COMPARISON AND EVALUATION OF EXTRACTION MEDIA AND THEIR SUITABILITY IN A SIMPLE-MODEL TO PREDICT THE BIOLOGICAL RELEVANCE OF HEAVY-METAL CONCENTRATIONS IN CONTAMINATED SOILS

The concentrations of heavy metals in agricultural soils are increasing gradually from various diffuse sources. The metals can transfer from contaminated soils into the growing plants or may affect the respiration rate of soil microorganisms. In the first part of this paper, the following simple model to predict the biorelevant metal concentration in anthropogenically or artificially contaminated soils is reported: log M(P) = alpha + beta log [M(NaNO3)] log M(r) = alpha' + beta' log [M(NaNO3] log M(P) = log Metal concentration in test plant (mg/kg dry weight) log M(R) = log percent Redution in the respiration or enzyme activity [M(NaNO3)] = Metal concentration in NaNO3-soil extract (mg/kg soil) alpha and beta & alpha' + beta' = Intercepts and slopes of the linear regression lines. The model has been tested in laboratory experiments, in growth experiments either in a greenhouse or in the field. Also, an attempt is made to present a theoretical interpretation. Critical concentrations are calculated with the help of the relationship between metal concentration in soil solution [M(NaNO3)] and metal concentration in plants. The critical concentration, which is able to induce either phyto- or zootoxic concentrations in the plant parts (productive or vegetative) or reduce their growth or to reduce significantly the growth of soil microorganisms, are found to be uniform in a wide range of soils (pH 3 to 8 and CEC 10-50 millimol/100g soil). the critical concentrations of NaNO3 in soil extracts are comparable with the values obtained with plant growth experiments in nutrient solution and also with the concentrations obtained from soil respiration experiments reported in literature. In the second part of this paper several salt extraction media such as NaNO3, CaCl2, NH4NO3, KNO3 along with strong acid and chelating agent are evaluated, compared and discussed with respect to prerequisites for selection of an extracting medium. Chelating agent and strong acid extraction media extract more metal from soil than salt extraction media and are unsuitable for the prediction of either the phyto-and/or zootxic concentration in the plants or the reduction in respiration of soil microorganisms. The evaluation clearly shows that in spite of the fact that the metal concentrations in NaNO3-extracts are lower than in other salt extracts, NaNO3 fulfils most of the prerequisites and is one of the best available choices to predict biorelevant metal concentration in soils and indicates that it is most suitable for the risk assement of metals in soils. Comparable data for other salt extraction media tested in this paper are not available in literature.