Submitted to: Agriculture and Forest Meterology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 27, 2011
Publication Date: December 15, 2011
Citation: Hernandez-Ramirez, G., Hatfield, J.L., Parkin, T.B., Sauer, T.J., Prueger, J.H. 2011. Carbon dioxide fluxes in corn-soybean rotations in the Midwestern U.S.: Inter- and intra-annual variations, and biophysical controls. Agriculture and Forest Meterology. 12:1831-1842. Interpretive Summary: Exchanges of carbon dioxide between the soil, the plant, and the atmosphere are a critical part of the global environment. With the current interest in climate change and the attention being given toward carbon dioxide levels in the atmosphere, there is a need for a more complete understanding of the role cultivated crops have in the overall carbon cycle. A long-term study has been conducted near Ames, IA to evaluate the yearly carbon budget of corn and soybean canopies grown in production fields. These measurements have been made to measure the carbon budget over these canopies and to also quantify the role of the soil as a carbon dioxide source or sink. These are basic research studies which provide insights into the dynamics of carbon dioxide exchanges as the plant grows during the year and then during the period which there is not actively growing plant. Understanding the processes and the changes during a day, during a growing season, and throughout the year provides a more rigorous insight into the overall carbon dynamics of agricultural systems in the Midwest. This information is of value to scientists attempting to understand the role agriculture has in the global carbon balance and to policy makers who require a more detailed value of the magnitude of the carbon exchanges.
Technical Abstract: Quantifying carbon dioxide (CO2) fluxes in terrestrial ecosystems is critical for better understanding of global C cycling and observed changes in climate. This study examined year-round temporal variations of CO2 fluxes in two biennial crop rotations during 4 yr of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production. We monitored CO2 fluxes using eddy-covariance (EC) and soil chambers in adjacent production fields near Ames, Iowa. Under the non-limiting soil water availability conditions predominant in these fields, diel and seasonal variations of CO2 fluxes were mostly controlled by ambient temperature and available light. Air temperature explained up to 79% of the variability of soil respiratory losses during fallow periods. In contrast, with full-developed canopies, available light was the main driver of daytime CO2 uptake for both crops. Furthermore, a combined additive effect of both available light and temperature on enhanced CO2 uptake was identified only for corn. Moreover, diurnal hysteresis of net CO2 uptake with available light was also found for both crops with consistently greater CO2 uptake in the mornings than afternoons perhaps owing to delay in peak of soil respiration relative to the time of maximum plant photosynthesis. Annual cumulative CO2 exchange was mainly determined by crop species with consistently greater net uptake for corn and near neutral exchange for soybean (-466 ± 38 and -13 ± 39 g C m-2 yr-1). Concomitantly, within growing seasons, CO2 sink periods were approximately 106 d for corn and 90 d for soybean, and peak rates of CO2 uptake were roughly 1.7-fold higher for corn than soybean. Apparent changes in soil organic C estimated after accounting for grain C removal suggested soil C depletion following soybean years and neutral C balance for corn. Overall, results suggest changes in land use and cropping systems have a substantial impact on dynamics of CO2 exchange.