Elucidating the chemical structure of pyrogenic organic matter by combining magnetic resonance, mid-infrared spectroscopy and mass spectrometry

Abstract

Fire-derived organic matter (pyrogenic organic matter, or PyOM), despite its apparent long term stability in the environment, has recently been reported to degrade faster than previously thought. Current studies have suggested that the composition and structure of PyOM can provide new insights on the mechanisms by which C and N from pyrolyzed biomaterials are stabilized in soils. To better understand the chemical structure of PyOM produced under typical fire conditions in temperate forests, samples of dual-enriched (13C/15N) Pinus ponderosa wood and the charred material produced at 450°C were analyzed by solid state nuclear magnetic resonance (ssNMR), electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy, and both isotopic and elemental composition (C, H, O, and N). Notably, the use of high magnetic field strengths in combination with isotopic enrichment augmented the NMR detection sensitivity, and thus improved the quality of molecular information as compared with previously reported studies of pyrogenic materials. The key molecular groups of pine wood and the corresponding PyOM materials were determined using magic-angle spinning (MAS) 13C, 15N, and 1H NMR. Together with DRIFT and EPR measurements, ssNMR revealed the formation of a free radical-containing disordered blend of nitrogenous aromatics and heat resistant aliphatics in the PyOM due to incomplete combustion of the precursor wood. 13C ssNMR and DRIFT analyses showed the removal of oxygenated aliphatics due to pyrolysis of the precursor wood and the dominant contribution of multiply-bonded and oxygenated aromatic structures in the resulting PyOM. However, the 18O isotopic analyses indicated selective retention of ligneous moieties during charring at 450°C. 15N ssNMR studies implied that the nitrogenous species in PyOM corresponded to thermally altered rather than heat resistant domains of the pine wood precursor. Our molecular characterization suggests that biomaterials pyrolyzed near 450°C may degrade in soils faster than those pyrolyzed at higher temperatures and may not represent a stable C sink in terrigenous ecosystems.