BACKGROUND

Prior to European colonization of the U.S. Great Plains, prairies were the dominant plant communities. The soils of the prairie landscapes contained relatively high amounts of organic carbon, possibly more than 50,000 kg of carbon per hectare stored in the topsoil, which is equivalent to the amount of carbon found in 20,000 gallons of gasoline (calculations based on 2.5% soil carbon, 20 cm deep topsoil and soil bulk density of 1 g / cm³).

In Figure 1a, Robertson and Grace (2004) redrew this graph from Haas (1957) to show how cultivation of crops for food decreases soil carbon. Soil carbon at two sites in Kansas was measured prior to initiation of cultivation and was monitored for over 40 years to track soil carbon losses. Cultivation (tillage, fertilization, long fallow periods) resulted in the oxidation of soil organic carbon and although the two sites differed in total carbon loss, both sites exhibited a negative exponential trend. It is assumed that soil carbon eventually reaches a steady state if cultivation continues for many years.