Nitrous oxide (N???O) is a potent greenhouse gas, an important driver of climate change, and its concentration in the atmosphere is rising at an unprecedented rate. Agriculture is the leading contributor of all the anthropogenic N???O sources, and the vast majority of agricultural N???O emissions originate from soil. Of all the natural N???O emissions, two-thirds originate from soil and temperate forests contribute approximately one-sixth of the natural soil emissions. Consequently, there is great interest in understanding the soil nitrogen processes responsible for N???O production so that effective policies and management practises can be implemented to successfully mitigate climate change.
The stable isotopes of nitrogen (N) and oxygen (O) in soil N???O emissions are hypothesized to be useful indicators of the biogeochemical processes that produce and consume N???O, and they may be used to apportion different environmental sources. The primary objective of this thesis was to assess the utility of ???????N and ???????O values to differentiate N???O produced by nitrification and denitrification.
Most of the previous research on N???O isotopes has utilized microbial cultures of single organisms; yet natural systems contain a consortium of N-metabolizing microorganisms so the relevance of this early work to natural environments is uncertain. This thesis presents the results of experimental incubations of soil from an agricultural site and a temperate forest located within Ontario, Canada. Two well-drained soils (upland), two poorly-drained soils (wetland), and one stream sediment were incubated under varying conditions (temperature, moisture, and N-availability) to achieve a wide range in the rate of N???O production. The ???????N and ???????O values of N???O produced from the different experiments were characterized and the isotope effects (??) of N???O production were calculated. Experiments were conducted in aerobic or anoxic atmospheres to stimulate N???O formation by nitrification and denitrification, respectively.
The ???????N-N???O produced by denitrification in all soils was 7???35??? lower than the ???????N-nitrate (NO??????). The ???????N-N???O produced by nitrification in the upland forest soil and the agricultural soils was 28???54??? lower than the ???????N-ammonium. Nitrification in the forested wetland soil yielded higher ???????N-N???O values (?? = ???16???), which was likely caused by an increase in the ???????N-substrate. With the exception of the latter soil, there was clear ?????N-separation between the nitrification- and denitrification-derived N???O in all soils. Consequently, ???????N values can be used to apportion different environmental sources of N???O on a site-by-site basis, provided that the rates of N metabolism are known and the isotopic endmembers are well-characterized.
A novel approach was employed in this thesis to help unravel the key controls of ???????O-N???O and ???????O-NO?????? formation. Different ?????O-labelled soil waters were used to demonstrate that the abiotic exchange of oxygen atoms between water and nitrite (in equilibrium) is an important control of the ???????O-N???O formed by nitrifier-denitrification and the ???????O-NO?????? formed by nitrification. O-exchange in these incubations was highly variable between soils (37???88%) and it appeared to be rate-related. Furthermore, the ???????O value of microbial NO?????? is partially controlled by ?????O/?????O fractionation that occurs during O-exchange (equilibrium fractionation) and the uptake of molecular oxygen (O???) and water (H???O) (kinetic fractionation). This research showed that the ???????O value of microbially-produced NO?????? cannot be successfully predicted in soils based upon the commonly used ???one third, two-thirds rule???, which only takes into account the ???????O values of O??? and H???O. Successful predictions of ???????O-NO?????? using this rule appear to be fortuitous and are because of the range of ???????O-H???O at natural abundance and the magnitude of the isotope effects involved.
Enzyme-catalyzed (biotic) O-exchange between water and nitrite/nitric oxide in denitrification was also quantified for the first time in soils. O-exchange during denitrification was significant and variable (39???95%), but uniquely confined to narrow ranges for each soil type. Almost complete O-exchange occurred in the well-drained agricultural and forested soils (86???95%); less O-exchange occurred in the agricultural and forested wetland soils (63???70%); and even less O-exchange occurred in the agricultural stream sediment (39???51%). The magnitude of O-exchange during denitrification was independent of soil temperature and moisture for a given soil, and it was not related to the rate of N???O production. This implies that the amount of O-exchange that occurs during soil denitrification is controlled by the dominant microbial community.
For the first time, estimates of the net O isotope effect were determined for N???O production by soil denitrifiers that accounted for the complicating effects of O-exchange. The net ?????O-discrimination (N???O???NO??????) ranged between +32??? and +60???, with the exception of one treatment that was cooled (?? = +17???). The O isotope separation (??) that is actually observed in natural systems is often much lower, and in some cases negative. This is because the atomic O-exchange between water and nitrite/nitric oxide effectively diminishes the net ?????O separation between NO?????? and N???O because ???????O values of environmental water are usually lower than the ???????O values of N???O-precursors.
The determinants of ???????O-N???O produced by nitrification pathways are complex and there is no holistic explanation of the O isotope dynamics in the literature. This thesis provides the first systematic model to describe ???????O-N???O formation by aerobic pathways. In addition to O-exchange between water and nitrite (at equilibrium), ???????O-N???O is controlled by ?????O/?????O fractionation that results from this O-exchange mechanism, and from fractionation that occurs during ammonia-oxidation and nitrite-reduction. Although explaining ???????O-N???O values produced by nitrification is complex, reports of nitrifier-derived ???????O-N???O in the literature and this thesis are narrowly confined between +13??? and +31??? (rel. VSMOW). This is distinct from much of the denitrifier-produced ???????O-N???O, which is often ?????O-enriched and higher than +33???.
In three out of the five different soils investigated in this thesis, ???????O-N???O could be used to separate N???O formed by nitrification and denitrification. There was poor ???????O separation between nitrifier- and denitrifier-derived N???O in the well-drained soils because high amounts of biotic O-exchange and reduced O isotope separations yielded lower (predicted) estimates of denitrifier-produced ???????O-N???O. On the other hand, ???????N values could be used to apportion nitrifier- and denitrifier-derived N???O sources in these soils. Thus, stable isotope ratios of N???O are a valuable and promising tool that may help differentiate nitrifier-N???O from denitrifier-N???O in natural soil environments.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:OWTU.10012/6053 |
| Date | January 2011 |
| Creators | Snider, David |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
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