The Copper Content in Soil of Olive Orchards from Dalmatia, Croatia

The goals of this study were to determine the concentrations of Cu in the soil of olive orchards grown on the Terra rossa and to examine how close the Cu concentration is to contamination limits defined by different soil quality standards. A total of 40 composite topsoil (0–25 cm) samples were collected in the traditional dry farming and highly fragmented olive orchards that covered an area of 3200 ha and located in the Middle Dalmatia, Croatia. The aqua regia Cu concentration was determined by ICP-OES; and basic soil properties (pH, carbonates, SOC, and particle size distribution), by standard analytical procedures. The mean value of Cu concentration of 68.4 mg kg–¹ was more than twice as high as the mean values of the naturally occurring concentration of Cu in the Terra rossa soil. High mean value and a wide range of copper in the soil (33.8–250 mg kg–¹) are due to the long-lasting application of Cu-based fungicides related to the fact that the olive growing area and vines overlap. The Cu contamination risk assessment in olive orchards was calculated by comparing the detected concentrations of Cu with the soil quality standards prescribed by the Finnish Decree and Croatian Ordinance and using the threshold value of the regional background data on Cu concentration in Terra rossa soil. According to the Finnish Decree, in 7.5% of cases, Cu concentration in soils exceeded the threshold limit of 100 mg kg–¹, which indicates the need for further assessment of the potential contamination of the area. In 7.5% of cases, Cu exceeded the critical limit of 150 mg kg–¹ that presents an ecological risk. By applying the Croatian Ordinance, in 10% of cases, the concentration of Cu exceeded the maximum admissible concentration of 120 mg kg–¹, so that the soil can be considered contaminated and unsuitable for agriculture. The regional threshold value for the background concentration of Cu in the Terra rossa soil amounted to 55 mg kg–¹, and its use in 47.5% of cases indicates the need for further assessment of the potential contamination in the area. The presented results showed significant differences between national soil guideline values (SGVs) and the regional threshold value for Cu and confirmed the fact that there is no general and accurate reference method for soil contamination assessment that can be applied at all sites. The use of threshold values derived for a particular soil type that dominates in a given area would be more reliable than the national soil quality standards. Research has pointed to the problem of soil contamination with Cu under olive orchards and has highlighted the need for studying the given issue in the areas, where olives share space with vines, and in particular, where mixed or consociate cropping system is present.