Oxidizable organic carbon fractions and soil aggregation in areas under different organic production systems in Rio de Janeiro, Brazil

The effect of organic management can influence positively the edaphic properties. The aim of this study was to evaluate the degree of organic carbon oxidation and the soil aggregation indexes in areas under organic management and different plant covers. The selected systems were: conventional tillage (corn - CT), no-tillage (eggplant - NT), passion fruit - Desmodium sp intercrops, fig cultivation, agroforestry system (AFS) and a secondary forest area. The soil samples were collected at 0-5 and 5-10 cm depths. The total organic carbon (TOC) was quantified and separated into four fractions (Fl, F2, F3 and F4) with decreasing degrees of oxidation, by the use of increasing quantities of sulfuric acid. Aggregate stability was determined by wet sieving, using the indices of mean weight diameter (MWD), geometric mean diameter (GMD) and sensitivity index (SI). The area under corn cultivation presented the lowest TOC concentrations and aggregation indices, in the two depths evaluated. The areas under eggplant and fig presented the highest TOC concentrations at the 0-5 cm depth, and the highest MWD at the 5-10 cm depth. There were SI values above 1 in the areas under fig and passion fruit. In general, the Fl fraction represented the largest proportion of TOC in all areas and in the two depths evaluated. The AFS presented the lowest proportions of TOC in the four fractions (0-5 cm). The area under eggplant (0-5 cm) presented the highest carbon concentrations in the Fl, F2 and F3 fractions. However, at the 5-10 cm depth, this behavior was observed in the area under corn, except for the Fl fraction. The results indicate that the management adopted in the area under corn did not favor soil aggregation and TOC. In the other areas, the SI determined indicates that the tillage practices associated to plant cover, in organic systems, preserved soil aggregation when compared to the forest area, at the 0-5 cm depth. The oxidizable carbon concentrations were influenced by the management systems adopted, with higher values in the areas where more plant residues were added to the soil.