Seasonal Variation and Uptake Pattern of Heavy Metals in Maize

Abdullahi et al. (2024) measured Zn, Pb, Cd, Ni, Co, Cu, Cr, and Hg in maize grain and root composites from 10 sampling sites within the Kano River Irrigation Project (KRIP) in northern Nigeria, sampling once during the 2022 wet season and once during the 2023 dry season, and computed root-to-grain Translocation Factors (TF = Rconc/Gconc) for each site-season pair. The principal finding is a pronounced seasonal effect on grain contamination: Cd, Cr, and Hg were below detection in wet-season grain, whereas dry-season grain showed detectable Cd at all sites and Cr at all sites, with location-mean concentrations of Zn, Pb, Cd, and Cr in dry-season grain exceeding the FAO/WHO permissible limits (PL) cited by the authors (compiled from Ogundele et al. 2015 and Chiroma et al. 2014). The seasonal effect also reorganized root-to-grain partitioning: in the wet season, Ni and Co accumulated preferentially in roots (mean TF > 1) while Zn, Pb, and Cu translocated preferentially to grain; in the dry season, only Cr accumulated preferentially in roots (mean TF 1.70) while every other measured metal translocated preferentially to grain.

Key numbers

Permissible limits (PL) cited by the paper (mg/kg, dry weight basis), per Table 1 and Table 3 footnotes:

• Zn 0.6 (Ogundele et al. 2015)
• Pb 2.0 (Ogundele et al. 2015)
• Cd 0.02 (Ogundele et al. 2015)
• Ni 10 (Ogundele et al. 2015)
• Co 50 (Chiroma et al. 2014)
• Cu 10 (Ogundele et al. 2015)
• Cr 1.3 (Ogundele et al. 2015)
• Hg PL not stated in tables

These PL values are the paper’s own comparators (the authors do not cite Codex or WHO documents directly; the cited compilations are tertiary). All concentrations below are mg/kg dry weight, AAS, triplicate, reported as Mean ± SD; “ND” = not detected at instrument detection limit (the paper does not publish LODs).

Wet season 2022 grain (Table 1, n=10 sampling sites)

• Zn: range 6.620–12.447; mean 9.204 (15.3× PL 0.6)
• Pb: range 1.260–2.673; mean 1.967 (overall mean below PL 2.0; 4 of 10 site means above PL — Dakasoye 2.500, Kura Tsauni 2.660, Imawa II 2.673, Yantomo 2.013)
• Ni: range 0.180–0.620; mean 0.366 (below PL 10)
• Co: range 0.080–0.287; mean 0.181 (below PL 50)
• Cu: range 1.613–3.673; mean 2.628 (below PL 10)
• Cd: ND in all 10 sites
• Cr: ND in all 10 sites
• Hg: ND in all 10 sites

Wet season 2022 root (Table 2, n=10 sampling sites)

• Zn: range 0.347–5.000; mean 2.873
• Pb: range 0.640–2.220; mean 1.284
• Ni: range 0.247–1.020; mean 0.557
• Co: range 0.067–0.373; mean 0.220
• Cu: range 0.307–2.653; mean 1.427
• Cd, Cr, Hg: ND in all 10 sites

Dry season 2023 grain (Table 3, n=10 sampling sites)

• Zn: range 11.540–20.927; mean 15.852 (26.4× PL 0.6)
• Pb: range 1.540–2.347; mean 2.092 (slightly above PL 2.0; 7 of 10 site means above PL)
• Cd: range 0.160–0.200; mean 0.179 (8.9× PL 0.02; every site mean above PL)
• Ni: range 1.147–1.613; mean 1.387 (below PL 10)
• Co: range 0.567–0.773; mean 0.686 (below PL 50)
• Cu: range 3.093–5.867; mean 4.145 (below PL 10)
• Cr: range 1.747–4.627; mean 2.529 (1.9× PL 1.3; 8 of 10 site means above PL)
• Hg: ND in all 10 sites

Dry season 2023 root (Table 4, n=10 sampling sites)

• Zn: range 1.833–7.053; mean 3.765
• Pb: range 0.993–1.793; mean 1.367
• Cd: range 0.127–0.187; mean 0.155
• Ni: range 0.827–1.293; mean 1.042
• Co: range 0.480–0.820; mean 0.601
• Cu: range 0.593–3.253; mean 1.205
• Cr: range 2.380–8.373; mean 3.695
• Hg: ND in all 10 sites

Translocation factors (Table 5; TF = Rconc / Gconc; >1 means root accumulates, <1 means grain accumulates)

• Wet season 2022 mean TF: Zn 0.32, Pb 0.67, Ni 1.77, Co 1.47, Cu 0.63. Ni and Co partition into roots; Zn, Pb, Cu translocate to grain.
• Dry season 2023 mean TF: Zn 0.24, Pb 0.66, Cd 0.88, Ni 0.76, Co 0.88, Cu 0.31, Cr 1.70. Only Cr partitions into roots; Zn, Pb, Cd, Ni, Co, Cu all translocate to grain.

The seasonal flip is most striking for Ni and Co, which switch from root-accumulating (wet) to grain-accumulating (dry), and for Cr, which is undetected in wet season but root-accumulating in dry season.

Methods (brief)

Sampling: 10 sites within the ~62,000-ha Kano River Irrigation Project, ~30 km from Kano City, with the Tiga Dam and Ruwan Kanya Reservoir as irrigation sources. Wet-season sites (Sept–Oct 2022): ‘Yantomo, Gafan, Rigada, Makuntiri, Yadakwari, Dakasoye, Kura (Tsauni), Kura (Gorubawa), Imawa I, Imawa II. Dry-season sites (May–Jun 2023): Gafan Bunkure, Gafan I, Gafan II, Kukoki, Kwalele, Bulan Wanzamai, Kura (Babbar Gada), Kura (Fegin Malu), Imawa (Gada Mailotso), Imawa (Rimin Kwarya). Note: wet and dry sampling sets are partially overlapping but not identical, limiting strict within-site seasonal comparison.

Sample preparation: cob and root separated; cobs dried, shelled, washed in deionised water; roots soaked and washed in excess deionised water; both oven-dried to constant weight; crushed to powder; ashed at 550°C for 5 hours in a muffle furnace; ash digested in HNO₃:HClO₄ 3:1 v/v (20 mL acid mixture per 1 g sample) in a heating digester; cooled, filtered through Whatman filter paper into 100 mL bottles, brought to mark with deionised water. Procedure follows Akinyele and Shokunbi (2015).

Instrument: atomic absorption spectrophotometer PerkinElmer PinAAcle 900H, triplicate measurements per sample, results reported as Mean ± SD.

Limitations relevant to wiki use:

• The paper does not publish per-element LODs/LOQs, so the “ND” call cannot be bounded.
• Dry ashing at 550°C is not a Hg-compatible preparation: Hg is volatile at this temperature and will be lost. The PinAAcle 900H supports cold-vapor Hg analysis when configured as such, but the paper does not describe a CV-AAS path. The Hg “ND” findings should therefore be read as method-limited, not as evidence of absence.
• As (arsenic) was not measured. The wiki page lists no As values for this source.
• Cr was measured as total Cr (no Cr-VI speciation), per AAS without a speciation step.
• The sampling sites differ between seasons, so the wet-vs-dry contrast confounds season with location.
• No within-site replication beyond analytical triplicates; the unit of inference is site-level composite means.

Implications

Certification. This source contributes dry-season occurrence data showing that maize grain from the Kano River Irrigation Project carries location-mean Cd and Cr well above the paper’s cited PL values, alongside Zn and Pb exceedances. The dataset is a single-project, single-year snapshot per season, with sampling-site discontinuity between wet and dry. It can inform geographic and seasonal stratification of West African irrigation-zone maize in HMTc workbench evidence pools at B-tier; it does not, on its own, support threshold-setting work.

Courses. Useful as a worked example of how growing-season variables (water source, evaporative concentration, dry-season agrochemical inputs) can shift grain contamination on the same crop and irrigation scheme. The TF inversion between seasons (Ni, Co accumulate in roots wet / translocate to grain dry; Cr undetected wet / root-accumulating dry) is a teachable demonstration that root-to-grain partitioning is not a fixed plant trait.

App. Supports a Nigeria / West Africa irrigation-zone geographic flag for maize on the Cd, Pb, Cr, and Zn axes; supports a “dry-season harvest” seasonal modifier that elevates grain contamination relative to wet-season harvest for the same site cluster. Strength: occurrence data with site-level granularity. Limit: single project, single year per season.

Wiki pages updated on ingest

• maize
• cadmium
• lead
• chromium
• nickel
• zinc
• copper
• cobalt
• mercury
• irrigation-water

Verification notes

Merge-enhanced 2026-05-18 from the prior 2026-05-14 page, which was in routing_malformed advisory (missing optional products field, which is correct to leave empty for this commodity-research source). Defects corrected against the source PDF (Tables 1–5, pp. 82–84):

• raw_handle: papers-cube placeholder → canonical PCMF_ajol-file-journals-579-articles-269230-66292f9b933 (per established PCMF handling).
• Added raw_sha256: 59bdcc92fcb7ad4c5134945226825a3fa91fd21976d1b2da0e282cf524e4c943 and access_url (DOI resolver).
• metals: expanded from [Pb, Cd, Cr, Ni] to [Pb, Cd, Cr, Ni, Zn, Cu, Co, tHg] to reflect all eight analytes the paper measured (the page previously omitted Zn, Cu, Co, and Hg even though all were quantified or assayed). Hg recorded as tHg per Part 14 (the paper measures total Hg without MeHg speciation; the “ND” finding is method-limited per the dry-ashing limitation noted in Methods).
• evidence_tier: A → B. Single-project, single-year, no within-site replication, no LOD reporting, tertiary PL comparators — this is solid primary data but does not meet A-tier criteria (national survey, prospective cohort, or multi-laboratory).
• sample_n: 20 → 40 with clarifying sample_population (the prior “20” appears to have counted site-season pairs and obscured the four-way crossing of 10 sites × 2 seasons × 2 tissues).
• Body — wet-season root means (Table 2): all five values were wrong on the prior page (prior Zn 5.553 / Pb 2.380 / Ni 0.523 / Co 0.347 / Cu 1.590 vs paper Zn 2.873 / Pb 1.284 / Ni 0.557 / Co 0.220 / Cu 1.427). Corrected to Table 2 values.
• Body — dry-season root means (Table 4): six of seven values were wrong on the prior page (prior Zn 4.403 / Pb 1.343 / Cd 0.164 / Ni 1.060 / Co 0.650 / Cu 1.947 vs paper Zn 3.765 / Pb 1.367 / Cd 0.155 / Ni 1.042 / Co 0.601 / Cu 1.205; only Cr 3.695 matched). Corrected to Table 4 values.
• Body — PL values: prior page cited “WHO limit of 5 mg/kg” for Zn and “WHO limit of 0.1 mg/kg” for Cd. The paper’s actual PL row (Ogundele et al. 2015) is 0.6 mg/kg for Zn and 0.02 mg/kg for Cd. Corrected; PL block in Key numbers now lists the paper’s full PL row with its citations.
• Body — translocation factors, dry season: prior page said “Dry season TF >1 for Zn, Pb, Cd, Ni, Co, Cu (grain > root); Cr TF <1 (root > grain)“. This had the TF-value bucketing inverted in both directions (the actual mean TFs are Cr 1.70 — the only metal >1 — and all others <1). The directional interpretation cancelled the inversion by accident. Corrected to match Table 5 means as printed in the paper, with the TF definition stated explicitly.
• Body — added per-site exceedance counts for dry-season Pb/Cd/Cr (e.g., “7 of 10 site means above PL” for Pb), which are derivable from Table 3 ranges and PL values and tighten the synthesis-readiness of the page.
• Methods — added explicit limitations note for Hg (dry-ashing volatilization), absent LODs, sampling-site discontinuity between seasons, and the absence of As measurement. These constrain how downstream synthesis can use the source.
• Implications — softened “supports a jurisdiction flag … in certification sourcing criteria” to “can inform geographic and seasonal stratification … in HMTc workbench evidence pools at B-tier; does not, on its own, support threshold-setting work” per Part 2 firewall.
• Wiki pages updated on ingest — expanded to include [[metals/zinc]], [[metals/copper]], [[metals/cobalt]], and [[metals/mercury]] (previously omitted despite being measured).

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.