Geochemical Assessment of Trace Elements in Agricultural Soils and Their Bioavailability to Crops in Delta State

The geochemical cycling of trace elements in agricultural soils and their phytoavailability determine both the nutritional quality of crops grown on these soils and the risk of food chain contamination, yet integrated geochemical and bioavailability assessments for Delta State, South South Nigeria, remain scarce. This study conducted a geochemical assessment of trace elements in agricultural soils and their bioavailability to food crops in three farming communities in Delta State affected by varying degrees of petroleum-related land use history. Composite soil samples from surface horizons were analysed for total and DTPA-extractable concentrations of cadmium, lead, nickel, chromium, copper, zinc, and selenium by inductively coupled plasma mass spectrometry. Sequential extraction using the Tessier five-fraction scheme was applied to determine geochemical partitioning of each element across exchangeable, carbonate-bound, iron-manganese oxide-bound, organic matter-bound, and residual fractions. Paired crop tissue samples of cassava, yam, and plantain were analysed for trace element content. Transfer factors and bioaccumulation coefficients were calculated for each crop-element combination. Cadmium and lead showed the highest proportions in mobile fractions, ranging from 18 to 36 percent, compared to 8 to 14 percent for chromium and nickel. Bioaccumulation coefficients for cadmium in cassava tubers exceeded 0.8 at oil-impacted sites. Sequential extraction data revealed petroleum-associated sites had greater organic matter-bound fractions for copper and zinc. These results demonstrate that petroleum contamination history significantly alters trace element geochemistry and phytoavailability in Delta State agricultural soils and recommends soil health rehabilitation before intensified food crop production.

Keywords: trace elements, geochemical assessment, bioavailability, sequential extraction, Delta State