Leaf litter mixtures alter microbial community development: mechanisms for non-additive effects in litter decomposition

Abstract

To what extent microbial community composition can explain variability in ecosystem processes remains an open question in ecology. Microbial decomposer communities can change during litter decomposition due to biotic interactions and shifting substrate availability. Though relative abundance of decomposers may change due to mixing leaf litter, linking these shifts to the non-additive patterns often recorded in mixed species litter decomposition rates has been elusive, and links community composition to ecosystem function. We extracted phospholipid fatty acids (PLFAs) from single species and mixed species leaf litterbags after 10 and 27 months of decomposition in a mixed conifer forest. Total PLFA concentrations were 70% higher on litter mixtures than single litter types after 10 months, but were only 20% higher after 27 months. Similarly, fungal-to-bacterial ratios differed between mixed and single litter types after 10 months of decomposition, but equalized over time. Microbial community composition, as indicated by principal components analyses, differed due to both litter mixing and stage of litter decomposition. PLFA biomarkers a15∶0 and cy17∶0, which indicate gram-positive and gram-negative bacteria respectively, in particular drove these shifts. Total PLFA correlated significantly with single litter mass loss early in decomposition but not at later stages. We conclude that litter mixing alters microbial community development, which can contribute to synergisms in litter decomposition. These findings advance our understanding of how changing forest biodiversity can alter microbial communities and the ecosystem processes they mediate.

Figure 1. Microbial decomposer biomass on single and mixed leaf litter.

The development of microbial communities on single and mixed litter types during leaf litter decomposition. In panels A and B, mixed litterbags had significantly higher total and fungal PLFA concentrations than single litterbags over the two litterbag harvest dates (p<0.01 in both cases). In panel C, bacterial PLFA changed significantly through time on single vs. mixed species litterbags time (p = 0.05). In panel D, there was a significant interaction between mixing effect and time for fungal:bacterial ratios (p = 0.01). Standard errors are indicated by bars on each point.

Figure 2. Litter microbial community composition changes due to mixing litter and stage of decomposition.

Principle components analyses of PLFA profiles (log₁₀ transformed mol%) on litter at two stages of decomposition (after 10 and 27 months in the field). Open symbols indicate single litterbags and solid symbols indicate mixed litterbags. Circles indicate the litterbags removed after 10 months and triangles indicate the litterbags removed after 27 months. Principle component (PC1) score was different between the two decomposition harvests (p<0.001) and between mixed and single litter (p<0.01). PC2 was significantly different for single litter vs. mixed litter (p = 0.01) but not between 10 and 27 months of decomposition.

Figure 3. Microbial decomposer biomass and litter decomposition.

Correlations between PLFA and litter decomposition for mixed (solid symbols, solid lines) and single litter types (open symbols, dashed lines). After 10 months of decomposition, total PLFA concentration significantly correlated with single litter decomposition (Pearsons coefficient (PC) = 0.67, p = 0.02) and tended to correlate with mixed litter decomposition though this correlation was not significant (PC = 0.33, p = 0.15; Panel A). There were no significant correlations between total PLFA concentration and litter decomposition at 27 months (Panel B). Fungal: bacterial ratios of PLFA showed a trend towards correlating with mixed litter decomposition at 10 months (PC = 0.36, p = 0.10) and 27 months (PC = 0.33, p = 0.14; Panel D) but not single litter decomposition at either time point.