Great question! As with many questions in ecology, it depends. The impact of fires on ecosystem soils tend to depend on the amount of soil heating that occurs, the amount and duration of vegetative cover lost, and the post-fire events that follow. Soils tend to be remarkably good insulators, and in many cases, elevated temperatures due to fire may only go a few inches underground. Because of that, low intensity fires often have minimal impacts on soils. On the other hand, high intensity fires or fires with heavy fuels concentrated on the surface and with long residence times, can have more significant impacts to the soils. In some cases directly volatilizing soil organic matter and soil nitrogen. As we see in more arid places in the western US, post-fire erosion can be a huge issue due to pre-fire heavy fuel loads and post-burn soil exposure due to delayed vegetative response. In the southeastern US, ecosystem vegetation can respond within days of a fire, protecting the soils from erosion. In more arid places, vegetation may not recover for months, leaving the soils exposed to erosion.