Effect of no-tillage on turnover of organic matter in a Rhodic Ferralsol

Soil organic matter (SOM) is considered to be key to sustainability of agriculture in the tropics. In southern Brazil, no-tillage has been adopted widely to control soil erosion, but its impact on the dynamics of SOM is not well established. We measured soil carbon (C) and delta(13)C in two crop rotations, one of which contained C4 maize (Zea mays L.), after 21 years of contrasting tillage (conventional tillage versus no-tillage). Adjacent sites that reflected historic land-uses were also sampled. In the tillage experiment there was no effect of tillage on the total amount of C in the 0-40 cm profile (even when contrasting bulk density was accounted for), and the concentration of C differed only in the 0-5 cm and 5-10 cm layers. However, the occasional input of C4 material in the maize rotation resulted in a significant effect of rotation on delta(13)C (P<0.001). Using C-13 as a tracer for the SOM formed since the start of the experiment, we estimated the abundance of 'recent' and 'old' C within each depth interval. We found the main effect of tillage was to increase the medium-term turnover of SOM, particularly in the subsoil (i.e. below 20 cm depth). Compared with no-tillage, there was almost five times more recent C in the subsoil, and 20% more recent C in the 0-40 cm profile as a whole.