Effect of biochar on microbial communities of a trace element contaminated soil

High concentrations of trace elements in soils can negatively affect soil microbiological communities and thus reduces their diversity and functionality. In addition, several recent studies have revealed that biochar can reduce bioavailability of trace elements in contaminated soils. The aim of this work was to evaluate the effects of biochar application on the soil microbial community for trace elements contaminated soils. For that purpose, Typic Xerofluvent soils affected by the toxic spill occurred in Aznalcóllar (SW Spain) on 1998 with two levels of contamination (moderately and highly polluted) were used. Biochar of rice husk (RHB) and olive pit (OPB) were produced in a continuously feed reactor at 500 °C and 12 min residence time and applied into 1 m2 plots on the contaminated soils at a rate of 8 ton ha-1. After determining the effects on soil properties (pH, water holding capacity, total carbon content), the effects on enzymatic activities (β-glucosidase and dehydrogenase), soil respiration (6400-09 Soil CO2 Flux Chamber LI-COR) and microbial communities (next generation sequencing-based 16S rRNA gene analysis) were tested at 1, 6 and 12 months after biochar application. Finally, FE-SEM-EDS were performed on handpicked biochar particles. The results showed that seasonal effects modified enzymatic activities. After 6 months, differences between control and biochar amended soils were found. However, these differences decreased drastically after 1 year. Soil respiration was greater in moderatly than in highly pollutued areas at the beginning of the experiment and was reduced with time. Microbial community was different both between the two levels of contamination as well as biochar amended and non-amended soils. FE-SEM images showed the presence of filamentous microbial structures on biochar surfaces, Ca-S particles (gypsum) in OPB and high content of Si in RHB