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DIVISION S-2—SOIL CHEMISTRY

Carbon Storage in Coarse and Fine Clay Fractions of Illitic Soils

^a Institut für Bodenkunde und Pflanzenernährung, Martin-Luther-Universität Halle-Wittenberg, Weidenplan 14, 06108 Halle, Germany
^b Earth Science Division, Lawrence Berkeley National Lab., MS 90-1116, Berkeley, CA 94720

^* Corresponding author (kahle{at}landw.uni-halle.de).

The storage of organic C in coarse (0.2–2 µm) and fine (<0.2 µm) clay fractions of illitic topsoils from loess was investigated in terms of the effect of particle size and mineral phase properties. We compared (i) C functional groups by ¹³C nuclear magnetic resonance spectroscopy (NMR), (ii) the stable C isotope ratio (¹³ C ratio) of organic pools, and (iii) residence time of C by ¹⁴C analyses. To investigate relationships between C storage and the size of mineral surface area or the amount of hydrous oxides, specific surface areas (SSAs, BET-N₂ method) and the content of dithionite-extractable Fe (Fe_d) were analyzed. The chemistry of the organic matter stored in clay subfractions was different. Compared with coarse clay, fine clay contained relatively (i) more ketonic/aldehyde, carboxyl and phenolic C, and (ii) less anomeric, O-alkyl, and methoxyl/N-alkyl C, and had (iii) a lower C content and C/N ratio and (iv) a higher ¹³C ratio. In 11 out of 14 fractions, C had turnover times of few centuries or less. In fine clay, the increase in SSA resulting from oxidation of organic matter explained 66% of the variation in C content, in coarse clay 97%. We calculated loadings of mineral surface area with C and Fe_d. Carbon loading exceeded Fe_d loading in coarse clay while it was of the same range in fine clay. The results may be interpreted as an indication that a certain portion of the mineral surface area controls the C content in both clay subfractions. The character of the important surface may differ between the subfractions.

Abbreviations: CPMAS, cross polarization magic-angle spinning • Fe_d, dithionite-extractable Fe • NMR, nuclear magnetic resonance • SSA, specific surface area • SSA(c), specific surface area of clay fraction after C removal • SSA, increase in SSA after removal of organic matter • ¹³C ratio, stable C isotope ratio

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