Response of nitrous oxide emissions to individual rain events and future changes in precipitation

Abstract

Changing precipitation has the potential to alter nitrous oxide (N₂ O) emissions from agricultural regions. In this study, we applied the Coupled Model Intercomparison Project Phase 5 end-of-century RCP 8.5 (business as usual) precipitation projections for the U.S. Upper Midwest and examined the effects of mean precipitation changes, characterized by increased early-season rainfall and decreased mid- to late-season rainfall, on N₂ O emissions from a conventionally managed corn (Zea mays L.) cropping system grown in an indoor mesocosm facility over four growing seasons. We also assessed the response of N₂ O emissions to over 1,000 individual rain events. Nitrous oxide emissions were most strongly correlated with water-filled pore space (WFPS) and soil nitrogen (N) status. After rain events, the change in N₂ O emissions, relative to pre-rain emissions, was more likely to be positive when soil NO₃ ^- was >40 mg N kg^-1 soil and soil NH₄ ⁺ was >10 mg N kg^-1 soil and was more likely to be negative when soil NO₃ ^- was >40 mg N kg^-1 soil and soil NH₄ ⁺ was <10 mg N kg^-1 soil. Similarly, hourly N₂ O emissions remained <5 nmol m^{- 2} s^-1 when combined NH₄ ⁺ + NO₃ ^- was <20 mg N kg^-1 soil or NH₄ ⁺ and NO₃ ^- were <5 and 20 mg N kg^-1 soil, respectively. Rain event magnitude did not substantially affect the change in N₂ O flux. Finally, growing-season N₂ O emissions, soil moisture, and inorganic N content were not affected by the future precipitation pattern. Near-optimal soil WFPS combined with soil N concentrations above the identified thresholds favor higher N₂ O emissions.