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

## TL;DR

This study shows how adding biochar to soil changes its properties, especially water retention, with the best results at moderate application rates.

## Contribution

The study introduces a combined spectroscopic and relaxometric approach to link molecular water dynamics to soil functionality.

## Key findings

- Biochar increases soil alkalinity, porosity, and water retention in a nonlinear dose-dependent manner.
- High biochar content leads to longer water proton T1 relaxation times, indicating less surface-driven water interactions.
- Water retention is primarily due to pore-space storage rather than surface adsorption.

## Abstract

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 T
1 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.

Dose–response of biochar in a clay soil: fifteen mixtures (f
BC 0–1) reveal nonlinear thresholds (pH/EC) and sharp WHC gains near f
BC ≈ 0.3–0.5. Coupled FT‐IR and FFC‐NMR (0.01–10 MHz; ModelFree τ
c distributions) link water dynamics to soil functionality. Systematic T1 lengthening indicates less surface‐constrained water and a pore‐storage‐dominated mechanism. Implication: moderate application rates deliver maximal, sustainable benefits.