Summary: We investigated the impact of biochar application on fungal (acetate incorporation into ergosterol) and bacterial (leucine incorporation) growth rates in two case studies: a temperate UK pasture soil and a Mediterranean Australian agricultural soil. We added biochar at similar rates per unit of soil organic carbon (SOC) and monitored both the immediate (after 1week equilibration) and longer-term (1-3years) effects. The immediate effect of the biochar applied to the UK soil was a decreased fungal-to-bacterial growth ratio, driven by greater bacterial growth. The immediate effect of biochar application to the Australian soils was subtle, only slightly increasing the fungal-to-bacterial growth ratio. In both case studies, the biochar effects were transient, and no long-term effects (1-3years) on microbial growth rates could be detected in either soil. The bacterial growth increase in the UK soil was probably related to a release of large amounts of labile C from the biochar, or as C released from the resident SOM caused by the biochar-induced pH increase. The increase in fungal-to-bacterial growth ratio could be related to a release of poor quality C in the Australian soils. There were immediate effects of biochar application on microbial growth in agricultural soils, but they were disparate between cases, making any generalization of mechanisms difficult. However, the microbial responses were consistently transient. Taken together, biochar application to agricultural soil appears to have an impact upon the decomposer community, suggesting limited resistance. However, the microbial functioning appeared resilient to these effects, stabilizing microbial communities to their initial state within 1-3years of application. © 2013 British Society of Soil Science.