Molecular heterogeneity in pyrogenic dissolved organic matter from a thermal series of oak and grass chars

Abstract

Pyrogenic organic matter (Py-OM), generated via the incomplete combustion of biomass, is well studied due to the presence of slow-cycling, condensed aromatic compounds (ConAC) known to sequester in soils and sediments. Recently, dissolved Py-OM (Py-DOM) has received interest, due to its higher mobility and potential to be transferred through watersheds to aquatic systems. Py-DOM quantities, molecular identities and importance to global carbon budgets and cycles are only beginning to be understood. Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) analyses were performed on Py-DOM isolated from oak and grass biochars produced over a range of temperatures (250–650 °C), and the data are compared to complementary 1H NMR spectroscopic and benzenepolycarboxylic acid biomarker (BPCAs) analyses. Py-DOM is revealed to be a heterogeneous mixture of compounds ranging in relative aromaticity and oxygenation. FTICR-MS analyses demonstrate a decrease in Py-DOM relative oxygen content and a concurrent increase in aliphatic character and heteroatomic (N, Cl) content. 1H NMR and BPCAs analyses detail low O/C dissolved ConAC not observed by FTICR-MS, demonstrating the necessity for a multiple proxy approach to Py-DOM characterization. Heterogeneous Py-DOM is explained as resulting from pyrolysis-initiated and radical-mediated functional group cleavage, aromatic condensation and aromatic ring-opening reactions. Oak biomass progresses faster along a char maturity continuum for a given pyrolysis temperature, perhaps due to its greater lignin content or radical quenching by grass cuticular material. Biomass species and pyrolysis temperature likely result in different Py-DOM compositions and fates and must be considered when evaluating the impacts of wildfires and biochar applications.