Toward a "molecular thermometer" to estimate the charring temperature of wildland charcoals derived from different biomass sources.

The maximum temperature experienced by biomass during combustion has a strong effect on chemical properties of the resulting charcoal, such as sorption capacity (water and nonpolar materials) and microbial degradability. However, information about the formation temperature of natural charcoal can be difficult to obtain in ecosystems that are not instrumented prior to fires. Benzene polycarboxylic acids (BPCA) are molecular markers specific for pyrogenic carbon (PyC) which can provide information on the degree of aromatic condensation in charcoals. Here we apply the BPCA molecular marker method to a set of 10 charcoals produced during an experimental fire in a Pitch pine-scrub oak forest from litter and bark of pitch pine and inkberry plants in the Pinelands National Reserve in New Jersey, USA. We deployed temperature-sensitive crayons throughout the burn site, which recorded the maximum air temperature and made comparisons to the degree of thermal alteration recorded by BPCA molecular markers. Our results show an increase of the degree of aromatic condensation with monitored temperatures for bark biomass, while for needles no clear trend could be observed. For leaf-derived charcoals at increasing monitored fire temperatures, decreasing degree of aromatic condensation was obtained. This suggests that molecular markers can be used to roughly estimate the maximum fire temperatures experienced by bark and wood materials, but not based on leaf- and needle-derived materials. Possible applications include verifying declared pyrolysis temperatures of biochars and evaluating ecosystem fire temperature postburn.