Summary Band 28

Frank Liebe: Trace elements in soils and plants of North Rhine-Westphalia - Contents of different chemical soil fractions and their relationship to soil pH and contents in plants


In order to evaluate soils with very different contents of trace elements, 335 soil samples - spread over all of North Rhine-Westphalia - were taken. This project was sponsered by the ministery of environement and agriculture of North Rhine-Westphalia (MURL). The location of the soil samples are characterized by different soil uses (arable fields, grassland, forest, gardenland, and dump sites, locations in city areas, playgrounds) and different soil contaminations (none or less polluted till extremly polluted locations). Furthermore the soil samples contain a wide spectrum of different parent materials, type of soils, and compositions. They are not representative for the area of North Rhine-Westphalia. For the determination of transfer of trace elements from soil to plant, different plant samples were taken from arable field locations 89 wheat grain samples - e.g. from gardenland 17 samples of green cabbage.

The total contents of the elements in the soil and plant samples were extracted by aqua regia (Thallium: HNO3 extraktion). The mobilizable (potentially available) fraction was determined by a cocktail of 0,025 M EDTA, 1 M NH4OAc and 0,1 M NH2OH.HCl (pH 4,6). The extraction with an unbuffered salt solution (1 M NH4NO3, soil-solution ratio 1 : 2,5) was used to characterize the mobile fraction in soils. The concentrations of elements in the soil solutions were specified by analysing the soil saturation extracts. 18 elements (Al, As, Be, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Sn, Tl, V, Zn) were analysed with different spectrometric methods (AAS, ICP-OES) in the different extracts.

The analysed elements vary widely in their total contents, in their extractable proportions of the total contents, and in their mobility and availability in soils. The proportion of the mobilizable fraction (EDTA-Cocktail) in percent of the total contents of Cd, Co, Cu, Mn and Pb are > 30 %. On the other hand the proportions of the NH4NO3 extractable contents in percent of the total contents of Al, As, Be, Bi, Co, Cr, Fe, Hg, Pb, Sb, Sn and V lie mostly below 0,1 %, except for Ni, Zn, Cu, Mn, Cd and Tl (max. 3,5%). The average proportions of the watersoluble fraction (saturation extraction) in percent of the total contents of all elements are below 0,1 %, except for Cd reaching up to 1,25 % in strongly acidic forest soils. The contents of the different trace elements from different plant samples have a relatively small variation.

The element contents of the different soil fractions were closely correlated, when the fractions extracted similar binding forms. Therefore, mobilizable (EDTA-Cocktail) pool of trace elements is closely correlated with the total content of the various elements (Cd, Co, Cu, Mn, Pb, Zn). Highly significant relations between the NH4NO3 extractable contents and the concentrations in the soil solutions were found for As, Cd, Co, Cr, Pb, Sb, Mn, and Zn. Furthermore, the mobile fraction (NH4NO3) of the elements Al, Be, Cd, Mn, Pb and Zn can be defined as a function of the total contents, respectively mobilizable fraction of elements, and the pH of the soils. The watersoluble fraction (soil saturation) is also closely related to the pH of the soils

The NH4NO3 extractable contents of As, Cd, Cu, Mn, Ni, Pb, Tl, V, and Zn were also significantly correlated with the contents of these trace elements in the analysed plants (corn of wheat, green cabbage, leek, and grass). Therefore the extraction of soil samples with 1 M NH4NO3 is a suitable procedure to forecast the load of plants on the basis of soil data or to derive threshold/trigger values for trace elements in soils.

The data of this project can also be used to derive background contents for trace elements. Therefore, frequency distributions of element contents in soils were established with the Kernel Density Estimation. With this procedure background levels can be determined per element for corresponding groups of soil samples being characterized by common features (e.g. parent material, region of origin, specific source of pollution). Typical NH4NO3 extractable contents of several trace elements can also be derived in the same way. These typical contents of the mobile fraction are given for different total contents of trace elements and different pH of the soils.