The isotope effect of benthic N removal in two Swiss lakes

Aquatic sediments play a critical role in moderating the availability of fixed nitrogen (N) in the biosphere. Microbial N cycling processes, such as denitrification and anammox, contribute to fixed‑N removal as N₂ gas from lakes and the ocean. N‑isotopic measurements of dissolved inorganic N (e.g., nitrate (NO₃^-)) can provide insights into the different sources, sinks, and pathways of N, if the associated N isotope signatures/effects are constrained. While substantial work has been done to resolve N‑loss using microbial metagenomic‑based approaches and rate measurements, how nitrogen‑loss processes imprint natural‑abundance isotopes of ¹⁵N and ¹⁸O of NO₃^- remains largely understudied in freshwater lake sediments. Current marine evidence suggests that water column denitrification involves high NO₃^- isotope effects (>20‰). In contrast, the marine NO₃^- isotope effect of sedimentary denitrification is suppressed at the level of the sediment‑water interface (apparent N/O isotope effect, ε_app <5‰). How anammox affects ε_app in either marine or freshwater systems, is completely unknown. This study aims to achieve a deeper understanding of NO₃^- N and O isotope fractionation during benthic N transformation and sedimentary N‑loss (including anammox), and its ultimate expression in the water column of freshwater lakes. We also investigate how ε_app values may vary with environmental conditions (e.g., trophic state) that affect the reactivity and amount of organic matter in the sediments, as well as the balance between benthic N‑cycle reactions.

The two study sites Lake Baldegg (eutrophic) and Sarnen (oligotrophic), were chosen because of their contrasting trophic states. We conducted a suite of experiments with sediment cores collected at different times of the year to assess the sedimentary ε_app in these two lakes. More specifically, we carried out whole‑core incubations under oxic/anoxic conditions and examined the change ¹⁵N/¹⁴N and ¹⁸O/¹⁶O of NO₃^- with net NO₃^- depletion in the overlying water. We integrated natural‑abundance N and O isotope measurements with ¹⁵N‑label based N transformation‑rate measurements, to understand how the phenology and differential combination of the different N transformation pathways may modulate ε_app. We demonstrate that nitrification, DNRA, denitrification, anammox and organic matter remineralization overlap spatially in the sediments of Lake Sarnen. In contrast, these processes are, in parts, spatially decoupled in Lake Baldegg. Moreover, the relative importance of anammox versus denitrification is significantly greater in Lake Baldegg. In both lakes the net N isotope effect of sedimentary NO₃^- consumption is strongly underexpressed at the ecosystem level, with NO₃^-‑N ε_app values systematically below 4‰. In Lake Sarnen the NO₃^- N‑vs.‑O isotope signature followed a 1:1 trend, whereas in Lake Baldegg a systematically higher ratio was observed. This suggests that, while in Lake Sarnen, the NO₃^- N and O isotope signatures are dominated by NO₃^- reduction, NO₃^- regeneration (e.g., by nitrification or anammox) overprints the NO₃^- isotopic signature of net denitrification under the more eutrophic conditions in Lake Baldegg.