Investigating spatial representation of micrometeorological flux measurements over agroecosystems

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Taylor, Amanda M.
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A challenge with micrometeorological flux measurements is determining whether they represent field spatial variability. Often only one tower characterizes agricultural fluxes, so we used three towers (co-located, 50 m, and 100 m apart) to evaluate the spatial capability of eddy covariance to measure net ecosystem exchange (NEE), latent heat flux (LE), and sensible heat flux (H) over a forage and a spring wheat crop. Regression comparisons of NEE decreased slightly as towers moved apart at the forage and was not reliable at the wheat because of senescence. LE varied little amongst separations, and H maintained a consistently high r2 at all tower locations on both fields whether examined over the short- or long-term. Georeferenced, gridded leaf area index values within tower footprints were representative of the whole field. Overall, H field variability was captured within 10 W m-2, LE ranged 15 – 60 W m-2, and NEE was 23 – 33% RMSE over the forage. Many farms have known heterogeneity that can come from forage, grains, and cattle operations within the same agroecosystem, and cattle movement is a challenge when measuring farm budgets. With two eddy covariance towers and static-vented chamber campaigns we measured NEE, methane (CH4), and nitrous oxide (N2O) fluxes for a year over a beef cattle farm. Enteric CH4 was used as a tracer to separate cattle respiration fluxes from the farm landscape NEE. Chamber measurements on bale locations after winter grazing were hotspots for N2O emission the following spring and summer, while pasture fluxes were minimal elsewhere. When all sources of greenhouse gas measurement were combined, field and cattle respiration and CH4 dominated, and the farm was a carbon source of 46 t CO2 equivalent ha-1 y-1. Flux research would benefit from quantifying field spatial variability to ensure that eddy covariance measurements are representative. Incorporating flux towers and chambers can help estimate greenhouse gas budgets of spatially variant agroecosystems.
eddy covariance, carbon, fluxes, sensible heat, latent heat, beef cattle, carbon dioxide, methane, nitrous oxide, spatial, micrometeorology, agroecosystem, agriculture, greenhouse gas, static vented chambers, Spatial variation, variability, Carbon budget, Flux