The origin of chemical complexity during plant litter decomposition

Background/Question/Methods

The process of plant litter decomposition is a critical step in the carbon cycle, and while the factors governing decay rate are well known, those that drive changes in litter chemistry remain poorly understood.  The current paradigm predicts that as chemically distinct litter types decompose their chemistries will eventually become indistinguishable.  This convergence in chemical composition is thought to occur regardless of initial differences in litter quality or in situ differences in biological communities.  We conducted a litter decomposition experiment over three growing seasons to determine how the chemical traits of qualitatively different litter types change during decomposition, and the degree to which these changes are linked to shifts in decomposer communities.  High and low quality litter types were decomposed for 730 days in three ecosystems with different decomposer communities.  During the course of decomposition, we tracked changes in microbial and microarthropod communities, litter enzyme activities, and litter chemistry using a high-resolution molecular method.

Results/Conclusions

In contrast to the current paradigm of chemical convergence, we observed that different litter types remain chemically distinct throughout decomposition.  Further, some novel chemical differences between litter types emerged during decomposition.  For example, lignin derivatives, which did not differ among litter types initially, differed markedly after extensive decomposition (~80% mass loss).  In addition, when decomposed in ecosystems with different decomposer communities, the chemistry of a single litter type diverged considerably.  Ecosystem-driven differences in litter chemistry persisted beyond 90% mass loss; however, their frequency and magnitude varied between high and low quality litter.  The divergence in chemical composition between litter types and among ecosystems was associated with strong differences in decomposer communities.  Our results suggest that initial litter quality and decomposer community characteristics have interactive effects on litter chemistry during decomposition and can ultimately lead to a divergence in chemical traits rather than a convergence as the current paradigm suggests.  Differences in decomposer community structure may, therefore, have long-term influence over the contributions of plant litter to soil organic matter.