Impact of biochar application on soil quality and microbial communities in a trace element polluted soil

Microorganisms play a key role on soil organic matter decomposition, nutrient cycling and soil health. High concentrations of trace elements in soils can negatively affect soil microbiological properties1. Recent studies have shown that biochar can act as an effective tool for the remediation of soils contaminated with trace elements2. This work evaluates the effects on soil microbial community of two different biochars used as amendment in two trace-element contaminated Typic Xerofluvent soils. They were sampled in a site affected by the breaching of a tailings dam in 1998, flooding 4.286 ha of lands close to Aznalcóllar (SW, Spain). The soils used for this study have two different levels of contamination: a) moderately polluted (MPS) and b) highly polluted (HPS). Rice husk and olive pit biochars (RHB and OPB respectively) were produced in a continuously feed reactor (500 ºC, average residence time 12 min; N2 atmosphere). The remediation potential of these biochars was tested by applying 0 and 8 t ha-1 to the topsoil layer in plots of 1 m2. Microbial communities were studied in the amended and unamended soils by DNA-based analyses. For comparison purposes, dehydrogenase (DHA), ß-glucosidase (BGA) enzymatic activities and soil respiration (6400-09 Soil CO2 Flux Chamber LI-COR) were measured for all the plots. Finally, FESEM-EDS analysis was performed to appraise changes produced on biochar particles throughout the field experiment time. FESEM images showed the presence of filamentous microbial structures on biochar surfaces, Ca-S particles in OPB and high content of Si in RHB. Amendment of the MPS with RHB decreased soil respiration and DHA. In contrast, OPB amendment increased BGA in MPS after 6 months. Preliminary results showed a greater stability of RHB than OPB, which may be of relevance for its use in the restoration of soils contaminated with trace elements. References: [1] Gans, J.; Wolinsky, M.; Dunbar, J.; 2005. Science, 309 (5739), 1387-1390. [2] Uchimiya, M.; Klasson, K.T.; Wartelle, L.H.; Lima, I.M.; 2011. Chemosphere 82, 1438-1447.The former Spanish Ministry of Economy, Industry and Competitiveness (MINEICO) and AEI/FEDER are thanked for funding the projects CGL2016-76498-R and CGL2015-64811-P. P. Campos thanks >Fundación Tatiana Pérez de Guzmán el Bueno> for funding her PhD.Peer Reviewe