Interactions between lignin, cellulose, and N are major controls over litter chemistry-decay relationships

Background/Question/Methods

Litter decay rates often correlate with the initial ratios of lignin:nitrogen (N) or lignin:cellulose in litter. However, the chemical and microbial mechanisms giving rise to these patterns are still unclear. We addressed this issue by studying the decomposition of a model plant system, Arabidopsis thaliana, in which plants were either down-regulated in the biosynthesis of either lignin or cellulose, or were grown with low or high levels of litter N. We decomposed plants in litterbags for one year in control and N-fertilized plots in an Alaskan boreal forest, then measured rates of total mass loss and the change in chemical composition of the litters. We hypothesized that 1) lignin offers protection of cellulose, hemicellulose, and N-rich protein from degradation, 2) cellulose is a labile C source for decomposers, and 3) litter N alleviates N limitation of decomposers. For these reasons, we expected total litter mass loss to increase in the order low N plant < low cellulose plant < unmodified wild type < low lignin plant. In addition, we hypothesized that fertilizer N should increase cellulose losses and reduce lignin losses by eliminating the need for microbes to degrade lignin to access litter N. 

Results/Conclusions

Plants differed in total decay rate (P = 0.0002) and each chemical component had significant effects on the decay rate of other components. Plants with low N content decomposed slower than control plants, due to lower losses of lignin, N, and soluble C. These data support the hypothesis that lignin, N, and soluble C degradation are limited by N availability. Low lignin mutants lost higher proportions of cellulose (P < 0.0001) and hemicellulose (P = 0.0040) from litter, supporting the hypothesis that lignin protects cell wall polysaccharides from decay. Low cellulose mutants lost lower proportions of lignin (P = 0.0005) but higher proportions of the initial cellulose (P < 0.0001), indicating that cellulose is required for lignin breakdown but lignin degradation is not required for cellulose breakdown. Collectively, our results indicate that lignin degradation is not controlled by access to C and N resources, but might instead be influenced by other factors such as competition between decomposers for space. N fertilization increased cellulose loss from litter (P = 0.0382) and tended to reduce lignin losses (P = 0.0528), suggesting a tradeoff between lignin degradation and cellulose degradation that is regulated by external N availability.