Greenhouse Gas Exchange above a Potato Field in the Lower Fraser Valley, BC, Canada

Agriculture is a significant source of global greenhouse gas (GHG) emissions, which leads to a serious concern about global climate change. To mitigate its effects, reliable estimates of actual GHG emissions from different cropping systems are critical. Because only infrequent and short-duration measurements have been attempted, no credible annual estimates of net GHG emissions are available for major cropping systems in BC. We made continuous year-round (2018-2019) eddy-covariance (EC) measurements of N₂O, CO₂, and CH₄ fluxes above a potato (Satina and Gold Rush varieties) field with silage corn and peas grown in the second year of the rotation. The EC system consisted of a 3D sonic anemometer (R3-50, Gill Instruments Ltd.), a closed-path continuous-wave laser spectrometer (913-1054, Los Gatos Research Inc.) for N₂O, CH₄ and H₂O, and an enclosed-path infrared gas analyzer (LI-7200, LI-COR Inc.) for CO₂ and H₂O. Due to the challenge of installing the EC system on a drainage ditch at the 3-m height with potatoes being grown on both sides, a flux footprint method is employed to correct the actual GHG fluxes from both sections of the field for the effect of the edge. This also uses the available manual non-steady-state chamber measurements of soil GHG fluxes near the edge using a portable Fourier-transform infrared spectroscopy gas analyzer (DX4040, Gasmet Inc).

In addition to EC fluxes, environmental and crop variables including wind speed and direction, radiation components, air temperature, precipitation, soil heat flux, soil moisture, soil temperature, leaf area index and crop height were also measured. Energy balance closure and relationships of different GHG fluxes to environmental variables were determined. Half-hourly N₂O fluxes were generally 0.5 nmol m^-2 s^-1 but reached approximately 5 nmol m^-2 s^-1 following rain events. Diurnal patterns in N₂O fluxes were observed with the changes in soil temperature, especially during the summer months. Diurnal net ecosystem exchange (NEE) varied between -20 and 4 μmol CO₂ m^-2 s^-1. CH₄ fluxes were generally extremely small despite the presence of the water-filled ditch. Annual GHG, carbon and water budgets are presently being calculated.