Measuring Field-Scale Nitrous Oxide Emissions Using the Eddy Covariance Technique

In the United States, the agricultural sector accounts for 10.2% of total annual greenhouse gas (GHG) emission budget, with methane (CH₄) and nitrous oxide (N₂O) account to 98% of the total agricultural GHG emissions. Accurate measurements of GHG from agriculture systems are crucial for calculating carbon credits owed to farmers, reducing uncertainties in national GHG inventories, improve prediction models, and evaluating mitigation strategies to reduce GHG emissions from agriculture. Soil N₂O emissions have high temporal and spatial variability making their quantification challenging. Recent advancements in N₂O sensors allow field-scale quantification of N₂O fluxes using the eddy covariance (EC) technique, which is a widely used micrometeorological method. The objectives of this study were to quantify N₂O emission in a winter wheat field using the EC technique and compare measured N₂O fluxes with a well-established biogeochemical model. The field study was carried out in a winter wheat field near the Konza Prairie Biological Station in Manhattan, Kansas from November 2020 to June 2021. Mixing ratios of N₂O and CO₂ were measured using tunable diode laser closed path gas analyzer (TGA200A, Campbell Sci.). In addition, the wind velocity orthogonal components were measured using a sonic anemometer (CSAT3B, Campbell Scientific) positioned on tower at 1.5m above the ground. The signals from the sonic anemometer and gas analyzer were recorded at 10 Hz using a datalogger. The high frequency data were analyzed using the EC package software EddyPro (v. 7.0.2, Licor). EC flux measurements were compared with estimates provided by the denitrification and decomposition (DNDC) model. The highest N₂O emission occurred right after precipitation event after nitrogen fertilization and the maximum flux value was 0.04 μmol/m²/s. The DNDC model underestimates 4% the N₂O emissions and missed the timing of some emission events.