Remediation potential of spent mushroom substrate on Cd pollution in a paddy soil

To investigate the remediation potential of spent mushroom substrate (SMS) on Cd pollution in a paddy soil, a rice pot experiment was conducted to study the effects of SMS addition on the availability of Cd in soil and the uptake of Cd in rice tissues. Five percent of SMS from Pleurotus eryngii (SMS-A, treatment: A), SMS from Agaricus bisporus (SMS-B, treatment: B), or SMS-A plus SMS-B (1:1, treatment: A+B) were added into a Cd-contaminated paddy soil before planting, respectively. The treatment of no SMS amendment was set up as the control (CK). At the four main growth stages of rice, the soils and plant samples were collected to detect the soil properties, Cd concentration in soils and rice tissues, and Cd fractions in soils. Results indicated that the application of SMS-A, SMS-B, and A+B significantly increased soil pH by 14.0–22.9, 23.9–32.9, and 22.7–30%, organic matter (OM) contents by 12.9–31.5, 22.1–34.5, and 26.1–36.9% comparing with CK. While cation exchange capacities (CECs) were increased by 3.6–8.5, 4.9–13.1, and 0.4–10.0% in A, B, and A+B treatments, respectively, except those at the maturation stage in A and B treatments. However, the CaCl₂-Cd concentrations in soils were significantly decreased by 64.8–77.9, 76.1–98.9, 73.2–98.9% in A, B, and A+B treatments, respectively, comparing with CK. The reduced availability of Cd was attributed to the changes of Cd from soluble to insoluble fractions in soils amended with SMS and resulted in the decreased Cd uptake in rice tissues. The Cd concentrations in roots significantly decreased by 22.8–36.9, 28.6–36.6, and 26.8–42.6%, while the Cd concentrations in straw decreased by 20.1–46.4, 9.3–41.6, and 16.0–49.1% in A, B, and A+B treatments, respectively. At the maturation stage, the Cd concentrations in brown rice were reduced by 17.7, 15.9, and 19.4% in A, B, and A+B treatments, respectively. Correlation analysis revealed that the Cd concentrations in rice roots, straws, and brown rice were all positively correlated with CaCl₂-Cd concentrations of soils. Moreover, soil pH and OM were significantly negatively correlated with the Cd concentration in rice tissues, except that between soil pH and the Cd concentration in rice straws. Therefore, the reduced Cd availability in soil and uptake in rice plant tissues together with better soil nutrient conditions by SMS application improved the biomass of root and straw at heading, filling, and maturation stages and the rice production by 32.9–38.8% at the maturation stage. The combined application of SMS-A and SMS-B can be used as a potential method for remediation of Cd-contaminated paddy soil.