Assessing agricultural nitrous oxide emissions and hot moments using mesocosm simulations

Nitrous oxide (N₂O) is a major greenhouse gas and the leading stratospheric ozone depleting substance emitted today. Effective mitigation strategies from agricultural soils, the largest anthropogenic source of N₂O, have remained elusive due to the occurrence of difficult to predict “hot moments,” or brief periods that contribute disproportionately to N₂O budgets. Future precipitation patterns may further complicate mitigation efforts by causing more favorable soil conditions that drive N₂O emissions. The objectives of this thesis, therefore, were to: 1) Assess the sensitivity of N₂O emissions to changes in precipitation; 2) Devise an approach to objectively identify hot moments; and 3) Identify the conditions that drive these hot moments. Six growing season simulations with nearly continuous N₂O measurements from an indoor mesocosm facility were used to address each of these objectives.

Four seasons comparing historical normal (1984-2014) versus end-of-century (2071-2099) precipitation patterns demonstrated that, for non-limiting soil nitrogen, the greatest N₂O emissions occurred when soil water-filled pore space (WFPS) was between 40 and 80% and that cumulative emissions increased with the number of days above ~60% WFPS. Consequently, any future changes in precipitation that contribute to these conditions will likely increase N₂O emissions. An assessment of 1350 rain events revealed that N₂O emissions consistently increased within 24 hours of rainfall and when soil moisture was near ~60% WFPS and NH₄⁺ was greater than 10 mg N kg^-1 soil. However, emissions were suppressed when WFPS was ~60% and soil NH₄⁺ was below 5 mg N kg^-1 soil, demonstrating that soil NH₄⁺ availability is an important determinant of the N₂O emission response to rain. Finally, a new approach to categorize hot moments from background N₂O emissions identified greater soil nitrate (NO₃^-), air temperature, and 10 cm WFPS among hot moments. Further, short-term pulses (up to 38 hours) of hot moments were sustained when nitrate (NO₃^-) was greater than 50 mg N kg^-1 soil and WFPS was above 50%, and were controlled by short-term changes in WFPS and air temperature. These findings have important implications for the development of N₂O emission models, agricultural management and mitigation strategies, and demonstrate the efficacy of conducting mesocosm experiments under controlled conditions.

In partial fulfillment of the requirements for the MS degree in the Graduate Program in Land and Atmospheric Science