Saturday, 15 July 2006
137-2

Scale Dependent Physical Protection of Organic Matter in Soils.

Axel Lamparter, Jörg Bachmann, Marc-Oliver Goebel, Susanne K. Woche, and Walter R. Fischer. Institute of Soil Science, Herrenhäuser Str. 2, Hannover, Germany

Predicting the impact that future land management will have on soil carbon content requires an understanding of the mechanisms controlling organic matter stabilization. In general, the stabilization of organic matter in soils is dependent on i) the chemical structure of the molecules (recalcitrance), ii) the chemical milieu (pH, nutrients), iii) the stability of soil structure, and iv) the availability of water, air, and the organic matter itself. The two last factors should be referred to as “physical stabilization”. The physical stabilization of organic matter in soils can be related to the soil properties, i.e. texture, bulk density, aggregation and wettability (hydrophobicity), which can be characterized in terms of the Contact Angle (CA) that forms at the three phase boundary (solid-liquid-gas). Physical stabilization processes are strongly scale-dependent. Especially processes regulated by the soil's wettability (e. g., capillary rise, preferential flow) have a different impact on the different scales regarded. On the micro-scale, i.e. the pore- and particle scale, hydrophobic surfaces are not wetted completely, leaving parts of the soil particle surface dry. In partly saturated porous media, discontinuous water films arise which reduce the availability of nutrients and confine the local diffusion of enzymes. Experiments indicate that hydrophobic soils have reduced respiration rates in comparison to hydrophilic soils (organic matter is protected). On the aggregate scale, two mechanisms are acting simultaneously. Hydrophobic SOM favours the formation and the protection of stable aggregates which, in turn, encapsulate the organic carbon. The organic matter, located inside the aggregate, is directly protected due to the inaccessibility of the small pores for microorganisms. Hydrophobic aggregates show lower water sorption rates than hydrophilic aggregates. The slower uptake of water keeps the air pressure low, the aggregate will be more stable and protected against slaking. By this effect the organic matter is protected additionally (indirect stabilization). On the soil profile scale hydrophobic soils have mostly been associated with preferential flow phenomena. It was found that sometimes more than 80% of the draining water is transported through preferential flowpaths, bypassing large areas of the soil which results in an incomplete wetting of the soil. In dry regions of the soil the organic matter is protected from decomposition due to the absence of water as a key factor in biodegradation. All the processes mentioned above are contact angle dependent and reduce potentially the decomposition of SOM in case of reduced wettability. Experiments were conducted to evaluate the importance of the CA in stabilizing the organic matter on different scales in soils. We tested the hypothesis that the composition of the Soil Organic Matter (SOM), characterized in terms of the wettability, protects organic carbon from decomposition by microorganisms. In our presentation we will propose a conceptual stabilization model describing physical protection processes and we will present examples for the stabilization on each scale. In real soils, however, these processes act simultaneously and may lead to a combined and enhanced effect, which may lead to a higher stabilization effect as can be estimated from analysis of each single process. We conclude that physicochemical properties of the SOM are essential for a quantitative understanding of the role of the organic phase in terrestrial ecosystems.


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