Effect of perfluorooctanesulfonate (PFOS) on the rhizosphere soil nitrogen cycling of two riparian plants

Here, we examined the effects of low and high concentrations of perfluorooctanesulfonate (PFOS) on rhizosphere soil N cycling processes in the presence of Lythrum salicaria and Phragmites communis over 4 months. Compared with the control group, the nitrate nitrogen (NO₃⁻-N) content of the bulk soil in the low PFOS (0.1 mg kg⁻¹) treatment significantly decreased (27.7%), the ammonium nitrogen (NH₄⁺-N) content significantly increased (8.7%), and the pH value and total organic carbon (TOC) content slightly increased (0.3% and 1.1%, respectively). Compared with the low PFOS treatment, the content of NO₃⁻N, NH₄⁺-N and pH value in the bulk soil of the high PFOS treatment (50 mg kg⁻¹) significantly increased (1.0%, 53.8% and 61.8%, respectively), and the TOC content significantly decreased (8.2%). Soil protease levels were high in the low PFOS treatment, but low in the high PFOS treatment. PFOS produced inverted U-shaped responses in the potential nitrification (1.5, 3.0, and 1.1 mg N d⁻¹ kg⁻¹ in no, low, and high PFOS, respectively), denitrification (0.19, 0.30, and 0.22 mg N d⁻¹ kg⁻¹ in no, low, and high PFOS, respectively), and N₂O emission rates (0.01, 0.03, and 0.02 mg N d⁻¹ kg⁻¹ in no, low, and high PFOS, respectively) of bulk soil. The abundance of the archaea amoA gene decreased with increasing PFOS concentration, whereas that of bacterial amoA increased; inverted U-shaped responses were observed for narG, nirK, nirS, and nosZ. In the PFOS-contaminated rhizosphere soil, the observed changes differed from those in the bulk soil and differed between treatments. P. communis tended to upregulate each step of the nitrogen cycle under low PFOS conditions, whereas L. salicaria tended to inhibit them. Under high PFOS conditions, both test plants tended to act as inhibitors of the soil N-cycle; thus, the effects of PFOS on soil N transformation were plant-specific.