Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 7, 2006
Publication Date: January 15, 2007
Citation: Hatfield, J.L., Prueger, J.H., Kustas, W.P. 2007. Spatial and temporal variation of energy and carbon fluxes in corn and soybean fields in central Iowa. Agronomy Journal. 99:285-296. Interpretive Summary: Carbon dioxide has become an important topic of discussion among all segments of society because of the interest in global warming. Changes in carbon dioxide levels and the impact on weather and crop production is a question often asked among producers and is related to their interest in changing farming practices to increase carbon storage. It is assumed that there is seasonal variation in carbon dioxide release and uptake by a crop; however, there is little known about the amount of variation present within a day and across corn and soybean fields that have similar practices. A study was conducted across central Iowa to use a series of detailed measurements of carbon dioxide and water vapor exchanges to measure the variation in these exchanges across corn and soybean fields. These studies were conducted during the summer of 2002. We observed that within a given day the variation among fields was due to changing patterns of clouds that shaded the sun, while among days the differences were caused by the uneven distribution of rainfall and differences in plant growth induced by different soil types. Understanding why these variations occur in an agricultural area that is assumed to be relatively uniform helps scientists improve the information base to assess the impact of changing farming practices on regional carbon budgets and helps policymakers understand the need for multiple measurement sites across regions to increase the confidence in policy decisions.
Technical Abstract: Energy balance and carbon dioxide exchange of agricultural crops has been investigated through limited field studies because of the expense of the monitoring equipment and availability of fields to place equipment. A study was conducted in central Iowa during 2002 as part of the Soil Moisture Experiment (SMEX02) to evaluate the Soil Moisture Energy Exchange (SMACEX) across the intensive corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) production area near Ames, Iowa (Latitude 41.98380985 and Longitude -93.75497316). Energy balance and carbon dioxide stations were placed in 12 corn and soybean fields across different soils and landscapes in and around the Walnut Creek watershed. These stations consisted of a net radiometer, soil heat flux plates, surface temperature, sonic anemometer, and fast response CO2/H2O sensor and were placed in fields to provide adequate fetch from the prevailing wind directions. The systems were installed shortly after planting and remained in place in most fields until near harvest. Prior to deployment and immediately after removal from the fields sonic anemometers and CO2/H2O sensors were compared for differences. There were major changes among the sites throughout the course of the study. There were no rainfall events for the first 20 days after deployment and differences among fields within a day were due to the presence or absence of cumulus clouds that caused a large variation in incident solar radiation and thus Q*. This variation affected G, LE, H, and CO2 uptake within a given day while across days the differences among fields was due to the spatial pattern of rainfall events. One event was on DOY 185 which caused a two-fold difference in rainfall totals across the study area. This affected the energy balance variation because of the differential soil drying. This effect was further exaggerated by the differences in ground cover induced by the interaction of soil water availability and crop growth. On the seasonal scale the primary difference among sites was attributed to the soil water holding capacity that affected the plant growth. The total rainfall for the overall growing season became more uniform, although the effect of early season growth pattern persisted in the differences among fields. Soil water holding capacity and differences in early season ground cover was correlated with energy balance differences (r = 0.80). Over the growing season LE and CO2 uptake were closely related (r = 0.90) for the different fields. Characterization of the spatial distribution of energy balance and CO2 uptake in an intensive cropping region provides guidance on the confidence that can be placed in interpreting single site measurements.