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Expression of benthic isotope effects associated with nitrogen elimination and regeneration in lacustrine sediments

English title Expression of benthic isotope effects associated with nitrogen elimination and regeneration in lacustrine sediments
Applicant Lehmann Moritz
Number 188728
Funding scheme Project funding (Div. I-III)
Research institution Institut für Umweltgeowissenschaften Universität Basel
Institution of higher education University of Basel - BS
Main discipline Hydrology, Limnology, Glaciology
Start/End 01.10.2020 - 30.09.2024
Approved amount 831'406.00
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All Disciplines (7)

Discipline
Hydrology, Limnology, Glaciology
Ecology
Environmental Research
Other disciplines of Earth Sciences
Experimental Microbiology
Geochemistry
Oceanography

Keywords (18)

Lake Lugano; benthic fluxes; sediment water exchange; nitrate ; isotope effects; microbial community structures; nitrous oxide; Skagerrak; isotope fractionation; nitrogen cycle; lakes; nitrite; porewater; anammox; denitrification; diagenetic model; nitrogen stable isotopes; sediments

Lay Summary (German)

Lead
Nitrat-Stickstoff ist ein wichtiger Nährstoff, der den Austausch von CO2 zwischen dem Weltozean und der Atmosphäre modulieren und die ökologische Gesundheit von Seen beeinträchtigen kann. Die wichtigsten Senken von Nitrat sind nach wie vor nur schlecht quantifiziert. Isotopenbilanzen können wichtige quantitative Erkenntnisse bezüglich dieser Senken liefern. Das Projekt wird Mechanismen der Nitrat-Isotopeneffekte der bei weitem wichtigsten Stickstoffsenke im Ozean, der Sediment-Denitrifizierung, untersuchen.?
Lay summary

Als wichtiger Nährstoff im Weltozean ist Stickstoff (N) in bioverfügbarer Form (z.B. Nitrat) ein wichtiger Kontrollfaktor der marinen Produktivität und damit wichtig für die CO2-Aufnahme des Ozeans via Algenphotosynthese. In Seen ist N oft Ursache von Überdüngung. Grosses Interesse gilt deshalb den verschiedenen Stickstoffflüssen, welche letztendlich die N-Konzentrationen im Oberflächenozean wie auch in Seen beeinflussen. Die mikrobielle Denitrifizierung ist die Hauptsenke für bioverfügbaren N in aquatischen Ökosystemen, aber die genaue Grösse dieser Senke vor allem im Ozean ist nach wie vor nur ungenau bekannt. Stickstoffisotopenbilanzen erlauben quantitative Rückschlüsse auf die Stickstoff-Flüsse, zumindest wenn man die 15N/14N-Isotopensignaturen, die mit diesen Flüssen bzw. Umsatzreaktionen einhergehen, kennt. In diesem Zusammenhang weiss man nur sehr wenig über den Isotopeneffekt der Denitrifizierung in Sedimenten, die bei weitem wichtigste Senke von N im Ozean wie auch in Seen. Dieser Isotopeneffekt variiert mit grosser Wahrscheinlichkeit mit den Sedimentationsbedingungen (Redox, Reaktivität der Sedimente) und kann von den «Isotopenfingerabdrücken» anderer N-Transformationsprozesse überprägt werden. In diesem Projekt werden Experimente, porenwasser-geochemische Feldstudien und Computermodellierungen genutzt, um Isotopeneffekte von N-Umsatz- und N-Eliminationsprozessen in marinen und Seesedimenten zu untersuchen sowie die wichtigen Kontrollmechanismen, welche diese Effekte beeinflussen, zu verstehen. Das Projekt wird wichtige Erkenntnisse liefern, welche für das Nutzen von N-Isotopenmessungen zur Abschätzung von regionalen oder globalen Stickstoffflüssen im Ozean und Stickstoffumsatzraten in Seen unabdingbar sind.

Direct link to Lay Summary Last update: 02.07.2020

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Associated projects

Number Title Start Funding scheme
170876 Advanced understanding of autotrophic nitrogen removal and associated N2O emissions in mixed nitritation-anammox systems through combined stable ISOtopic and MOLecular constraints (ISOMOL) 01.09.2017 Sinergia
160051 Copper availability, methanobactin production and methane oxidation in two Swiss lakes: Constraints on copper acquisition by methanotrophic bacteria 01.06.2016 Project funding (Div. I-III)
129491 Nitrogen elimination pathways and associated isotope effects in Swiss eutrophic Lake Lugano 01.04.2010 Project funding (Div. I-III)
153055 Seasonal Dynamics of Coupled Nitrogen, Sulfur, and Carbon Cycling in Redox Transition Zones of Lake Lugano 01.11.2014 Project funding (Div. I-III)
169552 The “methane paradox” in Lake Lugano - understanding methane production in oxygenated waters of lacustrine environments 01.01.2017 Project funding (Div. I-III)

Abstract

In many ocean regions, as a limiting nutrient, bioavailable nitrogen (N) controls marine primary productivity and thus the ocean’s capacity to fix and sequester atmospheric CO2 in its interior. In many lakes, N from both natural and anthropogenic sources is an important driver of lacustrine eutrophication. Therefore, both in the ocean and in lakes, it is crucial to understand the sources and sinks of fixed N. Denitrification, the microbial reduction of nitrate to dinitrogen (N2), and other modes of suboxic N2 production (e.g., the anaerobic oxidation of ammonium, or anammox), are the most important sinks of fixed N in aquatic environments, but particularly with regards to the N cycle in the ocean, there is a persistent debate regarding the overall size of, and the balance between, sinks and sources.Isotope ratios of nitrogenous species (e.g., 15N/14N) can provide important constraints on the natural N cycle. In order to use stable isotope measurements as a means to trace fluxes of N in aquatic systems, however, it is imperative to understand the isotope effects associated with these fluxes. While denitrification at the organism-level is known to be associated with a marked N isotope fractionation, the expression of the N isotope effect of benthic (i.e., sedimentary) denitrification in the water column above is only poorly constrained, and likely varies with the environmental conditions. Moreover, the possible impacts of other benthic N cycling processes on the N isotope exchange between the sediments and the water column (e.g., anammox, nitrate reduction to ammonium (DNRA), nitrate uptake, and/or nitrate regeneration) remain uncertain. Understanding the overall N isotope effect of net benthic N loss is a prerequisite for using N isotope measurements to infer its relative importance in the N cycle, in the global ocean or in a specific environment.Here we propose an in-depth investigation of the isotope effects of benthic fixed N elimination and nitrate regeneration in aquatic sediments. The prime goal of the proposed research is to build a thorough understanding of the modulating controls on the nitrate and nitrite N (and O) isotope signatures of denitrification and anammox (and the interacting effects from other benthic N cycling reactions), and the N isotopic composition of gaseous N (i.e., N2 and N2O) that is ultimately lost from the sediments. We predict that the expression of the biological isotope effect of benthic N elimination at the level of sediment-water exchange will vary across different environments, and will strongly depend on the reactivity of the sediments, the O2 penetration, the physical boundary conditions (i.e., diffusive transport), and on the extent to which other processes than denitrification contribute to the overall N cycling (nitrification, anammox, DNRA). Combining 1.) laboratory experiments with natural and artificial sediments, 2.) field investigations into the sediment porewater (N and O) isotope dynamics of distinct lacustrine and marine denitrifying benthic environments, and 3.) mathematical modeling, and making use of innovative multi-isotope techniques (natural abundance isotope analysis of NO3-/ NO2-, ammonium, dissolved organic N, and N2O, as well as 15N tracer experiments), we attempt to gain complementary information on how the combined isotope effects of benthic nitrate reduction and nitrate regeneration are expressed in the water column of lakes and the ocean. With the diagenetic porewater N isotope model that this project will deliver, the first of its kind that also includes N2O isotopologues, we will establish a quantitative framework for assessing benthic isotope fluxes and for verifying our hypotheses.The research proposed will result in the first comprehensive characterization of sediment pore-water N (and O) isotope dynamics in lacustrine settings, and will allow experimental constraints on the variability of N isotope effects during benthic nitrate reduction across different biogrochemical regimes. While the field component of the project focuses on lake sediments, the results expected will be directly pertinent to understanding of fixed-N elimination isotope effects in the ocean. It will thus be highly relevant for the use of N isotope measurements for local, regional, and even global N budgets, and will provide the basis for both paleolimnological and -oceanographic extrapolation.
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