Comparison of single extraction methods for assessing Cd availability in paddy soils in south China

Soil available Cd plays a key role in soil Cd pollution risk assessment and Cd-contaminated soil remediation. However, there was limited research on the suitable single extraction method for evaluating the Cd bioavailability from paddy soils to rice in south China. Therefore, 90 groups of ‘one-to-two’ soil and rice (both early rice and late rice) samples were collected and 9 single extraction methods were assessed. The extraction methods were 0.1 mol/L CaCl2 (T1), 0.01 mol/L CaCl2 (T2), DTPA (T3), NH4OAc (T4), TCLP (T5), HCl (T6), Mehlich III (T7), NaNO3 (T8) and Mehlich I (T9). The extraction capacity and relationship between extractable Cd and Cd contents in rice were analyzed. The results indicated that the 4.7% to 74.4% of the total Cd in soils could be extracted by different extractants and the sequence of extraction capacity from high to low was: T7, T6, T9, T3, T1, T5, T4, T2, T8. The correlation relationships between extracted Cd in soils and Cd contents in rice were better than that between soil total Cd and rice Cd contents. Among the 9 tested extractants, T1 has an intermediate extraction capacity (41.9%). In T1 treatment, significant correlations were detected between extractable Cd in soils and Cd in early rice, late rice and both early rice and late rice with r of 0.618, 0.338 and 0.363, respectively (P<0.01). Moreover, significant correlations were discovered between extractable Cd of T1 treatment in soils and rice Cd contents of major rice varieties (r = 0.376 to 0.793). In conclusion, the phytoavailability of Cd in paddy soils could be well described by extractable Cd from method T1 and it could be recommended as the extraction method of soil available Cd in south China.