Lignin is a major component of cell walls in terrestrial plants. Due to its aromatic structure it is believed to be highly resistant against microbial degradation. Short term decomposition studies indicated initial lignin enrichment in plant litter. The long-term fate of lignin in soils became clearer only very recently (Dignac et al. 2005; Heim & Schmidt, submitted). Long-term field experiments showed that in agricultural soils lignin turns over faster than bulk Soil Organic Carbon (SOC). Bulk SOC generally is strongly stabilized in the clay fraction, but several studies found clay fractions are depleted in lignin components (e.g. Amelung et al. 1999, Kiem & Kögel-Knabner 2003). To elucidate the fate of lignin and the processes of lignin stabilization in soils, we followed ¹³C labeled lignin in particle size fractions of arable and grassland soils. Samples from two arable soils were analyzed as bulk soil and after separation into particle size classes. At both sites, one plot has been under continuous maize (C4 plant) cropping for 23 years, while the control plot has been under continuous wheat (C3 plant). For comparison, a grassland soil, planted with ryegrass (Lolium perenne) was analyzed, where the ¹³C label had been introduced by fumigation with ¹³C labeled CO₂ for 10 years. The lignin content in these samples was determined by gas chromatography – mass spectrometry (GC-MS) after having been subjected to a CuO oxidation, which splits the macromolecule into its monomers. Compound specific isotope values of individual lignin monomers were measured by coupling a combustion oven and an isotope ratio monitoring mass spectrometer to the GC. In particle size fractions of arable and grassland soil we found that most of the new (i.e. ¹³C-labelled) lignin can be found in the sand (> 0.063 mm) fraction (43% in the arable soil, 57% in the grassland). The rest of the new lignin was about evenly distributed between coarse silt, fine silt, and clay fractions. The old (i.e. non-labeled) lignin is in the fine silt fraction (51% of all old lignin in arable soil, 38 % in grassland soil). The < 0.063 mm fractions combined, comprise about 90% of old (C3) lignin in the arable soil, and 78% in the grassland soil. The fine size fractions usually have a higher specific surface area, and this could point to the role of sorption processes, which may be involved in stabilizing lignin in soils. Contrasting, most of the lignin in the sand fraction probably is free particulate organic matter, which appears to be less stable than lignin in finer fractions. However, stabilization in fine fractions seems to be less important for lignin than for bulk SOC. In arable and grassland soil, the ratios of lignin oxidation products to total organic carbon decreased from sand to clay fractions consistently. Thus, although lignin in fine fractions is more stable than "free" lignin, lignin derived carbon contributes only small proportions to the stable carbon in clay fractions. Concluding, compound specific ¹³C isotope analysis showed that lignin derived carbon does not contribute to the strong enrichment of soil carbon in silt and clay-sized fractions in agricultural and grassland soil. References: (1) Amelung, W., Flach, K.-W. & Zech, W. (1999): Lignin in particle-size fractions of native grassland soils as influenced by climate. Soil Science Society of America Journal 63: 1222-1228. (2) Balesdent, J. & Mariotti, A. (1996): Measurement of soil organic matter turnover using 13C natural abundance. In: Mass Spectrometry of Soils (eds. Boutton, T.W. & Yamasaki, S.I.), 83-111. Marcel Dekker, New York.(3) Dignac, M.-F., Bahri, H., Rumpel, C., Rasse, D.P., Bardoux, G., Balesdent, J., Girardin, C., Chenu, C. & Mariotti, A. (2005): Carbon-13 natural abundance as a tool to study the dynamics of lignin monomers in soil: an appraisal at the Closeaux experimental field (France). Geoderma 128: 3-17. (4) Heim A., Schmidt M. W. I. European Journal of Soil Science submitted. (5) Kiem, R. & Kögel-Knabner, I. (2003): Contribution of lignin and polysaccharides to the refractory carbon pool in C-depleted arable soils. Soil Biology & Biochemistry 35: 101-118.