Oxygen and hydrogen stable isotope compositions of soil water in deep loess profile under different land use types of northern Shaanxi, China

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Investigation of stable isotope composition under different land use types is helpful for understanding soil water movement and hydrological effects of land use change. We collected soil samples in profiles ＞ 15 m deep under four land use types (i.e. farmland, grassland, Salix cheilophila and Populus sp.) in the loess deposits of northern Shaanxi. We measured hydrogen and oxygen stable isotope composition of soil water to explore the mechanism of soil water movement and the impacts of land use types. The isotope compositions of soil water under four land use types were significantly different. The δD values of soil water under farmland, grassland, S. cheilophila and Populus sp. were -81.1‰--60.1‰, -91.2‰--61.0‰, -87.4‰--63.6‰ and -73.5‰--62.2‰, while the δ18O values were -11.2‰--7.6‰, -12.6‰--8.2‰, -11.5‰--8.1‰ and -9.9‰--7.7‰, respectively. The soil water stable isotopes fluctuated across the profiles. The soil water isotope compositions in the layers of 0-3 m changed sharply, with the δD values being -80.2‰--61.8‰, -75.9‰--65.5‰, -76.0‰--63.6‰ and -73.5‰--62.2‰, respectively. In the layers of 3-12 m, the isotope profiles of farmland and grassland were parabolic, whereas those of S. cheilophila and Populus sp. were relatively stable. Soil water isotope compositions in the layers deeper than 12 m were generally stable with the δD values of -80.8‰--71.5‰, -83.0‰--67.5‰, -87.4‰--76.0‰ and -67.5‰--64.3‰, respectively. Across the four land use types, soil water stable isotope compositions were not significantly different either in the shallow layers or in the deep soil layers, but their differences in the layers of 3-12 m were significant. Soil moisture was mainly recharged from precipitation with piston flow as the main form of soil water movement. Soil water under four land use types might be recharged by wet events of different intensities. Soil water under farmland and grassland could be recharged by wet events of small intensity, but that under S. cheilophila and Populus sp. may be mainly recharged by the rainstorm in summer and autumn.