Changes in fire-derived soil black carbon storage in a subhumid woodland

Fire‐derived black carbon (BC) in soil, including charcoal, represents an important part in terrestrial carbon cycling due to its assumed long persistence in soil. Soil BC concentrations for a woodland in central Texas, USA, was found from study plots with a fire scar dendrochronology spanning 100 years. BC values were initially determined from 13C nuclear magnetic resonance (NMR) spectroscopy. The NMR‐based BC concentrations were used to calibrate midinfrared vibrational spectra (MIRS) for evaluation as a less expensive and expedient technique. However, unexpectedly high BC values from the MIRS method were found for sites without evidence of fire for the past 100 years. Estimation of BC from NMR technique showed mean BC concentration of 2.73 ± 3.06 g BC kg−1 (0.91 ± 0.51 kg BC m−2) for sites with fire occurrence within the last 40 years compared with BC values of 1.21 ± 1.70 g BC kg−1 soil (0.18 ± 0.14 kg BC m−2) for sites with fire 40–100 years ago. Sites with no tree ring evidence of fire during the last 100 years had the lowest mean soil BC concentration of 0.05 ± 0.11 g BC kg−1 (0.02 ± 0.03 kg BC m−2). Molecular proxies of stability (lignin/N) and decomposition (Alkyl C/O‐Alkyl C) showed no differences across the sites, indicating low potential for BC mineralization. Modeled soil erosion and time since fire from fire scar data showed that soil BC concentrations were inversely correlated. These results suggest that the addition of BC may be limited by topography and timing of fire. Key Points Changes in soil BC over time indicate transport of BC during soil erosionSoil BC concentrations are inversely related to soil erosion and time since fireNMR techniques provide the most reliable way to determine soil BC