Distribution of soil organic carbon in different size fractions, under pasture and crop rotations with conventional tillage and no-till systems

Soil organic carbon (SOC) is one of the principal indicators of soil quality. Its size fractions have been proposed as high sensitivity indicators in order to detect changes generated by different soil use and management intensities. The objective was to compare the impact of different soil management practices after 10 years on SOC distribution and its size fractions. Treatments consisted in two rotation systems (rotations of continuous annual crops and rotations of 3 years of crops and 3 years of pastures), performed with conventional tillage (CT) and no-till (NT). In 2000, NT treatments were additionally split into C3 or C4 summer crops. In 2003, soil was sampled at 0–3, 3–6, 6–12, 12–18, 18–40, 40–60 and 60–80 cm depths and SOC was determined. At the first four depths, SOC associated with particulate organic matter (POM-C) and with the soil mineral fraction (MAOM-C) were determined. Changes in carbon indicators (SOC and its size fractions) occurred mainly in the first 3 cm of soil, and with the exception of POM-C, were diluted when considering the 0–18 cm depth. Inclusion of pastures in the rotation was a better alternative to continuous cropping in CT systems, since it had better C indicator values. However, NT improved indicator values compared with LC, especially when C4 species were included in the rotation; no differences were found between continuous cropping or crop-pasture rotations. These results allowed discriminate different combinations of crops and tillage systems that contribute to maintain or increase SOC, suggesting a sustainable management of the soil resource.