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

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Supported by the National Natural Science Foundation of China(Nos. 41930865, 41877169, 42071145)

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    【Objective】The mechanisms of evaporation and infiltration during the processes of precipitation on soil layers is 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 and δ18O)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 as δD = 6.07δ18O-5.76(R2=0.86). The δD and δ18O 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 the δD and δ18O 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 and δ18O)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.

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JIANG Pengju, WU Huawu, MIN Leilei, SHEN Yanjun, TIAN Lihui, ZHANG Zhihua, BI Huitao. Isotopic Characteristics of Soil Water from Deep Vadose Zone of Different Typical Land Use Patterns in the North China Plain[J]. Acta Pedologica Sinica,2023,60(6):1626-1636.

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  • Received:October 18,2021
  • Revised:May 11,2022
  • Adopted:August 18,2022
  • Online: August 24,2022
  • Published: November 28,2023