Some studies suggest that some applications of biochar may reduce earthworm populations in the short term, but biochar seemingly has no long-term implications for earthworm populations. These results depend on how much biochar is used, and lower biochar concentrations are less likely to affect earthworms. Most likely, these results were due to the fact that biochar raises soil pH levels, so it’s best to use lower concentrations of biochar if you’re worried about the health of earthworms.