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Changes in isotopic ¹³C composition of solid residues and CO₂ evolved during decomposition of C₃ and C₄ plant materials were monitored over 10 months to determine carbon isotopic fractionation at successive stages of biodegradation. We selected plant materials of different chemical quality, e.g., Zea mays (leaves, stems, coarse roots, and fine roots), Lolium perenne (leaves and roots), Pinus pinaster (needles), and Cocos nucifera (coconut shell) and also characterized these by solid‐state ¹³C NMR. Roots were more lignified than aerial parts of the same species. Lignin was always depleted in ¹³C (up to 5.2‰) as compared with cellulose from the same sample. Proteins were enriched in ¹³C in C₃ plants but depleted in maize. Cumulative CO₂ evolved fitted a double‐exponential model with two C pools of different lability. During early stages of decomposition, the CO₂‐C released was usually ¹³C depleted …