Does lignin composition control litter decay rates?

Background/Question/Methods: Lignification of plant cell walls can slow rates of litter decomposition. While lignin concentration often has negative or neutral effects on decay rates, the role of lignin composition in controlling microbial degradation of litter is unknown. We tested the hypothesis that the chemical composition of lignin influences litter decay rates by controlling microbial access to the polysaccharide and protein components of plant cell walls. To test this hypothesis, we used three mutants of Arabidopsis thaliana that varied in the chemical composition of lignin. Mutants were either up-regulated or down-regulated in the synthesis of syringyl, 5-hydroxyguaiacyl, or cinnamylaldehyde lignin subunits relative to the wild type. Plants were decomposed for one year in the boreal forest of interior Alaska using a litterbag method. Decomposition rate was calculated as total mass loss over the decomposition period. Differential degradation of labile polysaccharides was determined by assaying the chemical composition of tissues before and after decomposition. Results/Conclusions: Across all plant types, lignin chemical composition had a significant effect on decomposition rate (P=0.01). However, the effect depended on the type of lignin modification. Plants deficient in syringyl units or containing an overabundance of 5-hydroxyguaiacyl units in their lignin decomposed at a rate similar to wild type plants, suggesting that the well-established differences in syringyl lignin content among plant clades do not explain differential rates of litter decay. By contrast, plants with high levels of cinnamylaldehydes incorporated into their lignin decomposed faster than wild type plants, which contained more cinnamyl alcohol units. The difference in decomposition rate was due to greater loss of cellulose and N in the high cinnamylaldehyde plants compared to the wild type. However, loss of lignin was similar among plant types. These results suggest that cinnamylaldehyde-rich lignins may offer less chemical protection of labile polysaccharides and protein via cross-linking. Analysis of cell wall structure in decomposed litter is required to determine how the incorporation of cinnamylaldehydes into lignin affects the structure and chemical properties of the lignocellulose matrix.