Microbes are key drivers of leaf litter decomposition, yet our understanding of their diversity and mechanisms remains in its infancy. Furthermore, it is unknown how microbes are affected by climatic gradients such as temperature and precipitation, two parameters affected by climate change.
Here, we first surveyed the bacterial community composition of decaying leaf litter along an elevation gradient in southern California, USA. The gradient included five sites with precipitation increasing, and temperature decreasing with elevation. To test the importance of climate for microbial composition, we further conducted a transplant experiment. We used “microbial cages” to transplant leaf litter communities to different climates while preventing microbial exchange with the environment. We inoculated transplant communities onto a common, irradiated, grassland litter. In contrast with observational data, this manipulation isolates the effects of climatic conditions versus that of litter substrate. To characterize bacterial community composition, we extracted DNA from the inoculum, intact litter at sites, and litter in the microbial cages, and amplified and sequenced part of the 16S rRNA region.
We hypothesized that communities from along the climatic gradient would differ in their abundance and composition, and that communities at the extremes of the gradient would be most affected by climate. We found that bacterial communities did indeed differ greatly between the five sites (PERMANOVA; R2 = 0.61, P < 0.01). The main axis of community separation appeared to be between the colder and wetter sites versus the hotter and drier sites. Variation due to climate overshadowed temporal variation; communities differed across three sampling dates, but this factor explained much less variation than site (R2 = 0.035, P < 0.05). Composition was also influenced by a site-by-date interaction (R2 = 0.13, P < 0.01). For example, the composition changed least over time for the two coldest and wettest sites.
After transplantation, communities were quickly influenced by local climate. Site was the strongest factor affecting bacterial composition (R2 = 0.40, P < 0.01), although inoculum source was also significant (R2 = 0.12, P < 0.01). However, a strong site by inoculum interaction effect (R2 = 0.18, P < 0.01) indicates that the strength of the inoculum effect varied by site. These results demonstrate that bacterial decomposers are highly sensitive to climate, but that different communities may respond faster to climatic change than others.