Fire exclusion reduces A‐horizon thickness in a long‐term prescribed fire experiment in Spodosols of northern Florida, USA.

Fire influences soils, but it is difficult to attribute long‐term changes in soils to fire, given the short duration, and irregular return interval of most fires. Prescribed fire, in which landscapes are intentionally burned, offer some experimental control, but study sites where prescribed fire has been applied systematically over decades are rare. We had the opportunity to investigate the effects of prescribed fire on soil horizon thickness in Spodosols from a long‐term study in northern Florida, USA, in which three prescribed fire frequency treatments (1, 2, or 4 years) and an unburned control have been continuously maintained in replicated experimental plots for the last 60 years. We found that the thickness of the A horizon in these soils was diminished in the unburned control plots, relative to those in plots receiving regular fire treatments. While conclusions based on one morphological measurement, thickness, certainly have limitations, drivers of this peculiar finding may have implications for carbon storage and vulnerability in these fire‐maintained landscapes. Core ideas: Fire impacts soils, but its temporary and irregular occurrence presents challenges to researchers.Prescribed fire, where it is applied systematically, offers an opportunity to understand fire effects on soils.At a prescribed fire study that has been maintained for 60 years, we measured mineral soil horizon thicknesses.Thickness of the A horizon was diminished by an average of 2.6 cm in unburned controls compared to burned plots. [ABSTRACT FROM AUTHOR]