Spectroscopic characteristics of dissolved organic matter during pig manure composting with bean dregs and biochar amendments

•The dissolved organic matter (DOM) was evaluated by various spectroscopic methods.•Bean dregs (BD) plus biochar (B) enhanced the humification and aromatization of DOM.•BD + B accelerated the decomposition of easily decomposed organic matter. In this study, the dissolved organic matter (DOM) derived from the compost of pig manure (PM) with the addition of biochar (B) and bean dregs (BD) was researched its composition and structure. No additives, 10%B, 15%BD and 15%BD + 10%B were mixed with raw materials for composting, which are defined as CK, B, BD and BD + B treatments, respectively. The spectroscopic characteristics of DOM were evaluated via various spectroscopy methods including fluorescence spectroscopy, Fourier transform infrared (FTIR) and UV–visible. The ratio of 1650 cm−1/1030 cm−1 and 1650 cm−1/2920 cm−1 in the FTIR spectra increased by 4.26–47.87% and 0.58–37.79% in the treatment with additives, respectively, and BD + B yielded the highest percent increase compared to the CK. Furthermore, the BD + B treatment increased the value of SUVA254 and SUVA280 by 23.11–63.08% and 21.13–56.36%, while declining the E250/E365 ratio by 0.54–6.17% compared to other treatments. Furthermore, the largest increased Pi,n in the fluorescence spectroscopy of excitation emission matrix (EEM) was observed in the BD + B treatment. Overall, the combined amendments of BD and biochar were more beneficial in enhancing the humification and aromatization of DOM during the composting.