Remediation of Cd-contaminated acidic paddy fields with four-year consecutive liming

Liming has been widely used to remediate Cd-contaminated acidic soils, but the effects of consecutive liming are still unclear. Four-year liming experiments were conducted to assess the remediation of Cd-contaminated acidic paddy fields in a double rice cropping system. With four-year consecutive liming (quicklime, 2.25 t ha^-1 per season), the soil pH was increased by an average of 0.57 units (0.10-1.16 units), while the soil DTPA-Cd and available Fe and Mn were reduced by 9%, 19% and 31% (p < 0.05), respectively. The exchangeable plus water-soluble Cd fraction in soil was reduced by 17%, while the soil carbonate-, Fe/Mn oxide- and organic-bound Cd fractions were increased by 23%, 41% and 10% (p < 0.05), respectively. The Cd in rice grain was reduced by 55% for early rice and 63% for late rice (p < 0.05) and in some cases was reduced to below the Chinese allowable limit (0.2 mg kg^-1). High annual fluctuations in rice grain Cd could be caused by variations in the field water regime and in rainfall. With consecutive liming, the soil pH, DTPA-Cd and rice grain Cd varied greatly in the first three seasons and then remained relatively less variable. Meanwhile, soil available nutrients (N, P and K) and rice grain yield were little affected by liming. Soil DTPA-Cd linearly decreased with increasing soil pH, while the reduction of Cd in rice grain logarithmically decreased with increasing soil pH and the reduction in soil DTPA-Cd in the heading stage, indicating potential implications for forecasting rice grain Cd content. Therefore, consecutive liming with quicklime can be recommended for the remediation of Cd-contaminated acidic paddy fields, though supplementary measures are still necessary.