SOIL ORGANIC CARBON AND NITROGEN STOCKS IN AN UNTILLED CROP-LIVESTOCK INTEGRATION SYSTEM UNDER DIFFERENT GRAZING INTENSITIES

The potential for adopting crop-livestock systems in southern Brazil is high, especially in untilled soybean areas with cover crops in the winter season. The long-term use of this system at different grazing intensities will result in different carbon and nitrogen stocks in the soil due to the different plant and animal residues. This research was conducted to evaluate alterations in total carbon and nitrogen pools and in organic matter fractions in a soil under different grazing intensities under no-tillage. The experiment on a Rhodic Hapludult (Oxisol) was initiated in 2001, after soybean harvest. The treatments were different cattle grazing (black oat + ryegrass) pressures (10, 20 and 40 cm sward height) and an ungrazed treatment in the winter and soybean in the summer, in a randomized block design. Soil samples were taken at the beginning of the experiment (May, 2001), after three years (May, 2004) and after six years (May, 2007) to evaluate the content and stocks of organic carbon and nitrogen. Moderate grazing intensities (20 and 40 cm plant height) resulted in an increase of total carbon and nitrogen and in the organic matter particulate fraction, similarly to the ungrazed area. However, at the most intensive grazing pressure (10 cm plant height) there were losses in the stocks of these elements and reduction in soil organic matter quality.