First "charosphere" view towards the transport and transformation of Cd with addition of manure derived biochar.

The specific area between biochar and soil surfaces is termed the "charosphere", which is analogous to the rhizosphere between plant roots and soil. Physicochemical properties of charosphere soil differ from natural soil. A double-layer mesh bag experiment was conducted to separate the charosphere soil (>2 mm) from biochar (derived from manure pyrolyzed at 300, 500, 700 °C) and the bulk soil. A 95-day experiment determined the availability, total concentration and speciation of Cd in charosphere soil and the total and available Cd in biochar. The availability of Cd decreased in the charosphere soils. Total Cd concentration increased in the biochars but decreased in charosphere soil, suggesting that Cd might transfer from charosphere soil to biochar. Cd in the acid soluble fraction decreased by 25-40% in different biochars while it increased in residue fraction, indicating that Cd tends to be more stable in charosphere soil. Instrumental analysis using SEM, FTIR were performed to provide further evidence of the transport of Cd and to enable interpretation of the mechanisms involved. The phenomena that Cd concentrations change markedly in the charosphere inspires a novel research perspective towards the study of transport and transformation of heavy metals in soil after biochar application.