Organic matter in particle-size fractions from A and B horizons of a Haplic Alisol M. W. I. S CHMIDT & I. K O È GEL- K NABNER Lehrstuhl fu Èr Bodenkunde, Technische Universita Èt Mu Ènchen, 85350 Freising-Weihenstephan, Germany Summary The organic matter in soils may be stabilized by its interactions with minerals. We have studied such interactions in a Haplic Alisol under forest in which clay and organic matter have migrated from an eluvial A horizon to accumulate in an illuvial B horizon. We have tried to trace the fate of organic matter in these horizons (Ah and Bvt) by determining clay mineralogy, carbon and nitrogen content, hydro- lysable amino acids, lignin signature by alkaline CuO oxidation and carbon species by 13 C CPMAS NMR of bulk soils and particle-size fractions. In both horizons, most of the organic matter was present in O±alkyl and methylene structures, each contributing one-third to the bulk organic matter. In the Ah horizon the ratios of carbon-to-nitrogen, and yields for lignin and hydrolysable amino acids decreased as the particle-size class decreased, but side-chain oxidation of lignin compounds increased with decreasing particle size. In contrast to previous observations, the proportions of O±alkyl carbon increased as particle size decreased, constituting a major proportion of the organic carbon in the clay-size fractions from both the Ah and Bvt horizons ( 38%), while proportions of methylene carbon decreased. Illite was the dominant mineral in the fraction  6 m, whereas the mobile fine clay fraction (<0.2 m) was rich in smectites ± minerals with large surface areas. Our results support the hypothesis that potentially labile organic matter, such as O±alkyl carbon typically present in polysaccharides, may be stabilized against further degradation in organomineral complexes. Introduction Soil organic matter comprises plant, animal and microbial residues in all stages of decay. The degree of decomposition of plant material in soil is controlled by its bioavailability. The microbial availability of plant material can be limited by the latter's binding to minerals, such as clay particles (Oades, 1990; Oades etal., 1988; Sollins etal., 1996). Generally, clay contents are positively correlated with soil organic matter concentrations, when other factors such as vegetation, climate and hydrology are similar (Davidson, 1995). However, recent research indicates that there is a protective capacity, characteristic of individual soils (Hassink & Whitmore, 1997). Some volcanic soils may have a greater stabilizing influence on organic matter than would be predicted from their clay contents, probably due to the presence of allophane and ferri- hydrite, both of which have a large specific surface capable of adsorbing organic molecules (Saggar etal., 1996; Parfitt etal., 1997; Torn etal., 1997). Mechanisms of interactions between organic materials and clay minerals are poorly understood from laboratory studies. We know even less about naturally occurring organomineral complexes, though we do know that they migrate down the profile in a broad range of soils; characteristically in Alisols, Luvisols, Acrisols and Lixisols in the FAO terminology. In such soils the A horizon is depleted of clay by percolating water, which transports dispersed colloids and fine clay down the profile to accumulate in a Bvt horizon, where clay and organic matter are intimately associated with metal oxyhydrox- ides typically forming clay skins or cutans. The clay con- tents in the argic B horizon are always greater than in the overlying A horizon, often by as much as 20%. Strong sorptive interactions between organic matter and minerals potentially stabilize organic matter against degradation. Thus, such soils may contain large proportions of natural organomineral complexes, providing a unique opportunity to study how organomineral interactions affect organic matter. Materials and methods We selected a much-studied Haplic Alisol from near Siggen, Schleswig±Holstein, Germany (Coordinates: R 44 398000, H 60 6017630, Topographical Sheet DGK5 SuÈssau). Previous Correspondence: M.W.I. Schmidt, University of ZuÈ rich, Department of Geography, Winterthurerstrasse 190, 8057 ZuÈ rich, Switzerland. E-mail: mschmidt@geo.unizh.ch Received 30 January 2001; revised version accepted 20 December 2001 European Journal of Soil Science, September 2002, 53, 383±391 # 2002 Blackwell Science Ltd 383