Functional differences among decomposer communities explain litter chemistry controls over decay

Background/Question/Methods

Linking community composition to ecosystem function is a central goal of ecology. Decomposition is one of the most important ecological processes that occurs in ecosystems, yet the mechanisms by which microbial community structure is linked to litter decay rates are poorly understood. We addressed this issue by studying the fungal community structure on decomposing litter of a model plant system, Arabidopsis thaliana. Plants were either down-regulated in lignin or cellulose biosynthesis, or had 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 and measured fungal communities on the litters with high-throughput sequencing. We hypothesized that 1) lignin reduces decay rates by restricting the growth of non-lignin decomposers, 2) cellulose increases decay rates by providing labile C to lignin and cellulose decomposers, and 3) litter N increases decay rates by alleviating N limitation of recalcitrant C decay. For these reasons, we expected fungal communities to be similar among wild type, low cellulose, and low N plants. However, we expected that fungal communities would be distinct between low lignin plants and wild type plants. In addition, we hypothesized that fertilizer N would increase competition between cellulose- and lignin-degrading fungi, leading to dominance of efficient cellulose- and N-degraders at later stages of decay. 

Results/Conclusions

Decomposer fungal communities clustered by Arabidopsis plant type with significantly different fungal communities occurring on wild type, low lignin, and low cellulose plants (P = 0.0004). Low lignin mutants lost higher proportions of cellulose (P < 0.0001) and hemicellulose (P = 0.0040) from litter, supporting the hypothesis that lignin excludes the growth of polysaccharide decomposers. However, distinct communities on the low cellulose plants do not support the hypothesis that decomposers are functionally redundant in cellulose use. Low cellulose mutants lost lower proportions of lignin (P = 0.0005) but higher proportions of the initial cellulose (P < 0.0001), indicating that strong competitors for cellulose displaced other decomposer taxa under low resource conditions. Collectively, our results indicate that lignin and cellulose controlled decay by feedbacks between resource supply and competitive interactions among decomposers. N fertilization caused a shift in decomposer community composition (P = 0.021) that coincided with increased cellulose loss from litter (P = 0.0382) and reduced lignin losses (P = 0.0528), suggesting that cellulose degraders were competitively dominant under high N availability.