Relation between lignins and aliphatic constituents in soils highlighted by an isolation procedure: implication for lignin degradation?
Résumé
Lignins are a major aromatic constituent of plant biomass. Lignins were suggested to contribute to stable C in soils. Recent studies, using CuO oxidation, identified two kinetic lignin pools in soils. However, the CuO method releases phenol monomers from the less condensed part of lignins only. The milled wall enzymatic procedure was developed to isolate a fraction of intact lignin polymer (MWEL). We used this technique to study the dynamics of isolated soil-MWEL in the experimental field of “les Closeaux”, where a C3/C4 vegetation succession induced a natural 13C labelling of soil organic matter. We isolated MWEL from maize (C4) and wheat (C3) plants, from a wheat-soil and a soil after 9-years maize cropping. Lignin yields, elementary composition, 13Cisotopic signature and chemical structure were analysed. 13C Nuclear Magnetic Resonance (NMR) spectroscopy revealed that soil-MWEL contained aliphatic compounds, while plant-MWEL were associated with polysaccharides. The presence of aliphatic moieties in soil-MWEL could explain the decrease of their O/C ratio and the increase of their H/C ratio compared to the plantlignins. Close association of lignins with hydrophobic aliphatic constituents in soils could limit their degradability. The evolution of the 13C-isotopic signature of the MWEL-lignins from soil was compared to those of the lignin monomers released by the CuO method and of the bulk SOC. We conclude that lignin fate in soil is not only dependent of their chemical structure but also related to their interactions in soil with other compounds, which nature and variability remain to be defined.