What happens in the Midwest that makes it a carbon sink? Growing crops that turn back into atmospheric CO2 within a couple of months would be carbon neutral at

What happens in the Midwest that makes it a carbon sink? Growing crops that turn back into atmospheric CO2 within a couple of months would be carbon neutral at best. "The strong atmospheric sink in the agricultural region is consistent with the difference in the spatial distribution of agricultural production and product consumption: a strong atmospheric uptake by crops during the growing season in the Midwest states, while much of the release of carbon associated with consumption of agricultural products occurs in other regions, in the United States and abroad. It must thus be pointed out that this apparent atmospheric sink in the Midwest does not imply a long-term carbon sequestration" https://www.pnas.org/content/107/43/18348 https://www.pnas.org/content/107/43/18348 , Discussion