We reviewed the potential of agricultural soils to sequester C and presented several examples of how management practices can increase C sequestration in agricultural soils. Our results suggest that a conversion of cropland to grassland will result in the largest reduction in net GHG emissions. Many of the practices that promote carbon accumulation, such as reduced tillage, and reduced summer fallow, will also reduce erosion thereby improving soil productivity and profitability of farming systems. Increasing organic matter in soil makes soil more productive, which is likely to lead to increased crop production and C sequestration. Soil carbon accumulation will however reach a saturation point because soils eventually become less responsive to increased carbon inputs. Hence, promotion of carbon sequestration in agricultural soils will be most relevant over the next few decades. Later on, GHG mitigation efforts will need to concentrate on maintaining the sequestered carbon in the soil and on reducing N2O emissions. Permanence of the sequestered C is, therefore, an important criterion to consider when we examine the net impact of management practices on C sequestration. Soil C in the surface layer can be quickly lost to the atmosphere by discontinuing some practices like conservation tillage or by increasing practices such as summer fallowing. This means that it is difficult to guarantee that sequestered carbon will never return to the atmosphere. Further, because C increases in soil are directly related to the amount of C inputs to the soil, and the latter is dependent on weather conditions, then the amount of C sequestered over time is very much a function of climatic conditions. We resisted the temptation of multiplying the amount of C sequestered for various management practices by the percent of arable land where this practice is employed to come up with the world carbon mitigation potential, because of the wide range of response of management practices to soil and climatic conditions. We also demonstrated that reducing N2O emissions is also extremely important because the global warming potential of N2O is about 300 times greater than that of CO2 and it provides an indefinite mitigation potential as compared to the C sequestration option where the mitigation potential is finite. In this paper we have not accounted for the absorption of methane, which is also an important GHG.

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