Year

2008

Degree Name

Doctor of Philosophy

Department

School of Chemistry, Faculty of Science

Abstract

Agriculture contributes 16% to Australia’s greenhouse gas emissions, of which 23% are the result of nitrous oxide (N2O) emissions. Global N2O emissions from agricultural activities have increased substantially since the beginning of the industrial revolution. This has been driven by the increase in the amount of nitrogen added to agricultural soils, stimulating the N2O-producing nitrification and denitrification processes. Currently there is a poor quantitative understanding of N2O emissions from nitrification and denitrification and their controlling parameters. This is difficult to quantify since N2O emissions from nitrification and denitrification occur simultaneously. As N2O produced from these microbial processes have different stable isotopic signatures, stable isotope measurements have shown promise as a method to partition N2O emissions. Additionally, the intramolecular 15N site preference of N2O presents a further stable isotopic measurement which can be used for studies of N2O emissions from nitrification and denitrification. In this thesis, the absolute intramolecular 15N site preference of N2O measured using high resolution FTIR. The FTIR system was fully calibrated for site preference measurements of 15N enriched N2O sample. In addition, using the FTIR system the absolute site preference value for the N2O working standard used as a reference for isotopic measurements was determined. Using this absolute site preference value, it was possible to determine a value of 19.9 (±2.1)‰ for the absolute site preference of tropospheric N2O. The absolute site preference value of the N2O working standard was used to report measurements of field collected N2O samples on the absolute scale. To determine the intramolecular site preference of N2O produced from nitrification and denitrification, a 2 week field campaign was completed at a dairy farm in south eastern Australia. Soil plots were treated with 15N-labelled NH4 + or NO3 -, either at application rates of 100 or 50 kg-N.ha-1, followed by irrigation. Following treatment, daily measurements were made of the N2O emissions, isotopic composition of N2O, soil mineral nitrogen concentrations and 15N composition, as well as soil moisture. From the interpretation of the collected data, intramolecular site preference signatures of -17 (±5)‰ was determined for nitrification and a range of 0.5 to 12.3‰ was attributed to denitrification. Using the 15N measurements of N2O, it was possible to estimate the N2O emissions produced from nitrification and denitrification. For all treatments, emissions produced from denitrification far outweighed those from nitrification. These measurements indicated that the reduction of N2O to N2 was a key process controlling N2O emissions. Additionally, there was a clear relationship between the intramolecular site preference of N2O, and the amount of N2O reduced to N2. This relationship was used to estimate that for treatments receiving NH4 +, 5 – 22% of the applied nitrogen was emitted as N2.

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