N2O Flux Short-Term Response to Temperature and Topsoil Disturbance in a Fertilized Crop: an Eddy Covariance Campaign

Using the eddy covariance technique, half-hourly N2O fluxes were measured over a sugar beet crop (ICOS Station, Lonzee, BE) from fertilization to harvest. Several parameters of the data quality control tests were adapted to suit the characteristics of N2O, No u* filtering threshold could be seen for N2O fluxes; therefore, it was determined based on CO2 data. The uncertainty of N2O fluxes was assessed for several aspects of data treatment (total random uncertainty, spectral correction, u* filtering, gap-filling), which were combined to determine the uncertainty of the N2O budget. Between fertilization and harvest, the crop emitted 1.83 ( +/- 0.21) kg N2O-N ha(-1) corresponding to 1.2% of N supplies. Flux variability was characterized by three episodes of high emissions across the experiment, interspersed with lower background fluxes. These peak events were driven by soil moisture and temperature, dependent on the time-scale. Soil water content at 5 cm was identified as the single trigger for N2O emission peaks given sufficient N availability, while intraday oscillations were positively correlated to the variations in surface temperature rather than deeper soil temperatures. For the first time, an inhibiting and short-term effect of topsoil disturbance (seed-bed preparation) on N2O fluxes was recorded, which interrupted the peak that followed fertilization, and delayed the start of the next high emission episode. This observation, along with the synchronicity found between surface temperature and diel oscillations of N2O fluxes, supports the hypothesis of a N2O-producing microbial community located in the topmost soil layer. Given that a third of the overall N2O emissions during the measurement campaign occurred between fertilization and seed-bed preparation, further investigation into the timing of farming operations as mitigation strategies is needed. The contribution of N2O emissions to the net greenhouse gas balance (which comprises CO2 and N2O fluxes) was estimated at between 20 and 66%. Our results stress the importance of including nitrous oxide when measuring gas exchanges in fertilized crops, and to do so at high temporal resolution for improved estimates.