Biochar application to soil has the potential to improve soil fertility under certain conditions. However, potential ecological effects remain largely unexplored and poorly understood, particularly those on soil biota. Six biochars were tested on two soil-dwelling invertebrates in short-term bioassays to determine effects on survival and reproduction. A pine wood gasification char increased collembolan adult mortality at high concentrations. Wood slow and fast pyrolysis biochars had a strong stimulatory effect on collembolan reproduction, but no strong effect on enchytraeids. A sewage sludge char was slightly stimulatory for both organisms, and a pine gasification char was inhibitory in both cases. Inhibitory effects were associated with biochars with high carbonate and Ca content and pH. In light of the high stimulation of collembolan reproduction, potential explanations such as soil microbial community shifts or gut symbiont use of biochar are suggested.