Inferred bioavailability of pyrogenic organic matter compared to natural organic matter from global sediments and surface waters

Wildfires are increasing in severity and extent, creating many negative consequences for aquatic ecosystems. Pyrogenic materials generated by wildfires are transported across terrestrial landscapes into inland waters, where ~10% of organic matter pools may be comprised of black carbon (BC), a major component of pyrogenic organic matter (PyOM). Yet, the heterogeneity of PyOM from various fuels and burn conditions complicates efforts to understand its bioavailability. We used a substrate-explicit model to predict the energy content, metabolic efficiency, and rate of aerobic decomposition of representative PyOM compounds. This enabled us to systematically evaluate a full spectrum of PyOM chemistries that is unfeasible with laboratory experiments. The model relies on the elemental stoichiometry, allowing comparison of known PyOM chemistry to formula assignments of natural organic matter (NOM) from a recent high resolution mass spectrometry assessment of global aquatic NOM. Overall, we found the range of predicted bioavailability of PyOM was similar to NOM. Phenolic and BC molecules had lower metabolic efficiency than other PyOM and NOM compounds, and BC metabolism was less negatively impacted by oxygen limitation. In total, our work supports the recent paradigm shift regarding PyOM bioavailability, highlighting its potential role in global C emissions as the prevalence of wildfires increases.