The relationship between denitrification and nitrous oxide flux from soil

Denitrification in soil is the major source of atmospheric nitrous oxide (N2O), a potent contributor to global warming and a regulator of stratospheric ozone. Flux of N2O to the atmosphere is poorly understood. There is a serious imbalance in the global N2O budget (missing sources), and N2O flux at the field scale is difficult to predict, even when the rate of denitrification has been adequately characterized. In this dissertation, I review the literature pertaining to the relationship between denitrification and N2O flux, emphasizing a major source of uncertainty: relative production of N2O and N2 (dinitrogen) during denitrification. I explore the ecological factors that influence the nitrous oxide mole fraction (N2O/[N2O + N2]), an aspect of denitrification that expresses relative production of N2O and N2. Second, I develop theory that facilitates the evaluation of N2O and N2 flux using 15N-labeled compounds and mass spectrometry. My heuristic model of labeled N-gas flux from soil simplifies the process of drawing inferences from isotope data. Third, I report new procedures for measuring fluxes of labeled N2O and N2 for the same incubation using mass spectrometry. My procedures could lead to improved estimates of the nitrous oxide mole fraction. I illustrate these procedures with field data. Finally, I describe a laboratory experiment in which I combine traditional and isotope methods to test for effects of moisture history (antecedent soil moisture) and ecosystem management history on nitrous oxide mole fraction, while controlling soil type and moisture. I find that response of mole fraction to differences in short-term (48 h) soil moisture history is different for soils from ecosystems with different management histories. A cropped soil had a high (∼0.9) mole fraction after rapid transition from air-dry to 85% water-filled pore space but a low (∼0.3) mole fraction when 80% of added moisture was applied 48 h in advance of the incubation. However, soil from a successional system generated a nitrous oxide mole fraction of about 0.3 regardless of short term moisture history. Progress in understanding the relationship between denitrification and nitrous oxide flux from soil seems still to be methods-limited.