Frequent burning causes large losses of carbon from deep soil layers in a temperate savanna

1. Fire activity is changing dramatically across the globe, with uncertain effects on ecosystem processes, especially belowground. Fire‐driven losses of soil carbon (C) are often assumed to occur primarily in the upper soil layers because the repeated combustion of aboveground biomass limits organic matter inputs into surface soil. However, C losses from deeper soil may occur if frequent burning reduces root biomass inputs of C into deep soil layers or stimulates losses of C via leaching and priming.

2. To assess the effects of fire on soil C, we sampled 12 plots in a 51‐year‐long fire frequency manipulation experiment in a temperate oak savanna, where variation in prescribed burning frequency has created a gradient in vegetation structure from closed‐canopy forest in unburned plots to open‐canopy savanna in frequently burned plots.

3. Soil C stocks were non‐linearly related to fire frequency, with soil C peaking in savanna plots burned at an intermediate fire frequency and declin...