Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

Yajun Wang; Xiaofeng Cao; Hongwei Yu; Yan Xu; Jianfeng Peng; Jiuhui Qu

doi:10.1016/j.jhazmat.2023.131527

Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

J Hazard Mater. 2023 Aug 5:455:131527. doi: 10.1016/j.jhazmat.2023.131527. Epub 2023 May 4.

Authors

Yajun Wang¹, Xiaofeng Cao², Hongwei Yu³, Yan Xu⁴, Jianfeng Peng², Jiuhui Qu⁵

Affiliations

¹ State Key Laboratory of Environmental Criteria and Risk Assessment, and State Environmental Protection Key Laboratory of Simulation and Control of Groundwater Pollution, Chinese Research Academy of Environmental Sciences, Beijing 100012, China; Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China.
² Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China.
³ State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
⁴ College of Marine Science and Technology, China University of Geosciences, Wuhan 430074, China.
⁵ Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China; State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.

PMID: 37163892
DOI: 10.1016/j.jhazmat.2023.131527

Abstract

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO₃^- isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ¹⁸O-NO₃^- (+13‰) compared with δ¹⁵N-NO₃^- (-2‰) on average during groundwater table rise was due to a combination of factors, including high ¹⁸O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and ¹⁸O-enriched O₂ produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate.

Keywords: Bacterial indicator; Dual isotope ratios; N transformation processes; Nitrate contamination; Seasonal shifting.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

China
Environmental Monitoring / methods
Groundwater*
Nitrates / analysis
Nitrogen / analysis
Nitrogen Isotopes / analysis
Oxygen Isotopes / analysis
Soil / chemistry
Water Pollutants, Chemical* / chemistry

Substances

Nitrogen
Nitrates
Nitrogen Isotopes
Oxygen Isotopes
Water Pollutants, Chemical
Soil