Changes in soil organic carbon stocks under agriculture in Brazil

Land use conversion from natural to agricultural ecosystems affects concentration and storage of soil organic carbon (SOC) depending on tillage frequency and depth, soil type, climate and other factors. Despite its importance, little is known about the magnitude and consistency of changes in SOC stocks (Mg ha−1) due to management of highly weathered soils in Brazil. From 37 uncultivated/cultivated paired sites in Brazil (most on Oxisols), SOC stocks were calculated for the depths of 0-20 and 0-40 cm. Changes in SOC stocks were calculated for intensive (with annual tillage) and non-intensive (pastures, conservation tillage and perennial crops) land use systems. Intensive systems caused significant (t-test, P < 0.05) SOC loss of 10.3% or 6.74 Mg ha−1 in the 0-20 cm depth, but not in the 0-40 cm depth. In general, non-intensive systems had no significant effect on SOC stocks in the 0-20 and 0-40 cm depths. However, in coarse-textured soils (≤200 g clay kg−1), non-intensive systems caused significant SOC losses of about 20% for both 0-20 and 0-40 cm depths (8.5 and 15.5 Mg SOC ha−1, respectively). No significant changes in SOC were detected in Oxisols (<500 g clay kg−1) and clayey Oxisols (≥500 g clay kg−1), in five arbitrary ecoregions or under the three different non-intensive land use systems. Average SOC losses under intensive and non-intensive systems were low in comparison to those reported for temperate ecosystems, probably due to: (a) lower SOC stocks in surface when compared to temperate soils, and (b) strong interaction of Al/Fe oxides in clay with SOC.