Investigation of uranium derived from phosphate fertilizers on plants uptakes and bacterial diversity in selected agricultural soils of east africa

Abstract

Different types of phosphate fertilizers (PFs) used in agriculture to increase soil fertility contain uranium (U) as an accompanying element. Uranium is both toxic and slightly radioactive. This study investigated the concentrations of U in phosphate rocks (PRs) and PFs used in growing tobacco (Nicotiana tabacum L.) and maize (Zea Mays L) plants from Burundi, Kenya, Rwanda, Tanzania, and Uganda. The results showed that the concentrations of U in phosphates from Burundi, Kenya, Tanzania and Uganda were statistically significant (P ≤ 0.01). The lowest U concentration was 10.7 mg kg−1 found at the Mrima Hill deposit in Kenya and the highest was 631.6 mg kg−1 found at the Matongo deposit in Burundi. Uranium concentrations for local and imported PFs was also statistically significant (P ≤ 0.01). Uranium concentration from PFs ranged from 107.88 ± 9.60 and 281.57 ± 15.82 mg kg−1 . The study also assessed the influence of U from PFs on the radioactivity of agricultural soils and uptake by tobacco plants. The results showed that applications of PFs in agricultural soils increased radioactivity in agricultural soil and tobacco crops. Tobacco smoking and snuffing behaviors resulted in an annual effective dose that was 2.41-6.53 and 1.14-2.45 times greater than annual recommended effective doses for snuffers and smokers. This work also investigated the influence of fertilizer derived U on maize plant uptake and bacterial diversity in soil after application of fertilizers with varying U concentrations. The pilot field experiments showed that application of PFs with different U concentrations influenced bacteria abundance and diversity in maize crops. Applications of Nafaka plus (NP) (3.93 mg kg-1 ) and Minjingu Powder (MP) (3.06 mg kg-1 ) PFs in soil increased bacteria abundance and diversity. Some bacteria were abundant on NP treated soil (high U content) because of their ability to tolerate higher U concentrations. Uptake from soil to crop for maize after applications of PFs of varying U concentrations was investigated by amending soil with Eucalyptus globulus ssp maideii bark and kaolin clay. The soil amendments in reduced U uptake from soil to plant in pot experiments. It is believed that the reduced U uptake is associated with eucalyptus globulus ssp maideni carboxyl groups that can reduce mobile hexa-uranyl ions to immobile tetra uranyl ions that can be absorbed by the kaolin clay. The study recommends further work in understanding the mechanisms of Eucalyptus ssp maidenii bark powder and kaolin U uptake reduction in soil complex matrix.