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 a griculture 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 Fie ld, one of the oldest experimental fields in the United Slates, increa sed after 1950, and this was accompanied by C accumulation in soils. U nder 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 ma nure 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 S tates, approximated on the basis of these rates, indicates that at lea st 32 Tg C was sequestered annually during the last 40-50 years.