The carbon isotope composition of semi-labile and stable pyrogenic carbon in a thermosequence of C3 and C4 derived char

To better elucidate the reactions forming pyrogenic carbon (PyC) during pyrolysis, we investigated the carbon isotope fractionation trends in thermosequences for biomass types utilizing C3 and C4 photosynthetic pathways. PyC remaining after pyrolysis was treated using hydrogen pyrolysis to isolate the stable polycyclic aromatic carbon (SPAC) component and the semi-labile carbon component (PyCSL) was estimated from mass balance (the component that survived pyrolysis, but was not SPAC). C isotope fractionation trends as a function of pyrolysis temperature were determined for each of these three components (PyC, SPAC and PyCSL) relative to the C isotope composition of the bulk raw biomass. Although the isotope fractionation patterns for all materials were similar for total PyC, differences were noted between C4 and C3 biomass for isotope fractionation patterns of SPAC. The δ13C values of SPAC were higher than the original biomass for C3 material, yet similar for C4 material at formation temperature 300–700 °C. The δ13C values of PyCSL for all materials displayed distinct and progressively lower isotope composition relative to original biomass at higher temperature. The results, in combination with Fourier transform infrared patterns, indicated that the dominant source of SPAC is cellulose, and that lignin decomposes at higher temperature with very low δ13C MeO-rich lignin moieties preferentially surviving in PyCSL. The δ13C values of SPAC were 0.2 ± 1.2‰ of the starting material, suggesting that this component might be used to determine the dominant source of PyC, although environmental mixtures of PyC in natural settings are more complex than those studied.