Dose‐Dependent Effects of Biochar on Soil Revealed by Fast Field‐Cycling (FFC) NMR: From Molecular Water Dynamics to Soil Functionality

Biochar is a multifunctional soil amendment that improves soil structure, enhances water-holding capacity, and contributes to carbon sequestration. However, the dose-response relationship between biochar addition and soil behavior remains underexplored, particularly at high application rates. In this study, fifteen soil-biochar mixtures were prepared with biochar mass fractions from 0 to 1 (f_BC = 0-1) to evaluate in detail the changes induced in a Sicilian clay soil. The mixtures were investigated for pH, electrical conductivity, bulk density, water-holding capacity, and water activity (Aw). Biochar addition caused pronounced increases in alkalinity, porosity, and water retention, following nonlinear dose-response trends with clear thresholds beyond f_BC ≈ 0.3-0.5. FT-IR spectroscopy revealed the progressive appearance of oxygenated and aromatic functional groups, accompanied by a reduction in signals from adsorbed water and native soil polar groups. Fast Field-Cycling NMR relaxometry provided molecular-scale insight into soil-water interactions. At high biochar contents, water proton T₁ relaxation times were markedly lengthened, indicating a reduced overall efficiency of surface-driven relaxation. Correlation-time (τ_c) analysis further revealed the emergence of water populations with longer correlation times and a redistribution of relaxation pathways toward outer-sphere dominated mechanisms. Overall, the results indicate that biochar improves soil water retention not by strong surface adsorption but through effective pore-space storage, keeping water available for biological use. The combined spectroscopic and relaxometric approach establishes a direct link between molecular-level water dynamics and macroscopic soil properties, highlighting the value of FFC-NMR as a powerful tool for studying natural porous systems.