Using eddy covariance flux measurements, gross primary production (GPP) and ecosystem respiration (Re) of irrigated maize and soybean fields were quantified and examined in terms of relevant controlling variables. Even though the peak green leaf area index (GLAI) of the maize and soybean canopies was comparable (about 5.8 m2 m−2), the maize GLAI increased more rapidly during the vegetative growth stages and peaked a month earlier compared to the soybean canopy. In both fields, seasonal changes in GLAI explained about 95% of the variability in GPP (at a given incident PAR = 1400–1500 μmol m−2 s−1). On a day-to-day basis, during the peak growth period in both fields, the hourly daytime values of GPP showed a strong hyperbolic relationship with incident photosynthetically active radiation (PAR). Peak hourly GPP was 3.2 and 2.0 mg m−2 s−1 for the maize and soybean crops, respectively. At night, within narrow ranges of green leaf area throughout the growing season, exponential relationships using air temperature explained between 39 and 88% of the variability in Re of both crops. The growing season cumulative GPP of maize was 1744 g C m−2, larger than most natural ecosystems except a few very productive forests. Cumulative ecosystem respiration of maize was 1154 g C m−2. The cumulative GPP and Re of soybean were both substantially smaller than maize (966 and 826 g C m−2, respectively). Differences in GPP and Re between the two fields were largely related to greater photosynthetic capacity of the maize crop, which also resulted in a more rapid canopy development compared to soybean.