基于稳定同位素技术的塔里木河下游不同林龄胡杨的水分利用来源

Water utilization sources of Populus euphratica trees of different ages in the lower reaches of Tarim River

水分是制约很多陆地生态系统植物生长和繁殖的重要因素,在干旱地区尤为明显。利用稳定同位素技术探究塔里木河下游不同林龄胡杨(Populus euphratica)的水分来源情况,了解生态输水背景下荒漠河岸林的水分利用循环与利用策略,可为生态输水提供科学依据,同时也可对同类地区的生态恢复提供借鉴。本研究通过测定塔里木河下游胡杨茎干水和各潜在水源(土壤水、地下水)的稳定氢氧同位素值(δD、δ^(18)O),应用多源线性混合模型(Iso Source)分析了各潜在水源对不同林龄胡杨的贡献比例,并结合3种林龄胡杨不同土壤深度含水量的变化,分析了胡杨的主要吸水层位。结果表明:(1)不同林龄胡杨样地的不同深度区间上的土壤水δ^(18)O值存在显著差异(P<0.05):胡杨幼龄木、成熟木、过熟木木质部δ^(18)O分别为-7.83±0.07‰、-8.53±0.11‰、-9.36±0.21‰;而δD值不存在显著差异(P>0.05)。可据此来推断胡杨的主要吸水层位。(2)总体上,三种林龄胡杨土壤水δ^(18)O值随土壤深度增加而减小,并趋于接近地下水的δ^(18)O值。其中,0–60 cm土壤水受蒸发影响比较大,其同位素组成经历了强烈的蒸发分馏过程,土壤含水量极少,土壤水δ^(18)O值偏正。(3)不同林龄胡杨所利用的水分来源不同:胡杨幼龄木对于地表80 cm以下的土壤水以及地下水均有一定程度的利用,对80–140 cm、140–220 cm和220–340 cm的土壤水平均利用比率依次为16.2%、21.4%和24.6%,对地下水平均利用比率为24.5%;成熟木主要利用220–340 cm的土壤水及地下水,平均利用比率分别为36.9%和42.3%;过熟木主要利用140–340 cm的土壤水及地下水,平均利用比率分别为32.8%和49.3%。

Water availability is one of the most important factors affecting the growth and reproduction of terrestrial plants, particularly in arid regions. Understanding the strategies of desert riparian plants to circulate and utilize water can reveal the scientific basis for ecological water conveyance engineering and contribute to planning ecological restoration of similar areas. In this study, we examined whether Populus euphratica trees of different ages in the lower reaches of Tarim River differed in their water sources by using the stable hydrogen and oxygen isotope technology. Specifically, for P. euphratica of different ages in the same habitat, we measured hydrogen and oxygen stable isotope ratios（δD and δ18O） values of xylem water and potential water sources（soil moisture and ground water）. Then, we used multi-isotope mass balance analysis（Iso Source） to calculate the possible contributions of potential water sources to the total water content in plants. We found significant differences of δ18O value for soil moisture at different soil depths. Also, the δ18O values of young wood（–7.83 ± 0.07‰）, mature wood（–8.53 ± 0.11‰） and over-mature wood（–9.36 ± 0.21‰）, which were significantly different, while δD values did not differ statistically among the forest types. Furthermore, δ18O values of P. euphratica in three kinds of forest age became lower while the distance was deeper from soil surface and gradually approached to ground water on the whole. The shallow soil water in 0–60 cm was significantly affected by evaporation. The isotopic composition of soil water had experienced a strong evaporation and fractionation process. And soil water content was small, while δ18O values were large. Finally, our results showed that water utilization of P. euphratica of different ages came from different sources. The young wood of P. euphratica used an average of 16.2% of its water from 80–140 cm of the soil, 21.4% from 140–220 cm, and 24.6% from 220–340 cm and the average utilization ratio of ground water was 24.5%. In comparison, mature wood of P. euphratica mainly utilized the water from 220–340 cm of the soil and the ground water, with the utilization rate being 36.9% and 42.3%, respectively. Over-mature wood had water utilization ratios of 32.8% from 140–340 cm of soil and 49.3% of ground water.