure soil water isotopic compositions at 20?;by calibrating soil clay and water content,the isotopic value measured by direct liquid-vapour equilibration was close to that obtained by CVD;high-speed centrifuge combined with CVD could be used to obtain the distribution of isotopic composition in soil pore space;the isotopic bias caused by CVD did not affect the calculation of evaporative water loss.