Kolton, M., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, PO Box 15159, Rishon Lezion, Israel, Department of Plant Pathology and Microbiology, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel
Graber, E.R., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, PO Box 15159, Rishon Lezion, Israel
Tsehansky, L., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, PO Box 15159, Rishon Lezion, Israel
Elad, Y., Department of Plant Pathology and Weed Research, The Volcani Center, Agricultural Research Organization, PO Box 15159, Rishon Lezion, Israel
Cytryn, E., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, PO Box 15159, Rishon Lezion, Israel

The ‘biochar effect’ depicts a phenomenon in which biochar soil amendment enhances plant performance by promoting growth and suppressing disease. Although this phenomenon has been observed in numerous studies, the mode of action that explains it is currently unknown. In order to elucidate mechanisms responsible for the ‘biochar effect’, we comprehensively monitored tomato plant development and resistance to the foliar fungal pathogen Botrytis cinerea, in biochar-amended and nonamended soils using native biochar and washed biochar, striped of labile chemical constituents. We concomitantly assessed bacterial community succession in the rhizosphere by high-throughput 16S rRNA gene amplicon sequencing and carbon-source utilization profiling. Biochar had little impact on plant physiological parameters. However, both native and washed biochar treatments were characterized by higher rhizosphere bacterial diversity and enhanced carbohydrate and phenolic compound utilization rates coupled to stimulation of bacteria known to degrade phenolic compounds. This study indicates that the ‘biochar effect’ is at least partially dictated by increased diversity and changes in metabolic potential in the rhizosphere microbiome, which is primarily triggered by the recalcitrant carbon backbone of the biochar and tightly bound compounds. It corresponds to the growing consensus that soil amendments which enhance microbial diversity have important benefits to ecosystem functioning. © 2016 The Authors. New Phytologist © 2016 New Phytologist Trust