Sugarcane bagasse biochar modulates metal and salinity stresses on microbial functions and enzyme activities in saline co-contaminated soils

Biochar can reduce salinity stress and metal toxicity to soil microbial community and enzymatic activity, hence would improve soil biological fertility and quality. However, the effects of sugarcane bagasse biochar (SCB) on microbial and biochemical responses to salinity stress in metal co-contaminated soils still remain unknown. The aim of this study was to assess the impact of SCB application on microbial activity, biomass and enzymatic activities in a soil co-contaminated with cadmium (Cd) and lead (Pb) when simultaneously exposed to NaCl salinity stress during an incubation experiment. Soil samples were initially co-contaminated with cadmium (10 mg Cd kg(-1)) and lead (150 mg Pb kg(-1)) solutions, then pre-incubated for 30 days and finally salinized with three levels of NaCl solution (0, 20 and 40 mM NaCl). Two slow pyrolysis SCBs prepared at 400 and 600 degrees C were applied to the saline polluted soils at 1% (w/w) and the mixtures were incubated for 120 days under laboratory conditions. Soils amended with raw bagasse and without amendment were also used in the experiment. Results showed that soil amendment with SCBs decreased the availability of Cd by 17-19% and Pb by 11-18%, and increased the content of soil organic carbon (SOC) by 96-104% and dissolved organic carbon (DOC) by 14-164% under saline conditions. High-temperature SCB decreased metal availability and enhanced SOM content in the soil more than low-temperature SCB. Application of SCBs increased soil microbial and biochemical properties from 27 to 180%, depending on the pyrolysis temperature, salinity level and the assay itself. Biochar reduced the adverse influences of metal toxicity and salinity stresses on soil microbial and biochemical functions, most largely through immobilizing metals and improving SOC and DOC. Low-temperature SCB promoted soil microbial quality more than high-temperature SCB. This study indicated that low-temperature SCB could be used as an amendment in metal co-contaminated soils to alleviate the potential risks associated with the combined effects of metal pollution and salinity stresses on microbial and biochemical indicators of soil quality/health under arid and semi-arid conditions. The findings would have some useful implications for the soils co-contaminated with toxic metals under the stress of salinity and for rehabilitation of salt-affected soils using SCB as a cost-effective source of organic matter.