Multi-Year N2O Flux Measurements with Multiple Methods: A Comparison.

Nitrous oxide emissions from agricultural lands occur as pulses at short intervals with the timing and magnitude dependent on management, soil, and climatic conditions. The multi-plot flux gradient (FG) method has the advantage of providing side-by-side comparisons of total N₂O emissions from separate treatments under similar climatic and soil conditions using field-scale plots with one gas analyzer. FG measurements provide spatially integrated fluxes at high temporal resolution, semi-continuously over multiple seasons, but are less common than eddy covariance (EC) or chamber techniques.

Several methods for measuring N₂O fluxes were evaluated as part of an FG-based N₂O study in Elora, Ontario, Canada. This project, operating since May 2015, uses the multi-plot FG technique to measure total N₂O emissions from four 4-ha plots within a homogeneous 30-ha cornfield, each with a different fertilizer treatment. EC fluxes of N₂O are being measured simultaneously using a tunable diode laser adsorption spectrometer (Campbell Scientific TGA100A) in one plot throughout the 3-year study. A field test of a thermo-electrically cooled TDLAS (CS TGA200A) for N₂O EC flux measurements was included at this tower until Fall 2016. Automated chambers measured N₂O fluxes in the growing seasons of 2015 and 2016. In spring of 2017 a new low-powered (<30 W) N₂O FG system (FG-LP) was installed at the site.

Preliminary results showed that temporal patterns measured by each method matched for N₂O emission events. N₂O fluxes measured by the four-plot FG system correlated with the EC fluxes (r² ~ 0.8) but magnitudes were slightly lower than the EC fluxes. Chamber measurements were beneficial for confirming N₂O emission in relatively low flux periods, but the lack of spatial coverage resulted in poor correlations between the chamber and EC and FG measurements. Results of the comparison of the four-plot FG, FG-LP and EC N₂O fluxes from 2017 will be presented.