Minimizing salinity-induced Pb toxicity to microbial N cycling processes in saline Pb-polluted soils amended with biochar

Biochar may affect nitrogen (N) cycling processes and, therefore, plant-available forms of soil N by modifying the activity and composition of the soil microbial community. However, it is unclear how biochar addition can influence microbial N cycling and availability in saline polluted soils. The present study investigated the impact of sugarcane bagasse biochars (SBBs) produced at 400 and 600 °C on potential net ammonification, nitrification and N mineralization, microbial biomass N (MBN), and urease activity in a calcareous soil polluted with lead (Pb) under different levels of NaCl salinity (6 and 10 dS m^-1 ) during a 120-day incubation period. NaCl salinity increased soil available Pb concentration, with a greater increase at high than low salinity level. The application of SBBs increased soil organic carbon (SOC, 96–101 % relative to controls), dissolved organic carbon (DOC, 14–110 %), total nitrogen (TN, 6–25 %), cation exchange capacity (CEC, 12–20 %), and immobilized soil Pb (10–25 %). Lead immobilization, which largely resulted from increased soil CEC, was greater with addition of 600 °C (20–25 %) than 400 °C (15–20 %) biochar in saline soils. Addition of SBBs decreased the availability of soil Pb that was enhanced by salinity. Biochar application stimulated potential net ammonification (177–218 %), nitrification (70–83 %) and N mineralization (92–110 %), MBN (114–221 %) and urease activity (66–79 %) in saline Pb-polluted soils. Redundancy analysis revealed that increased DOC, increased TN and Pb immobilization were the most significant factors contributing to the enhanced N bio-transformations, MBN, and urease activity in saline polluted soils amended with SBBs. Our highlights the potential benefits of low-temperature SBBs as an amendment to minimize the impact of metal toxicity associated with salinity on microbial N cycling processes in Pb-polluted soils. In conclusion, biochar application could be a useful practice for enhancing the N turnover rate in saline Pb-polluted soils by increasing substrate availability and decreasing Pb mobility.