Global warming is considered a major threat to Earth’s lakes water budgets and quality. However, flow regulation, over-exploitation, lack of hydrological data, and disparate evaluation methods hamper comparative global estimates of lake vulnerability to evaporation. We have analyzed the stable isotope composition of 1257 global lakes and we find that most lakes depend on precipitation and groundwater recharge subsequently altered by catchment and lake evaporation processes. Isotope mass-balance modeling shows that ca. 20% of water inflow in global lakes is lost through evaporation and ca. 10% of lakes in arid and temperate zones experience extreme evaporative losses >40 % of the total inflow. Precipitation amount, limnicity, wind speed, relative humidity, and solar radiation are predominant controls on lake isotope composition and evaporation, regardless of the climatic zone. The promotion of systematic global isotopic monitoring of Earth’s lakes provides a direct and comparative approach to detect the impacts of climatic and catchment-scale changes on water-balance and evaporation trends.

Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Isotopes (δ¹⁵N, δ¹⁸O) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO₃⁻). We synthesized nitrate isotope data (n = ~5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO₃⁻ (0.3 ± 0.2 mg L⁻¹, n = 2902) compared to aquifers (5.5 ± 5.1 mg L⁻¹, n = 2291) and slightly lower δ¹⁵N values (+7.1 ± 3.8‰, n = 2902 vs +7.7 ± 4.5‰, n = 2291), but were indistinguishable in δ¹⁸O (+2.3 ± 6.2‰, n = 2790 vs +2.3 ± 5.4‰, n = 2235). The isotope composition of NO₃⁻ was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher δ¹⁵N and δ¹⁸O values in wintertime, suggesting waste-related N-source signals are better preserved in the cold seasons. Isotopic assays of nitrate biogeochemical transformations are key to understanding nitrate pollution and to inform beneficial agricultural and land management strategies.

Stable isotope ratios of nitrogen and oxygen (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) of nitrate (NO₃^–) are excellent tracers for developing systematic understanding of sources, conversions, and deposition of reactive atmospheric nitrogen (N_r) in the environment. Despite recent analytical advances, standardized sampling of NO₃^–) isotopes in precipitation is still lacking. To advance atmospheric studies on N_r species, we propose best-practice guidelines for accurate and precise sampling and analysis of NO₃^– isotopes in precipitation based on the experience obtained from an international research project coordinated by the International Atomic Energy Agency (IAEA). The precipitation sampling and preservation strategies yielded a good agreement between the NO₃^– concentrations measured at the laboratories of 16 countries and at the IAEA. Compared to conventional methods (e.g., bacterial denitrification), we confirmed the accurate performance of the lower cost Ti(III) reduction method for isotope analyses (¹⁵N and ¹⁸O) of NO₃^– in precipitation samples. These isotopic data depict different origins and oxidation pathways of inorganic nitrogen. This work emphasized the capability of NO₃^– isotopes to assess the origin and atmospheric oxidation of N_r and outlined a pathway to improve laboratory capability and expertise at a global scale. The incorporation of other isotopes like ¹⁷O in N_r is recommended in future studies.