Geochemical isotopic composition in the Loess Plateau and corresponding source analyses: A case study of China's Yangjuangou catchment

Isotopic fractionation technology is widely used in identifying sources and the speciation of geochemical isotopic elements. With the increase in human activity, geochemical element transport by soil erosion has become the most critical environmental problem in the Loess Plateau, so tracing the geochemical element source would help in the identification and management of local soil erosion. In this study, we investigated the spatial distribution of carbon (C), nitrogen (N), oxygen (O), and hydrogen (H) isotopes in water and then further analyzed 13C and 15N in soil and vegetation to better understand the C and N sources and their biogeochemical cycling function in the Loess Plateau. Results showed that mean dual isotopic values of δ15N-NO3− and δ18O-NO3− in the watershed streams were 11.44 ± 6.15‰ and − 11.29 ± 2.52‰, respectively, and that N wet deposition, fertilizers, and manure were the three main pollution sources. The mean dual isotopic value of δ13C in the water was − 5.36 ± 0.28‰, indicating that δ13C-DIC in the Yangjuangou Catchment of China's Loess Plateau is mainly controlled by carbonate weathering or soil erosion. The severe erosion in this region has typically occurred in grassy (C4) land-use types devoid of woody vegetation (C3), and this has led to a discrepancy in δ13C between soil and water. We found δ18O and δ2D in water to be − 7.87 ± 0.85‰ and − 61.49 ± 3.25‰, respectively, and to show a high positive correlation (r2 = 0.81). This suggests that summer rainstorms lead to soil erosion and runoff, which cause a wide range of isotopic values to occur across the Loess Plateau.