*2.2. Sampling Procedure*

Soil sampling was performed according to ASTM E1727-04/05, Sampling and analysis of soil, part IIIa recommendations [27]. Soil sampling was carried out in November of each year. Samples were collected following a concentric profile around a tree in which leaves were also collected for analysis. The soils were collected according to the four cardinal points (north, south, east and west) at 1, 3 and 5 m radius circle. The total area covered by this systematic sampling procedure was 100 m2. Soil samples were collected at a depth of 20 cm, using plastic (PVC) tubes with 3.5 cm in diameter and stored in black plastic sampling bags. Sample handling was always performed with plastic instruments in order to ensure non-contamination of the soils by metals from usual sampling tools. Each collected sample was identified with a label encoding information of the sampled site, year, cardinal point and distance to the tree, in order to ensure full traceability of the samples. A total of 648 soil samples were collected in the six sampling sites of the city and for nine years.
