Assessing Factors Controlling Structural Changes of Humic Acids in Soils Amended with Organic Materials to Improve Soil Functionality

Humic acids (HAs) regulate soil chemical reactivity and improve many functions. The amendment of with organic materials increases matter (SOM) content promotes the formation HAs. However, effect type, frequency duration amendment, pedoclimatic conditions on SOM transformation HA structural changes remains unclear. Herein, four experimental field sites (S1–4) short-to-long-term fertilisation schemes were used to assess effects such factors, i.e., S1: loamy sand amended once farmyard manure (FYM), brown coal waste (BCW), biochar (BIO) for 0.5 1.5 years; S2: silt loam BIO 8 S3: every 5 years FYM 94 S4: clayey 2 116 years. All HAs extracted analysed differences by elemental analysis (EA), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR), solid-state cross polarisation magic angle spinning nuclear magnetic resonance (CP/MAS 13C-NMR), differential scanning calorimetry (DSC). Results from EA, FTIR, NMR showed that long-term samples S3 (treatments, T9–T10) S4 (T11–T12) had greatest aromatic characteristics, which increased (T10 T12). These agreed DSC data, indicated lower aliphatic contents compared other samples. Samples S2 (T7–T8), receded effects, less greater characteristics short-term samples, S1 (T1–T6). In S1, limited, but aromaticity (T3 T6) corresponding (T1 T4) BCW (T2 T5) amendments due inherently high groups in former. Overall, results site (due conditions), age OM, main factors influenced structure, hence transformation. Regular, HAs, can functionality, improvements are achievable only when amending material is rich compounds.