USING MICROMETEOROLOGICAL METHODS AND MODELING TO DETERMINE GREENHOUSE GAS BUDGETS OVER AGRICULTURAL SYSTEMS IN THE INLAND PACIFIC NORTHWEST
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Cropping systems can be net sources or net sinks of greenhouse gases (GHGs) depending on climate and management. Agriculture can be a sink for carbon dioxide (CO2) if climate and management conditions are conducive for the build-up of soil organic matter. At the same time, agricultural soils are the main anthropogenic source of nitrous oxide (N2O), which is both a GHG and an ozone depleting substance. The work presented in this dissertation investigates the dynamics of CO2 and N2O fluxes over terrestrial systems in the Pacific Northwest (PNW) using measurements and modeling. The first study used the eddy covariance micrometeorological method to monitor fluxes of CO2 for two years at two agricultural sites representing the low- and high-end of the regional precipitation gradient. Combining the measured net ecosystem exchange of CO2 with information on carbon (C) export during harvest, we found that both sites were net carbon sinks. The high-rainfall site had a two-year C balance of 202 ± 60 g C m-2, and the low-rainfall site had a C balance of 444 ± 34 g C m-2. The second study used the flux gradient micrometeorological technique and automated static chambers to measure N2O emissions over one year at two agricultural sites, one under no-tillage management (CAF-NT) and one that had conventional tillage (CAF-CT). The chamber results indicated total annual emissions of 2.7 ± 1.6 kg N2O-N ha-1 and 4.4 ± 3.2 kg N2O-N ha-1 from CAF-NT and CAF-CT, respectively, while the flux gradient results indicated that CAF-NT and CAF-CT had total annual emissions of 2.1 ± 0.4 and 1.7 ± 0.3 kg N2O-N ha-1, respectively. The third study investigates coupling the GHG processes in the Community Land Model (CLM) with a PNW regional air quality model. Results from two exploratory model runs are presented, which indicate that using CLM can strengthen monitoring and improve emission inventories in the PNW. The policy chapter is a review of California’s plan to reduce elevated drinking-water nitrate levels, which are due to agricultural fertilizer use. This is a case where open-access to publications and data sets would expedite the formulation of good policy.