Evaluation of Metal(loids) Concentrations in Soils of Selected Rice Paddy Fields in Malawi

Abstract

The aim of this study was to obtain baseline data for heavy metal(loids) concentrations of rice paddy fields to evaluate the impacts of soil metal(loids) concentrations on quality of rice, and to identify sources of metal(loids) pollution of paddy fields in Malawi. In total, 66 soil samples were collected from 22 different smallholder rice farmers’ paddy fields (sites) in Malawi. Concentrations of metal(loids) (arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), manganese (Mn), lead (Pb), uranium (U), and gallium (Ga)) were measured using inductively coupled plasma mass spectrometry (ICP-MS) after acid block digestion with a mixture of concentrated nitric acid (70% HNO3) and hydrogen peroxide (30% H2O2). Measured soil metal(loids) concentrations were compared with Soil UK CLEA soil metal(loids) guidelines, maximum allowable limits (MAL) recommended by the World Health Organization (WHO), the Chinese Environmental Quality Standards (CEQS) for soil metal(loids) concentrations in agricultural soils, and the normally reported soil metal(loids) concentrations in agricultural fields worldwide. Results indicated that mean soil As (2.2 mg As kg−1), Cd (0.044 mg Cd kg−1), Pb (11 mg Pb kg−1), Co (14 ± 6 mg Co kg−1), Mn (601 mg Mn kg−1), U (2.02 mg U kg−1), and Ga (24 mg Ga kg−1) concentrations were at least three times lower than the respective guidelines and MAL recommended by WHO, UK CLEA, and CEQS (20 mg As kg−1, 0.3 mg Cd kg−1, 32 mg Pb kg−1, 50 mg Co kg−1, and 2000 mg Mn kg−1). The values obtained in this study were also within the normally reported metal(loid) concentrations for unpolluted agricultural soils worldwide. However, the mean Cr concentration (78.0 mg Cr kg−1) obtained in this study was higher than that reported for agricultural soils in China (27 ± 5 mg Cr kg−1; range: 22–39 mg Cr kg−1) but was within the normally reported Cr concentrations (1–100 mg Cr kg−1) for unpolluted soils. These findings suggest that metal(loids) concentrations of Malawian rice paddies pose no threat to production to elevated metal(loids) accumulation in rice, and that rice paddies should be safeguarded from contamination. However, further research is required to investigate the impacts of factors such as source of irrigation of water, water management regimes, soil cation exchange capacity, organic manure/composts amendments, and the application of inorganic fertilizers on uptake, transfer, and translocation of soil metal(loids) to various parts of rice plants, even when the soil metal(loids) concentrations are low.