Carbon Storage in Coarse and Fine Clay Fractions of Illitic Soils

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 13 C nuclear magnetic resonance spectroscopy (NMR), (ii) the stable C isotope ratio (δ 13 C ratio) of organic pools, and (iii) residence time of C by 14 C 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‐N 2 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 δ 13 C 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.