Comparison of N2O fluxes measured using flux-gradient, eddy-covariance, and chamber methods from an agricultural site

Abstract:

Nitrous oxide emissions from agricultural lands occur as pulses at short intervals during various times throughout a given year, 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 large temporal and spatial scales. A study investigating the effect of 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. Four 2-ha plots within a homogeneous 30-ha area were each subject to different nitrogen fertilizer source and timing treatments. N₂O fluxes were measured using eddy-covariance (EC), multi-plot flux gradient (FG), and chamber techniques. Each method has advantages and disadvantages. Eddy-covariance is a standard method for measuring fluxes at the resolutions required to assess trace gas emissions, but the erratic nature of agricultural N₂O fluxes necessitates testing of N₂O analyzers, and the application of the EC method to N₂O fluxes. This study acted as a field test of the Campbell Scientific TGA200A tunable diode trace gas analyzer. Testing the TGA200A against a TGA100A provided two simultaneous EC-flux measurements of N₂O for one plot. Multi-plot FG measurements have the advantage of providing year-round, spatiality 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 winter measurements are often impossible. Preliminary results showed that temporal patterns measured by each of the methods matched for three post-fertilizer N₂O emission events of one plot. EC fluxes of N₂O measured by each of the TGA analyzers correlated well (r² = 0.90) and values were on average within 7% of one another. N₂O fluxes measured by the FG system correlated with the EC fluxes (r² = 0.75) 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.