So what are some of the agricultural management practices that have the potential to bring about a small annual percent increase in the amount of carbon stored in the soils of the world's agro-ecosystems?  One is conservation tillage, which includes no-till, ridge-till and mulch-till farming.  These techniques reduce soil organic carbon losses to the atmosphere by minimizing the amount of disturbance to the upper layers of the soil, where carbon accumulates as a consequence of aboveground plant litter, root material and root exudates produced during the growing season.  Prior to the advent of conservation tillage, large-scale cultivation disrupted the long-term carbon balance of most agricultural soils; and much of the carbon in their stored organic matter was lost to the atmosphere via oxidation upon exposure to the air.  Such practices made U.S. cropland soils major carbon sources, until the adoption of conservation tillage and other soil management strategies began to turn the situation around in the mid 1980s (Allmaras et al., 1999).

A second major strategy for increasing agricultural carbon sequestration is to increase crop yields through breeding and genetic improvement programs.  Higher yields result in a greater abundance of crop residue left in the field after harvest.  Ultimately, this residue decomposes and a portion of its carbon makes its way into the soil organic carbon pool.  According to Follet, "the importance of crop yields and the accompanying increases in crop residue yields must be recognized for their providing current and future potential of U.S. cropland soils to sequester soil organic carbon."

Another way of enhancing soil carbon sequestration is to increase the amount of plant cover on agricultural lands.  This feat could be accomplished in two different ways: by increasing cropping intensity or by converting seasonal cropland into permanent cover.  The former strategy could include initiating rotations with winter cover crops, while the latter approach could include setting aside more cropland for the U.S. Conservation Research Program.

A fourth major soil carbon enhancing strategy is to improve soil fertility, as fertility affects biomass production and thus is directly related to the amount of carbon sequestered by the soil.  One method for improving soil fertility is to apply fertilizers, such as nitrogen or livestock manure.  Additionally, the application of irrigation water is important for increasing biomass and yield.  It must be remembered, however, that there are significant carbon costs associated with the production and application of fertilizers, as well as the transport of irrigation water (Schlesinger, 2000); and the CO2 emissions associated with these activities must be counted against the extra CO2 removed from the atmosphere via these intensified agricultural operations.

The following table, derived from the work of Follett et al. (2001) and Lal et al. (1998), provides the results of a number of quantitative analyses of how these and several other "best management practices" could enhance soil carbon sequestration rates.

Table. Potential rates of carbon sequestration (kilograms carbon per hectare per year) due to improved land management practices.

Improved rangeland management ........................ 50 to 150
Improved pastureland management
   Commercial fertilizer applications ...................... 100 to 200
   Manure applications ........................................... 200 to 500
   Use of improved plant species .......................... 100 to 300
Improved grazing management ............................ 300 to 1300
Nitrogen fertilization of mountain meadows ...... 100 to 200
Restoration of eroded soils .................................. 50 to 200
Restoration of mined lands .................................. 1000 to 3000
Conversion of cropland to pasture ..................... 400 to 1200
Conversion of cropland to natural vegetation .. 600 to 900
Conversion from conventional to conservation tillage
   No till ................................. 500
   Mulch till ........................... 500
   Ridge till ............................ 500

Jawson and Shafer (2001) estimate that the successful implementation of these farm and rangeland management techniques could boost the current overall U.S. farm and rangeland carbon sequestration rate by a full order of magnitude (factor of ten), which is an amount sufficient to sequester 13% of the country's yearly carbon emissions (Comis et al., 2001).  There thus appears to be a great potential for the U.S. agricultural community to play a major role in helping to slow the rate of rise of the air's CO2 concentration.  And because enhancing soil organic matter content has a host of beneficial impacts on plant growth and development, such an endeavor would have multiple positive implications.

References
Allmaras, R.R., Wilkins, D.E., Burnside, O.C. and Mulla, J.D.  1998.  Agricultural technology and adoption of conservation practices.  In: Advances in Soil and Water Conservation.  F.J. Pierce and W.W. Frye (Eds.).  Ann Arbor Press, Chelsea, MI, pp. 99-158.

Comis, D., Becker, H. and Stelljes, K.B.  2001.  Depositing carbon in the bank.  Agricultural Research 49 (2): 4-7.

Follett, R.F.  2001.  Soil management concepts and carbon sequestration in cropland soils.  Soil & Tillage Research 61: 77-92.

Follett, R.F., Kimble, J.M. and Lal, R.  2001.  The Potential of U.S. Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect.  Lewis Publishers, Boca Raton, FL.

Jawson, M.D. and Shafer, S.R.  2001.  Carbon credits on the Chicago Board of Trade?  Agricultural Research 49 (2): 2.

Lal, R., Kimble, J.M., Follett, R.F. and Cole, C.V.  1998.  The Potential for U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect.  Sleeping Bear Press, Chelsea, MI.

Schlesinger, W.H.  2000.  Carbon sequestration in soils: Some cautions amidst optimism.  Agriculture, Ecosystems and Environment 82: 121-127.