Assessment of heavy metal transfer factors from soil to edible crops in peri-urban agricultural lands

Introduction

Picture a typical peri-urban landscape in southern England: allotment gardens, smallholdings, and community farms nested between roads, former industrial sites, and residential areas [1] . These spaces produce food consumed locally, often without formal soil quality testing. Heavy metals including Cd, Pb, Zn, and Cu accumulate in peri-urban soils from vehicle emissions, industrial fallout, and sewage sludge application [4,6] . Whether these metals pose a food safety risk depends on their transfer from soil to the edible portions of crops a pathway governed by soil chemistry, plant species, and metal speciation [2,10] . The soil-to-plant transfer factor (TF = plant concentration/soil concentration) is the most widely used metric for assessing this pathway [3] . TFs vary enormously: Cd may transfer 10-50 times more efficiently than Pb due to differences in root uptake mechanisms [8,13] . Soil pH is generally the strongest predictor, as metal solubility increases exponentially below pH 6.5 [1,11] . Organic matter can either increase or decrease bioavailability depending on the metal and soil conditions [14] . UK peri-urban soils have received limited attention compared with Chinese [7] or Indian [5] studies. Alexander et al. [8] measured vegetable metal uptake in Reading allotments but covered only two metals. This research expanded the scope to six metals across 20 sites, identified the soil factors controlling TFs, and assessed food safety compliance against EU regulations.

Materials and Methods

Study area description

Twenty peri-urban agricultural sites were selected across Berkshire, UK (51.45° N, 0.98° W), including 8 allotment gardens, 6 community farms, and 6 smallholdings within 5 km of major roads or former industrial areas. Sites ranged in elevation from 35 to 92 m on Thames Valley alluvial and London Clay soils. Mean annual temperature was 10.4 °C with 640 mm rainfall [9] .

Soil and water quality analysis

At each site, five composite soil samples (0-20 cm depth, each from 10 subsamples) were collected at crop harvest (July-September 2021 and 2022). Samples were air-dried, sieved to <2 mm, and digested with aqua regia (HCl:HNO₃, 3:1). Metal concentrations were determined by ICP-OES (Cd, Pb, Zn, Cu, Ni, Cr). Soil pH was measured in 1:2.5 soil:water suspension, organic matter by losson-ignition (550 °C), and CEC by ammonium acetate method. Plant samples (lettuce leaves, spinach leaves, carrot roots) were washed, oven-dried at 70 °C, ground, and acid-digested for ICP-OES analysis [3] . TFs were calculated as: TF = metal concentration in plant tissue (mg/kg DW)/metal concentration in soil (mg/kg DW).

Methods

Relationships between TFs and soil properties were assessed by Pearson correlation and stepwise multiple regression R 4.2. Food safety assessment compared fresh-weight plant concentrations (converted from dry weight using measured moisture against EU Commission Regulation 1881/2006 maximum levels. Spatial patterns were mapped using inverse distance weighting interpolation in QGIS 3.28 [9] .

Results

Comprehensive interpretation

Cadmium had the highest TF (mean 0.31), while Pb was lowest (0.017; Table 1, Figure 1). Soil pH was the strongest predictor of Cd TF (r = -0.78, p<0.001), with values rising sharply below pH 6.0 (Figure 2). Organic matter showed a weaker negative correlation with Cu TF (r = -0.54). At 4 of 20 sites, lettuce Cd exceeded the EU limit of 0.20 mg/kg FW; all four had soil pH < 5.8 and total Cd > 4.0 mg/kg. No sites exceeded Pb limits in edible tissue despite high soil Pb concentrations, reflecting Pb's low mobility.

Discussion

The Cd TF of 0.31 is consistent with the 0.10-0.50 range reported for leafy vegetables in contaminated European soils [2,16] . The strong pH dependence matches the established mechanism: below pH 6.0, Cd desorbs from Fe/Mn oxides and organic complexes, increasing its soluble fraction available for root uptake [1,13] . Pb's low TF (0.017) reflects its strong binding to soil organic matter and phosphate minerals, limiting root absorption [4,10] . The four sites exceeding Cd limits all combined acidic pH with elevated soil Cd, suggesting that liming to raise pH above 6.5 would be a practical remediation strategy [11] . Alexander et al. [8] found similar pH thresholds for Reading allotments. The lack of Pb exceedances despite high soil concentrations (up to 418 mg/kg) confirms that total soil concentration alone is a poor predictor of crop contamination for this metal [15] .

Conclusion

Cadmium poses the greatest food safety risk in peri-urban Berkshire agriculture due to its high soil-to-plant transfer efficiency, particularly below soil pH 6.0. Four of twenty sites exceeded EU Cd limits in lettuce. Lead, despite high soil concentrations, showed negligible transfer to edible tissue. Soil pH management through liming is the most practical intervention to reduce Cd uptake. Peri-urban food growers should test soil pH and Cd levels before planting leafy vegetables, especially on allotments near roads or former industrial land.

References

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