Stocks and fractions of organic carbon in an Oxisol under different management systems

The objective of this work was to determine the effects of soil management systems on the stocks and fractions of organic carbon of an Oxisol (Xantic Ferralsol), in the cerrado of the state of Piaui, Brazil. The following systems were evaluated: a three-year-old conventional tillage; a three- and a five-year-old no-tillage system, using millet as cover crop; and a nine-year-old no-tillage, using millet as cover crop for seven and forage (Urochloa brizantha) for two years. A native cerrado area was used as a reference. Soil samples were collected in the wet and dry seasons from 0.00-0.05, 0.05-0.10, 0.10-0.20, and 0.20-0.40 m depths for determining: total organic carbon and nitrogen contents and stocks; humic substances content; and humification index. The highest carbon contents were observed in the soil under cerrado, at 0.00-0.05 m depth, in both seasons. However, the highest stocks of carbon and nitrogen as well as the highest contents of humic substances were verified in the soil with no-tillage system for five and nine years, in both seasons, at all evaluated depths. The highest humification index was also observed in these soils, in the dry season. No-tillage associated with cover crops decreases soil organic matter mineralization, increases soil carbon stocks in depth, and favors carbon sequestration in cerrado areas of Piaui state.