Forskning ved Københavns Universitet - Københavns Universitet


Trace Gas Evolution in the Present and Past Atmosphere

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

  • Malte Nordmann Winther
Nitrous oxide (N2O) is a very important trace greenhouse gas in the atmosphere. With anincreasing atmospheric concentration of 327 ppb at present, and a warming potential 300times that of CO2, the signicance of N2O has been rapidly increasing since the 1950s. It isgenerally known that N2O primarily originates from microbial production in terrestrial andaquatic ecosystems. In this thesis I present measurements of the intramolecular distributionof 15N in N2O given as site preference (difference in abundance of the isotopomers), σ15Nbulk(average abundance of the isotopomers), and measurements of σ18O-N2O. The isotopes ofN2O are used in investigations of both the specic reactions pathways influencing N2O emissions and the general evolution of N2O.In Part II, I presents continuous incubation experiments with the two primary microbialcommunities responsible for N2O production. The two microbial communities have beenstudied numerous times, but to the best of our knowledge never continuously. Continuousmeasurements of the microbial evolution of N2O is possible due to the recent instrumentdevelopment of a cavity ringdown spectroscopy-based analyzer, which allows for continuousposition dependent 15N measurements.Continuous incubation experiments are presented with nitrifying bacteria Nitrosomonasmobilis revealing strong indications of N2O production from different chemical reactions.The measurements revealed a three step site preference pattern in the range of nitricationand denitrication and we therefore suggest that Nitrosomonas mobilis is of the bacterialcommunity nitrifier denitrication. The experiments furthermore revealed the bacterialpotential for cyclic use of the most abundant substrate rather than the most energy effcient.Continuous incubation experiments with denitrifying bacteria are also presented withPseudomonas fluorescens (producing and reducing N2O) and Pseudomonas chlororaphis(only producing N2O). Measurements revealed a clear transient pattern of KNO3 to N2Oand KNO3 to N2O to N2 for the two bacterial species, respectively. A Rayleigh type distillationmodel modied for isotopomers and simultaneous reduction, allowed for determinationof isotopic fractionation values during both production and reduction of N2O, comparableto previous studies.In Part III, I present measurements of ice core samples analyzed for isotopes of N2O. Icesamples from three time periods of the Holocene and one from the glacial were selectedand measured for isotopic composition of N2O using isotope ratio mass spectrometry. Theanalysis resulted in isotopic variations comparable with previous findings of site preference,σ15Nbulk, and σ18O-N2O. Source identification analysis using the isotopic composition resultedin a discussion of the primary source region being nitrification in aquatic ecosystems,while inclusion of the stratospheric sink effect leads to heavy depletion in all isotopes, henceshifting the primary source region towards denitrication in terrestrial ecosystems.In Part IV, I present N2O measurements from eld studies performed 1) on the Arctictundra and 2) on an inclined temperate slope. 1) Previous studies has shown that largeamounts of N2O is being emitted after thawing of permafrost. We investigated a downslopesite covering a moisture gradient area in the arctic tundra. Moss-covered sites revealed highnitrication potential, low nitrate levels, and emission of denitrication marked N2O. Thenatural nitrogen cycle in the arctic is therefore hypothesized to reduce the atmospheric N2Oconcentration by denitrication, which contrasts the opening nitrogen cycle in temperate andtropical areas. 2) On an inclined temperate slope N2O emission variations were investigatedon a mesoscale range. A temperate changing interface with oxic nitrification enhancingconditions upslope and anoxic denitrification enhancing conditions downslope was discoveredwith high N2O production.
ForlagThe Niels Bohr Institute, Faculty of Science, University of Copenhagen
StatusUdgivet - 2016

ID: 173288122