Major research achievements

1. Major research achievements DFG priority programme 1090 Soils as sink and source of CO2 - mechanisms andregulation of organic matter stabilisation in soils Final workshopSchloss Thurnau, Bayreuth, Germany18 - 21 March 2006

2. Working hypothesis 1 Organic C is stabilized in soils by selective preservation of recalcitrant molecules We found: • The biotic community able to degrade any OM of natural origin and we have no indication for the existence of an inert OM pool • Long term stabilization of potentially labile compoundsshow the importance of active stabilization mechanisms • No indications for polymerisation Kalbitz et al., 2003; Hamer et al., 2004; Kleber et al., 2004; Rumpel et al., 2004; Vetter et al., 2004; Ekschmitt et al., 2005; Metz et al., 2005; Rethemeyer et al., 2005; Kramer & Gleixner, 2006; Fox et al., 2006

3. Working hypothesis 1 Organic C is stabilized by selective preservation of recalcitrant molecules • Recalcitrance is only important during early stages of decomposition • Recalcitrance can not explain long term stabilization and is not the major driving force of passive C pool formation • This implies a reconsideration of the basic concepts underlying most actual compartment and cohort models

4. Working hypothesis 2 Organic C is stabilized by organo-mineral interactions and by complexation We found: • Precipitation of DOM by Al results in C-stabilization within the intermediate pool • Preferential precipitation of aromatic compounds Schwesig et al., 2003; Scheel et al., 2006 • Sorption to the mineral phase stabilizes OM Preferential sorption of carboxylic and aromatic groups Highest stability of the clay and medium silt fraction Ludwig et al., 2003; Kalbitz et al., 2005; John et al., 2005; Leinweber & Kandeler in prep.; Eusterhues et al., in prep.

5. Working hypothesis 2 Organic C is stabilized by organo-mineral interactions and by complexation • Proportion of the mineral-bound OM increases with soil depth • 14C age of the mineral-bound OM increases from modern to >1000 years with increasing soil depth • In some soils the mineral bound OM is younger than not mineral associated OM • Relevance of other stabilization mechanisms? Kaiser et al., 2002; Kaiser & Guggenberger, in prep.; Rumpel et al, 2002; Eusterhues et al., in prep.

6. Working hypothesis 2 Organic C is stabilized by organo-mineral interactions and by complexation • Pedogenetic processes of mineral formation control strength of bonding and the amount of OM sorbed • Microporous oxide phases efficiently stabilizes OM, especially in acid subsoils • More than one bonding mechanism may operate in neutral soils: ligand exchange? + cation bridges Kahle et al., 2002, 2003, 2004; Eusterhues et al., 2005; Kleber et al., 2005, 2006; Kaiser & Guggenberger, 2003, 2006; Mikutta et al., 2006

7. Working hypothesis 2 Organic C is stabilized by organo-mineral interactions and by complexation • Surface coverage is discontinuous and specific surface area is not always a good predictor for C stabilization • Conceptual models Spatial orientation of organo-mineral interactions under different OC contents Self-assembly of OM into multilayered structures on mineral surfaces Kahle at al., 2002, 2003; Mikutta et al., 2005; Eusterhues et al., 2005; Ellerbrock et al., 2005; Kleber et al., subm.

8. Working hypothesis 2 Organic C is stabilized by organo-mineral interactions and by complexation • Stabilization by organo-mineral interactions operates at long-term scales and dominates during late decomposition phases and in subsoils • In the same soil/horizon several stabilization processes may be operative simultaneously on the long-term time-scales (e.g. spatial inaccessibility)

9. Working hypothesis 3 Organic C is stabilized through spatial inaccessibility for decomposing organisms We found: • Increasing stability due to occlusion in aggregates • Increasing stability of occluded OM with decreasing aggregate size • With decreasing aggregate size the stabilized OM shows also a higher recalcitrance • Aggregation is promoted by interactions with long-chain fatty acids Jandl et al., 2004; John et al., 2004, 2005; Yamashita et al., subm.; Helfrich et al., subm.

10. Working hypothesis 3 Organic C is stabilized through spatial inaccessibility for decomposer organisms • Most soil particles are hydrophobic • Hydrophobicity protects against degradation • Hydrophobicity is a major factor in aggregate formation and thus contributes to stabilization by occlusion of OM Goebel et al., 2002, 2004, 2005, Woche et al., 2005; Jasinska et al., in press; Jasinska et al., in prep. • C enrichment factors in the <6.3µm fractions are negatively related to the 14C activity indicating that C-stabilization in the subsoil horizons occurs in the fine particle size fractions within the passive pool • No indications of intercalation of OM were found Kaiser et al., 2002; Eusterhues et al., 2003; Rumpel et al., 2004

11. Working hypothesis 3 Organic C is stabilized through spatial inaccessibility for decomposer organisms • Reduced access for decomposer organisms due to their specialization on microhabitats and substrates • Decomposition in biologically active microsites is restricted by transport processes and gradients Ekschmitt et al., 2005, Poll et al., 2003, 2005; Poll et al., subm.

12. Working hypothesis 3 Organic C is stabilized throughspatial inaccessibility for decomposer organisms • Different stability of occluded OM results from simultaneously acting stabilization mechanisms (aggregation, hydrophobicity, recalcitrance) • Spatial inaccessibility becomes more relevant in subsoils within the turnover time frame of the passive pool • Some postulated stabilization mechanisms are not supported by analytical evidence (encapsulation, intercalation)

13. Management options We found: • Biomass of earthworms, microbial biomass and its activity are increased by organic fertilization indicating an increased active OM pool • Microbial 14C assimilation is more efficient under organic fertilization and thus causes less CO2 losses during mineralization (C-sink) • From the management perspective a large and efficient active pool is useful to stabilize OM and at the same time to profit form OM decay (nutrient cycling). PhD by Vogt, Marhan & Scheu, 2005

14. Management options We found: • Long-term fertilization (organic and mineral) results in enrichments of long-chain fatty acids from plant residues in clay and fine silt fractions and promotes aggregation • Innovative management practices with continuous residue input promote aggregation • Another management strategy to stabilize C in soils is the enhancement of the stable OM pool, e.g. through increasing the hydrophobicity of soil OM (reforestation, production of back carbon, organic fertilization) Jandl et al., 2004; Kaiser & Ellerbrock, 2005; John et al., 2005; Jasinska et al., in prep.

15. Methodological contributions • Functional identification of decomposer organisms (Egert et al., 2003, 2004; Selesi et al., 2005; Kramer & Gleixner, subm.; Kindler et al., 2006; Miltner et al., 2004) • Molecular approach to evaluate the gene expression of laccases (Luis et al., 2005) • Method to quantify Black Carbon in soils (Brodowski et al., 2005) • Quantification and identification of soil lipids (Wiesenberg et al., 2004; Jandl et al., 2002) • Identification of molecular lipid markers for C3/C4 plants (Wiesenberg et al., 2004; Wiesenberg & Schwark, 2006) • Isotope-selective sensing of soil-respired CO2(Hörner et al., 2004; Hörner & Löhmannsröben, 2006)

16. Methodological contributions • Qualitative and quantitative characterization of operational fractions by their pool size, composition and turnover time Sequential extractions (Kaiser & Ellerbrock, 2005; Ellerbrock & Kaiser, 2005; Ludwig et al., 2003; Wiesenberg et al., 2004; Rethemeyer et al., 2005) Mineral associated fractions (Eusterheus et al., 2003, 2005; Rumpel et al., 2002; Kaiser & Guggenberger, 2003, 2006 in prep.; Kleber et al., 2005) Literature reviews on the functionality of available operational fractions (Mikutta et al., 2005; v. Lützow et al., subm.) C-assimilation by microorganisms amounts 10% of the microbial biomass (Miltner et al., 2005)

17. Improvements in the parameterization of the Roth-C model: predictive modeling • Comparison of modeled pools with measured fractions • Evaluation of approaches to calculate the passive pool • Testing yield-dependent approaches for the estimation of C inputs Ludwig et al., 2003, 2005, 2006 New model approaches • C turnover model approach CIPS (Carbon turnover in pore spaces) relates C turnover to soil structure and thus to accessibility • Parametrization of a two compartment model approach to calculate the particle density of soils by considering properties of OM and the mineral matrix PhD by Kuka, 2005; Rühlmann et al., 2006

18. Development of a conceptual model • Integration of recent findings and differentiation of the passive pool Evaluation of the time scales of stabilization mechanisms in relation to conceptual model pools Identification of key stabilization mechanisms in different horizons Linking processes for pedogenesis to stabilization mechanisms v. Lützow et al., 2006; v. Lützow et al., subm.

19. Acknowledgements • DFG for financial support  All participants in the SPP  The review panel