Mean residence time of soil organic matter associated with kaolinite and smectite

To gain insight into the effect of clay mineralogy on the turnover of organic matter, we analysed the C-14 activity of soil organic matter associated with clay in soils dominated by kaolinite and smectite in natural savanna systems in seven countries. Assuming that carbon inputs and outputs are in equilibrium in such soils, we took the C-14 age as mean residence time of the organic matter. We corrected the C-14 activity for the Suess effect, Bomb effect and difference between date of sampling and date of C-14 measurement. Organic matter associated with kaolinite turned over fast (360 years on average). Organic matter associated with smectite turned over relatively slowly, with an average mean residence time for the whole clay-size fraction of 1100 years. Multiple linear regression indicates that clay mineralogy is the main factor explaining differences in the mean residence time of the organic matter extracted.