Insights into variations on dissolved organic matter of bauxite residue during soil-formation processes following 2-year column simulation

Bauxite residue, an industrial alkaline solid waste, has a low organic carbon content which hinders plant growth. Dissolved organic matter (DOM) drives many biogeochemical processes including carbon storage and soil formation in soils. Input of exogenous organic materials may provide organic carbon and accelerate soil formation processes in bauxite residue. However, the potential effects of ameliorants on the quantity and quality of DOM in bauxite residue are still poorly understood. Here, the integration of ultraviolet-visible (UV-Vis) spectra, fluorescence spectra, and parallel factor (PARAFAC) analysis were used to investigate the vertical characteristics of DOM in bauxite residue treated by PV (the combined addition of 2% phosphogypsum and 4% vermicompost, w/w) and BS (6% w/w including 4% bagasse and 2% bran) with 2-year column experiments. The content of DOM in untreated residues ranged from 0.064 to 0.096 g/kg, whilst higher contents of DOM were observed in PV (0.13 g/kg) and BS (0.26 g/kg) treatment. Meanwhile, with the increase of residue depth, the aromaticity and hydrophobic components of DOM in residue decreased, which indicated that the degree of humification of the treated residues in the upper layer was higher than that in the lower layer. Compared with BR, BS and PV treatment accumulated the related content of fulvic acid-like substance from 36.14% to 71.33% and 74.86%, respectively. The incorporation of vermicompost and biosolids increased the content of humic-like substances, whilst decreasing the content of protein-like substances in the surface layer, which may be due to the enrichment of the microbial community. During soil formation processes, the application of organic amendments reduced both salinity and alkalinity, enhanced microbial community diversity, and changed the quantity and quality of DOM in bauxite residue. These findings improve our understanding of the dynamics of DOM and response of DOM to soil formation processes in bauxite residue.