Abdullahi et al. (2024) measured Zn, Pb, Cd, Ni, Co, Cu, Cr, and Hg in maize grain and roots from 10 sites within the Kano River Irrigation Project (KRIP) in northern Nigeria during wet season (2022) and dry season (2023), computing translocation factors to examine root-to-grain metal movement. The key finding is a pronounced seasonal effect: Cd and Cr were undetected in wet-season grain but present and above WHO/FAO permissible limits in dry-season grain, and Zn exceeded limits in both seasons. The dry season produced consistently higher grain concentrations for Pb, Cd, and Cr. Translocation factors showed that Ni and Co accumulated preferentially in roots (TF >1) during wet season, while dry season saw Cr accumulating in roots and all other metals preferentially translocating to grain.
Key numbers
Wet season 2022 maize grain (mg/kg, n=10 locations, AAS):
- Zn: 6.620–12.447 (mean 9.204) — exceeds WHO limit of 5 mg/kg
- Pb: 1.260–2.673 (mean 1.967) — below WHO limit of 2 mg/kg in some locations, above in others
- Ni: 0.180–0.620 (mean 0.366)
- Co: 0.080–0.287 (mean 0.181)
- Cu: 1.613–3.673 (mean 2.628)
- Cd: not detected
- Cr: not detected
- Hg: not detected
Dry season 2023 maize grain (mg/kg, n=10 locations):
- Zn: mean 15.852 — exceeds WHO limit
- Pb: mean 2.092 — exceeds WHO limit (2 mg/kg)
- Cd: mean 0.179 — exceeds WHO limit (0.1 mg/kg)
- Ni: mean 1.387
- Co: mean 0.686
- Cu: mean 4.145
- Cr: mean 2.529 — exceeds limit
- Hg: not detected in any grain sample
Wet season roots (mg/kg means): Zn 5.553, Pb 2.380, Ni 0.523, Co 0.347, Cu 1.590; Cd, Cr, Hg not detected.
Dry season roots (mg/kg mean of means): Zn 4.403, Pb 1.343, Cd 0.164, Ni 1.060, Co 0.650, Cu 1.947, Cr 3.695.
Translocation factors: Dry season TF >1 for Zn, Pb, Cd, Ni, Co, Cu (grain > root); Cr TF <1 (root > grain). Wet season: Ni and Co TF >1 in roots.
Methods (brief)
AAS (PerkinElmer PinAAcle 900H). Dry ashing: muffle furnace at 550°C for 5 hours; acid digestion with HNO₃:HClO₄ (3:1 v/v). Samples oven-dried, crushed to powder before ashing. Results in mg/kg dry weight, reported as Mean ± SD (triplicates). FAO/WHO permissible limits used as comparators. Limitation: AAS method does not speciate As (none reported); Hg may be lost during dry ashing at 550°C, making “not detected” for Hg unreliable as a true absence finding. Single irrigation project; n=10 geographic locations but no replication within location.
Implications
Certification: Dry-season maize from Nigerian irrigation schemes shows Cd, Pb, Zn, and Cr above FAO/WHO limits. Cd mean of 0.179 mg/kg dry weight in dry-season grain is nearly double the WHO limit (0.1 mg/kg). This supports a jurisdiction flag for maize from West African irrigation zones in certification sourcing criteria.
Courses: Excellent demonstration of the seasonal effect on grain metal concentrations: same irrigation scheme, same crop, markedly different contamination depending on season. Practical relevance for supply-chain timing and lot-level testing protocols.
App: Supports a Nigeria/West Africa irrigation zone flag for maize as a Cd, Pb, and Zn risk elevating factor. Seasonal modifier (dry season = higher risk) is supported by data.