Carbon-13 natural abundance as a tool to study the dynamics of lignin monomers in soil: an appraisal at the Closeaux experimental field (France)

Nine years of maize cropping influenced neither the total organic carbon (OC) content nor the lignin content and biodegradation degree in soil. The total SOM after 9 years of maize cropping was significantly enriched in 13C by 1.4‰ compared to the wheat soil. In the CuO-derived lignin phenols, the variations of 13C contents after 9 years of maize cropping ranged from 4.9‰ to 10.0‰, with an average value for total lignin of 7.3‰. These variations were well above the precision of the analytical method, calculated for maize plant and soils to be in the range 0.2-0.8‰. This study thus demonstrates that the CuO oxidation technique is applicable to the determination of the natural isotopic abundance variations in lignin monomers of soils of a C3/C4 chronosequence. Finally, we could calculate the proportion of newly derived OC after 9 years of maize cropping. This proportion was 9% for total SOM and 47% for lignin, which displayed faster dynamics in this soil than total OC. This study confirms, using in situ labeling technique in combination with lignin monomers analysis after CuO oxidation, that lignin macromolecules are not stabilized as such in these soils.