Direct molecular evidence for the degradation and mobility of black carbon in soils from ultrahigh-resolution mass spectral analysis of dissolved organic matter from a fire-impacted forest soil

Abstract

The molecular composition of water-soluble products generated by the natural degradation of charcoal particles over a period of 100 years in a temperate forest soil has been investigated by ultrahigh resolution mass spectrometry with electrospray ionization. The detectable products are condensed aromatic ring structures extensively substituted with oxygen-containing functional groups, indicating that oxidation and dissolution of charcoal black carbon occurs on a centennial timescale. Many of the same species are also detected within the dissolved organic matter (DOM) of the forest’s soil pore waters. We introduce the calculation of carbon normalized double bond equivalents (DBE/C) as a structural determinant for the empirical formulas obtained by mass spectral analysis. A threshold DBE/C value of 0.7 serves as a criterion for identifying species with condensed aromatic ring structures (CARS). A comparison with ultrahigh resolution mass spectra from previous studies shows that many of the CARS extracted directly from soil BC have the same mass (within 1 ppm) and empirical formulas as CARS detected in volcanic ash soil humic acid (HA) from Japan, and Amazonian Rio Negro DOM. The similarity of water-soluble condensed aromatics present within, and exported from fire-impacted soils of geographically and climatically disparate ecosystems indicates that the CARS reported herein are the molecular fingerprint of black carbon degradation in soils. Understanding the production mechanisms, reactivity, and fate of these molecular species should provide new insight to BC degradation and cycling. The soil charcoal particles at this site are infiltrated by filamentous microorganisms, suggesting that saprophytic fungi may be important to soil BC degradation processes.