Effects of biochar and compost on the bioavailability of heavy metals in soil aggregates with varying particle sizes

PurposeApplication of compost and biochar might lower the amount of heavy metals presented in soil, but their effects on the heavy metals in soil aggregates were ignored. Therefore, the main research contents of this study included the changes of physicochemical properties and nutrients contents in soil aggregates, morphological distribution of heavy metals (Cu, As, and Cd), and the relationships between heavy metal speciation and physicochemical parameters of soil aggregates. The purpose of this study was conducted to investigate the effects of compost and biochar in decreasing heavy metals toxicity and morphological distribution, and identify the key factors influencing heavy metals toxicity.Materials and methodsBiochar was derived through the pyrolysis of rice straw. Compost was obtained by composting of straw, fresh soil, discarded vegetable leaves, and wood bits. Four groups were set in the experiment: Group S with no additives; Group S+B incorporating 5% biochar; Group S+C incorporating 5% compost; and Group S+B+C simultaneously incorporating 5% compost and 5% biochar. Soil aggregates were isolated by a modified dry sieve method. Heavy metal speciation was determined using the BCR method.ResultsBiochar and compost effectively mitigated the toxicity of Cd. The potential ecological risks (RI) of Cu, As and Cd all decreased with the inputs of the amendment, especially for combined addition of biochar and compost. The soil organic amendment of biochar and compost facilitated the formation of larger aggregates, thereby transferring heavy metals from smaller aggregates with higher potential risks to more stable larger aggregates, indirectly decreasing the bioavailability of heavy metals. Additionally, both Cd and Cu were negatively correlated with soil organic matter (SOM) and water-soluble organic carbon (WSOC), whereas As exhibited a positive correlation with these variables.ConclusionThis study revealed that biochar and compost effectively reduced the bioavailability and toxicity of heavey metals in aggregates, and the properties of aggregates could also influence heavey metals bioavailability. This study contributes valuable theoretical agroecosystem services for the optimization of heavy metal pollution by compost and biochar.