The isotope effect of benthic N removal in two Swiss lakes
Authors: Paulus Tim J. , Mazzoli Alessandra , Frey Claudia , Zopfi Jakob , Callbeck Cameron , Lehmann Moritz F. 
Published: February 2023

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&#8209;N removal as N2 gas from lakes and the ocean. N&#8209;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&#8209;loss using microbial metagenomic&#8209;based approaches and rate measurements, how nitrogen&#8209;loss processes imprint natural&#8209;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&#8240;). In contrast, the marine NO3- isotope effect of sedimentary denitrification is suppressed at the level of the sediment&#8209;water interface (apparent N/O isotope effect, &#949;app <5&#8240;). How anammox affects &#949;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&#8209;loss (including anammox), and its ultimate expression in the water column of freshwater lakes. We also investigate how &#949;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&#8209;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 &#949;app in these two lakes. More specifically, we carried out whole&#8209;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&#8209;abundance N and O isotope measurements with 15N&#8209;label based N transformation&#8209;rate measurements, to understand how the phenology and differential combination of the different N transformation pathways may modulate &#949;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-&#8209;N &#949;app values systematically below 4&#8240;. In Lake Sarnen the NO3- N&#8209;vs.&#8209;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.