Carbon Storage in Coarse and Fine Clay Fractions of Illitic Soils

The storage of organic C in coarse (0.2-2 (mu)m) and fine (<0.2 (mu)m) 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 ((delta)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 dithioniteextractable Fe (Fed) 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 (delta)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 Fed. Carbon loading exceeded Fed 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.