Fire removes preexisting pyrogenic organic matter from the ecosystem through the mechanisms of both direct combustion and increasing mineralizability

Pyrogenic organic matter (PyOM) produced during fires plays an important role in the global carbon (C) cycle due to its low microbial availability and resulting high persistence. However, PyOM can be combusted or chemically altered in subsequent fires. To explore the effects of subsequent fire on PyOM, we used a mass loss calorimeter to simulate reburning by delivering realistic heat fluxes to jack pine (Pinus banksiana Lamb.) PyOM produced at 350 °C. We tested three heat flux profiles (High, Low, and Control) and three burial depths (Surface, 1 cm, and 5 cm) arranged in a full-factorial design, with the “high” heat flux profile consisting of heating for 4.72 h with a peak of 85 kW m−2 and “low” heating for 2.5 h with a peak of 45 kW m−2. We found that both variables affected the C remaining after the simulated reburn (p < 0.05), with greater C losses at higher heat fluxes and shallower exposure depths. Consistent with our predictions, treatments with high heat exposure (HFHigh + Surface, HFHigh + 1 cm, HFLow + Surface) showed increases in pH and decreases in total dissolved organic carbon (DOC) and mineralized C after the simulated reburn. Counter to our predictions, treatments with intermediate reburn conditions (HFHigh + 5 cm and HFLow + 1 cm) caused decreases in pH and increases in DOC, mineralized C, modelled decomposable C, and modelled C decomposition rate (p < 0.05), compared to the unburned controls. These findings highlight that, despite its high persistence in fire-free conditions, PyOM can also be readily combusted and altered in subsequent fires, with important implications for changing fire regimes and the global C cycle. Subsequent fires, represented by simulated reburns with realistic heat fluxes, not only consume PyOM produced in previous burns, but can also increase its susceptibility to microbial decomposition, which affects the potential for PyOM to contribute to long-term carbon storage.