Charcoal Carbon in U.S. Agricultural Soils

High levels of charcoal C resulting from repeated historical burning of grasslands, open woodlands, and agricultural crop residues have been reported in soils from Australia and Germany. In this study, five U.S. soils were selected from long‐term research plots in widely different agricultural areas. The charcoal C content was estimated on each soil using a combination of physical separation, high energy photo‐oxidation and solid‐state 13 C nuclear magnetic resonance (NMR) spectroscopy. These analyses showed that all five soils contained measurable amounts of charcoal C, <53 μm in size and ranging from 1.8 to 13.6 g C kg −1 soil and constituted up to 35% of the soil total organic C (TOC). Scanning electron microscopy showed that the charcoal material had a plant‐like morphology but were blocky and had fractured edges. These particles were similar in morphology to those separated from Australian and German soils. The implications of this material, which must be highly resistant to microbiological decomposition, to the soil C cycle are discussed.