Effects of mineral surface iron on the CPMAS 13 C-NMR spectroscopic detection of organic matter from soil fractions in an agricultural topsoil with different amendments J.-M. S E ´ QUARIS , H. P HILIPP , H.-D. NARRES & H. V EREECKEN Agrosphere, ICG 4, Forschungszentrum Ju ¨lich GmbH, D-52425 Ju ¨ lich, Germany Summary The decrease of NMR visibility of the C signal in soil samples due to the association between organic carbon (OC) and the topsoil mineral surface was investigated. CPMAS 13 C-NMR spectra were obtained for soil particle-size fractions (< 2 mm, 2–20 mm, > 20 mm) and bulk soils from an agricultural topsoil (Chernozem) that had received three different amendments (no fertilization, mineral fertilization (NPK), mineral (NPK) and organic (cattle manure) fertilizations) at Bad Lauchsta¨dt, Germany. The soil organic carbon content of the three soils depended on the degree of soil fertilization. There was no constant relationship between the total NMR signal intensity and the total amount of organic carbon (TOC) for all size fractions. Indeed, a key role played in the C signal intensity by the paramagnetic ferric ion from the clay content in soil fractions and bulk soils was confirmed. Thus, we describe the variations of C signal intensity by taking into account the distribution of clay-associated OC and non- associated OC pools. Depending on the amendment, the C signal visibility was weakened by a factor of 2–4 for the clay-associated OC. This estimation was rendered possible by combining mineral specific surface area (SSA) measurements with the N 2 gas adsorption method (BET method) and determination of TOC and iron concentrations. This approach contributes to the quantitative evaluation of the CPMAS 13 C-NMR detection. Introduction 13 C-NMR analysis with cross-polarization and magic angle spinning (CPMAS) of heterogeneous soil organic matter is usually applied to determine the relative amounts of carbon in broad classes of chemical structures. However, in the presence of the soil mineral matrix, large decreases in NMR signal intensity are observed, which are mainly related to the detri- mental effects of ubiquitous paramagnetic ferric ion Fe(III) that reduces the relaxation time of proton nuclei in the rotating frame (T 1q H) (Pfeffer et al., 1984; Vassallo et al., 1987; Wilson, 1987; Arshad et al., 1988; Preston et al., 1994; Schmidt et al., 1997; Smernik & Oades, 2000a,b; Keeler & Maciel, 2003; Conte et al., 2004; Scho¨ ning et al., 2005). The effects can affect a specific functional group (Pfeffer et al., 1984) or reduce the sensitivity of all the carbon nuclei present (Vassallo et al., 1987). It was thus proposed that the intimate associations between OC and the mineral surface containing the iron make the mineral-associated OC fraction from SOM less ‘visible’ to NMR (Randall et al., 1995). In the present work, we measured the CPMAS 13 C-NMR mass-total intensities (Smernik & Oades, 2000b) of various par- ticle soil fractions (< 2 mm, 2–20 mm, > 20 mm) and bulk soils from three Bad Lauchsta¨ dt topsoils. The signal variations can be related to the total amount of organic carbon (TOC) while the mineral content remains constant. Variations of the recor- ded signals were detected and are discussed here with regard to establishing correlations with the specific surface area (SSA) of the soil clay fraction and its surface iron content. We attempted to model the C visibility from the distribu- tion of TOC between the clay-associated OC and (clay) non- associated OC which is based on the relationships between the TOC and the surface-area values obtained by the N 2 gas method (J.-M. Se´quaris, G. Guisado, M.R. Magarinos, C. Moreno, S. Sa´ nchez, P. Burauel, H.-D Narres, & H. Vereecken, submitted). Correspondence: J.-M. Se´quaris. E-mail: j.m.sequaris@fz-juelich.de Received 18 September 2007; revised version accepted 25 February 2008 592 # 2008 The Authors Journal compilation # 2008 British Society of Soil Science European Journal of Soil Science, June 2008, 59, 592–599 doi: 10.1111/j.1365-2389.2008.01044.x