The following are extracts from an article posted by Thayer Tomlinson (to the link below).
Amending Healthy Soil
…”Until recently, it was believed that biochar’s beneficial soil impacts were seen primarily in soils with significant constraints, but two recent publications examine its impact on more fertile Midwestern agricultural soils, showing that biochar can benefit even healthy, fertile soils.”
…
“An interesting study came out in 2010 that examines the impact of biochar not in terms of nutrient addition but of stimulating plant growth. The paper “Biochar impact on development and productivity of pepper and tomato grown in fertigated soilless media” by Graber et al[4] found that when biochar-treated pots were compared against controls, plant development was enhanced. The impacts of biochar on plant development were not due to direct or indirect effects on plant nutrition as both the biochar-treated pots and the control had the same leaf nutrient content (the biochar was a nutrient-poor biochar to ensure the same nutrient content). The research team found two alternatives to explain the improved plant performance under biochar treatment; first, that the biochar “stimulated shifts in microbial populations towards beneficial plant growth promoting rhizobacteria or fungi, due to either chemical or physical attributes of the biochar” and/or that the “low doses of biochar chemicals, many of which are phytotoxic or biocidal at high concentrations, stimulated plant growth at low doses (hormesis).”

http://www.ecolandscaping.org/05/biochar/recent-research-on-biochar%E2%80%99s-potential-in-soils/