Letter to Science:

The 276 Tg of grain produced on US farms in 1995 (USDA Agricultural Statistics) contained about 124 Tg C. Much of this C returned to the atmosphere after the grain was eaten, but the 45 Tg C exported as grain reduced net CO2 flux to the atmosphere from North America. C in crop residues at least equal to that in grain were returned to the soil, but microbial respiration released much of this C to the atmosphere. Still, the net C sink from production and export of crops may rival annual storage of 79 Tg C by US forests . Farms and forests together can explain only a small fraction of the 1700 Tg sink in North America recently suggested .

R. Ford Denison

Director, Long Term Research on Agricultural Systems

University of California

Davis, CA 95616