A range of experiments was conducted between 2011 and 2013 in several sites within and nearby the city of Bologna (Fig. 1). Prior to experimentation, soil samples were collected from all study sites and analyzed as follows. Soil samples were air-dried and sieved (<2 mm). pH (pHmeter, Crison, Barcelona, Spain) measures were performed with distilled water on 1:2.5 w/v. Total organic carbon was measured by Dumas combustion with a EA 1110 CHN elemental analyzer (Thermo Fisher Scientific, Waltham, MA, USA) after dissolution of carbonates with 2 M HCl, and the organic matter was obtained using 1.72 factors. Soil particle size distribution was determined by the pipette method (Gee and Bauder 1986) and total carbonates (CaCO3) were quantified by a volumetric method, according to Dietrich-Fruehing. The sites used for experimentation were as follows:
A rural control (coordinates 44° 28′ 33″ N, 11° 40′ 45″ E) from now on called CONTROL/RURAL, located nearby the small town of Medicina (about 16,000 inhabitants, 35 km from Bologna, known as a vegetable crop cultivation area). The soil is UDIC CALCIUSTEPT Fine Silty, Superactive, Mesic (SSS 2014), Cambic CALCISOL, Siltic, Hypocalcic (IUSS 2014), with the following physico-chemical properties: sand (2–0.05 mm Ø) = 120 g kg−1, silt (0.05–0.002 mm Ø) = 580 g kg−1, clay (>0.002 mm Ø) = 300 g kg−1, pH = 7.8, organic matter = 23.2 g kg−1, total CaCO3 = 90.7 g kg−1.
A traditional garden within the old city center (coordinates 44° 29′ 16″ N, 11°20′ 51″ E, from now on called CENTRE), in the old district where few ancient traditional gardens still exists. The soil is UDIFLUVENTIC HAPLUSTEPT Fine Silty, Superactive, Mesic (SSS 2014), Terric, Calcaric, CAMBISOL, Siltic, (IUSS 2014), with the following physico-chemical properties: sand = 190 g kg−1, silt = 560 g kg−1, clay = 250 g kg−1, pH = 7.9, organic matter = 18.3 g kg−1, total CaCO3 = 120.5 g kg−1.
Two gardens nearby the main railway (about 800 trains per day): a traditional one (coordinates 44° 30′ 17″ N, 11° 21′ 28″ E, from now on called RAILWAY/SOIL), and a nearby rooftop soilless garden (coordinates 44° 30′ 17″ N, 11° 21′ 20″ E, from now on called RAILWAY/SOILLESS). Aerial distance between these two gardens is about 200 m. In traditional garden, the soil is UDIFLUVENTIC HAPLUSTEPT Fine Silty, Superactive, Mesic (SSS 2014), Fluvic, Eutric, CAMBISOL, Siltic (IUSS 2014), with the following physico-chemical properties: sand = 250 g kg−1, silt = 550 g kg−1, clay = 200 g kg−1, pH = 7.5, organic matter = 16.8 g kg−1, total CaCO3 = 10.9 g kg−1. The soilless garden uses coir as substrate (features provided by the supplier: bulk density = 0.06 g cm−3, pH = 7.4, EC = 1.7 dS m−1).
Two gardens nearby a main road of the city (via San Donato, 103–104 vehicles day−1): two traditional gardens, placed 10 m from the street (coordinates 44° 30′ 54″ N, 11° 23′ 29″ E, from now on called ROAD/10) and 60 m from the street (coordinates 44° 30′ 55″ N, 11° 23′ 33″ E, from now on called ROAD/60). Aerial distance between these two gardens is about 80 m. The soil in both gardens is UDIFLUVENTIC HAPLUSTEPT Loamy, Superactive, Mesic (SSS 2014), Irragric, Fluvic, Calcaric, CAMBISOL, Loamic, (IUSS 2014), with the following physico-chemical properties: sand = 150 g kg−1, silt = 620 g kg−1, clay = 230 g kg−1, pH = 8.0, organic matter = 21.6 g kg−1, total CaCO3 = 181.7 g kg−1.
During years 2011 and 2012, plant samples were collected from crops already grown in the gardens, according to their availability in each site. In these years, plant species considered included vegetable and aromatic plants, namely, tomato (Lycopersicon esculentum), zucchini (Cucurbita pepo L.), chicory (Cichorium intybus L.), strawberry (Fragaria × Ananassa), eggplant (Solanum melongena L.), sage (Salvia officinalis L.), basil (Ocimum basilicum L.), rosemary (Rosmarinus officinalis L.), and chilli pepper (Capsicum annuum L.), as well as some tree species, such as cherry (Prunus avium L.), peach (Prunus persica L. Batsch), poplar (Populus alba L.), lime (Tilia L.), and maple (Acer campestre L.).
In 2013, plantlets of three species, namely, tomato (cv Caramba 281, Seminis Inc., Oxnard, CA, USA), lettuce (cv Brasiliana, Eurosementi, Avellino, Italy), and basil (cv. Aromatico della Riviera Ligure, Arcoiris, Modena, Italia), were purchased from a local nursery (LACME, Medicina, Bologna, Italy). Transplanting was conducted in CONTROL/RURAL, ROAD/10, ROAD/60, RAILWAY/SOIL, and CENTRE on April 15th (tomato and lettuce) and May 15th (basil). Each garden was provided with nine plants per species.
Test#1: determination of pollutants as function of the distance of the road. Plant samples were collected in 2011 and 2013 in ROAD/10 and ROAD/60 sites. Species considered in both gardens were tomato and zucchini in 2011 and tomato, lettuce, and basil in 2013.
Test#2: comparison between different sources of pollution. Two sites were selected for different sources of pollution: ROAD/10 and RAILWAY/SOIL. In 2011, leaves of lime, poplar, and maple were sampled in ROAD/10, while in the RAILWAY/SOIL, leaves of cherry, peach, and poplar were collected. In 2013, tomato, lettuce, and basil were collected from both gardens.
Test#3: comparison between the pollutants of horticultural crops grown in soilless and in soil. In 2012, samples of sage, tomato, strawberry, basil, eggplant, rosemary, and chilli pepper grown in RAILWAY/SOIL and RAILWAY/SOILLESS were collected.
Test#4: urban versus rural horticulture. In 2013, according to the promising results obtained in the first 2 years and in order to overcome possible errors linked to differential mineral uptake due to species/cultivar, the analysis was extended to a great number of gardens within the city (CENTRE, RAILWAY/SOIL, ROAD/10, and ROAD/60). Furthermore, with the aim of having a reference value in a rural environment, also the CONTROL/RURAL site was included. In all sites, same cultivars of tomato, lettuce, and basil obtained from the same nursery were simultaneously grown.
Leaf leaching test
The leaves of different species were sampled in glass jars of known tare weight. In the laboratory, leaves were weighed and then washed with water acidulated with HCl (pH ~5) (Vittori Antisari et al. 2012). Samples were shaken for 15 min and then water samples were collected in polyethylene beakers, evaporated in ventilated oven, and brought to 100 ml. Samples were then filtered, acidified with HNO3 (65 % Suprapur, E. Merck, Germany; 1:100 v/v), and stored at 4 °C until analysis. The major and trace elements were determined by inductive coupled plasma optical emission spectrometry (ICP-OES, Spectro Ametek, Arcos). The ICP-OES setting followed multi-standard solutions (CPI International, Amsterdam) that reproduce the matrix effect present in samples and allow the lowering of detection limits (DLs). Instrument response was assessed by measuring a standard sample (CRM 609 - Community Bureau of Reference, BCR).
In order to evaluate the deposition rate of pollutants, the leaf area was determined in function of the leaf weight for three leaf samples. The calculated area/weight ratio ranges from 5.5 to 3.9 m2 kg−1 (Rutter et al. 2011).
Analysis of leaf samples
Clean leaves were dried in ventilated oven (T < 40 °C) and ground in a blender with blades made of pure titanium, carefully avoiding to introduce any further metal contamination to the samples (Vittori Antisari et al. 2012). Briefly, approximately 0.4 g of leaf sub-sample, weighted in Teflon bombs, was dissolved in 8 ml of H3NO3 (suprapure, Merck, Roma, Italy) + 2 ml of H2O2 (Carlo Erba, Milano, Italia) using a microwave oven (Milestone 2100, Sorisone, Bergamo, Italy). After cooling, solutions were made up to 20 ml with Milli-Q water and then filtered with Whatmann 42 filter paper. The accuracy of the instrumental method and analytical procedures used was checked by triplication of the samples, as well as by using reference material, which was run after every 10 samples to check for drift in the sensitivity. The analytical quality of the results was checked against the following reference materials, which certify values of the studied elements close to the measured ones: CRM 060 (aquatic plants) and CRM 062 (Olive leaves) provided by the European Commission Institute for Reference Materials and Measurements.
The experimental data were treated statistically using software packages (i.e., Excel, Statgraphic plus 5.0, and Systat 12.0). The used one-way analysis of variance (ANOVA) test (Tukey’s test, p ≤ 0.05) is a general technique that can be used to test the hypothesis that the means among two or more groups are equal. This is a non-parametric test used to determine if one of several groups of data tends to have more widely dispersed values than the other.