Isotopic constraints on active nitrification in a eutrophic artificially oxygenated lake: Implications for nitrate regeneration and nitrous oxide production

Abstract

Nitrification is a key process in the aquatic nitrogen (N) cycle, but its products, nitrate (NO 3 − ) and nitrous oxide (N 2 O), contribute to eutrophication and greenhouse gas emissions, particularly in eutrophic lakes. Variations in in‐lake N cycling and N 2 O production pathways, as a function of seasonality and artificial oxygenation, remain poorly understood. We investigated nitrification in the artificially oxygenated eutrophic Lake Baldegg, by analyzing NO 3 − and N 2 O concentrations and isotope ratios, and measuring ammonium oxidation rates via 15 N tracer incubations over one year. An N isotope mass‐balance model revealed that nitrification sustained only 5.3 ± 0.7% of total NO 3 − consumption in the epilimnion, where external N loadings were influential, and considerably more in the hypolimnion (81.6 ± 18.5%) during stratification. Dual NO 3 − isotope signatures (Δδ 18 O : Δδ 15 N ~ 1.5–1.73) and associated negative NO 3 − isotope anomalies confirmed epilimnetic nitrification, though external inputs partly obscured this signal. During stratification, relatively high hypolimnetic nitrification rates correlated with organic matter export, and seemed linked to sediment resuspension and artificial oxygenation. While sedimentary denitrification/DNRA dominated hypolimnetic NO 3 − reduction (with negligible effects on δ 15 N‐NO 3 − and δ 18 O‐NO 3 − ), transient suboxic conditions enabled water column denitrification during stratification (24.1–30.2% of the total hypolimnetic denitrification). High N 2 O isotope site‐preference values (30–35‰) confirmed hypolimnetic ammonium oxidation as the main N 2 O production pathway. During winter overturn, N 2 O transport from the hypolimnion caused epilimnetic N 2 O oversaturation and atmospheric emissions up to 3.52 μ mol m −2 d −1 . Comparison with other lakes suggests that artificial oxygenation enhances N turnover, manifesting in greater ambient N 2 O backgrounds and fluxes to the atmosphere.

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