Evaluation of N2O Emissions from Multiple Fertilizer Treatment Plots Using a Scanning Open-Path FTIR.

Near-continuous N2O emissions were measured after nitrogen fertilization treatments from multiple adjacent fields in 2013 through 2015 using a micrometeorological approach. Continuous measurements of N2O concentration and emission over agricultural fields are difficult due to limited sensitivity of gas analyzers and the interference of the absorption bands with water vapor and carbon dioxide with the absorption bands of N2O. Here we have approached the measurement of integrated N2O concentrations along multiple paths over multiple fields using closed and open-path optical methods. Emissions were determined using a backward Lagrangian stochastic emissions model (WindTrax®) combined with the gas concentration and on-site turbulence measurements. The measurement configuration included an open-path Fourier transform infrared spectrometer (FTIR) scanning along 6-7 optical paths (OP) across four treatments of anhydrous ammonia on maize fields over three years. A bias in the derived N2O from the FTIR spectra was corrected using a Difference Frequency Generation (DFG) laser-based N2O analyzer sampling air collected along a line collocated with an FTIR OP. Upwind background N2O concentrations were also measured. Soil temperature and moisture measurements, atmospheric pressure, temperature and humidity, and turbulence were also measured on-site to correlate with emissions events. Emissions measurements were not possible during most night times due to the low wind speeds. N2O emissions were highly variable from fields with between 100 kg and 200 kg of nitrogen applied per hectare; ranging up to 20 ugm-2s-1. N2O emissions were correlated more with wind speeds than air temperature or soil moisture. In some years N2O emissions from fields that had already received some nitrogen in the prior fall showed significant emissions the day of the spring application suggesting high populations of either denitrifying or nitrifying bacteria surviving from the prior fall. In two of the three years, the N2O emissions from a spring-only nitrogen application increased steadily after application.