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Fluid Journal : Fluid Journal 1993-1995
3 Fluid Journal Fall 1993 Table 1. Average carbon unit for energy associated with crop production. Operation Conventional Chisel plow Disk No-till ---------------lbs carbon/acre/year--------------- Moldboard plow 11.3 Chisel plow 7.3 Disk 4.0 4.0 4.0 Herbicide application and second disking 4.6 4.6 4.6 Apply herbicides 0.6 Plant 2.7 2.7 2.7 3.3 Single cultivation 2.7 2.7 2.7 Herbicide manufacture 10.6 12.6 14.6 17.9 Machinery and repair 11.3 10.0 7.9 4.0 Annual total 47.1 43.9 36.6 25.9 % of conventional 100.0 93.2 77.7 54.9 Tab le 2. Changes in mean soil organic carbon and fossil fuel carbon emissions (tons C x 106) for three scenarios of conservation tillage (CT) usage to the year 2020. Scenario 1 Scenario 2 Scenario 3 27% CT 57% CT 76% CT Tillage system Soil Fuel Soil Fuel Soil Fuel Conventional-till -45 -133 -26 -96 -14 -74 Minimum-till 0 -33 0 -57 0 -73 No-till 0 -7 115 -11 415 -14 Totals -45 -173 88 -164 401 -161 Net loss or gain -218 -76 240 Shifting carbon balance Widespread conversion of major field crop production from conventional to conservation tillage would change the entire system of soil and soil manipulation from a source of atmospheric carbon to a sink for scenario three (Table 2). Increasing the amount of no-till management would change cropland soil from a possible source of atmospheric carbon to a sink for both scenarios two and three. It is unclear how much of the 34 to 55 million tons of carbon that are projected to be lost from cropland soils for scenario one, as a result of continued conventional tillage, would become atmospheric carbon, because some of it would be eroded and not oxidized. Some of the eroded carbon would be deposited in low-lying areas within the same fields. The remainder of the eroded soil would be deposited by water and wind to other land, drainage channels, streams, and water bodies. The ultimate fate of eroded soil organic carbon has not been extensively studied and this is a topic of great importance. Of all the tillage types, conventional tillage systems produce the greatest soil organic carbon and fossil fuel carbon losses. Minimum tillage apparently does not lead to additional carbon being held in the soil, but it does prevent a net loss of soil organic carbon. Minimum tillage fuel requirements are not much less than those of conventional tillage. Our estimates show no-till systems increase the amount of carbon held in the soil, as well as reduce fossil fuel carbon emissions. In our studies, soils with the greatest potential for increasing carbon already had high carbon levels in the upper six inches, such as those soils found in the Pacific Northwest, the Upper Midwest, and northern New England. The regions with low soil organic carbon also had a low amount of field crop production. Thus, there was a net gain rather than a net loss of soil organic carbon for the 3 to 6-inch layer of soil converted to no- till. Net gain in soil organic carbon held in both scenarios of reduced tillage (Table 2) is a positive step toward reducing the amount of carbon in the atmosphere. Our highest estimate of soil organic carbon benefit from a tillage conversion would offset approximately 0.7 to 1.1 percent of the U.S. fossil fuel emissions for the same period. Reducing impact The release of soil organic carbon globally from agriculture has been estimated at 880 million tons of carbon per year. If alternative tillage practices could prevent this loss, then approximately 16 percent of the annual global fossil fuel emissions would be offset. This offset would be even greater if soil organic carbon increased, as is possible with no-till agriculture. Many additional benefits would be derived from increasing the carbon content of soil, such as increased fertility, stable soil structure, and increased water- holding capacity. The magnitude of conservation tillage adoption projected in this study is within what appears to be economically and agronomically feasible and is likely to occur regardless of the importance society places on carbon sequestration in soils. Conversion of land to conservation tillage alone is not likely to sequester sufficient carbon to offset the impact of carbon released by non-agricultural fossil fuel consumption. When combined with other strategies of carbon sequestration and fossil fuel emission reductions, widespread implementation of conservation tillage practices may be significant in reducing the impact of global climate change. References Kern, J.S.; Johnson, M.G.; "Conservation tillage impacts on national soil and atmospheric carbon levels." Soil Science of America Journal, 57:200-210, 1993. Kern and Dr. Johnson are soil geographer and soil chemist, respectively, at Man Tech En Environmental Technology, Inc., U.S. EPA Environmental Research Laboratory in Corvallis, Oregon. q
Fluid Journal 1996-1998