Comparison of N2O Fluxes Measured Using Flux-Gradient, Eddy-Covariance and Chamber Methods from an Agricultural Site Subjected to Different Nitrogen Fertilizer Treatments

Nitrous oxide fluxes from agricultural lands occur over short-lived emission events with the timing and magnitude dependent on management, soil, and climatic conditions. A thorough assessment of N2O emissions from fertilized fields requires methods capable of measuring fluxes at detailed temporal scale while integrating over large spatial scales. A study investigating the effect of nitrogen fertilizer treatment on the total annual N2O emissions from cornfields in Southern, Ontario, Canada provided the setting to analyze three methods for measuring N2O fluxes from May 2015 to April 2016. Four 4-ha plots within a homogeneous 30-ha area were each subject to different nitrogen fertilizer source and timing treatments. N2O fluxes were measured by eddy-covariance (EC) using a new tunable diode laser trace gas analyzer (TGA200A, Campbell Scientific), by multi-plot flux gradient (FG) using a earlier version of the same analyzer (TGA100A), and automatic chambers interfaced with a Fourier Transform Infra-red analyzer (FTIR, Gasmet). Eddy-covariance is a standard method for measuring fluxes at the resolutions required to assess trace gas emissions, and was deployed in one of the four plots. Multi-plot FG measurements have the advantage of providing year-round, spatially integrated, semi-continuous fluxes for side-by-side comparisons of N2O fluxes from separate treatments under similar climatic and soil conditions, but is a less common practice. Chambers have the advantage of being the most direct means of measuring soil fluxes; however, spatial resolution is low, and in our case was limited to 2 of the 4 plots. Preliminary results showed that temporal patterns measured by each of the methods matched for three post-fertilizer N2O emission events of one plot. N2O fluxes measured by the FG system correlated with the EC fluxes but magnitudes were slightly lower than the EC fluxes. Chamber measurements confirmed the emission of N2O, but the lack of spatial coverage resulted in poor correlations between the chamber and the EC and FG measurements.