EGU21-16166
https://doi.org/10.5194/egusphere-egu21-16166
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Seasonal variation of nitrogen biogeochemical processes constrained by nitrate dual isotopes in cascade reservoirs, Southwestern China 

Sainan Chen1, Fu-Jun Yue1,3, Xiao-Long Liu2, Jun Zhong1, Yuan-Bi Yi1, Wan-Fa Wang1, Yulin Qi1,3, Hua-Yun Xiao1, and Si-Liang Li1,3
Sainan Chen et al.
  • 1Tianjin University, Institute of Surface-Earth System Science , China (chensainan@tju.edu.cn)
  • 2Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, 300387,China
  • 3Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Tianjin University, Tianjin, 300072, China

The increase of affected river reaches by reservoirs has drastically disturbed the original hydrological conditions, and subsequently influenced the nutrient biogeochemistry in the aquatic system, particularly in the cascade reservoir system. To understand the seasonal variation of nitrogen (N) behaviors in cascade reservoirs, hydrochemistry and nitrate dual isotopes (δ15N-NO3andδ18O-NO3) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO3–N accounted for almost 90% of the total dissolved nitrogen (TDN) concentration with high average concentration 3.8 ± 0.4 mg/L among four cascade reservoirs. Higher N concentration (4.0 ± 0.8 mg/L) and larger longitudinal variation were observed in summer than in other seasons. The relationship between the variation of NO3–N and dual isotopes in the profiles demonstrated that nitrification was dominated transformation, while assimilation contributed significantly in the epilimnion during spring and summer. The high dissolved oxygen concentration in the present cascade reservoirs system prevented the occurrence of N depletion processes in most of the reservoirs. Denitrification occurred in the oldest reservoir during winter with a rate ranging from 18 % to 28 %. The long-term record of surface water TDN concentration in reservoirs demonstrated an increase from 2.0 to 3.6 mg/L during the past two decades (~ 0.1 mg/L per year). The seasonal nitrate isotopic signature and continuously increased fertilizer application demonstrated that chemical fertilizer contribution significantly influenced NO3–N concentration in the karst cascade reservoirs. The research highlighted that the notable N increase in karst cascade reservoirs could influence the aquatic health in the region and further investigations were required.

How to cite: Chen, S., Yue, F.-J., Liu, X.-L., Zhong, J., Yi, Y.-B., Wang, W.-F., Qi, Y., Xiao, H.-Y., and Li, S.-L.: Seasonal variation of nitrogen biogeochemical processes constrained by nitrate dual isotopes in cascade reservoirs, Southwestern China , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16166, https://doi.org/10.5194/egusphere-egu21-16166, 2021.

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