Carbon isotopic fractionation during decomposition of plant materials of different quality

Changes in isotopic 13 C composition of solid residues and CO 2 evolved during decomposition of C 3 and C 4 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 13 C NMR. Roots were more lignified than aerial parts of the same species. Lignin was always depleted in 13 C (up to 5.2‰) as compared with cellulose from the same sample. Proteins were enriched in 13 C in C 3 plants but depleted in maize. Cumulative CO 2 evolved fitted a double‐exponential model with two C pools of different lability. During early stages of decomposition, the CO 2 ‐C released was usually 13 C depleted as compared with the initial substrate but enriched at posterior stages. Consequently, with ongoing decomposition, the solid residue became 13 C depleted, which could only partly be explained by an accumulation of lignin‐C. The extension of the initial 13 C depleted CO 2 ‐C phase was significantly correlated with the labile substrate C content, acid‐detergent soluble fraction, and total N, pointing to a direct influence of plant quality on C isotopic dynamics during early stages of biodegradation. This isotopic fractionation can also lead to an underestimation of the contribution of plant residues to CO 2 ‐C when incubated in soils. We discuss possible implications of these mechanisms of 13 C fractionation in ecosystems.