Soil carbon sequestration in traditional farming in Sudanese dry lands

Do altered land management practices offer possibilities to sequester carbon in the soil and thereby mitigate increasing atmospheric CO2 as well as improve local soil fertility? This study investigates the impact of fallow periods on soil organic carbon in semiarid subsistence agroecosystems on sandy and poor soils in Kordofan, Sudan. The area is characterized by low-input cultivation of millet and sorghum in combination with livestock grazing. Recently, cultivation intensity has increased and the fallow periods have been shortened. Soil carbon contents were assessed for sites that have been under various cultivation intensities, ranging from 30 years of fallow to 30 years of continuous cultivation. Soil organic carbon showed a significant negative relationship with cultivation intensity. Measurements indicate a mean increase of approximately 4 g soil organic carbon (SOC) per square meter per year during fallow periods. The possibilities of increasing soil organic carbon by land management were also estimated through simulations using the Century model. Modeling suggested that reverting an intensely cropped millet site to permanent grassland would sequester approximately 1-2 g SOC/m2/yr, with higher rates during the early part of the period. Continuous intense cultivation could decrease the currently low soil carbon levels even further. These results indicate that altered land management could contribute to transforming degraded semiarid agroecosystems from a source to a weak sink for atmospheric CO2. Possible data improvements and uncertainties are discussed.