Rhizosphere Effect on Physicochemical Properties and Immobilization Performance of Biochar

The application effect of biochar in soil is often affected by environmental factors, but the effect of rhizosphere processes on the properties and functions of biochar remains unclear. The purpose of this study was to explore the effect of the rhizosphere process on biochar characteristics and heavy metal immobilization through different wheat rhizosphere simulation experiments. After aging in the wheat rhizosphere, the physicochemical properties of rice husk biochar (RH BC) and sludge biochar (Sl BC) changed significantly. In general, ash content, specific surface area (SAA), and thermal stability were significantly reduced, while the oxygen (O) content, polarity, and surface O–containing functional groups content were significantly increased. Meanwhile, the increase of C = O and O = C–OH on the aged biochar surface promotes its adsorption capability for Cd 2+ and Pb 2+ in solution. However, in heavy metal–contaminated rhizosphere soils, the immobilization effect of RH BC on heavy metals was significantly weakened by the rhizosphere process, which may be due to the elevated dissolved organic matter (DOM) content of the rhizosphere soil, but that of Sl BC was not affected. Therefore, although the rhizosphere process may enhance the adsorption performance of biochar for heavy metals, the actual immobilization effect of biochar in the rhizosphere soil was still at risk of being weakened, depending on the type of biochar. An in-depth understanding of how biochar properties change in soil–plant systems will help to scientifically evaluate the long-term effectiveness of biochar.