Tracing of Nitrogen Using Stable Isotopes: From Precipitation to River and Groundwater Systems

Dear Colleagues,

The nitrogen cycle (N cycle) is the circulation of nitrogen among the atmosphere and the hydrosphere through conversion into multiple nitrogen chemical forms via (biogeo)chemical processes. Excess nitrogen loading has altered the global N cycle, often resulting in the eutrophication of rivers and lakes (often witnessed as algal blooms and hypoxia), and health-related contamination issues of groundwater, particularly when interacting with surface waters and used for drinking water purposes. Anthropogenic activities including fossil-fuel combustion and agricultural practices have significantly perturbed the global N cycle. Trace N gases emitted, including nitrogen oxides (NO_x) and ammonia (NH₃), are transformed in the atmosphere and undergo wet and/or dry deposition, introducing fixed forms of nitrogen to often sensitive ecosystems. In addition to atmospheric inputs, nitrogen is introduced in the terrestrial environment through natural (e.g., soil) but mostly through anthropogenic point (e.g., wastewater treatment plants) and non-point sources (fertilizers, animal breeding facilities). N species (e.g., NO₃^-, NH₄⁺) undergo biogeochemical changes in aquatic systems (e.g., nitrification, denitrification) depending on the physical–chemical conditions, which makes nitrogen tracing a challenge. Stable isotopes of nitrate (δ¹⁵N and δ¹⁸O), in conjunction with other indicators (e.g., chemical ions, emerging contaminants, water isotopes), constitute a reliable tool to identify N pollution sources in river/ground water bodies, including wet deposition, and aid in unveiling the complexity of the (biogeo)chemical processes nitrogen undergoes in the atmosphere and the aquatic environment, including soils. Moreover, Bayesian-type source apportionment models (e.g., SIAR, MixSIAR, Simmr) are used to estimate the contribution of different N pollution sources to water bodies by taking into account fractionation processes, such as denitrification, in many cases.

This Special Issue encourages the submission of works aiming at studying the atmospheric and terrestrial aquatic compartments of the N cycle through the use of nitrogen isotope tracers. Research on the following topics is promoted: (i) source contributions to atmospheric nitrate; (ii) N atmospheric chemistry processes and wet deposition; (iii) sources of nitrogen pollution in rivers and groundwaters in relation to land-use practices; (iv) biogeochemical processes in ecosystems and their resilience to nitrogen pollution; (v) statistical modeling techniques to quantify N contribution sources in different compartments of the N cycle; (vi) N cycling in rivers on the spatial and temporal scale; and (vii) evaluation of the N-cascading impact of nitrogen deposition and pollution on water resources and ecosystems. Examination of these N aspects is critically important to enhance the implementation of beneficial land management and mitigation strategies toward a sustainable management and protection of water resources.

Dr. Ioannis Matiatos
Dr. Wendell Walters
Dr. Viviana Re
Guest Editors

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• sources of N pollution
• biogeochemical processes
• nitrogen wet deposition
• land uses
• eutrophication
• stable isotopes
• bayesian modelling
• surface water–groundwater interaction
• N cycling
• sustainable management