华北平原典型土地利用类型下包气带土壤水稳定同位素变化特征

Isotopic Characteristics of Soil Water from Deep Vadose Zone of Different Typical Land Use Patterns in the North China Plain

基于华北平原典型土地利用类型(梨园、农田)包气带(>18m)土壤水同位素测定结果,分析了华北平原深层土壤水稳定同位素(δD、δ18O)特征,揭示了不同土地利用类型下包气带土壤水补给过程中蒸发和入渗的规律。结果表明,研究区大气降水线δD=6.07δ18O–5.76(R2=0.86),土壤水δD、δ18O值均落在大气降水线下方,表明降水入渗补给土壤水过程中经历了强烈的蒸发作用;除梨园Ⅰ,土壤水同位素值变异系数浅层>中层>深层,表明浅层土壤水δD、δ18O波动较大,主要由于其易受到降水和蒸发的影响,随土壤剖面深度的增加,蒸发和降水的影响逐渐变弱;梨园Ⅰ深层土壤水同位素变异系数较大,表明该样点深层土壤水受到地下水波动的影响;梨园浅层土壤水氘盈余(d-excess)较农田大,说明农田浅层土壤水蒸发强度大于梨园;0.25~0.5m深处土壤水均出现δD、δ18O的明显富集,主要受土壤质地分层影响导致土壤水入渗受阻,同位素较为富集的土壤水在此深度层积聚;而梨园2~5 m出现δD、δ18O的贫化现象,主要是梨树根系埋深使得降水以优先流形式补给至此土壤层。梨园和农田包气带土壤水δD、δ18O垂直剖面上差异显著,表明了华北平原不同土地利用方式的包气带土壤水入渗过程有明显差异,梨园包气带土壤水入渗过程主要以优先流补给影响。本研究为深入了解华北平原农田区厚包气带水分运动、氮素迁移转化与地下水水质之间的关系提供了理论依据。

【Objective】The mechanisms of evaporation and infiltration during the processes of precipitation on soil layers are still not clear.Thus,this study was designed to investigate the spatial variations of soil water isotopes from deep vadose zones(>18 m)of pear orchard and farmland plots in the North China Plain.【Method】Stable isotopes(δD andδ18O)in soil water,soil water content and soil texture were explored in the pear orchard and farmland plots of the North China Plain.【Result】Results showed that the local meteoric water line(LMWL)was established asδD=6.07δ18O–5.76(R2=0.86).TheδD andδ18O of soil water from different soil layers fell below the LMWL,indicating that soil water replenished from precipitation experienced strong evaporation.Except for pear orchard(I),the coefficient of variation(CV)of soil water isotopic values was in the order of shallow layer>middle layer>deep layer,indicating that theδD andδ18O of shallow soil water fluctuated greatly.This implies that shallow soil water was more impacted by precipitation and evaporation compared with other soil layers.As the profile depth increased,the effect of precipitation and evaporation gradually became weaker.The larger isotopic CV of deep soil water in pear orchard(I)indicated deep soil water was affected by groundwater at this sampling site.The d-excess values of shallow soil water were greater in pear orchard plots than those in farmland plots,indicating that the evaporation intensity of shallow soil layers was less in the pear orchard plots than in farmland plots.The obvious isotopic enrichment(δD andδ18O)was observed in the soil layer of 0.25–0.5 m,which was associated with soil texture stratification and hindered infiltration into deep soil layers.This resulted in enriched soil water accumulating in the soil layer.The phenomenon of isotopic depletion was found in the soil layers(2–5 m)from the pear orchard plots and resulted from preferential flow induced by pear roots.【Conclusion】The remarkable spatial differences of isotopic variations in soil layers were observed in the pear orchard and farmland plots,indicating that the infiltration processes of soil water differed in various land-use patterns of the North China Plain.The preferential flow of soil water during the infiltration processes was dominated in the pear orchard plots.These findings provide a theoretical basis for understanding the relationship between water movement,nitrogen migration and transformation,and groundwater quality in the deep vadose zone of the North China Plain.