The study links growth period and plant part to heavy metal bioaccumulation in crops, finding frequent exceedances in leaves and short-term species, with seeds and fruits often safer. Evidence guides HMTC sampling, limits, and safer-crop recommendations.
Authors
- Awino FB, Maher W, Lynch AJJ, Asanga Fai PB, Otim O. — Integrated Environmental Assessment and Management. 2022
What was studied
This original study examined heavy metal bioaccumulation in crops grown on an urban dumpsite in Mbale, Uganda, quantifying 11 metals and 2 metalloids across eight common food crops and their edible parts to assess public-health risk and regulatory relevance. The investigators compared heavy metal bioaccumulation in crops by growth period short-term (≤6 months; Zea mays and Amaranthus cruentus) versus long-term (>6 months; Manihot esculenta, Colocasia esculenta, Musa acuminata, Carica papaya, Coffea arabica, Saccharum officinarum) and by consumable part (leaves, seeds, fruits, flowers, tubers, stems). Methods included acid digestion and ICP-MS with a 0.1 mg/kg MDL, and results were benchmarked against WHO/FAO and other food safety limits.
Who was studied
Plant samples (n=80 composite crop/part samples) were collected from two spatial zones of the Mbale municipal dumpsite—the dump center and the adjacent slope—representing realistic exposure for low-income urban consumers who rely on “dumpsite farming.” Short-term crops like leafy Amaranthus and maize, and long-term crops including cassava, taro, banana, papaya, coffee, and sugarcane were sampled during a drought-affected period (Nov 2016–Jan 2017), with leaves, seeds, fruits, flowers, tubers, and stems processed separately. The design allows inference about population-level dietary risk where alternative food sources are limited and about how crop maturity windows influence contaminant transfer to edible tissues.
Most important findings
| Critical point | Details |
|---|---|
| Widespread exceedances vs standards | Nine metals (Al, Fe, Zn, Mn, Cu, Ni, Cr, Pb, Hg) exceeded food safety limits in at least some crop–part combinations; Hg, though trace, was above the 0.001–0.03 mg/kg range |
| Leaves are the highest-risk part | Leaves consistently had higher concentrations than seeds, fruits, and tubers; linear correlations show leaf levels predict other parts |
| Growth period matters | Short-term crops accumulated more Fe, Al, Pb, Co; long-term crops accumulated more Mn, Ni, Cr, indicating transporter/root-depth and transpiration effects |
| Short-term leafy crops are critical | Amaranthus leaves frequently exceeded limits for Zn, Fe, Mn, Pb, Hg, Cr; maize leaves showed multiple exceedances, whereas maize seeds were often within limits. |
| Long-term crops not exempt | Cassava leaves and roots showed exceedances for Fe, Mn, Ni, Zn, Pb; banana leaves exceeded Fe and multiple other metals. |
| Translocation and enrichment | Cassava showed high leaf/root translocation for Mn (56.8), Fe (26.1), Al (17.2), Zn (10.7); taro showed limited transfer and even leaf→root preference for some metals. |
| Seeds and fruits often safer | Relative Accumulation Factors indicate lower metal fractions in papaya fruit and maize seed compared with leaves, though papaya seeds and banana fruit can enrich specific metals. |
| Site and pH effects | Dump-center crops generally higher than slope; variable soil pH (6.4–8.7 center; 7.4–8.0 slope) likely increased mobility and uptake in more acidic sections |
| Aerial deposition is material | Unwashed Amaranthus flowers had elevated metals; proximity to roads, industry, and open burning supports foliar deposition pathways. |
| Risk heterogeneity by part | Certification decisions must be part-specific; for Zea mays, seeds mostly pass while leaves do not; for papaya, fruits and seeds differ from leaves in both direction and magnitude. |
Key implications
For regulators, heavy metal bioaccumulation in crops at dumpsites demands part-specific limits and growth-period stratification for surveillance. HMTC certification should prioritize leafy vegetables from short-term crops for routine testing, require leaf-focused sampling for long-term species, and recognize fruits and seeds as lower-risk but not uniformly safe. Industry can apply safer-crop substitution, buffer zones, and soil pH management. Research gaps include soil–plant correlations and seasonal dynamics. Practically, HMTC should mandate validated ICP-MS screening panels, leaf-dominant composites, and clear pass/fail thresholds aligned to WHO/FAO.
Citation
Awino FB, Maher W, Lynch AJJ, Asanga Fai PB, Otim O. Comparison of metal bioaccumulation in crop types and consumable parts between two growth periods. Integrated Environmental Assessment and Management. 2022;18(4):1056–1071. doi:10.1002/ieam.4513
Heavy metals are high-density elements that accumulate in the body and environment, disrupting biological processes. Lead, cadmium, arsenic, mercury, nickel, tin, aluminum, and chromium are of greatest concern due to persistence, bioaccumulation, and health risks, making them central to the HMTC program’s safety standards.