Application of oyster shell powder for five consecutive years effectively controlled soil acidification and reduced cadmium accumulation in rice grains

Oyster shell powder enhances soil quality by mitigating acidification and improving fertility, boosting crop yields. However, its efficacy in immobilizing cadmium (Cd) in paddy soils remains uncertain. We performed a field experiment from 2014 to 2018 and assessed four treatments of oyster shell powder: 0 kg/ha (L0), 750 kg/ha (L750), 1,500 kg/ha (L1500), and 2,250 kg/ha (L2250). From 2014 to 2018, grain yield among all treatments was assessed at maturity, and Cd of grain, soil pH, and available Cd content were measured. Our results revealed that oyster shell powder significantly increased rice yields compared to L0, with L750, L1500, and L2250 treatments enhancing yields by 0.53-3.07% (average value of 1.80%), 3.20-7.61% (average value of 5.41%), and 4.19-10.96% (average value of 7.58%), respectively. Simultaneously, the Cd concentration in rice grains decreased by 3.57-5.51%, 7.14-16.34%, and 9.91-19.84% for L750, L1500, and L2250, respectively, with L2250 demonstrating the most effective Cd reduction. Additionally, soil pH increased by 0.12-0.39, 0.20-0.60, and 0.38-0.79 units, while soil bioavailable Cd decreased by 5.71-13.04%, 10.39-15.46%, and 14.73-18.84% for L750, L1500, and L2250, respectively. Further analysis indicated that over five years of continuous application, a 0.1-unit increase in soil pH correlated with a reduction of 0.42-0.82 mg/kg in Cd levels in rice grains, while a 0.1 mg/kg reduction in bioavailable Cd in the soil corresponded to a 0.07-0.16 mg/kg decrease in Cd in rice grains. In conclusion, five years of continuous application of oyster shell powder effectively mitigated soil acidification and reduced Cd bioavailability, increasing rice yields and diminishing Cd accumulation in rice grains. Higher application rates of oyster shell powder (up to 2,250 kg/ha) resulted in more pronounced acid control, Cd immobilization, and yield enhancements.