Study of the copper distribution in contaminated soils of hop fields by single and sequential extraction procedures

Abstract

Hop growing in Bavaria requires the use of copper containing fungicides against mildew, which results in an accumulation of copper in the upper soils to concentrations of up to approximately 450 mg kg −1. The naturally occurring concentration of copper is approximately 10–15 times lower. Several single and sequential extractions were performed in parallel to the determination of total copper content after aqua regia digestion. By these means copper mobility and retention, its availability to plants as well as its distribution in soils was assessed. Three agricultural soils and two soils from fallow land have been analysed to elucidate whether the copper distribution changes in relation to agricultural activities and to predict environmental risks due to the high copper concentrations. This work also focuses on the difficulties arising from different results when using single and sequential extraction procedures. EDTA, acetic acid, deionised water, ammonium nitrate and calcium chloride were used as single extractants for the functionally defined speciation. EDTA and acetic acid can be regarded as extractants for the plant-available species, while deionised water, NH 4NO 3 and CaCl 2 only release electrostatically weakly bound metals to estimate the mobile species. The extraction behaviour of copper in these procedures is discussed with respect to the quantities extracted from the contaminated and non-contaminated soils. The studies indicate that no vertical copper translocation is observed in the ground, but the main amounts are highly available to plants. Sequential extraction procedures were used for operationally defined speciation, quantifying copper in specific soil phases. The ‘five-step extraction scheme’ first applied, suggested by the Standard Measurement and Testing Programme of the European Community, overestimated the amounts of copper bound to the residual phase. This was mainly due to the non-specific nature of the reagents for the organic/sulfidic and crystalline iron oxide fraction. A second reason could be the fact that this scheme was developed for sediment and not for soil analysis. Therefore, the sequential extraction has been applied using an adapted method of Zeien and Bruemmer, using different extractants for the two phases. The copper distribution was different, showing that approximately 40–50% of the total copper was bound to the organic fraction and approximately 15–30% could be extracted with the iron oxides. The amounts in the residual fraction were much lower. After comparing the results of the different extraction procedures the environmental behaviour of copper and its risks for plants, micro-organisms and groundwater is discussed in brief.