Objective  The water management model plays an important role in remediation of cadmium (Cd) polluted paddy soils. The reasonable water management regime is explored to promote the immobilization of Cd by palygorskite in contaminated paddy soils and investigate the effects of palygorskite addition on physicochemical property and environmental quality of soil and resistance of rice plant to Cd oxidative stress.

  Method  There were totally 18 treatments in the pot experiment, which included 3 water managements of continuous flooding (5-7 cm surface water during the whole growth period of rice plant, with a scale line on the sidewall of pot), traditional irrigation (moist soil surface during the late tillering state and grain filling stage, and 5-7 cm surface water during the other growth stages of rice plant) and wetting irrigation (moist soil surface during the entire growth period of rice plant. The deionized water (2.3 kg) was added in the pot according to soil moisture content of 75% field water capacity and 8.0 kg of soil sample added in the pot) and six palygorskite treatments included the doses of 0, 0.5%, 1.0%, 1.5%, 2.0% and 2.5% in paddy soils. The effects of palygorskite addition on pH, zeta potential and contents of chemically extractable Cd (0.025 mol L⁻¹ HCl extractable Cd and Toxicity Characteristic Leaching Procedure, TCLP extractable Cd) were determined in paddy soils, and biomasses and concentrations of Cd in brown rice were tested in the pot experiment, as well as the activities of enzymes in soils and antioxidant enzymes in leaves of rice plant.

  Result  After the palygorskite was added to paddy soils, the pH increased by 0.49-1.24 units under continuous flooding, 0.49-1.47 units under traditional irrigation, and 0.42-1.64 units under wetting irrigation (P < 0.05). The absolute values of zeta potential in amended soils rose significantly. The contents of chemically extractable Cd in treated soils declined significantly, with reductions of 15.4%-46.2%, 11.5%-39.7% and 11.4%-32.4% for the 0.025 mol L⁻¹ HCl extractable Cd and maximum decreases of 47.4%, 42.4% and 40.2% for the TCLP extractable Cd under 3 water managements (P < 0.05). In paddy soils untreated with palygorskite, the biomasses of brown rice reduced by 11.2% under continuous flooding and 19.3% under wetting irrigation in contrast to traditional irrigation (P < 0.05). The concentrations of Cd in brown rice in amended soils reduced by 21.9%-75.0% under continuous flooding, 17.8%-70.2% under traditional irrigation, and 17.4%-66.5% under wetting irrigation (P < 0.05). The activities of phosphatase in paddy soils amended with palygorskite under 3 water managements rose significantly with maximum increases of 40.0%, 57.1% and 40.9%, and the activities of superoxide dismutase (SOD) in leaves of rice plant upon palygorskite treatment increased by 33.9%, 50.2% and 37.4% maximally (P < 0.05).

  Conclusion  The continuous flooding promotes the Cd immobilization by palygorskite in heavy metal contaminated paddy soils with a maximum reduction of content of chemically extractable Cd among 3 water managements. The 1.0% palygorskite treatment causes the concentration of Cd in brown rice lower than Chinese standard of 0.20 mg kg⁻¹ (GB 2762—2012, Maximum permissible concentration of contaminants in food in China). The palygorskite addition combined with continuous flooding management, could be recommended as a practical measure for immobilization remediation of Cd-polluted paddy soils.