Carbon dioxide fluxes in corn–soybean rotation in the midwestern U.S.: Inter- and intra-annual variations, and biophysical controls

Quantifying carbon dioxide (CO₂) fluxes in terrestrial ecosystems is critical for better understanding of global carbon cycling and observed changes in climate. This study examined year-round temporal variations of CO₂ fluxes in two biennial crop rotations during 4 year 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 81% 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 primarily 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±39gCm⁻²year⁻¹). Concomitantly, within growing seasons, CO₂ sink periods were approximately 106 days for corn and 90 days for soybean, and peak rates of CO₂ uptake were roughly 1.7-fold higher for corn than soybean. Apparent changes in soil organic carbon estimated after accounting for grain carbon removal suggested soil carbon depletion following soybean years and neutral carbon balance for corn. Overall, results suggest changes in land use and cropping systems have a substantial impact on dynamics of CO₂ exchange.