*2.3. Sample Preparation*

In the laboratory, soil samples were dried for 12 h in an oven at about 50 ◦C, followed by manual cleaning using a plastic tweezer to remove roots, stones, glasses, snails, earthworms and insects. Thereafter samples were sieved for 20 min on a 2 mm nylon sieve using a Retsch AS200 shaker (Retsch, Düsseldorf, Germany). Only the fraction of soil

with particles size less than 2 mm was taken. The resulting sample was homogenized and divided by the quartile method into four parts.

Two portions of each quartile were dried at 105 ◦C, for 24 h in porcelain capsules. After complete drying, soils were stored in desiccants until grinding. Grinding was performed on a Retsch S100 ball mill of agate grinding jars and agate balls for 5 min at 400 rpm. After milling, the diameter of the soil particles was less than 1 μm. About 50 cm<sup>3</sup> of each of the 12 samples from each local and each year were stored and used as the composite sample of that local and that year. Two portions of the composite sample, of about 100 g each, were used to perform the duplicate analysis of the studied metals.

## *2.4. Soil Characterization*

Composite samples of the first (2004) and last (2011) years, from each site were characterized in terms of pH, conductivity, humidity and organic matter content.

The procedure for pH measurement was conducted according to ISO 10390:2015 [28] and involved the preparation of a 10 g of soil in 25 mL of CaCl2, 0.01 M suspension, followed by pH measurement with a calibrated pH meter. For conductivity measurement a 20 g of soil in 100 mL of ultrapure water suspension was used. The humidity of the samples was determined according to ISO 11465:1993 [29] with the samples to be heated at 105 ◦C until constant weight. The humidity was then calculated by the weight difference between the initial and the dried sample. The determination of organic matter according to European Standard CEN/TC [30] involved heating and calcining the samples at 550 ◦C for the complete destruction of organic matter. This procedure was performed after the determination of sample humidity and the samples were calcined until constant weight. The organic matter contents were calculated by the weight difference between the dried and the calcined samples.
