Effects of agro-climatic zones, soil orders, and irrigation types on the exchangeable cadmium in paddy soils

Even though the concentrations of the total cadmium (Cd) in paddy soils from different countries have been reported, the exchangeable Cd (Ex-Cd) concentration in these soils are unknown despite its importance in agriculture. This study was conducted with a total of 5,460 soil samples collected in Sri Lanka, representing six agro-climatic zones (ACZ), six soil orders, and three irrigation types. The exchangeable Cd concentration in soil samples was extracted using 0.01 M CaCl2 and analyzed using an inductively coupled plasma-mass spectrophotometry (ICP-MS). The Ex-Cd ranged between <0.31-163 μg kg−1 with mean and median concentrations of 14.05 and 8.98 μg kg−1, respectively, and was affected by both agro-climatic and soil conditions. Samples collected from the Wet zone, particularly the Wet zone Low country, had higher Ex-Cd (24.1±1.0 μg kg−1) than that those from the Dry zone Low country (11.6±0.2 μg kg−1) (p<0.05). Among the soil orders, Histosols (21.3±3.0 μg kg−1) and Inceptisols (19.5±0.9 μg kg−1) had the highest Cd concentration while Vertisols had the lowest (6.3±0.4 μg kg−1) (p<0.05). The irrigation types only affected Ex-Cd in Dry zone Low country (p<0.05), but not in other agro-climatic zones. Overall, the consideration of agro-climatic zones, soil orders, and irrigation types is needed when implementing agronomic strategies to mitigate the risk associated with Cd accumulation in paddy fields.