Acid–base properties of humic acid from soils amended with different organic amendments over 17 years in a long-term soil experiment

The acid–base properties of soil organic matter are crucial in regulating plant nutrient availability in agricultural soils. This study examined the effect of long-term application of different forms of manure: liquid swine manure (LSM), solid swine manure (SSM), and swine manure compost (SMC), applied biennially over 17 years, on the intrinsic charge characteristics of humic acid (HA) extracted from soils. Potentiometric titration and a continuous distribution pKa model assessed buffer intensity, surface charge excess, pKa distribution, and proton binding site capacities. The HA formed in soils amended with SMC (SMC-HA) showed higher proton neutralization, particularly in the pH range critical for plant nutrient availability, correlating with phenol content. Variations in pKa distribution highlighted stronger acidic sites in SMC-HA, attributed to phenols and sulfur content, while LSM-HA (HA formed in soils amended with LSM) and SSM-HA (HA formed in soils amended with SSM) displayed weaker acid strength due to lower phenol content and molecular configurations. Acidic/basic site ratios revealed dominance of acidic functional groups in LSM-HA and SSM-HA, whereas SMC-HA exhibited relatively higher basic site content and a lower acid/basic ratio. These findings underscore the heterogeneity of binding sites and differences in binding strengths among HA samples from various manure forms. The enhanced buffering capacity and distinct charge characteristics of SMC-HA suggest a greater potential to improve nutrient availability and overall soil health. SMC amendment produces HAs with properties conducive to improved nutrient management in agricultural soils.