Challenges with measuring greenhouse gas fluxes over annual cattle operations in Manitoba, Canada

Measurement of greenhouse gas emissions from a whole farm poses a spatial and temporal challenge. The spatial challenge is matching the farm scale to the footprint of the emissions, which is typically at a scale that can be addressed using eddy covariance flux towers that need to consider a distribution of area sources of different intensities. The temporal challenge is seasonality, which has both a climate and management factor. In addition, the dynamics of greenhouse gas net fluxes entails accounting for carbon dioxide, methane and nitrous oxide, each of which has a different process and source. Our objective was to measure the greenhouse gas balance from a beef cattle farm over a 12-month period in Manitoba, Canada, and to follow the seasonal aspects of the operation. The 125-hectare farm supported 100 steers that were rotated among quadrants that included bale-grazing in winter, spring confinement, summer pasture, and strip swath-grazing in autumn. Eddy covariance measurements of carbon dioxide and methane were made continuously from a central 8-m-high tower, with a second mobile 3-m-high tower used to supplement measurements from specific parts of the farm. We followed cattle position with two to four motion-triggered trail cameras that were moved between fields as needed. Static-vented chambers were deployed to measure nitrous oxide efflux on the bale-grazing site in spring and summer, and on the swath-grazing site following planting and fertilizer application until cattle were allowed on field. The flux footprints sampled different parts of the farm and a combination of flux measurements and footprint modelling were used to build an estimate of net farm emissions. Enteric methane emissions from cattle-dominated parts of the landscape were high when the cattle congregated near the water source, with fluxes exceeding 2000 nmol m^-2 s^-1. This signal was especially obvious in winter, when carbon dioxide and nitrous oxide fluxes are very low. Insufficient sampling from all wind directions was an issue, as well as being able to account for the position of cattle within the flux footprint. Although we are arriving at an estimate of emissions for this farm, additional towers would have improved the spatial coverage.