Effects of ant mounts (Formica exsecta) on subsoil properties, in a heathland

Ants are undisputed masters at transforming the local environments they inhabit, with subsequent vast effects on soil chemical and hydrological processes. Yet, it remains unclear how deep into the subsoil these effects range, as most ant-soil studies focus on the topsoil. Furthermore, studies quantifying these effects on podzolized, and nutrient-poor heathland soils remain scarce. We excavated 15 Formica exsecta ant mounds on a long-term, unmanaged heathland in Denmark. We sampled soil moisture, soil penetration resistance (SPR), pH, total phosphorous content, and the thickness of each soil horizon at three positions at each mound: directly below the mound, at the edge of the mound, and an adjacent undisturbed reference soil. Results revealed that ant activity reduced soil moisture, loosened the soil, and increased the flow of total phosphorus to the deeper layers. Importantly, the cemented spodic horizons (hardpans) with waterlogging properties were penetrated by ant digging, resulting in potentially higher water infiltration into the subsoil. The ant activity within the otherwise undisturbed sandy subsoil below the hard pan caused a slight alteration in the thickness of each soil horizon and chemistry. These patchy, small-scale disturbances (mounds covered 0.06 % of the site) increase heathland soil heterogeneity and affect subsoil properties in time. We conclude that ant mounds may play a previously overlooked role in heathland soil dynamics by penetrating the heathland hardpans and manipulating soil chemistry and soil moisture. We argue that a viable mound-forming ant community is valuable for the soil heterogeneity of dry heathland ecosystems.