Variation of humic carbon and microbial communities in bauxite residue following co-application of straw and phosphogypsum

Humification is one of the critical processes in the ecological restoration of bauxite residue deposit areas. Straw addition is widely used strategy to increase organic carbon in bauxite residue. However, the effect of straw application on the humic carbon fractions in bauxite residue is largely unknown. In this study, the accumulation of humic fractions and associated microbial communities in bauxite residue following straw application were evaluated by humus fractionation and high-throughput sequencing technology. The results showed that straw application significantly increased humic carbon fractions (humic acid and fulvic acid) and humification degree in bauxite residue. The content of humic acid and fulvic acid increased by 27.1% and 22.9% in straw-amended bauxite residue after phosphogypsum addition, respectively. The glucosidase, cellulolytic enzyme, polyphenol oxidase and peroxidase increased by 7.15–8.76 times, 5.64–7.12 times, 2.69–4.57 times and 2.59–4.24 times following the straw application. High-throughput sequencing results indicated that the operational taxonomic unit (OTU) numbers and Shannon index of both bacterial and fungal communities significantly increased following co-application of straw and phosphogypsum. In addition, co-application of straw and phosphogypsum significantly increased the relative abundance of Devosiaceae, Rhizobiaceae, Flavobacteriaceae, Caulobacteraceae, and Cellvibrionaceae in bauxite residue. These findings provide us with a biological perspective of straw on the humification process in bauxite residue.