Total Organic Carbon and Aggregate Size Distribution in Soils Under Natural Vegetation and No Tillage.

Organic carbon and soil structure are closely related and influenced by soil management systems. The aim of this study was to evaluate changes in total organic carbon (TOC) and the distribution of soil aggregates under 3 different soil management systems i.e. 10 and 20 years of no-tillage cultivation as compared with soil under natural vegetation (Cerrado). Undisturbed soils (0-5cm depth) were collected from Central-West Region, Brazil. The soils, Oxisols from Cerrado, were collected from a field under Natural Vegetation-Cerrado (NV), and from fields that were under conventional tillage since 1970s, and 10 and 20 years ago were changed to no-tillage cultivation system (NT-10; NT-20 respectively). The undisturbed samples were gently sieved (4mm) and the aggregates retained were further fractionated by wet sieving through five sieves (2000, 1000, 500, 250, and 50 μm) with the aggregates distribution expressed as percentage retained by each sieve. The TOC was determined for each aggregate size by combustion (Thermo-Finnigan). The organic C concentration for NT-10 and NT-20 were 10 and 14 mg.g^-1 soil, respectively, considerably lower than that of the NV (20 mg.g^-1 soil). A predominance of aggregates >2000 μm were observed under NV treatment (92%), NT-10 (64%) and NT-20 (71%). However, there are greater amounts of aggregates in sizes 1000, 500, 250 and 50 μm under treatments NT-10 and NT-20, explaining the lower aggregate stability compared to the soil under NV, even when analyzing the first 5 cm of soil layer, which receives higher contribution of organic residues in no-tillage cultivated areas. These results suggest that 20 years no tillage were insufficient to recover the soil from 20 years of conventional tillage, with regard to total organic carbon content and distribution of aggregates indicating that the recovery process is slow.