The transformation and mobility of charcoal in a fire-impacted watershed

The incomplete combustion of fossil fuels and biomass has resulted in the global-scale distribution and accumulation of black carbon (BC) in the environment. Recently, the molecular identity of BC in the dissolved phase has been distinguished from that of natural organic matter. However, many of the processes that control BC cycling remain unidentified. We investigate changes in soil charcoal particle morphology and chemical composition using surface area analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy, chemical oxidation, and ¹³C NMR spectroscopy. A comparison of soil charcoals differing in age by 100 years shows that aged charcoal has lower specific surface areas, higher BC/OC ratios, direct associations with soil minerals and microbial biomass, and a greater abundance of non-aromatic carbon. The water-soluble portion of soil charcoal and dissolved organic matter (DOM) from the watershed were also characterized by electrospray ionization mass spectrometry. Aqueous charcoal extracts are comprised mostly of condensed aromatic ring structures (CARS) which are also present in soil pore, river, and ground water samples. We present indirect evidence and a chemical rationale for a microbial BC dissolution mechanism. Furthermore, the speciation of CARS in the soil solution versus river and ground water provides molecular evidence of reactivity in the dissolved phase. The dissolution and export of soil BC are presently unmeasured fluxes with important implications for the global carbon cycle.