Links between soil organic matter input and microbial community structure and function following volcanic eruption on Kasatochi Island, Alaska

Background/Question/Methods

In August 2008, Kasatochi volcano erupted and buried a small lush island in pyroclastic deposits and fine ash; since then, microbes, insects, plants and birds have begun to recolonize the initially sterile surface.  In August 2013, samples of pre- and post-eruptive soil surfaces were collected from sites with contrasting inputs of organic matter (OM): plant or seabird recolonization (or both), and “legacy” (buried pre-eruptive) soils.  Reference soil samples from a nearby island with no impact from the 2008 Kasatochi eruption were also collected.  A selection of extracellular enzyme activity potentials (hydrolytic and oxidative enzymes), bacterial and fungal population sizes (quantitative PCR of bacterial 16S rRNA genes and fungal ITS) and bacterial community composition (Illumina MiSeq libraries of bacterial 16S rRNA genes) was measured on all samples.  We predicted that levels of microbial recovery from volcanic disturbance would be correlated with levels of OM accumulation since eruption.

Results/Conclusions

The size of bacterial and fungal populations and magnitude of microbial extracellular enzyme activity on post-eruptive surfaces was strongly related to the presence and amount of OM input.  Bacterial 16S rRNA gene and fungal ITS copy numbers and beta-glucosidase enzyme activity potentials were one to two orders of magnitude greater in pyroclastic materials with OM inputs relative to those without, despite negligible accumulation of OM (≤ 0.15 ± 0.06 %C) on post-eruptive surfaces, and microbial population size and activity increased with soil OM up to 17 %C (in buried pre-eruptive soils).  When normalized to the amount of organic matter, many enzyme activities were highest in post-eruptive soils with OM inputs, and microbial populations in post-eruptive soils with OM inputs approached those of reference soils.  In contrast, bacterial community structure was not correlated with OM accumulation, microbial growth or activity levels; instead, the dominance of plants versus birds as OM-input vectors was the strongest factor determining bacterial community composition.  While soil pH ranged from 3.9 to 7.0 among all samples, and %C ranged 100x, differentiation between plant- and bird-associated microbial communities suggested that cell dispersal or nutrient availability are more likely drivers of assembly than pH or OM content.