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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
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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
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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

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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.
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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 gt1000 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.
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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
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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.
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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)

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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.
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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 lt6.3µm 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
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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.
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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)

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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
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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.
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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)

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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)
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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
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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.
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Acknowledgements

• DFG for financial support

? All participants in the SPP
? The review panel