Monday, 10 July 2006

Evaluation of Carbon Stabilization in Soils: A Conceptual Model Approach.

Margit v. Lützow1, Ingrid Kögel-Knabner1, Egbert Matzner2, Klemens Ekschmitt3, Georg Guggenberger4, Heiner Flessa5, Bernd Marschner6, and Bernard Ludwig7. (1) Technische Univ München, Lehrstuhl für Bodenkunde, Wissenschaftszentrum Weihenstephan, München, D-85350, Germany, (2) Lehrstuhl für Bodenökologie, Univ Bayreuth, Bayreuth, D-95440, Germany, (3) Justus Liebig Univ Giessen, Giessen, D-35392, Germany, (4) Univ Halle, Institut für Bodenkunde und Pflanzenernährung, Halle, D-06108, Germany, (5) Univ Göttingen, Institut für Bodenkunde und Waldernährung, Göttingen, D-37077, Germany, (6) Ruhr Univ, Geographisches Institut, Bochum, D-44780, Germany, (7) Univ Kassel, Dept of Environmental Chemistry, Witzenhausen, D-37213, Germany

Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. The mechanisms for C stabilization in soils are still not well understood and the ultimate potential for C stabilization in soils is unknown. Current soil OM turnover models are not fully process-orientated and thus the simulation of ecosystem response to environmental changes like management and the impact of changing climate still is difficult. We developed a conceptual model that integrates recent findings and a range of concepts of stabilization mechanisms especially within the passive pool that are described in the literature. The evaluation of stabilization mechanisms is demonstrated by regarding data of pool sizes and turnover times (14C) of operational soil OM fractions in relation to total soil OM and by linking environmental conditions for processes in pedogenesis to stabilization processes. In different soil horizons only a few key stabilization mechanisms are operative and this knowledge is important to exclude alternative interpretations for the evaluation of composite fractions. Fractionation concepts include: Light fraction (LF <1.6 g cm-3) that is controlled by recalcitrance & aggregation; demineralisation with hydrofluoric acid (HF): HF insoluble OM that is also controlled by recalcitrance & spatial accessibility; Dense fraction (DF >1.6 g cm-3) and HF soluble fraction as mineral associated fractions; OM resistant to hydrogen peroxide oxidation (H2O2) to describe spatial inaccessible OM. Results are demonstrated for a Dystric Cambisol soil from Germany. The analyses show that in the A-horizon stabilization of OM within the active and intermediate pool is dominated by selective preservation and resynthesis and occlusion in aggregates while the relevance of organo-mineral interaction is low. In B- and C-horizons the importance of organo-mineral interactions increases and is in the time frame of the passive pool. pH 4.3-1.0 in B- and C-horizons is optimal for ligand exchange. Highest amounts of pedogenic oxides are found in the Bw-horizon. The pool size of not mineral-associated fractions decreases in subsoils (HF insoluble fraction and LF) while in the case of the HF-insoluble fraction this not mineral-associated fraction is even older than the mineral associated HF-soluble fraction. The fraction of OM resistant to H2O2-oxidation increases with increasing soil depth. The results of HF-demineralisation and H2O2-oxidation demonstrate the increasing importance of spatial inaccessibility for the stabilization of OM within the passive pool in subsoils. Changes of the relevance of different stabilizing mechanisms in dependence of soil forming factors are discussed and must be considered in new soil specific model structures.

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