Variations in fluorescence properties of humic acids from calcareous soils amended with different swine manures in a long-term soil experiment

Molecular properties of soil humic acid (HA) can play an important role in the mechanisms regulating plant nutrient availability. This study explores how the structure of HA is altered by long-term treatment with different forms of swine manure and how these changes may influence nutrient availability. Liquid swine manure (LSM), solid swine manure (SSM), and swine manure compost (SMC) were applied to a calcareous soil over 17 years in a long-term soil fertility study. HA was extracted from site soil samples and analyzed using fluorescence spectroscopic techniques, including a Cu²⁺ quenching experiment, in order to assess differences in the structure and functionality of the soil organic matter (SOM) resulting from these different treatments. Emission spectra of the SSM-HA and SMC-HA are similar, while the LSM-HA is distinct. Procedures such as parallel factor analysis (PARAFAC) decomposition of emission–excitation matrices showed that structures in the SSM-HA and SMC-HA samples have lower complexity, whereas the structures of LSM-HA are of higher complexity. Interactions with Cu²⁺ at different pH levels indicate that the LSM-HA shows more dynamic conformational changes as well as stronger interactions and higher quenching efficiency compared to the other treatments. Conversely, SMC-HA demonstrates relatively stable binding constant (K_a) values across different pH levels. The binding constants and quenching efficiency of SSM-HA are significantly affected by changes in pH. This study shows distinct structural characteristics of HA formed under different manure management systems and provides valuable insights into how these variations may impact nutrient dynamics in soils.