Evaluation of single extraction procedures for the assessment of heavy metal extractability in citrus agricultural soil of a typical Mediterranean environment (Argolida, Greece)

Abstract

Heavy metals are natural soil constituents; however, the intense use of agrochemicals can increase total contents above background levels as well as the available fractions of the more toxic elements. In this study, the occurrence of some metals was investigated in agricultural soils from an intensely farmed rural area of Greece (Argolida) aiming to evaluate the various available pools and examine their relationship with the reactive and pseudototal soil fraction. Thirty soil samples were selected from a large database of a previous geochemical survey in the study area. The pseudototal (aqua regia), reactive (0.43 M HNO3), potentially phytoavailable (0.05 M EDTA), mobilizable (0.43 M HAc), and mobile (0.01 M CaCl2) pools of Cu, Zn, Cd, and Mn were determined. Soil properties were also examined including pH, total organic carbon, calcium carbonate content, and amorphous Al, Mn, and Fe oxides. In order to combine all geochemical information and elucidate the association between available metal pools and soil chemical characteristics, multiple linear regression was employed. The various proportions of available metal pools (%) were expressed as a function of the pseudototal or reactive metal content and general soil properties. The mobile pool of trace elements in the studied soils was quite low, while notable amounts of Cu were released by EDTA indicating a greater tendency for complexation. The acetic acid extraction yielded increased percentages of Cd and Mn and for these elements the reactive pools were quite representative of the pseudototal content. Extractable amounts of Zn by all the applied reagents were very low indicating that a considerable part of this metal is of geogenic origin. The empirical regression models showed that prediction of the available pools of Cu, Zn, and Cd from the pseudototal and reactive content is feasible. Although pH is known to be a key factor of at least exchangeable metal pools, its narrow range blurs its influence in the present study. The relatively low extraction yields of the studied elements show that the potential hazard of metal transfer to plants in the area is minimal despite some elevated pseudototal concentrations. The geochemical reactive and various available pools of Cu, Zn, and Cd in Mediterranean calcareous citrus soils can be determined by a single soil extraction test using dilute nitric acid.