Tuesday, November 14, 2006

Turnover of biomarkers in a cropland soil: the cutin and the suberin.

Mercedes Mendez-Millan, Marie-France Dignac, Cornelia Rumpel, and Sylvie Derenne. BioEMCo, Bldg EGER, Thiverval-Grignon, 78850, France

Plant residues and microorganisms are the main sources of organic matter in cropland soils. However humification and degradation processes turning the fresh plant material into stabilised soil organic matter (SOM) remain unclear. One major question concerns the contribution of aboveground versus belowground plant residues.
To improve knowledge on degradation and stabilization processes in the soil matrix, we studied the fate of the two plant biomarkers the cutin and the suberin, present respectively in the aboveground and the belowground tissues of higher plants.
The aim of this study was to identify and quantify these biopolyesters as well as to follow their incorporation into the SOM pool thanks to their monomeric composition. The cutin and the suberin derived monomers were recovered after saponification of plant and soil samples.
The conceptual approach included the analysis of two cropping plants with two different 13C contents: wheat, a C3 plant (d13C ≈ -27 ‰) and maize, a C4 plant (d13C ≈ -12 ‰). In addition the soil from the Closeaux experimental field was analysed. This soil consists of a twelve year chronosequence of maize crop replacing the previous wheat crop. The analysis of the natural 13C labeling of biopolyester monomers allows following their incorporation into the SOM pool.
Analysis of the aboveground and the belowground biomass showed a similar monomeric composition. Differences between the two biopolyesters were recorded with regards to the relative abundance of each monomer. Leaves contain a high concentration of isomeric compounds (7,16-, 8,16-, 9,16- and/or 10,16-)-dihydroxy hexadecanoic acids and 18-hydroxy octadec-9-enoic acid whereas in roots the major monomer is the 22-hydroxy docosanoic acid. Variation of the natural 13C labeling of monomers in soil does not exceed 4 ‰ after 12 years of maize cropping. These results indicate a progressive incorporation of cutin and suberin into SOM.