Objectives

1. Depth C Fe ox Al ox Horizon pH Clay Silt Sand CaCl2 % cm % g kg-1 Oi+e+a 3 - 0 n.d. A 0 - 5 3.2 6.7 14 39 47 1.3 0.6 Bw1 5 - 24 3.7 1.1 10 35 55 1.6 1.0 Bw2 24 - 50 4.0 0.4 10 38 52 0.5 0.9 Bw3 50 - 80 4.0 0.2 10 33 57 0.6 0.7 2C 80 - 85 n.d. 0.1 10 38 52 - 1.2 6 3C 85 - 115 4.1 0.1 37 57 0.3 n.d. 4C 115 - 140 4.3 0.1 2.4 13 84 0.1 0.9 + LF DF = 100 % + HF insoluble HF soluble = 100 % + H2 O2 oxidable H2 O2 resistant = 100 % 100 100 80 80 60 60 40 40 20 20 100 80 60 40 20 A conceptual model of organic matter stabilization in soils M. v. Lützowa, I. Kögel-Knabnera, E. Matznerb K. Ekschmittc, G. Guggenbergerd, B. Marschnere , H. Flessaf & B. Ludwigg aTUM, Lehrstuhl für Bodenkunde,WZW, Department für Ökologie, TU München,Germany; bLehrstuhlfür Bodenökologie, Universität Bayreuth,Germany; cIFZ– Tierökologie, JustusLiebig Universität, Giessen,Germany; dInstitutfür Bodenkunde und Pflanzenernährung, Universität Halle, Germany;eGeographischesInstitut, Ruhr-Universität,Bochum,fInstitut fürBodenkunde und Waldernährung, Universität Göttingen, Germany;gDepartment of EnvironmentalChemistry, Kassel University,Witzenhausen, Germany Material and Methods Results and Discussion Objectives Table 1: Selected properties of the Dystric Cambisol at Steinkreuz, Germany • Evaluation of the relevance of stabilization mechanisms in different horizons • Relating time scales of stabilization mechanisms to conceptual pools Investigating pool sizes and turnover (14C) of available operational fractions in relation to bulk soil organic matter (OM) 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 Conclusions: conceptual model of OM stabilization A C T I V E P O O L 1 - 10 yr  Stabilization of OM within the active and intermediate pool (modern bulk soil age) in the surface soil plant residues &exudates Selective preservation &resynthesis microbial / faunal biomass & residues • Selective preservation & resynthesisand • Occlusion in aggregates are important as also indicated by a typically strong crumb structure. • Relevance of organo-mineral interactions is evaluated as low I N T E R M E D I A T E P O O L 10 - 100 yr Operational fractions:(abbreviations see Figure 1) LF <1.6 g cm-3:controlled by recalcitrance & aggregation HF insoluble OM: controlled by recalcitrance & spatial accessibility DF >1.6 g cm-3 : mineral associated fraction HF soluble fraction: mineral associated fraction OM resistant to H2O2-oxidation: spatial inaccessible OM decomposed residues Biogenic aggregation occluded particulate OM 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 microbial / faunal biomass & residues Complexation with Fe3+, Al3+, Ca2+ organo-mineral associations P A S S I V E P O O L > 100 yr A horizon SOC: modern charcoal Production of charcoal by fire [%]  Increasing importance of organo-mineral interactions in subsoils. pH 4.3 - 4.0 in B- and C-horizons is optimal for ligand exchange. Highest amounts of pedogenic oxides are found in the Bw horizon. humic polymers Polymerisatiation fractions Formation of hydrophobic surfaces 0 >100 500 1000 2000 3000 4000 >5000 Transport of DOM & colloids Bw2 horizon SOC: 655 ± 25 yr pseudo-macromolecules Encapsulation of SOC in different • 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. intercalated OM Intercalation OM in clay microstructures Abiotic microaggregation >100 500 1000 2000 3000 4000 >5000 0 amount 3C horizon SOC: 1756 ± 56 yr organo-mineral associations Interactions with mineral surfaces • OM that is oxidized by H2O2 decreases (from 90% to 70% SOC in A- to C-horizons). OM resistant to H2O2 oxidation: hydrophobic OM - intercalated OM - OM in clay microstructures. H2O2 has a low dispersing effect. Relative 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. 0 >100 500 1000 2000 3000 4000 >5000 C age [ years ] 14  Increasing importance of spatial inaccessibility in subsoils. Figure 1: Pool sizes and 14C 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, DF = dense fraction >1.6 g cm-3, H2O2 = 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). LITERATURE: a Sollins, P., Homann, P. & Caldwell, B.A. 1996. Stabilisation and destabilisation of soil organic matter: mechanisms and controls. Geoderma: 65-105. Rumpel, C., Kögel-Knabner, I. & Bruhn, F. 2002. Vertical distribution, age, and chemical composition of organic carbon in two forest soils of different pedogenesis,. Org. Geochem. 33:1131-1142. Eusterhues, K., Rumpel, C. & Kögel-Knabner, I. 2006. Stabilisation of soil organic matter by oxidative degradation. Organic Geochem. In press. Eusterhues, K., Rumpel, C. & Kögel-Knabner, I. 2005. 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. 2006. 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. 2005. Radiocarbon dating of soil organic matter fractions: How effective is stabilization by organo-mineral associations? Deutsche Forschungsgemeinschaft DFG Priority Programme 1090 Soils as sink and source of CO2 - Mechanisms and regulation of organic matter stabilization in soils