Carbon isotopic fractionation during decomposition of plant materials of different quality

Changes in isotopic C-13 composition of solid residues and CO2 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 C-13 NMR. Roots were more lignified than aerial parts of the same species. Lignin was always depleted in C-13 (up to 5.2%) as compared with cellulose from the same sample. Proteins were enriched in C-13 in C-3 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 C-13 depleted as compared with the initial substrate but enriched at posterior stages. Consequently, with ongoing decomposition, the solid residue became C-13 depleted, which could only partly be explained by an accumulation of lignin-C. The extension of the initial C-13 depleted CO2-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 CO2-C when incubated in soils. We discuss possible implications of these mechanisms of C-13 fractionation in ecosystems.