Changing role of cultivated land in the global carbon cycle

 The carbon balance is ill defined for agricultural lands so that their role in global C balance cannot be accurately estimated. Changes in agriculture in the last 50 years have resulted in a general increase in grain yields, total net annual production (TNAP), and C input to the soil. Amounts of C returned annually with crop residues on Sanborn Field, one of the oldest experimental fields in the United States, increased after 1950, and this was accompanied by C accumulation in soils. Under wheat monocrop (with mineral fertilizer), C accumulated at a rate of 50 g m^–2 year^–1. A 3-year rotation (corn/wheat/clover) with manure and nitrogen applications sequestered 150 g m^–2 year^–1 of C. Total C balance for the wheat and corn production area in the United States, approximated on the basis of these rates, indicates that at least 32 Tg C was sequestered annually during the last 40–50 years. Received: 1 December 1997