Distribution of exchangeable cobalt in low-land rice cultivated soils in Sri Lanka as affected by the differences in climate, soil and water source

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Cobalt (Co) is considered a beneficial element for plants. However, when soils contain excessive amounts of Co, it could cause phytotoxicity. Despite this, the current status of Co in Sri Lankan rice-cultivated soils is not known. Therefore, this study was conducted to (i) determine the distribution of exchangeable Co concentration, and (ii) examine the interactive effects of climatic zone (CZ), agro-climatic zone (ACZ), soil order, water source, and their interactions in determining exchangeable Co concentration in lowland rice fields in Sri Lanka. A total of 8,292 soil samples representing six ACZs, six soil orders, and three water sources were collected using a stratified random sampling approach. Cobalt was extracted in 0.01 M CaCl2 and measured using Inductively Coupled Plasma Mass Spectrophotometry. Exchangeable Co concentration ranged between 0.03-2,409 μg kg−1 with a mean value of 185.9 μg kg−1. Over 77.5% of the soil samples tested were Co deficient, i.e. <250 μg kg−1. Samples collected from the Intermediate zone, particularly Intermediate zone Mid country, had higher Co concentration than that reported in other ACZs (p<0.05). Among the soil orders, Histosols had higher (232 μg kg−1) and Vertisols had lower (91 μg kg−1) Co concentrations (p<0.05). Moreover, Co concentration was negatively correlated with soil pH (r=−0.3391, p<0.0001) and rice crop productivity (r=−0.1512, p<0.0001). Although exchangeable Co concentration in rice cultivated soils was low, it is important to implement strategies such as proper waste management, treatment of industrial effluents, and the use of safer and more sustainable practices in the chemical and mining industries to minimize further accumulation of Co exceeding the critical limit (i.e. 30 - 40 mg Co kg−1 rice grain), and to ensure the safety of rice production in situations where geological and anthropogenic activities can increase soil exchangeable Co concentration.