Predicting a ceiling for soil carbon sequestration on variable landscapes under no-till in eastern Canada

Much of the crop production in eastern Canada occurs on landscapes where erosion/deposition has occurred. The potential to sequester C by reducing tillage will be greatest in those parts of landscapes where the organic carbon (OC) stocks are below a ceiling (OC c ). However, the physical/biochemical basis for OC c is not understood and therefore it is difficult to predict where C sequestration will occur in landscapes with variable topography. In this research we tested two hypotheses proposed as the physical/biochemical basis for OC c : (1) OC c coincides with the steady state OC (OC ss ) stocks on non-eroded sites and (2) OC c coincides with a critical proportion of the capacity of the clay and silt fraction to absorb and retain OC (i.e., a critical saturation ratio). Comparison of data from sites with level and variable topography disproved the first hypothesis; OC stocks on level sites were, on average, 14 Mg ha -1 larger than OC c 15 yr after implementing no-till (NT) on variable landscapes. Further analyses of data from sites with variable topography indicated the saturation ratio in the surface 10 cm of soil must be less than 0.45 before NT results in C sequestration in the profile. Although the analyses are not incompatible with the second hypothesis, the critical saturation ratio is surprisingly small compared with values obtained from level sites. Additional tests of the second hypothesis are warranted on sites with variable topography in which C sequestration has been documented. Key words: Erosion, C capacity, saturation ratio, spatial variability, C sequestration