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Title: CARBON DIOXIDE DYNAMICS DURING A GROWING SEASON IN MIDWESTERN CROPPING SYSTEMS

Author
item Prueger, John
item Hatfield, Jerry
item Parkin, Timothy
item Kustas, William - Bill
item Kaspar, Thomas

Submitted to: Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2003
Publication Date: 10/10/2003
Citation: Prueger, J.H., Hatfield, J.L., Parkin, T.B., Kustas, W.P., Kaspar, T.C. 2003. Carbon Dioxide Dynamics During a Growing Season in Midwestern Cropping Systems. Environmental Management. 33(1):330-343.

Interpretive Summary: The Upper Midwest is a major corn and soybean production region for the U.S. This region also represents a significant land transformation from native prairie to production agriculture. The role of this large spatial region in terms of carbon dioxide uptake from the atmosphere and subsequent sequestration into the soil is not well defined. In addition estimates of carbon dioxide for large regional areas are often based on limited point measurements that fail to capture significant differences across landscape regions that are functions of soil, water availability and local meteorological conditions. Within a typical corn/soybean production watershed in central Iowa, multiple towers were erected to measure carbon dioxide and water vapor transport during the course of a growing season. Results clearly show considerable spatial and temporal variability in carbon dioxide and water uptake. This illustrates the need to increase our understanding of the factors that contribute to the variability so that more accurate estimates of carbon uptake can be made for a major agricultural production region. This will in turn aid in interpreting the role of the Upper Midwest region on carbon sequestration and regional climate changes.

Technical Abstract: Dynamics of the carbon dioxide exchanges is a critical component in the evaluation of the Net Ecosystem Exchange (NEE) of terrestrial ecosystems. The spatial and temporal dynamics of CO2 across Midwestern cropping systems have not been well documented and quantifying these dynamics in one of the most intense cropping systems in the world will enable a more complete understanding of the C exchanges. This study was designed to evaluate the spatial dynamics across a growing season within production fields of corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) in central Iowa and to quantify the above canopy, within canopy, and soil components in the C balance of these crops. These fields are located in the Walnut Creek watershed 5 km south of Ames, Iowa. An energy balance approach using eddy covariance instrumentation was placed across different fields and year-around measurements in both corn and soybean fields to quantify the exchange of CO2 and H2O between the crop canopy and the atmospheric boundary layer. Within canopy concentrations of CO2 and H2O vapor were measured with an 8 port CO2 /H2O vapor analyzer. Soil respiration was quantified with soil chambers at various landscape positions and throughout the growing season. Fluxes of CO2 and H2O vapor throughout the day were dependent upon net radiation and the stage of canopy development. Diurnal variations in CO2 and H2O vapor fluxes revealed that the magnitude of the fluxes is large and variation in CO2 fluxes among fields had a standard error of 0.5 mg CO2 m-2 s-1 with a mean uptake of 1.25 mg CO2 m-2 s-1. Variation in water use for these same fields was similar. Integration of the daily fluxes into seasonal totals showed large differences among crops and fields. The field differences were a result of soil type and the impact of soil type on water holding capacity. The integrated values are lower than estimates derived from biomass samples collected within the fields and the measurement of the C content of the biomass. Within canopy recycling of soil CO2 may resolve this discrepancy. The techniques are available to quantify the CO2 and H2O vapor fluxes across different management systems and landscapes to help refine our understanding of the magnitude of the C and water dynamics in cropping systems.