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Die Bedeutung des Bodenskeletts als Speicher für kurzfristig verfügbare Nährelemente


Abstract:

For soil chemical analyses, the soil skeleton is normally rejected because this size fraction is considered to have no significant short-term nutritional potential. In order to revise this practice, the short-term potential for ion storage and ion mobilization of the isolated and cleaned soil skeleton was investigated by model experiments, using undisturbed and homogenized soil samples as references. The cleaned skeleton was embedded in an inert quartz-silt-matrix ("fine earth substituted soil systems"). The study considered different soil profiles on granite, gneiss and sandstone bedrock from Black Forest, Germany.

The method allowed for the investigation of soil columns at a water status near field capacity. After the extraction of water soluble ions with deionized water, cation exchange properties were determined by percolation of the soil cores with ammonium chloride (NH4Cl).

The results revealed site-specific ion mobilization potentials of the soil skeleton. Below the A-horizon, the skeleton fraction of the gneiss site plays the dominant role as a source for short-term base cation supply. The fine earth of the corresponding soil horizon had lost this function, since the base saturation was less than 5 %. More than 80 % of the exchangeable Ca and Mg in naturally layered soil cores originate from the skeleton. The skeleton of the granite site had much lower ion mobilization rates, but nevertheless, due to the high skeletal contents in soil the importance for ion mobilization must not be neglected. The soil skeleton of the sandstone site showed cation exchange capacities which were comparable to the gneiss site, but its ecological importance is less because of the low skeleton content in soil.

Mineralogical studies of the stones supported the findings: similar to decayed teeth, we found cavities in the gneiss stones filled with vermiculized fine material of high base saturation. With micropedological approaches we could show that fungal hyphae grow into these micro habitats. But the occurrence of hyphae in stones was not limited to the weathered zones. Hyphae were also found growing inside stones without the existence of any recognizable weathered zones (rock eating fungi?). Furthermore, hyphae clusters were noticeable in the transitional area between stones and bordering soil matrix, and macropore spaces respectively. We think that these findings close the gap of plausibility which often arises if most unfavorable chemical properties of the fine earth are related to a normal tree nutrition.


Fig.: Thin section of soil skeleton from the B-Horizon (depth 60 cm) of the ”Conventwald” site (particle size class 7-15 mm; stereoscopic photograph, 50 times magnified). The net-shaped weathering zones (black) are clearly visible. Along these zones the clay mineral “Vermiculite” or transition minerals between “Chlorite” and “Vermiculite” were identified by using x-ray diffraction.

In further model experiments, we studied the influence of different CO2-partial pressures in soil air on ion release of skeleton-rich, naturally layered soil samples from the Cv-horizon at the ‘Conventwald’ site. For the control of the CO2-partial pressure, an advanced percolating system was used. Equilibrium soil pore solution (ESPS Aqua dest. 48h) was obtained for the investigation of short-term ion release. Long-term studies were additionally led, in which an annual percolation rate of 800mm was simulated in an open percolation system.


Fig.: Percolation system with controlled CO2-concentrations in the soil atmosphere.

The short-term as well as the long-term investigations demonstrate which key-role the CO2-partial pressure plays for the ion mobilization in the transition area between the pedosphere and lithosphere. Weathering rates were calculated for the assessment of ion release in long-term studies. These rates were considerably greater than the published silicate weathering rates. The rate of mobilization of Ca2+ and Mg2+ can be explained not only by silicate weathering but also by the dissolution of fast-dissolving solid phases and exchange processes. This could be shown with help of the first order reaction models which parametrized the ion release observed in long-term investigations. With help of the ion balance, an approach was derived where the percentage of the fast-dissolving solid phases and silicate weathering can be quantified. In this case, it appears that the mobilization of fast-dissolving solid phases as a fixed value proceeds, to a large extent, independently of CO2-partial pressure in the soil air. A silicate weathering rate of 0.5 kmol ha-1 a-1 was determined for the control treatment (PCO2 0.00 bar). A distinctly higher release rate was calculated for the treatment with soil-typical partial pressure (PCO2 0.01 bar) with ca. 6 kmol ha-1 a-1. This value cannot be exclusively described by silicate weathering. Rather, it can also be explained by the displacement of Mb-cations with Al on exchanger places.

In conclusion, possibilities of integrating ion storage and mobilization potential of the skeleton in the soil chemical steady-state models, PROFILE and BODEN is shown in the “Conventwald” site example. Until now, the skeletal fraction was left out of consideration in both models. It was considered as an inert matrix that “thinned” the chemical properties of the fine earth. The integration of the skeletal fraction led to a change of the modelling results in both models. In comparison to conventional procedures, PROFILE calculated a weathering rate increased by 30 %. Using the BODEN model, it was shown that at the actual acidic deposition the acidification front advances distinctly slower within the subsoil when the skeletal fraction is considered.

Budget: DM 220 000.-


Publications:

KOHLER, M. (2001): Ionenspeicher- und Ionenmobilisierungspotentiale der Skelettfraktion von Waldböden im Schwarzwald. Freiburger Bodenkundliche Abhandlungen, Heft 39, 158 S.

KOHLER, M., K.v.WILPERT, E.E. HILDEBRAND (2000): The Soil Skeleton as a Source for the short-term Supply of "Basic Cations" in Forest Soils of the Black Forest (Germany).Water Soil Air Poll, Vol 122, 37-48.

KOHLER, M. and HILDEBRAND, E. E., (2000): Micropedological investigations for the opening up of soil by mycorrhizal fungi with special consideration of the skeleton fraction on a Black Forest gneiss site. Swiss Society of Agronomy. Abstract of the SSA-Workshop “Mycorrhiza and Root Research in Switzerland”, 2000; Bulletin 14

KOHLER, M. and HILDEBRAND, E. E., (2000): Influence of CO2-partial pressure in soil atmosphere on ion mobilisation at a gneiss site in the Black Forest, Germany. Proceedings, International IUFRO-Conference “Forest Ecosystems Restoration 10.-12. April 2000 in Vienna.

KOHLER, M. und HILDEBRAND, E. E.(1999): Zur Bedeutung des Bodenskeletts von Waldböden des Schwarzwaldes als Speicher und Quelle für kurzfristig verfügbare Neutralkationen. Berichte Freiburger Forstliche Forschung. Heft 7, S. 115-120.

HILDEBRAND, E. E. und KOHLER, M. (2001): Bestimmung der Silikatverwitterung aus Modellversuchen? Berichte Freiburger Forstliche Forschung (in press).


Related Project: Effects of Rock Powder Application on Soil Solution Chemistry:


Publications:

E.E.HILDEBRAND, H. SCHACK-KIRCHNER (2000): Initial Effects of lime and rock powder application on soil solution chemistry in a dystric cambisol - results of model experiments. Nutrient Cycling in Agroecosystems, 56, 69-78.