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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Mean Residence Time of Kaolinite and Smectite-Bound Organic Matter in Mozambiquan Soils

Lab. of Soil Science and Geology, Dep. of Environmental Sciences, Wageningen Univ., P.O. Box 37, 6700 AA Wageningen, The Netherlands

^* Corresponding author (wattel{at}hotmail.com).

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.

Abbreviations: AMS, accelerator mass spectroscopy • ECEC, effective cation-exchange capacity • MRT, mean residence time • SOM, soil organic matter • XRD, X-ray diffraction

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