Molecular dynamics of shoot vs. root biomarkers in an agricultural soil estimated by natural abundance C labelling

The aliphatic biopolyesters cutins and suberins have been suggested to significantly contribute to the stable pool of soil organic matter (SOM), and to be tracers for the above- or belowground origin of plant material. Contrary to other plant-derived aliphatic molecules found in the lipid fraction of soils, the stable isotope derived estimates of turnover of cutins and suberins have never been studied in soils. The aim of this study was to analyse the dynamics of shoot- and root-derived biomarkers in soils using a wheat and maize (C₃/C₄) chronosequence, where changes in the natural ¹³C abundance can be used to evaluate the incorporation of new carbon into SOM at the molecular level. The relative distribution of aliphatic monomers in wheat and maize roots and shoots suggested that α,ω-alkanedioic acids can be considered as root-specific markers and mid-chain hydroxy acids as shoot-specific markers.The contrasting distribution of the plant-specific monomers in plants and soils might be explained by different chemical mechanisms leading to selective degradation or stabilization of some biomarkers. The changes of the ¹³C isotopic signatures of these markers with years of maize cropping after wheat evidenced their contrasted behaviour in soil. After 12 years of maize cropping, shoot markers present in soil samples probably originated from old C₃ vegetation suggesting that new maize cutin added to soils was mostly degraded within a year. The reasons for long-term stabilization of shoot biomarkers remain unclear. By contrast, maize root markers were highly incorporated into SOM during the first six years of maize crop, which suggested a selective preservation of root biomass when compared to shoots, possibly due to physical protection.