Nitrous oxide (N2O) is both an important greenhouse gas and as a source of NO in the stratosphere. The amount of N2O in the atmosphere has increased during the past century, though the reasons for that increase remain uncertain. Different sources adding N2O to the atmosphere may be distinguished if they have different stable isotope signatures. In soils, the largest natural source of N2O, we have shown that the isotopic signature of N2O depends on both the availability of N substrates and whether the main process producing N2O is nitrification or denitrification. The microbial processes in turn are linked to soil moisture and temperature conditions. Based on our previous work, we hypothesize that much of the increase in N2O over the past few decades has been fueled by increased use of nitrogen fertilizers in agriculture, which in turn stimulates the emission of N2O, primarily through nitrification. This work will explore the role of fertilizer addition to agricultural soils on N2O emissions, using the isotope signatures of N2O and its precursor compounds in urea-amended soil to understand which microbial pathways are responsible for producing N2O, and how those pathways vary with soil moisture conditions. We will work in a temperate (North Carolina) and a tropical (Venezuela) agricultural setting. Specific hypotheses to be tested include: (1) nitrification and denitrification have very different isotopic fractionation factors (including positional dependence of the 15N within the N2O molecule), allowing us to quantify in a given soil what fraction of the N2O emitted was produced by each process; (2) soils fertilized with urea versus nitrate or organic nitrogen will have very different isotopic fingerprints in emitted N2O, even from the same soil; (3) temperate and tropical agricultural soils will differ in both the rate and isotopic signature of N2O emissions following fertilizer use. To test these hypotheses, we will: (1) develop and apply methods to determine the isotope fractionation factors characteristic (15N and 18O contents, as well as the bond in the N2O molecule which contains the 15N); (2) develop the method for measuring the positional dependence of 15N in the N2O molecule at the stable isotope facility at UC Irvine; and (3) make initial assessments of how the isotopic signatures of N2O emitted from agricultural soils in temperate (North Carolina) and tropical (Venezuela) sites differ when the same type of fertilizer is applied (urea).to soils with different soil water content. Our results will provide a greater understanding of the processes resulting in N2O emissions, and hence will be of use for managing fertilizer application so as to minimize future N2O emissions.
