Summary
To gain insight into the effect of clay mineralogy on the turnover of organic matter, we analysed the 14C 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 14C age as mean residence time of the organic matter. We corrected the 14C activity for the Suess effect, Bomb effect and difference between date of sampling and date of 14C 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.
To gain understanding about the process of global warming, it is essential to study the global C cycle. In the global C cycle, soil organic matter (SOM) is a major source and sink of atmospheric C. Turnover times of C in these soil organic compounds vary from hours to thousands of years. Clay minerals can stabilize SOM through the formation of organo‐mineral bonds. The aim of this research was first, to determine the mean residence time (MRT) of organic matter that is bound to different clay mineral surfaces, and second, to explain the variance in the measured MRTs using multilinear regression. We especially studied organic matter that is bound to kaolinite or smectite. We analyzed the 14C activity of organic matter in the whole and heavy clay‐size fraction of kaolinite‐ and smectite‐dominated soils from N'Ropa, in northern Mozambique. The soils originated from natural savanna systems and bamboo forest. We assumed that C inputs and outputs are in equilibrium in such soils, so that the 14C age equals the MRT of the organic C. For both kaolinite‐ and smectite‐dominated soils, the organic matter in the whole and heavy clay‐size fraction and extracts had a fast turnover (400–500 yr on average). The MRT of kaolinite‐bound organic matter did not differ significantly from that of smectite‐bound organic matter. Multiple linear regression indicates that the effective cation‐exchange capacity (ECEC) is the main factor to explain variance in the MRT of the extracted SOM. These results agree with previously found trends in organic matter turnover of kaolinite and smectite‐associated clay.
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