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Abstract "Heft 13"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 13

Ernst Segatz

Pedologisch-ökologische Untersuchungen zur Problematik der Rekultivierung von
Trockenbaggerungsflächen im Hardtwald des Oberrheintals

Freiburg im Breisgau 1984

ISSN 0344-2691


The sands and gravels of the upper Rhine valley are valuable raw materials for the construction industry and so are excavated in ever increasing amounts from forest areas. The recultivation of such excavated sites has proven to be a problem.
In order to determine the changes in the soil and its suitability as a forest site from the time before dredging up to completed recultivation, 15 profiles of undisturbed soils and 27 profiles of recultivated soils were selected for investigation. These profiles were located within four areas of heavy dredging on the lower terrace of the Rhine river in the region Rastatt - Karlsruhe -Philippsburg (Hardtwald).
The undisturbed soils on the lower terrace were developed from predominantly sandy parent material. On originally more calcareous substrate clay translocation occurred leading to the typical profile of a "Parabraunerde" (grey brown podzolic soil - Hapludalf) with an accumulation of clay (and silt), sesquioxides, and nutrient elements in the illuvial horizon. "Braunerden" (acid brown forest soil -Inceptisol) with typical clay, sesquioxide and available nutrient contents decreasing with depth, developed on the weakly calcareous or non-calcareous sands.
The recultivated soils can be roughly divided into the main types I and II: Type I developed without re-use of the displaced soil, i.e. without mixing of the raw soil with pedogenetical1y altered material. On these sites recultivation occured directly on the sands and gravels of the excavated surfaces. Hence, these are raw soils with predominantly high pH values in the initial stages of soil development. The extent of their weathering is slight as is shown by the low clay content, the minimal proportion of ses-quioxides and of readily available nutrient elements in relation to the total contents (with the exception of Ca and Mg).
The more widespread Type II soils developed from formerly excavated soil material anthropogenetically deposited onto the raw excavated surface of the dredged areas. These two layers differ sharply in many chemical and physical properties. This type of "Kultosol" will be here designated as a two-layer recultivated soil.
Apart from Type I and Type II there exist all possible transitions even to the extent that the different substrates are almost blended; multi-1ayered soils are also frequently found.
The natural soils are mainly free of coarse fragments; the proportion of fine fragments reaches a maximum value of 25% by weight in the subsoil.
The raw soils (Type I) or the "subsoil" of the two-layered profiles (Type II) often have a coarse fragment content of ca. 20% by weight and a fine fragment content of 30-4 0% by weight (total fragment content 50-60% by weight).
The topsoils of the natural soils contain about 70% by weight sand within the fine earth fraction (of this ca. 70% by weight is medium sand fraction), about 20% by weight silt, and 10? by weight clay. In contrast the raw soils (Type I) and the IlCn horizons (subsoils) of the two-layered soils (Type II) are characterized by proportions of sand in part up to 98% by wei ght.
The density of undisturbed soils is 1,5 g/cm3 at 50 cm depth, whereas the top re-deposited layer of Type II shows a density of more than 1,6 g/cm3 at the same depth; densities of up to 1,9 g/cm3 were also found. The volume proportions of the large and fine pores are correspondingly reduced here in conjunction with an absolute and a relative decrease in the large pore volume with depth. This reduction as well as the discrepancy in pore size at the boundary between the deposited layer and the excavated surface (Type II) increase the usuable field capacity, but also lead to poor drainage after heavy precipitation.
The pH values of the natural soils lie generally around 4, while those of the raw soils of Type I and the subsoils of
Type II are in part higher than 8 due to their calcium carbonate content.
The change from the acid to the alkaline layers is often limited to a span of a few centimeters so that the soil depth is physilogically limited.
While the humus contents of the top soil of the natural soils and of the deposited layer of Type II soils average around 1% by weight, humus is generally lacking in the raw soils (Type I) and the raw subsoils (Type II). The total nitrogen contents of 1 g/kg in the developed soils compare to only traces of nitrogen in the raw material (Type I and subsoil of Type II). The supply of nitrogen increases with the amount of deposited material.
The nutrient element phosphorus yields no clear difference in total contents for the topand subsoils. The total contents of calcium, magnesium, and potassium (sodium) are in part higher in the sediment of the subsoil as in the pedogenetically altered substrate (leaching). The supply of citric acid soluble nutrient elements varies over a wide range, usually lying above the levels of the naturally developed soil. If the substrate is calcareous (Type I, subsoil of Type II and blended material) the well-known difficulties in nutrient element availabilities can occur (K : Ca antagonism, reduced availability of certain elements).
In order to gain information for future recultivation, the root development among the young pine trees was investigated. The following parameters were measured, root count and root density, total root cross section area, "root core" according to BARNER, sum of root depth, sum of root spacing and "degree of root utilization" according to BARNER.
Relationships between the soil analysis data and the corresponding root parameters taken every decimeter were tested for the natural and recultivated soils. Highly significant correlations exist between the root parameters and the contents of organic matter as well as the contents of clay (water retaining capacity) and phosphorus (stimulation of root development).
It was shown, that the more widespread recultivation soils of Type II allowed only an incomplete root development in comparison to the undisturbed soils, due to the mechanical and physiological barrier effect at the boundery of the deposited layer and subsoil.
Soils with a more or less thorough mixing of the various substrates show a better root development at greater depth than did the two-layered soils.
No relationship between stand age and depth related root parameters turned up for the collective of 5 - 15 years old stands. However, significant correlations occurred between various root parameters and the thickness of the deposited layer. For 19 recultivation stands a significant positive correlation was found between height growth and deposited layer thickness and its resultant greater available field capacity. The available water supply is the key factor in the success of any recultivation.
Various guidelines for future recultivation practice can be concluded from the root parameters:
Along with the A and B horizons of the naturally developed soils, the upper pedogenetical1y altered parts of the C horizons (BvC and Cv) should be conserved, if technically feasible, and re-deposited, after dredging is completed. The separation of horizons upon removal and redeposition of soils is not required and may even have negative effects due to the unavoidable production of more layer boundaries within the soil.
If possible, excavation methods should be employed that minimize driving across the surface of the excavation pit. Any additional planing of the excavated surface should be avoided. Before re-deposition of removed soil, the underground of the excavation pit should be loosened as deeply as possible.
At the same time, a large scale ameliorative blending of the redeposited soil with the raw sediments of the underground should be attained. This practice should be most successful on sandy subsoils with low fragment contents.
Theoretically the greatest possible re-deposition layer thickness should be promoted as regards improvement of the water budget and stand stability. In practice this is usually not possible due to lack of sufficient material. The minimum thickness of the deposited layer - as is only seldom attained in practice - can be estimated from the relationship between the potential retaining capacity of water available to plants (nFK) and height growth. This corresponds to about a depth of 1 m (120-140 mm nFK) for the dominant texture in the investigated area.
The height growth of stands on Type II recultivated soils with such a thick deposited layer is comparable to that for the same aged stands occuring on natural sites.
The originally proposed goal of improving dry "Hardtwald" sites by tapping groundwater proved unattainable due to the great fluctuations in ground water level. Groundwater related demages occur even years after a recultivation has been considered successfully established. Therefore sufficient rooting depth above the fluctuating ground water level must be present for recultivation to succeed.
The soil of the deposited layer should be loosened before recultivation. Here, the normal agricultural techniques are insufficient because they only result in surface loosening; the lower layers remain compacted and impede drainage. Hence, deep loosening methods are required.
A final reforestation with timber trees should be preceded by methods of biological soil preparation. For example, the planting of root intensive, nitrogen fixing legumes prior to planting the timber crop should be considered. A longer pre-forest period taking potential natural succession into consideration would be optimal. The production losses occurred when observing site improvement techniques must be taken into account and be balanced by the re-establishment of site productivity before a decision to excavate is made.

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