Distribution and availability of copper in aggregate size fractions of some calcareous soils

Purpose The difference in copper (Cu) availability between soils can be a result of different distribution of Cu forms in various size fractions of aggregates. This study aimed to determine different Cu forms in bulk soils and aggregate size fractions of some heavy metal-contaminated soils from Isfahan Province, Iran and to examine the relationship between Cu forms associated with different soil aggregates and plant indices. Materials and methods Bulk soil of five contaminated soils was partitioned into four aggregate size fractions (2.0 to 4.0 mm (large macro-aggregates), 0.25 to 2.0 mm (small macro-aggregates), 0.05 to 0.25 mm (micro-aggregates), and <0.05 mm (mineral fraction)) by dry sieving. Copper was fractionated into soluble and exchangeable (F1), carbonate-bound (F2), Fe-Mn oxide-bound (F3), and organic-bound (F4) by Tessier's method. Copper concentration and dry weight of shoots and roots of corn (corn indices) were determined in a pot experiment to assay the Cu availability in the five studied soils. Relationship between the Cu bound to different chemical fractions in different size aggregates and corn indices was assessed using cluster analysis. Results and discussion The results showed that the 0.25-2.0-mm fraction, with the highest mass percentage in the soils, had higher contribution to the total content of Cu in the bulk soils. Copper was mainly associated with the organic-bound and residual fractions in the bulk soils and aggregates. Principal component analysis (PCA) represented that the distribution patterns of Cu chemical fractions in different aggregates were strongly related to the soil type. The study of relationship between Cu fractions and the corn indices demonstrated that the organic-bound fraction of Cu in 2.0-4.0 mm aggregates was remarkably correlated with the Cu concentration in corn root and suggested that the organic-bound fraction of Cu in larger aggregates constitutes the chief plant-available Cu pool in the soils. Conclusions Soil type and aggregate size distribution were important factors controlling availability and distribution patterns of Cu fractions in studied soils. The organic-bound fraction of Cu in the larger aggregate fractions appeared to be more readily available for plant than in the smaller aggregate fractions. Therefore, soil aggregate size fractionation can be used to assess the distribution, bioavailability, and environmental hazard of Cu in soils.