Date of this Version
Following a 6-year study of net ecosystem exchange of carbon (NEE) in a maize–soybean system, Hollinger et al. (2005) concluded that the system was a net sink of 90 g C m-2 year-1. This estimate was erroneous because a wrong equation for computing grain carbon removal (Eq. 1) was used and because the authors computed average annual values for each of the two crops (average C gain under maize: 184 g C m-2; average C loss under soybean: -94 g C m-2), then added them together, as if both crops were present each year. Hollinger et al. (2006) have recently corrected those calculation errors, now suggesting that (1) maize acted as a C-sink of 184 g C m-2, (2) soybean was a C-source of 124 g C m-2, and (3) the system was a net sink of 30 g C m-2 year-1. Although we agree with the corrections of the major calculation errors, we disagree with the authors’ conclusions about the carbon sequestration potential of the maize–soybean rotation.