Organic fractions and soil organic carbon management index in an Oxisol under soybean cultivation in the cerrado region of Goiás, Brazil

Abstract

Organic matter dynamics influences the main chemical, physical and biological processes in soils, and often determines its natural fertility. The aim of this study was to evaluate the organic fractions and the carbon management index (CMI) of the soil in soy bean areas (Glycine max L.) grown on straw from Brachiaria ruziziensis and from sorghum (Sorghum bicolor L. Moench). Samples were collected in an Oxisol at the municipality of Montividiu, Goiás, Brazil. Samples were taken at five depths (0-5, 5-10, 10-20, 20-40 and 40-60 cm) in soils under the following management conditions: area with grass growing after and before soybean crops (SB); area with sorghum crop in soybean off-season (SS); and area under native vegetation. Organic carbon amounts (OC) and soil organic matter particle size fractions were determined. From these values, carbon stocks in each particle size fraction were quantified and the CMI was calculated. The CMI is a relative measure of the changes caused by soil management, in comparison to an original situation, evaluated by the changes in OC stocks and taking under consideration the aspects of the soil OC lability. Significant differences were found in the OC contents and in the respective stocks at all evaluated depths. The particulated organic matter fraction proved to be an effective parameter to demonstrate differences between management systems. The CMI showed a positive effect in SS and SB systems in relation to the natural condition (cerrado), indicating that the carbon management systems studied were effective in maintaining, or even increasing, soil organic carbon contents.