The role of lignin in the saprotrophic degradation of plant biomass in boreal forest soil.

The amount of carbon stored in boreal forests soil as Soil organic matter (SOM) is significant. Carbohydrate polymers such as cellulose and hemicellulose constitute 40-50% of the SOM mass in the surface mor layer, even in SOM that has been decomposed for decades to centuries. This is in contrast to conceptual decomposition models assuming aromatic and aliphatic polymers to constitute the fraction of recalcitrant SOM. One prevailing view for support is that lignin manifest itself as a factor in the stabilization of carbohydrate polymers as SOM. However, detailed elucidation of how the complex array of molecular moieties making up SOM decompose over time is lacking. Here we investigated the effect of lignin content and composition during the progressive degradation of polymeric carbohydrates, lignin, and lipids in the lab during a year-long soil decomposition study using Aspen (Populus tremula) wood as a model substrate. To specifically address lignin decomposition we used a range of Aspen clones that varied naturally in their lignin content (high lignin 30% - low lignin 25%) with boreal coniferous forest soil obtained from the surface moor layer (O-horizon). The decomposition of the different molecular moieties of the model substrate was evaluated by Two-dimensional (2D) liquid state 1H–13C nuclear magnetic resonance (NMR) spectroscopy. In addition, the CO₂ production during decomposition was monitored continuously and assays for exo-enzymatic activity was carried out at selected time points.

The NMR spectroscopy revealed that for different periods of decomposition, saprotrophic microorganisms preferred different monomers of polymeric lignin, carbohydrates, and lipids. The relative degradation of resinol, spirodienone, and cinnamyl alcohol were higher among lignin interlinkages and the relative degradation of p-hydroxybenzoate and syringyl were higher among lignin subunits. For carbohydrates, the relative degradation of mannose and glucose were higher than that of e.g. xylose. The relative degradation of unsaturated fatty acids was higher among lipids. The lignin: carbohydrates ratio decreased linearly over the period of decomposition. This showed that the initial degradation of lignin compounds was greater compared to the decomposition of carbohydrate compounds. The significant difference in the relative degradation of mannose among model substrate with different lignin content showed that lignin had no effect on cellulose degradation but may have had an effect on the preferential degradation of hemicelluloses. The high-resolution decomposition patterns we observe are crucial for obtaining a detailed mechanistic understanding of plant polymer decomposition by soil microorganisms during the initial stages of SOM genesis.

 

Keywords: Soil organic matter (SOM), Lignin, Carbohydrates, 2D NMR, Decomposition