Impacts of Repeated Fire on Physical, Chemical, and Biological Characteristics of Pyrogenic Organic Matter in the Boreal Forest System

Excessive fire suppression in the last century and modern climate change have exacerbated the risk of wildfire events in many regions of the US due to the accumulation of fuels, extending warm seasons, and drought events. Prescribed fire can be a useful landscape management tool because of its high utility for ecological restoration and relatively low disturbance to the soil. Both prescribed fire and wildfire can produce pyrogenic organic matter (PyOM), which can be a highly persistent form of organic matter, with implications for the C cycle and mitigating global warming. In order to understand the net effects of fire on the C cycle, we must understand the effects of fire on the PyOM produced during previous fire events. We investigated the effects of burning on the physical, biological, and chemical properties of PyOM. Pinus banksiana (jack pine) PyOM produced at 350 °C was used to simulate the preexisting PyOM in the boreal forest system. 45-55 mm quartz sand was used to simulate a soil matrix typical of the region, holding the samples in a columnar steel container. Nine treatments with variables of fire intensity (high intensity, low intensity, and control) and exposure depth (surface, 1 cm, and 5 cm) were used to dose the PyOM samples. The heat fluxes generated under a cone calorimeter were chosen and quantified based on two experimental burns of two logs prior to the treatment burns. We hypothesize that higher fire intensity and shallower exposure depth will trigger more changes in the physical, chemical, and biological characteristics of PyOM. At the same fire intensity, PyOM on the surface will be subject to more mass loss, C loss, and pH increases than PyOM at both 1 cm and 5 cm depth. At the same depths, high-intensity fire will consume more PyOM and cause greater increases in pH and C persistence than low-intensity fire. Our findings will have implications for understanding the complex interactions between repeated fire, PyOM production and transformation, and the C cycle.