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 effect of subsequent fire on PyOM, we used a mass loss calorimeter to deliver realistic heat fluxes to jack pine (Pinus banksiana Lamb.) PyOM produced at 350 {degrees}C, testing heat flux profiles (High, Low, and Control) and burial depths (Surface, 1 cm, and 5 cm) in a full-factorial design. We found that both variables significantly affected the C remaining after burns (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+1cm, HFLow+Surface) showed increases in pH and decreases in total dissolved organic carbon (DOC) and mineralized C after the subsequent fire. Counter to our predictions, treatments with intermediate heat exposure (HFHigh+5cm and HFLow+1cm) caused significant 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 is readily combusted and altered in subsequent fires, with important implications for changing fire regimes and the global C cycle. Synopsis StatementSubsequent fires not only consume PyOM produced in previous burns, but can also increase its susceptibility to microbial decomposition, which challenges the potential for PyOM to contribute to long-term carbon storage.