Multiple isotopes reveal the driving forces of nitrogen cycling from freshwater to brackish water

Anqi Zhang; Yan Wang; Yi Li; Yingyu Tan; Pengxia Liu; Xubo Lv; Kun Lei

doi:10.1016/j.scitotenv.2023.165396

Multiple isotopes reveal the driving forces of nitrogen cycling from freshwater to brackish water

Sci Total Environ. 2023 Nov 1:897:165396. doi: 10.1016/j.scitotenv.2023.165396. Epub 2023 Jul 10.

Authors

Anqi Zhang¹, Yan Wang², Yi Li³, Yingyu Tan⁴, Pengxia Liu⁵, Xubo Lv², Kun Lei⁶

Affiliations

¹ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China; Key Lab of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling 712100, PR China.
² State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China.
³ Key Lab of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling 712100, PR China.
⁴ Key Laboratory of Environmental Pollution Control Technology of Zhejiang Province, Eco-Environmental Science Research & Design Institute of Zhejiang Province, Hangzhou, Zhejiang 310007, PR China.
⁵ Ecology and Environment Monitoring and Scientific Research Center of Taihu Basin & East China Sea Ecology and Environment Supervision Authority, Ministry of Ecology and Environment, Shanghai 200120, China.
⁶ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China. Electronic address: leikun@craes.org.cn.

PMID: 37437639
DOI: 10.1016/j.scitotenv.2023.165396

Abstract

Rivers play a crucial role in global nitrogen (N) cycling, but revealing the driving mechanism of N cycling remains challenging because of the complex natural background gradients. The Qiantang River Basin provides an opportunity to elucidate the driving mechanism due to the complex climatic and hydrological conditions. In this study, the multiple stable isotopes suggested that the conservative mixing of two end members was insufficient to explain the complex behavior of N in both seasons. In-soil processes were the primary N cycling processes that controlled riverine N loading during the wet season, whereas in-stream N biological transformation processes (nitrification and assimilation) were more prevalent during the dry season. The results of MixSIAR revealed that soil sources (soil N and N fertilizer) contributed the most to NO₃^- during the wet season, accounting for 64.3 %, followed by manure and sewage (31.6 %) and atmospheric precipitation (4.1 %). During the dry season, manure and sewage were the predominant contributors to NO₃^- (52.1 %), followed by soil N (26.6 %), N fertilizer (18.8 %), and atmospheric precipitation (2.5 %). The relationships between d-excess and δ¹⁵N-NH₄⁺ or δ¹⁵N-NO₃^- suggested that both climatic and hydrological conditions would be the driving forces regulating the N transportation and transformation in this basin, leading to the high spatiotemporal heterogeneity in N loading and isotopic compositions. In the wet season, precipitation patterns served as the primary driving forces regulating in-soil biological processes and soil leaching. While the hydrological conditions, especially water residence time, were the crucial factors controlling in-stream biological processes during the dry season. This study elucidates N sources, biotransformation processes, and their driving forces from freshwater to brackish water, which has applications for understanding the N fate from terrene to ocean.

Keywords: Climatic and hydrological factors; Freshwater-brackish water; Isotopes; Nitrogen cycling.