The impact of fire on soil-dwelling biota: A review

Fire has always been a driving factor of life on Earth. Now that mankind has definitely joined the other environmental forces in shaping the planet, lots of species are threatened by human-induced variation in fire regimes. Soil-dwelling organisms, i.e., those organisms that primarily live in soil, suffer the numerous and different consequences of fire occurrence that are, however, often overlooked compared to those on vegetation and wildlife. Most of these organisms live in the uppermost soil layer, where fire-imposed temperatures on the ground are the highest insofar as they are lethal or dangerously upset natural habitats. This contribution is a reasoned collation of findings from a number of works conducted worldwide that aims to gain insight into the immediate and longer-term impacts of single or repeated wild or prescribed fires on one group of soil-dwelling organisms or more. In fire-prone ecosystems, fire is a controlling factor of soil biota biodiversity and activity, but also where it is infrequent its ecological footprint can be substantial and lasting. Generally, the immediate fire impact on soil biota is strictly related to the peak temperatures reached on the ground and their duration, and on a set of soil properties and water content. Vertebrates can escape overheating death by running away, searching for wet niches or burrowing deep into soil. Invertebrates and microorganisms, which have little or no mobility, succumb more easily to fire, but make up for this intrinsic vulnerability thanks to their greater fecundity at the population level. Fire or burn severity, which can generally be defined as loss of organic matter aboveground and belowground, is the key factor of the indirect fire effects on soil-dwelling biota; whereas controlled burns do not often imply any substantial and lasting shift from the original situation, extreme and vast wildfires can have major consequences that may be severer than direct killing. In fact lairs are devastated, nutrient pools are heavily affected, food webs are upset, soil temperature and moisture regimes change, and toxic pyrogenic compounds remain in soil. All types of organisms can recolonise the burned area from their sanctuaries, provided that land use does not change, e.g., to pastures or arable fields, and prompt enough vegetation re-sprouting and/or encroachment prevent substantial soil erosion. Each major taxon has genera or species with useful traits and behaviours to resist fire or to recover from its unwelcome environmental legacy sooner than others. If burned soil does not undergo other fires that occur too closely together for the typical fire regime of that particular area, most of its living components are generally capable of returning to pre-fire levels in times that depend on a series of factors, such as fire severity and post-fire rainfall.