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• Extracellular enzymes adsorption by biochar is driven primarily by electrostatic interactions. • Enzyme adsorption decreases as pH increase and follows the Langmuir isotherm. • Enzyme adsorption is affected by surface chemistry and pore structure of biochar. • By adding O-containing groups, oxidation reduces a biochar’s affinity for enzymes. • Biochar addition can enhance or reduce enzyme adsorption by a soil. Extracellular enzymes play a key role in mediating organic matter decomposition in soils and the mobility of enzymes is largely controlled by their interaction with soil surfaces. The introduction of pyrogenic products, including biochar produced for the purpose of carbon sequestration or soil health management, may alter the ecological functioning of soil. In this work, we studied the adsorption of four representative soil extracellular enzymes (urease, invertase, α-amylase and protease) to biochar (derived from wood biomass and wheat straw produced at different pyrolysis temperatures, and a wildfire pine char) and soil mixed with biochar. A pH-edge adsorption experiment showed that, for all biochar/enzyme combinations, adsorption of all extracellular enzymes decreased as pH increased from 4 to 9. This pH dependency suggests that electrostatic interaction was the primary adsorption mechanism. Equilibrium enzyme adsorption data was best fit by the Langmuir isotherm and adsorption capacity varied significantly with enzyme type, ranging from 67 to 232 mg·g −1 for urease and 0 to 11 mg·g −1 for the others at pH 5.0. Enzyme adsorption also differed among biochars with or without surface oxidation treatment. Correlations between enzyme adsorption data and biochar properties demonstrated the relevance of enzyme sizes, biochar surface porous structure, and surface chemical functionality in determining biochar adsorption capacity and affinity for enzymes. Soil adsorption experiment showed that biochar addition can enhance or reduce soil adsorption of enzymes, depending on the relative enzyme affinity between the soil and biochar. These findings indicate that pyrogenic organic matter has varying impacts on the mobility of soil extracellular enzymes through direct adsorption and potentially affect the activity and stability of enzymes, and ultimately soil carbon and nutrient cycling.
Zeng et al. (Fri,) studied this question.