Increasing the amount of C in soils may be one method to reduce the concentration of CO2 in the atmosphere. We measured organic C stored in southern Idaho soils having long term cropping histories that supported native sagebrush vegetation (NSB), irrigated mold-board plowed crops (IMP), irrigated conservation-chisel-tilled crops (ICT), and irrigated pasture systems (IP). The CO2 emitted as a result of fertilizer production, farm operations, and CO2 lost via dissolved carbonate in irrigation water, over a 30-yr period was included. Net organic C in ecosystems decreased in the order IP>ICT>NSB>IMP. In this study if NSB were converted to IMP, 0.15 g C m-2 would be emitted to the atmosphere, but if convered to IP 3.56 g C m-2 could be sequestered. If IMP land were converted to ICT, 0.95 g C m-2 could be sequestered in soil and if converted to IP 3.71 g C m-2 could be sequestered. There are 2.6 x 10 ^8 ha of land worldwide presently irrigated. If irrigated ariculture were expanded 10% and the same amount of rainfed land were converted back to native grassland, an increase of 3.4 x 10^9 Mg C (5.9% of the total C emitted in the next 30 yr) could potentially be sequestered. The total projected release of CO2 is 5.7x 10^10 Mg C worldwide during the next 30 yr/ Converting rainfed agriculture back to native vegetation while modestly increasing areas in irrigated agriculture could have a significant impact on CO2 atmospheric concentrations while maintaining or increasing food production.
