Road warriors diminish the home-field advantage: Does the functional breadth of decomposer communities explain variation in leaf litter decomposition rates?

Background/Question/Methods

Leaf litter decomposition is a rate-limiting step in the remobilization of previously-fixed carbon and nutrients. Rates of litter decomposition vary with plant and microbial community composition; this fluctuation is variously attributed to home-field advantage or substrate- and matrix-quality interactions. However, these explanations neither account for variation in nutrient availability nor recognize empirical evidence that home-field advantage is often stronger for low-quality than high-quality leaf litter. Here, we test the Functional Breadth hypothesis (FBH), which posits that microbial communities exposed to low-quality litter have a greater functional breadth than those exposed to high-quality litter. FBH predicts that high-quality litter decomposes rapidly regardless of litter matrix quality. In contrast, low-quality litters are expected to decompose more rapidly in low-quality litter matrices than in high-quality matrices because microbes therein uniquely able to decompose both high and low quality litter, i.e. have greater functional breadth. To test FBH, we performed a leaf litter decomposition experiment across 20 plots spanning a gradient in the distribution of ecto- (ECM) and arbuscular mycorrhizal (AM) trees in southern Indiana. In October 2011, we set out 10 low-quality (oak; ECM) and 10 high-quality (maple; AM) litter bags in each plot and periodically harvested them to assay litter decay rates.

Results/Conclusions

In support of the FBH, oak litter exhibited positive home field advantage and substrate quality-matrix quality interactions while maple litters did not. Decomposition of oak litter was positively correlated with the relative abundance of ECM litter (p<0.001; R²=0.63) and oak litter (p=0.001; R²=0.46) at a site. In contrast, mass loss of maple litter was driven by neither the relative abundance of AM litter (p=0.450) nor the relative abundance of maple litter (p=0.097), but decomposed more rapidly than oak leaf litter (p=0.004). Overall, these patterns support our Functional Breadth hypothesis.

Across the gradient of AM and ECM trees, litter quality and inorganic nitrogen availability increase with the abundance of AM trees. Thus, microbial communities in ECM-dominated sites exposed to low quality leaf litter may degrade complex C compounds as a side effect of mining organic matter for nitrogen. In contrast, our results suggest that the ability to decompose labile carbon may be a ubiquitous trait. As such, microbial communities exposed to low quality litter have a greater functional breadth than microbial communities exposed to high quality litter. FBH provides a robust framework for predicting variation in leaf litter decomposition within ecosystems.