Using Micrometeorological and Chamber Methods to Characterize Spatial and Temporal Trends in Nitrous Oxide Emissions over a Wheat Field in the Inland Pacific Northwest

Characterizing the dynamics of greenhouse gas (GHG) fluxes in agricultural systems is becoming increasingly important in the face of population growth and climate change. Monitoring baseline fluxes of both nitrous oxide (N2O) and carbon dioxide (CO2) over agroecosystems allows for a better understanding of how climate and management practices affect the GHG budget of a given field. In this study, fluxes of N2O were measured and compared using the eddy covariance (EC) technique, the flux gradient (FG) technique, and sixteen automated static chambers. The site, a winter wheat field under no-tillage management, is outfitted with an EC tower to continuously measure fluxes of CO2, H2O, and energy as well as auxiliary meteorological variables. The chamber system and micrometeorological instrumentation for measuring N2O were deployed to following seeding and fertilization in the fall of 2013. The N2O system made continuous measurements for two months month following seeding, was employed periodically during the winter, and then again continuously for a two-month period in the spring.

The overall goal of this study was to characterize the cumulative long-term emissions of N2O for this site. This empirical information is needed for parameterization and validation of process-based models, and to inform emission factors for this type of agricultural system. In the two months following fertilization alone 0.47 ± 0.5 kg N2O-N/ha was emitted from the field. Continued monitoring will determine what portion of the overall emissions this “hot moment” constituted.

A secondary objective of this study was to optimize the integrated use of the static chambers and tower measurements. It is useful to use both chamber and micrometeorological techniques to characterize cumulative fluxes, as the methods complement each other in terms of sensitivity and spatial coverage. The chamber flux detection limit is an order of magnitude smaller than that of the tower techniques, but chambers constitute point measurements while tower based methods (EC and FG) integrate over the field scale. N2O has high spatial variability evidenced in the chamber results, which displayed up to ten-fold differences in N2O emission. To test the detection limit of the micrometeorological methods we conducted an N2O tracer release study where the emission rate could be carefully controlled and measured. Our experience suggests that the FG method is a better choice based upon detection limits, ease of operation, and compatibility with the chamber operations. Overall, combining high sensitivity automated static chamber measurements of N2O with field integrated and low maintenance (relative to EC) FG measurements provided a high quality data set that gave insight into both N2O emission dynamics and total field-scale emissions.