PhD defense by Malte Winther

Title: Trace gas evolution in the present and past atmosphere

Abstract: Nitrous oxide (N2O) is a very important trace greenhouse gas in the atmosphere. With an increasing atmospheric concentration of 327 ppb at present, and a warming potential 300 times that of CO2, the significance of N2O has been rapidly increasing since the 1950s. It is generally known that N2O primarily originates from microbial production in terrestrial and aquatic ecosystems. In this thesis I present measurements of the intramolecular distribution of ¹⁵N in N2O given as site preference (difference in abundance of the isotopomers), δ¹⁵N^bulk (average abundance of the isotopomers), and measurements of δ¹⁸O-N2O. The isotopes of N2O are used in investigations of both the specific reactions pathways influencing N2O emis­sions and the general evolution of N2O.
In Part II, I presents continuous incubation experiments with the two primary microbial communities responsible for N2O production. The two microbial communities have been studied numerous times, but to the best of our knowledge never continuously. Continuous measurements of the microbial evolution of N2O is possible due to the recent instrument development of a cavity ringdown spectroscopy-based analyzer, which allows for continuous position dependent ¹⁵N measurements. Continuous incubation experiments are presented with nitrifying bacteria Nitrosomonas mobilis revealing strong indications of N2O production from different chemical reactions. The measurements revealed a three step site preference pattern in the range of nitrification and denitrification and we therefore suggest that Nitrosomonas mobilis is of the bacterial community nitrifier denitrification. The experiments furthermore revealed the bacterial potential for cyclic use of the most abundant substrate rather than the most energy efficient.Continuous incubation experiments with denitrifying bacteria are also presented with Pseudomonas fluorescens (producing and reducing N2O) and Pseudomonas chlororaphis (only producing N2O). Measurements revealed a clear transient pattern of KNO3 to N2O and KNO3 to N2O to N2 for the two bacterial species, respectively. A Rayleigh type distilla­tion model modified for isotopomers and simultaneous reduction, allowed for determination of isotopic fractionation values during both production and reduction of N2O, comparable to previous studies.
In Part III, I present measurements of ice core samples analyzed for isotopes of N2O. Ice samples from three time periods of the Holocene and one from the glacial were selected and measured for isotopic composition of N 2O using isotope ratio mass spectrometry. The analysis resulted in isotopic variations comparable with previous findings of site preference, δ¹⁵N^bulk, and δ¹⁸O-N2O. Source identification analysis using the isotopic composition re-sulted in 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, hence shifting the primary source region towards denitrification in terrestrial ecosystems.
In Part IV, I present N2O measurements from field studies performed 1) on the Arctic tundra and 2) on an inclined temperate slope. 1) Previous studies has shown that large amounts of N2O is being emitted after thawing of permafrost. We investigated a downslope site covering a moisture gradient area in the arctic tundra. Moss-covered sites revealed high nitrification potential, low nitrate levels, and emission of denitrification marked N2O. The natural nitrogen cycle in the arctic is therefore hypothesized to reduce the atmospheric N2O concentration by denitrification, which contrasts the opening nitrogen cycle in temperate and tropical areas. 2) On an inclined temperate slope N2O emission variations were investigated on a mesoscale range. A temperate changing interface with oxic nitrification enhancing conditions upslope and anoxic denitrification enhancing conditions downslope was discovered with high N2O production.

Supervisor
Thomas Blunier, Centre for Ice and Climate, NBI

Assessment committee
Anders Svensson, Centre for Ice and Climate, NBI
Barth F. Smets, DTU Environment
Joachim Mohn, Laboratory for Air Pollution / Environmental Technology, Empa