Hatfield, Jerry (USDA-ARS Ames, Iowa, National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, IA, 50011; Phone: 515-294-5723; Fax: 515-294-8125; Email: hatfield@nstl.gov)

Spatial Variation of Carbon Dioxide and Energy Balance Fluxes among Corn and Soybean Fields in Central Iowa

J.L. Hatfield *, J.H. Prueger, W.P. Kustas

Estimates of fluxes from cropped surfaces are generally conducted in fields that are assumed to be under uniform management and similar soil types. There is little information available on the spatial or temporal variation of carbon dioxide or energy balance fluxes across production-sized fields of corn (Zea mays L.) or soybean (Glycine max (L.) Merr.). This study was conducted in 2002 as part of a larger scale experiment on surface energy balances across central Iowa. Twelve eddy covariance/energy balance stations (6 in corn 6 in soybean) included measurements of water vapor, carbon dioxide, sensible heat, net radiation and soil heat fluxes, air temperature, surface temperature, wind speed and precipitation. The stations were placed in the fields shortly after planting and remained until near harvest. Diurnal variations in CO₂ and H₂O vapor fluxes revealed that the magnitude of the fluxes is large and that variation of the fluxes among fields was consistent throughout the season. Integration of the daily fluxes into seasonal totals showed considerable differences among crops and fields. Flux differences were the result of the effect of varying soil types on water holding capacity. Seasonal integrated values were lower than estimates derived from biomass samples collected within the fields and the measurement of the C content of the biomass. Understanding patterns of spatial variability of carbon dioxide and energy balance fluxes enhances our ability to characterize regional scale fluxes.