Denitrifying bacteria agent together with composite materials enhanced soil chemical properties and denitrifying functions in rare earth tailings: A field study

The exploitation of ionic rare earth ore using ammonium sulfate extractant in China caused serious soil degradation and nitrogen compounds pollution in surrounding water. It was critical to improve soil properties and eliminate the nitrogen compounds and prevent their diffusion from the rare earth tailings. Here, we addressed this issue by conducting a field experiment for six months through four different treatments including control (CK), denitrifying bacteria agent mainly consisted of Bacillus (DBA), composite materials (CM) and denitrifying bacteria agent together with composite materials (DBA+CM). Besides, the treatments except CK were also amended with basic soil conditioners. DBA+CM could significantly increase soil pH from 5.01 to 6.84 (p ≤ 0.05). Cation exchange capacity in DBA+CM increased from below detection limit to 2.79 cmol+/kg. DBA+CM possessed the highest removal rate of soil NH4+ (95.14 %) and soil NO3− (66.46 %). Compared to CK, DBA+CM significantly increased the absolute abundance of nirS genes and relative abundance of denitrification, nitrate respiration, and nitrite respiration the most (p ≤ 0.05). Denitrification, nitrate respiration and nirS genes were negatively correlated with soil NO3− (p ≤ 0.05). This study demonstrates denitrifying bacteria agent together with composite materials can be a promising approach to control the pollution of nitrogen compounds in ionic rare earth tailings.