Spectroscopic and molecular insights into humic substances in compost amended with varying biochar levels

This study explores the effects of biochar (BC) amendments at varying levels (0%, 1%, 2%, and 5%) on the molecular and structural characteristics of humic substances (HS), specifically humic acids (HA) and fulvic acids (FA), in compost. The addition of BC significantly increased the HA content by 13-16% while reducing the FA content by 12-32%, with the most pronounced effects observed at a 5% BC addition. Spectroscopic analyses, including UV-Vis and fluorescence, revealed enhanced aromaticity and molecular weight (MW) of HA, driven by microbial activity and polymerization facilitated by BC amendments, whereas FA aromaticity remained largely unchanged. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of HA increased significantly, indicating improved humification and stabilization. Fourier-transform infrared (FTIR) spectroscopy showed that BC addition had minimal impact on the functional group composition of HA and FA but altered the proportions of carbon types. Electron paramagnetic resonance (EPR) analysis demonstrated higher persistent free radical (PFR) levels in HA, correlating with increased molecular size and a higher degree of condensation. These findings underscore the potential of BC amendments to enhance compost quality by promoting the formation of stable and complex humic substances with improved reactivity.