Evapotranspiration and Carbon Dioxide Exchange Rates in a Corn-Soybean Rotation System and a Reconstructed Prairie Using a Portable Canopy Chamber.

Measurements of evapotranspiration (ET) and CO₂ exchange rate (CER) for crop canopies are valuable for agricultural management, where ET indicates plant water use, and CER relates to photosynthesis and plant productivity. Although portable canopy chambers have been used for direct measurements of ET and CER fluxes in various cropping systems, seasonal patterns of chamber measured ET and CER fluxes have not been fully studied, and the soil CO₂ emission fluxes (SCE) have not been separated from CER. The objectives of this study were to quantify seasonal ET and CER fluxes in corn, soybean and reconstructed prairie; to measure cropping system SCE and combine SCE with CER to determine daily and seasonal patterns of CO₂ fluxes and plant photosynthesis rates (PNS); and to compare the CER fluxes with eddy covariance results. Crop canopy chamber measurements were performed at the Comparison of Bio-fuel Systems (COBS) research site near Ames, IA. Canopy chambers were used to measure ET and CER fluxes in the different cropping systems. Three different chamber heights were used to match crop heights during different crop growth periods. During the 2014 growing season, diurnal measurements were performed approximately once a week, and data from 15 measurement days were obtained. Eddy covariance measurements were made at corn and soybean fields near Ames, IA (northeast of the COBS research site). The pattern of ET for the three crops was linearly correlated to the pattern of CER, with correlation coefficients larger than 0.8. The chamber measured ET and CER fluxes were consistent with the eddy covariance measurements, with differences of seasonal cumulative values less than 5%. For diurnal measurements, the chamber CER values were consistent with the day time eddy covariance results, and the combination of chamber SCE and the night time eddy covariance values well represented daily CO₂ emission fluxes. For seasonal results, the chamber measured ET and CER fluxes in prairie were the largest among the cropping systems in the early growing season, and the ET and CER fluxes in corn were the largest among the cropping systems in the late growing season. The portable canopy chamber appears to provide reliable measurements of ET and CER fluxes in the field.