Differential isotopic characteristics of eco‐hydrologic processes in a subtropical watershed, China

Abstract

In order to better understand isotopic characteristics of eco‐hydrologic processes in subtropical zones, this study selected a typical subtropical watershed in China to investigate and determine the complex differential mechanisms inherent to this water body type. Results showed that stable isotopes in precipitation within a Local Meteoric Water Line in the Xiangxi River basin (where δD = (8.23 ± 0.27)δ18O + (17.97 ± 1.65); R2 = 0.94) weighted δD and δ18O average precipitation values were −22.11‰ and −5.23‰, respectively. Furthermore, enrichment trends in river water δ18O (−4.21‰ to −6.91‰) and δD (−25.11‰ to −38.13‰) and soil solution δ18O (−6.30‰ to −11.21‰) and δD (−11.21‰ to −34.57‰) were greater compared with precipitation δ18O (−0.23‰ to −12.06‰) and δD (0.54‰ to −98.52‰) and throughfall δ18O (−1.11‰ to −4.72‰) and δD (−2.11‰ to −24.95‰). This study found obvious seasonality and higher deuterium excess (d‐excess) values in precipitation during the dry season and lower values during the rainy season. Moreover, the orchard was more enriched and the coniferous and broad‐leaved mixed forest was more depleted in δD and δ18O compared with other land cover types. Precipitation was an important climate variable in subtropical watersheds under the influence of monsoon systems. According to results from the isotopic mixture model, precipitation input fractions were greater than 88% in the soil solution and greater than 21% in river water.

Tracking the fate of deposited nitrogen and its redistribution in a subtropical watershed in China

“…The δ 15 N source in baseflow mainly derived from soil erosion, local fertilizer application, livestock, plant litter, and deposition from industry, vehicle transportation, and agriculture, but ammonium fertilizers were a minor source of NO 3 − ‐N in the Xiangxi River watershed (Hao, Gao, et al, ), which was similar to results from other studies conducted around the Yellow River (Yue, Li, Liu, Zhao, & Ding, ). Our study also showed that the NO 3 − consumption rate that derived from denitrification was higher than the NO 3 − production rate derived from nitrification, which led to changes in δ 15 N enrichment in soil solutions (Cheng, Fang, Yu, Zhu, & Zheng, ; Fang et al, ; Yue, Li, CQi, Lang, & Ding, ).…”

“…The dual isotopic approach also enables us to trace denitrification, because this process generates δ 18 O to δ 15 N between 0.5 and 1 values. Certain sampling points in the study area were between the 1:1 and 1:2 ratio, which means that denitrification occurred in certain sampling points in the Xiangxi River watershed (Hao, Gao, Sun, & Wen, ). The δ 15 N‐NO 3 and δ 18 O‐NO 3 showed seasonal changes can be explained by that nitrates would be enriched in 18 O during chemical reactions with tropospheric O 3 .…”

Using δ13C to reveal the importance of different water transport pathways in two nested karst basins, Southwest China

“…however, with n (A, B, C, …) isotope systems and >n+1 sources, we were still able to apply the requirement for mass balance conservation to determine multiple combinations of source proportions that offer feasible solutions (Phillips and Gregg, 2003;Hao et al, 2018).…”