Biochar Impacts on Soil Silicon Dissolution Kinetics and their Interaction Mechanisms

Effects of biochars on soil silicon dissolution kinetics remain unaddressed. Si-rich rice husk (RH) and rice straw (RS), and Si-deficient wood sawdust (WB) and orange peel (OP) were applied to prepare biochars at 300–700 °C. The silicon dissolution of Si-rich biochars was relatively high in comparison with Si-deficient biochars, and increased with the pyrolysis temperature. The mechanism of silicon release is suggested to be controlled by a protective carbon-silicon interaction, as accompanied by carbon release. After mixing with soil, the addition of Si-rich biochar leads up to 72.7–121% improvement in silicon dissolution in a high-silicon soil (HSS) compared to 147–243% improvement in a low-silicon soil (LSS). The total cumulative amount of silicon dissolved decreased compared to the theoretical value due to the adsorption of silicic acid by the biochar. The addition of WB700 or OP700 as Si-deficient biochars leads to a cumulative Si dissolution decrease of 15.7 and 12.1%, respectively. The adsorption of silicic acid in the biochar and the protection of soil dissolved Fe make biochar a reservoir of soil silicon. Thus, Si-rich biochar could serve as a source of Si with slow release, while Si-deficient biochar could serve as an extra Si sink in agricultural paddy soil.