Combing δ15N and δ18O to identify the distribution and the potential sources of nitrate in human-impacted watersheds, Shandong, China

Abstract

Identifying the anthropogenic and natural sources of nitrate emissions contributing to surface water continues to be an enormous challenge. It is necessary to control the water quality in the watershed impacted by human disturbance. In this study, water chemical parameters including nitrate (NO₃⁻) concentrations, δ¹⁵N–NO₃⁻, δ¹⁸O–NO₃⁻, and δ¹⁸O–H₂O were analyzed to investigate the contamination and sources of NO₃⁻ in two watershed rivers (Jinyun, JYN and Jinyang, JYA), Jinan, Shandong, China. Results indicated NO₃⁻ concentrations in the JYN were significantly higher than those in the JYA (P < 0.05), probably because of high N input of the extensive farmlands or orchards in the drainage basin. δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻, associated with Cl⁻, indicated that nitrate-nitrogen (NO₃⁻–N) was not derived from atmospheric deposition but came principally from manure/sewage and soil organic matter in these two watersheds. The microbial nitrification took place in the nitrate of manure/sewage and soil nitrate. The combination of NO₃⁻ concentration and nitrogen and oxygen isotope suggested that NO₃⁻ had undergone microbial denitrification after entering the rivers. Furthermore, NO₃⁻ concentrations had significant temporal and spatial variation highlighting differential sources and fates. These results expand our understanding of mechanisms driving NO₃⁻ retention and transport and provide strategies in managing NO₃⁻ contamination in different land use watersheds around the world.