Decoding the Sustainability Code: Enzyme Thermodynamic and Kinetic Parameters Reveal the Efficacy of Straw, Biochar, and Nanocarbon in Black Soil

For sustainable soil management, the link between carbon amendment structure and soil health is paramount, yet how the particle size of carbon governs hydrolase activity through kinetic and thermodynamic mechanisms remains poorly understood. A three-year field experiment with four treatments, including Control, Straw, Biochar, and Nanocarbon, was conducted in black soil. After harvest, the activities of invertase (INV), urease (URE), and acid phosphatase (ACP) were assayed from 15 to 55 °C. Kinetic parameters—including half-saturation constant ( K m ), maximal reaction rate ( V max ) and catalytic efficiency ( K a )—and thermodynamic parameters—including Gibbs free energy (Δ G ), enthalpy (Δ H ) and entropy (Δ S )—were determined. INV and ACP activities increased with temperature, peaking at 55 °C, whereas URE peaked at 45 °C. The V max , K a , and Δ G of the enzymes also increased with temperature. With straw, INV activity remained stable, whereas INV- K a , INV-Δ H , and INV-Δ S increased with decreased INV- K m . URE activity declined with thermodynamic elevation. For ACP, ACP- K m and ACP- V max increased, whereas ACP- K a and ACP-Δ G decreased. With biochar or nanocarbon, the enzyme activities, V max , and K a decreased, whereas ∆ G increased, with stronger inhibition by nanocarbon. Correlation analysis revealed ∆ G as the dominant factor for activity after carbon addition, while redundancy analysis identified organic carbon (OC) and total phosphorus (TP) as the key regulators. Overall, straw, biochar, and nanocarbon had different sustainable values on hydrolase systems, with thermodynamic parameters, especially ∆ G , better reflecting system shifts than kinetic traits.