Turnover of carbon in the free light fraction with and without charcoal as determined using the 13C natural abundance method

Charcoal fragments have been reported frequently in the light fraction (LF) and this suggests that charcoal C is an important constituent of LF of North American soils. The LF is considered to have a rapid turnover but charcoal is highly resistant to biological degradation, hence it will have significant implications for studies of LF composition, dynamics and modeling exercises. This study aimed to quantify the contribution of charcoal to free LF (density ≤ 1.8 g cm− 3) C, and its effect on the turnover of C in that fraction using the δ13C technique. Duplicate free LF samples were obtained from the 0–20 cm depth of no-tilled and conventionally tilled soils, each under corn and tobacco/rye cropping. Based on morphological properties, charcoal and plant fragments were handpicked under a light microscope from one set of free LF samples and their δ13C were measured. The δ13C of whole free LF samples were also measured. The chemical properties of charcoal were characterized using solid 13C NMR spectroscopy technique, and these were compared with those documented for thermally generated charcoal. A two end-member mixing model was used to estimate the proportion of free LF C derived from non-charcoal and charcoal residues in continuous corn plots.Light microscopy of charcoal fragments showed a particulate morphology consistent with charcoal and charred plant residues. The chemical properties of charcoal fragments were also consistent with charcoal. The δ13C of charcoal (− 26 to − 25‰) showed that charcoal C was derived entirely from C3 vegetation. Charcoal and non-charcoal C3–C accounted for between 72 to 75% and 25 to 28%, respectively, of the total C3–C in the free LF, indicating that charcoal C was responsible for the persistence of a sizable portion of native C in free LF. The turnover of C3–C in free LF with charcoal was slower than that of C3–C in free LF without charcoal by 2.5 times. These results provide supporting evidence that charcoal C is likely to lead to misinterpretation of dynamics of the LF and suggest that concepts about soil C dynamics and LF turnover may have to be adjusted to account for charred C, and that charring may be an under-appreciated stabilization mechanism.