Assessing Carbon and Water Dynamics in Multiple Wheat-based Cropping Systems in the Inland Pacific Northwest Using the Eddy Covariance Method

Carbon and water fluxes at five agricultural sites (11 site-years) covering a large precipitation gradient and a variety of management practices and crop species were measured in the inland Pacific Northwest US using the eddy covariance method. Three winter wheat fields were net CO₂ sinks, with annual net ecosystem exchange of CO₂ (NEE) ranging from -450 ± 32 to -525 ± 28 g C m^-2. Spring crop fields can be a net CO₂ source, CO₂ sink, or close to CO₂ neutral. Partitioning of NEE and evapotranspiration (ET) showed that the low-rainfall winter wheat field had lower total ecosystem respiration (R_eco), gross primary productivity (GPP), evaporation (E), and transpiration (T) compared to the high-rainfall area. No-till practice resulted in lower R_eco and E compared to conventional tillage, indicating no-till as a sustainable farming strategy to help mitigate greenhouse gas emissions from agriculture. No-till practice also caused slightly lower GPP for winter wheat, so long-term measurements are still needed to assess the no-till benefits of reducing carbon and water loss, maintaining crop production, and sequestering more carbon into soils. Irrigation applied in the dry area maintained good crop production, but resulted in large amounts of carbon and water losses into the atmosphere. Seeding dates, tillage fallow, weeds, and water stress also affected agricultural carbon and water budgets in these cropping systems via different physical and biological processes. In summary, agricultural ecosystems can be net CO₂ and carbon sinks and the sink strength heavily depends on crop species, management practices, and climatic conditions.