Project

Discipline

ammonia oxidation; Lake Lugano; nitrogen isotopes; denitrification; nitrification; nitrous oxide; Benguela; upwelling zones; nitrifier denitrification

Lead
Lachgas (N2O) ist ein wichtiges und langlebiges Treibhausgas, dessen anthropogene und natürliche Quellen recht gut bekannt sind (7 bzw. 11 Tg N/yr). Grosse Unsicherheiten bestehen jedoch bezüglich der jeweiligen Anteile terrestrischer gegenüber aquatischer Quellen an den Gesamt-Lachgasemissionen sowie der biogeochemischen Mechanismen welche die N2O Produktion in aquatischen Systemen steuern.
Lay summary
Unser übergeordnetes Ziel ist, zu einem verbesserten Verständnis von Lachgasproduktion durch verschiedene Mikroorganismen in marinen und lakustrinen Umweltsystemen mit saisonal stark variierenden Umweltbedingungen beizutragen. Unsere Studiengebiete sind: der Luganersee (Schweiz) sowie das hoch-produktive Auftriebsgebiet vor der Küste Namibias. Im Detail sollen folgende Fragen beantwortet werden: Welche Anteile der Gesamt-Lachgasproktion können jeweils den Prozessen Ammoniumoxidation, Nitrifikanten-Denitrifizierung und Denitrifizierung in den beiden Studiengebieten  zugeordnet werden?  Welche biogeochemischen Faktoren (e.g., pH, O2) sind primär für die Steuerung von Nitrifikanten-Denitrifizierungsraten verantwortlich, und gibt es sytematische Unterschiede zwischen marinen und lakustrinen Milieus? Gibt es direkte und diagnostisch verwertbare Zusammenhänge zwischen den Konzentrationen und Isotopenzusammensetzungen von N2O, NO2-, NO3-, and NH4+ und den gemessenen N2O Umsatzraten in den Studiengebieten? In beiden Studiengebieten werden mittels 15N-Tracermethoden N2O-Produktionsraten sowie Schlüsselparameter wie  pH, Chlorophyll a, Sauerstoffkonzentration (O2) und Konzentrationen und Sickstoffisotopenverhältnisse anorganisch-gelöster Stickstoffverbindungen gemessen mit dem Ziel, Lachgasproduktionsmechanismen  und relevante Kontrollfaktoren besser zu beleuchten. Unsere Arbeit wird neue und wichtige Informationen bezüglich der Rolle von Auftriebsgebieten und produktiven Seen in globalen N2O Budgets liefern. Auch wird sie neue Erkenntnisse über einen bisher von wissenschaftlichen Studien eher wenig berücksichtigten aber im globalen Stickstoffkreislauf vermutlich überaus wichtigen Prozess, der Nitrifikanten-Denitrifizierung, erlauben.

Direct link to Lay Summary

Last update: 25.03.2013

Active participation

Number	Title	Start	Funding scheme

Nitrous oxide (N2O) is now the third largest contributor to radiative forcing of the long-lived greenhouse gases. Human inputs of nitrogen (N), mainly as fertilizers, have stimulated the microbial N cycle transformations that are the dominant source of N2O. The partitioning between the anthropogenic and natural N2O sources is relatively well constrained at 7 and 11 Tg N/yr, respectively. However, large uncertainties remain in the relative contributions of terrestrial versus aquatic environments and with regards to the underlying biogeochemical controls on microbial N2O production. Such uncertainties present challenges for those devising and implementing N2O emissions policies, and make it difficult to interpret prehistoric atmospheric N2O fluctuations and predict the response of N2O production rates to future climate change.Aquatic N2O fluxes are often highly variable through time and space. The variation is likely modulated by fluctuations in the biogeochemical conditions, which may affect microbial N2O production pathways differentially. N2O can be produced by several processes, such as microbial nitrification (ammonia oxidation), denitrification, and nitrifier-denitrification, and multiple groups of microorganisms are involved. Yet, the exact environmental controls on temporal/spatial variations in net N2O production and the balance between the different pathways are still poorly constrained. It is highly probable that changes in the microbial processes that generate N2O are closely linked to seasonal changes in water productivity, organic matter remineralization rates, and in turn water-column redox-conditions.In this study we propose using incubation-based stable N-isotope tracer methods and natural N isotope measurements in dissolved N2O to identify and quantify specific N2O production pathways in two aquatic environments with strong seasonal N cycle dynamics: eutrophic Lake Lugano in southern Switzerland and the highly productive Benguela Upwelling region along the coast of southwestern Africa. Our main goals will be to shed light on the dynamics and controls on N2O production in two comparable environments. Within the frame of one postdoctoral project we propose to address the following research questions:•How much do ammonia oxidation, nitrifier-denitrification, and denitrification, respectively, contribute to N2O formation in Lake Lugano and the Benguela Upwelling?•Which biogeochemical factors control nitrifier-denitrification rates, and are there systematic differences between the marine and freshwater environment?•How do the concentrations and stable isotopic signatures of N2O, NO2-, NO3-, and NH4+ observed in Lake Lugano and the Benguela Upwelling zone relate to the rates of the studied pathways?In both environments, key variables, such as water pH, chlorophyll a, oxygen (O2), and dissolved inorganic N concentrations, will be measured, and some of these parameters will be manipulated in incubations to study their impacts on N2O production rates. Combining natural abundance N2O isotope measurements and N2O turnover measurements, and integrating the results in a 1-D geochemical model (for Lake Lugano) we attempt to gain information on the N and O isotope effects that are associated with N2O cycling processes. Measurements of the isotope effects and isotopic signatures associated with N2O production by natural microbial communities are valuable biogeochemical information because they allow detection of N cycle transformation processes in environments where direct rate measurements cannot be made. Our study is designed to capture the temporal dynamics associated with N2O production in both aquatic environments. The University of Basel has been invited to join two research cruises (R/V Meteor) to the Benguela Upwelling zone that are scheduled for the seasonal maximum in upwelling intensity (September 2013) and the minimum (February 2014). The upcoming cruises will provide a unique opportunity to identify seasonal change in biological N2O production mechanisms and rates, as well as the physical parameters that affect sea-to-air fluxes of N2O.The research proposed here will result in a detailed characterization of seasonal cycles in N2O biogeochemistry. The study in Lugano has been designed to produce clear information for water resource managers describing the aspects of water quality that most influence N2O emissions in nutrient-impacted lakes. The Benguela cruises will provide maximum and minimum N2O flux constraints on this geochemically important upwelling region. Both studies will be used to provide new information about the controls on aquatic N2O that are needed to accurately model the global dynamics of this powerful greenhouse gas.