1. Abstract We report results from an on going multiannual measurements of CO2 and energy fluxes above soybean and winter wheat crop canopies at the Winfred Thomas Agricultural farm, Alabama Agricultural and Mechanical University. Across northern Alabama, soybean and winter wheat, in double cropping systems, are planted between May and September and from October to late April, respectively. Measurements were made in rainfed soybean-winterwheat rotation since fall 2006. Preliminary results indicate that the overall daily and monthly energy partitioning (latent, sensible and soil heat flux, W/m2) is influenced by soil water avilabity and crop type. Because of the unusually dry summer of 2007, energy dissipation via sensible heat was higher than latent heat. The trend for the Bown ratio across the measured months exhibited two peaks in May and June suggesting higher proportion energy dissipation was via sensible heat. During the cooler months (winter wheat canopy) carbon assimilation values ranged from a maximum of -14.5  mol m-2 s-1 (11:00 -12:00 local time) in March, to a minimum of about - 5  mol m-2 s-1 in January. In contrast, during the warmer months, CO2 flux over soybean canopy reached a maximum of -15.5  mol m-2 s-1 in May to a minimum of -8.5  mol m-2 s-1 in August. Similar reduction in CO2 assimilation was also observed during June and July and is mainly attributed to the unusually sever summer drought the region has experienced. Introduction Understanding how environmental variables regulate C and energy flux over an agricultural farm land ecosystem is critical to our understanding as to how these variables controls crop phenology and productivity. Over the last three decades, studies on global C cycle research have predominately focused on forest CO2 and energy fluxes because forests are thought as major global C regulator (Running et al., Grace et al., 1995, Saleska et al., 2003). Consequently, less effort has been devoted to estimate C and energy balances for agriculture systems. However, studies have shown that cropland of the world representing about 12 % of the earth's surfaces are as equally productive as some of the major natural ecosystems (Barford et al., 2002; Wood et al., 2000; Hollinger et al., 2004). Agricultural ecosystems are ecologically complex systems and their management could impact important ecosystem exchange processes at various temporal and spatial scales. Thus, understanding the dynamics of carbon dioxide and energy exchanges of these major cropping systems at multiple time scales is a critical component in the evaluation of Net Ecosystem Exchange (NEE) particularly in the face of a changing climate and such measurements could be the foundations for predicating ecosystem responses to multiple environmental changes (Loescher et al., 2003) and for modeling ecosystem process (Law, et al., 2002). Objectives The main objective of the objectives the study were to: 1) understand the environmental controls on the diurnal and monthly patterns of CO2 and energy fluxes during the two growing seasons and 2) understand and compare how energy is partitioned among the various components with minimum soil disturbance. 3) What are the most important atmospheric and soil variables that control interannual variation in carbon dioxide and energy exchanges? Materials and Methods The eddy covariance (EC) flux station is at Winfred Thomas Agricultural Research site (34 54’ N, 86 32’ W, 191m), 13 miles North of the Alabama Agricultural and Mechanical University, Alabama. Before we started measurements, different kinds of crops (cotton, winter wheat, canola and corn) and tillage types had been practiced. Beginning fall 2004, however, the area (~ 18 ha) surrounding the EC station is under no-till farming practice. To quantify the effects of environmental factors on ecosystem exchange processes a standard eddy covariance station will be used along with all established micrometeorological measurement protocols, data collection and analyses. The 3.04m tall EC tower is established approximately in the middle of the 15 ha of agricultural research site, with a fetch length of about 200-220 meters in all four directions from the center. Measurements of the vertical flux of CO2, water vapor and momentum will be made using a 3D sonic anemometer (CSAT 3, Campbell Sci. Logan UT) and an open path infrared gas analyzer (Li7500 LiCor, Nebraska, LN). Appendix 2 contains the description of all the sensors currently operating at the station. Incoming and reflected radiation will be measured using an upward and down ward facing (Kipp and Zonen, radiometers mounted on a horizontally extended boom 2.45 m above the surface. In addition, the upwelling and down welling terrestrial radiation (thermal long wave radiation) will be measured using (Epply laboratories, Newport, RI Inc.,) mounted at the same height described above. Data analysis The micrometeorological station is equipped with datalogger (model CR 5000, Campbell Scientific Inc., Logan, UT) capable of storing high frequency data from the open-path CO2 /H2O infra-red gas analyzer (IRGA, model Li-Cor 7500, Li-Cor Inc., Lincoln, NE) and sonic anemometer (model CSAT-3, Campbell Scientific Inc.) and other ancillary weather and soil data (averaged over 30 minutes) (ref. Table 2). M.Gebremedhin 1, T.Tsegaye 1, H. Loescher 2, M.Wagaw 1, M.Silitonga 3, O.Mbuya 4, and A. Johnson 5. (1) Department of Plant and Soil Science, Alabama A&M University, P.O. Box 1208, Normal, AL 35762, (2) NEON Inc., Boulder Colorado, (3)Alcorn State University, 1000 ASU Drive #750, Alcorn State, MS 39096, (4) Florida A&M University, 119 Perry Paige Building, Tallahassee, FL 32307. March, 2007 Result and Discussion Energy balance closure Energy fluxes calculated from EC allow direct comparisons to incoming net radiation flux and for the computation of Bowen ratio (β = H/LE). As shown in Fig. 1, the evaporative fraction of net radiation (Rn) is quite low for all the measured months (except February when available energy was small along with wetter soil) with well). During May and June of 2007, H was largest from early morning (09:00-12:00) to late afternoon (14:00-18:00) and the average fractions of available energy used for H were consistently high (i.e., β >1, for during the daytime, Fig. 2) suggesting that more available energy was partitioned to sensible heat flux than LE because of water limitation. Summery and Conclusions * Diurnal CO 2 and energy flux of the two canopies investigated demonstrated clear differences in daytime magnitudes. * Published studies on whole-ecosystem gas exchange in drought stressed agricultural canopies are rare and this study attempts to fill the gap in our understanding how major environmental factors (e.g. summer droughts) impact CO 2 uptake and surface energy partitioning among various components. * We are currently working in refining data analysis procedures (screening, filtering, gap filling etc) to reduce uncertainties associated with nighttime fluxes and lack of energy closure for agricultural ecosystems. References Aubinet, M., (et al.) 2000. Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology. Advances in Ecological Research 30, 111-175. Bladocchi, D.D., Hicks, B.B., Meyers, T. D. 1988. Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology, 69: 1331-1340. Barford, C.C., Wofsy S.c., Goulden, M.L., Munger, J.W., Hammond, P. E.,, Urbanski, S.P., Hutyra, L., Saleska, S. R., Foley, J. A., et al.,. 2005. Global Consequences of Land Use. Nature, 309: 570-574. Law, B. E., et al.,. 2002. Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation. Agri. For. Meterol.131: 77-96. Lee, X., Massman, W., and Law, B. 2004. Handbook of Micrometeorology: A Guide for Surface Flux Measurement and Analysis. Kluwer Acad. Publ., Dordrecht, Boston, London Ed: 250pp. Loescher, H. W., B. E. Law, L. Mahrt, D. Y. Hollinger, J. Campbell, and S. C. Wofsy 2006. Uncertainties in, and interpretation of, carbon flux estimates using the eddy covariancetechnique, J. Geophys. Res., 111, D21S90, doi:10.1029/2005JD006932 Table 1. Micrometeorological Instrumentation at WTARS, Fall 2006. MeasurementInstrumentModelUnit CO 2 densityInfrared gas analyzerLiCor7500mmol mol -1 H 2 O densityInfrared gas analyzerLiCor7500mmol mol -1 Atmospheric pressureBarometreic pressureVaisala CS105mb Air temperaturePT1000Vaisala HMP45CºC Relative humidityCapacitive RH sensorVaisala HMP45C% Incoming/reflected/emitted shortwave/longwave radiationNet radiometer pyranometer/pyrgeometerKipp & Zonen CNR1W m -2 Soil water content (volumetric)Water content reflectometerCampbell CS616 m -3 m -3 Soil temperatureThermistorCampbell CS107ºC Soil heat fluxThermopile gradientREBS HFT3 2W m -2 Rainfall amountTipping bucketTexas Instruments TE525mm Wind speedCup/propeller anemometers RM Young Wind Sentry/ Monitor (05103)m s -1 Typical Bowen ratio values over well-watered grass or moist soil is about 0.2. In August 2007, LE surpassed H (i.e., β < 1) only in the early morning hours. During period when the soil was moist, i.e., in February of 2007, LE was higher than H during the daylight hours (8:00-18:00) suggesting a larger proportion surface energy dissipation was via latent heat. The validation of EC technique can be examined using the surface energy budget closure (Baldocchi, et al., 1988). The half-hourly energy balance was determined using linear regressions between the hourly values of (H + LE) and (Rn – G) (Fig. 3). Although close The half-hourly energy balance was determined using linear regressions between the hourly values of (H + LE) and (Rn – G) (Fig. 3). Although close (88%), agreement between the two components was found, the EC underestimated turbulent energy fluxes (ref. Loescher et al. 2006). Discrepancies may be related to various sensor locations and mis-matched source areas (e.g., Rn sensors over non representative areas) and to potential horizontal and vertical advection of heat and water vapor (Aubinet et al., 2000). Seasonal CO2 flux trend Typical diurnal cycles of CO 2 fluxes for the two canopies are presented in Fig. 5 and 6. The daily flux trend comparisons were made only for cloud free and warm days. The longest diurnal carbon uptake rates were observed during the warmer months (May to August, 2007). Large differences in daily CO 2 uptake was observed during the cooler months (January to April) over winter wheat canopy while differences were small during summer time. The trend in CO 2 uptake was consistent with changes in day length. Over winter wheat canopy, the carbon assimilation values ranged from a maximum of -14.5  mol m -2 s -1 (11:00 -12:00 local time) in March, to a minimum of about - 5  mol m -2 s -1 in January. During the warmer months, soybean CO 2 flux reached a maximum of -15.5  mol m -2 s -1 in May during the early morning hours to a minimum of -8.5  mol m -2 s -1 in August. Mid-summer drought occurred during the main growing season (June-August) had a strong impact on canopy CO 2 uptake which resulted in reduced day time carbon assimilation. February, 2007 Figure 4. Diurnal trend in CO 2 flux (negative values denote C canopy uptake) at WTARS over winter wheat and b) soybean canopy. Figure 1 Half-hourly diurnal course of energy components: sensible heat (H), latent heat (LE), net radiation (Rn) and soil heat flux (G). Raw data Convert data Convert covar. & flux to physical units Check off range values & despikes Pre- rotate Online 30 min flux average & auxiliary data Rotate data from inst. to natu. coo. Compute NEE Calculate seasonal & yearly NEE Gap filling Table 3. Flow chart for analyzing time series data to calculate covariance, flux and NEE values. a) b) YearPlanting-harvestingCrop typeTillage/herbicide 2006 season 1 2006-2007 season 2 season 3 2007-2008 season 4 season 5 12 May - 20 Sep, 2006 Oct, 20 - Apr 20 May 15 - Oct 30, 2007 Nov, 07 - Apr, 08 May - Sep, 2008 1 Soybean (round up) Winter wheat (cover) Soybean (round up) Winter wheat (cover) Soybean (round up) Minimum tillage/herbicide Table 2: Planting schedule and tillage treatments for the study period. Treatment include minimum tillage (MT) and crop rotation (CR) involving soybean and winter wheat cover crop. Measuring CO 2 and Energy Exchange Over a No till Agriculture Using Eddy Covariance Technique Figure 2. Daytime trend in Bowen ratio for the months of February March, June and August of 2007. Figure 3. Half-hourly sums of LE +H against available energy (Rn- G) for the month of September, 2007.