Coconut shell-derived biochar and oyster shell powder alter rhizosphere soil biochemical properties and Cd uptake of rice (Oryza sativa L.)

Accumulation of Cd in the soil is a worldwide issue intimidating agricultural productivity and human health. Therefore, developing low-cost, environmentally sound, and readily available technologies is required to restore Cd-polluted agricultural soils and assure food safety. In the present field study, changes in rhizosphere soil biochemical property and Cd content of rice grains with biochar and oyster shell treatment were evaluated in two rice cultivars with contrasting Cd-accumulation potential (high = HA and low = LA). The results revealed that applied biochar and oyster shells effectively immobilized Cd. The amount of Cd extractable by DTPA and CaCl2 was reduced by 34–46% and 11–28%, respectively. Moreover, rhizosphere soil biochemical properties, including pH, MBC, and enzyme activity, were enhanced by the remarkable increases in soil pH after treatment (5.6–7.3). Applied biochar and oyster shell significantly reduced Cd uptake and accumulation in rice cultivars; a more significant reduction was observed in LA rice type with oyster shell treatment. Grain Cd concentration was reduced considerably with oyster shells in both HA (by 58%) and LA (by 70%), meeting the permissible Cd limits in rice grains. Furthermore, the reduced Cd availability and improved soil biochemical properties resulted in improved rice crop productivity. This study indicates that integrating readily available and low-cost oyster shell material with low Cd-accumulation rice cultivars could effectively reduce the transfer of soil Cd to the grain and alleviate the associated human health risks.