Season and location–specific nitrous oxide emissions in an almond orchard in California

Almonds are an important commodity in California and account for around 15% of the state’s fertilizer nitrogen (N) consumption. Motivated by strong correlations typically observed between fertilizer N inputs and emissions of the potent greenhouse gas and ozone depleting molecule nitrous oxide (N2O), this study aimed to characterize spatial and temporal patterns in N2O emissions in an almond orchard under typical agronomic management. N2O fluxes were measured for a total of 2.5 years, including 3 growing seasons and 2 dormant seasons. Measurements targeted two functional locations, defined as tree rows and tractor rows. In conjunction with the flux measurements, we determined driving variables including soil ammonium (NH4 +) and nitrate (NO3 −), dissolved organic carbon (DOC), soil water-filled pore space (WFPS), soil pH, air temperature and precipitation. Cumulative annual N2O emissions were low (0.65 ± 0.07 and 0.53 ± 0.19 kg N2O–N ha−1 year−1 in year 1 and 2, respectively), likely due to the coarse soil texture and microject sprinkler irrigation and fertigation system. Emission factors (EF), conservatively calculated as the ratio of N2O emitted to fertilizer N applied, were 0.25 ± 0.03% and 0.19 ± 0.07% for year 1 and 2, respectively, which is below the IPCC EF range of 0.3–3%. Correlation analyses between N2O and driving variables suggested that overall N2O production was limited by microbial activity and nitrification was likely the major source process, but specific drivers of N2O emissions varied between seasons and functional locations.