Objectives Conclusions:conceptual model of OM stabilization  Evaluation of the relevance of stabilization mechanisms in different horizons  Relating time scales of stabilization mechanisms to conceptual pools Material and Methods Operational fractions: (abbreviations see Figure 1) LF <1.6 g cm -3 : LF <1.6 g cm -3 :controlled by recalcitrance & aggregation HF insoluble OM: HF insoluble OM: controlled by recalcitrance & spatial accessibility DF >1.6 g cm -3 : DF >1.6 g cm -3 : mineral associated fraction HF soluble fraction: HF soluble fraction:mineral associated fraction OM resistant to H 2 O 2 -oxidation: OM resistant to H 2 O 2 -oxidation: spatial inaccessible OM Results and Discussion Figure 1: Pool sizes and 14 C ages of soil OM fractions in different horizons of a Dystric Cambisol, Germany. SOC = bulk soil organic carbon, LF = Light fraction 1.6 g cm -3, H 2 O 2 = hydrogen peroxide oxidation, HF = demineralisation with hydrofluoric acid. Dates from Eusterhues et al. (2005) and (2006), Rumpel et al. (2002) and Kaiser and Guggenberger (in preparation). A horizon: 0.77 LF * 112,4 pMC = 77% of bulk soil pMC 0.90 HF insoluble * 113 pMC = 90% of bulk soil pMC 0.23 DF * 112 pMC = 23% of bulk soil pMC 0.10 HF soluble * 98 pMC = 10% of bulk soil pMC  Selective preservation & resynthesis and  Occlusion in aggregates are important as also indicated by a typically strong crumb structure.  Relevance of organo-mineral interactions is evaluated as low  Stabilization of OM within the active and intermediate pool (modern bulk soil age) in the surface soil  Pool sizes of not mineral associated fractions decrease in subsoils (HF insoluble fraction to 30% and LF to 12%). In B and C-horizons the HF insoluble fraction is even older than the mineral associated HF-soluble fraction.  Increasing importance of spatial inaccessibility in subsoils.  OM that is oxidized by H 2 O 2 decreases (from 90% to 70% SOC in A- to C-horizons). OM resistant to H 2 O 2 oxidation: hydrophobic OM - intercalated OM - OM in clay microstructures. H 2 O 2 has a low dispersing effect. Bw2 horizon: 0.55 HF soluble * 101 pMC = 60% of bulk soil age 0.8 DF * 84 pMC = 73% of bulk soil pMC 3C horizon: 0.7 HF soluble * 91 = 56 % of bulk soil pMC 0.8 DF * 70 pMC = 70% of bulk soil pMC  Increasing importance of organo-mineral interactions in subsoils. pH in B- and C-horizons is optimal for ligand exchange. Highest amounts of pedogenic oxides are found in the Bw horizon. M. v. L ü tzow a, I. K ö gel-Knabner a, E. Matzner b K. Ekschmitt c, G. Guggenberger d, B. Marschner e, H. Flessa f & B. Ludwig g a TUM, Lehrstuhl f ü r Bodenkunde, WZW, Department f ü r Ö kologie, TU M ü nchen,Germany; b Lehrstuhl f ü r Boden ö kologie, Universit ä t Bayreuth,Germany; c IFZ – Tier ö kologie, Justus Liebig Universit ä t, Giessen,Germany; d Institut f ü r Bodenkunde und Pflanzenern ä hrung, Universit ä t Halle, Germany; e Geographisches Institut, Ruhr-Universit ä t, Bochum, f Institut f ü r Bodenkunde und Waldern ä hrung, Universit ä t G ö ttingen, Germany; g Department of Environmental Chemistry, Kassel University, Witzenhausen, Germany A conceptual model of organic matter stabilization in soils Investigating pool sizes and turnover ( 14 C) of available operational fractions in relation to bulk soil organic matter (OM) LITERATURE: a Sollins, P., Homann, P. & Caldwell, B.A Stabilisation and destabilisation of soil organic matter: mechanisms and controls. Geoderma: Rumpel, C., Kögel-Knabner, I. & Bruhn, F Vertical distribution, age, and chemical composition of organic carbon in two forest soils of different pedogenesis,. Org. Geochem. 33: Eusterhues, K., Rumpel, C. & Kögel-Knabner, I Stabilisation of soil organic matter by oxidative degradation. Organic Geochem. In press. Eusterhues, K., Rumpel, C. & Kögel-Knabner, I Organo-mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores. Eur. J. Soil Sci in press. Kaiser, K. & Guggenberger, G Long-term stabilisation of organic carbon in acid soil by association with the mineral matrix, in preparation. Poster of Eusterhues, K., Rumpel, C. & Kögel-Knabner, I Radiocarbon dating of soil organic matter fractions: How effective is stabilization by organo-mineral associations? Figure 2: Italic: mechanisms; 3 process groups of mechanisms according to Sollins et al. (1996) a : primary and secondary recalcitrance, spatial inaccessibility, organo-mineral interactions. Pools within broken lines indicate postulated pools but their existence is not verified by direct measurements. DOM = dissolved OM. Table 1: Selected properties of the Dystric Cambisol at Steinkreuz, Germany DepthC Oi+e+a3 - 0n.d. A Bw Bw Bw C n.d.0.1 3C HorizonpH 4C cm% CaCl SandClaySilt % Fe oxAl ox n.d g kg C age [years] Relative amount of SOC in different fractions [%] SOC:modern SOC: 655  25yr SOC: 1756  56yr A horizon Bw2 horizon 3C horizon 0 > > > > > >5000 LF HFinsoluble H2H2 O2O2 oxidable = 100 % HFsoluble resistant DF H2H2 O2O plant residues & exudates A C T I V E P O O L yr microbial / faunal biomass & residues decomposed residues I N T E R M E D I A T E P O O L yr Transport of DOM & colloids OM in clay microstructures P A S S I V E P O O L > 100 yr microbial / faunal biomass & residues Interactions with mineral surfaces Production of charcoal by fire intercalated OM humic polymers pseudo-macromolecules organo-mineral associations Selective preservation &resynthesis Biogenic aggregation organo-mineral associations Complexation with Fe 3+, Al 3+, Ca 2+ occluded particulate OM Formation of hydrophobic surfaces Polymerisatiation Intercalation Abiotic microaggregation Encapsulation charcoal Deutsche Forschungsgemeinschaft DFG Priority Programme 1090 Soils as sink and source of CO 2 - Mechanisms and regulation of organic matter stabilization in soils