Investigation of carbon sequestration in maize-based agroecosystems

Carbon pools and fluxes have been measured in three maize-based agroecosystems in eastern Nebraska in an effort to better understand the potential for these systems to sequester carbon in the soil. Landscape-level fluxes of carbon, water and energy were measured using the eddy covariance technique. In order to better understand the landscape-level results, measurements at smaller scales, using techniques promoted by John Norman, were made and scaled up to the landscape level. Single leaf gas exchange properties (CO2 assimilation rate and stomatal conductance) and optical properties, direct and diffuse radiation, and incident, reflected and transmitted photosynthetically active radiation (PAR) were measured at regular intervals throughout the growing season. In addition, soil surface CO2 fluxes were measured with chambers. From leaf measurements, the responses of net CO2 assimilation rate and stomatal conductance to relevant biophysical controlling factors were quantified. Single leaf gas exchange data were scaled up to the canopy level using a simple radiative model that considers direct beam and diffuse PAR penetration into the canopy. Canopy level photosynthesis and transpiration were estimated, coupled with the soil surface CO2 fluxes and compared to measured NEE values from the eddy covariance approach. Estimated values of canopy level intercepted and transmitted PAR were also compared to measured values. The agreement between estimated and observed values increases our confidence in the measured carbon pools and fluxes in these agroecosystems and enhances our understanding of biophysical controls on carbon sequestration. John Norman was influential in the development of many of the techniques involved in this research; his contribution is acknowledged and will be highlighted.