Postmortem Hot Spots: Soil Microbial Community Succession below Decomposing Vertebrate Mortalities.

In terrestrial ecosystems, dead animals generally decompose rapidly, resulting in a flux of organic matter and nutrients into the soil.  Despite the fact that this is common and natural, we know little of the microbes in these decomposition hot spots.  The objectives of this study were to 1) determine responses of soil microbial communities in terms of compositional and functional changes, and 2) determine the consistency of these responses between seasons and species.  We found that microbes in soils below decomposing cadavers were significantly altered.  First, we saw significant shifts in bacterial community composition, with a proliferation of Firmicutes and Actinobacteria and a decrease in Acidobacteria and Verrucomicrobia.  Second, we observed a change in microbial functions:  early decay communities had a mean growth efficiency of 0.57%, similar to background soils.  By late decay, the growth efficiency dropped to 0.003%; respiration rates were elevated, but bacterial production rates dropped, suggesting a shift to anaerobic respiration.  Shotgun metagenomic sequencing revealed changes in the community’s functional potential: as decay progressed, we observed decreases in the community’s capacity for carbohydrate and aromatic compound metabolism and increases in stress response, protein biosysnthesis and sporulation related genes.   Metagenomic analysis also revealed shifts in the relative abundance of nitrogen cycling genes indicating changes in N cycling capacity.  We have begun to assess if these responses are universal for vertebrate mortalities.  We observed that the soil responses are not always similar between species (humans compared to pigs) or between seasons (winter compared to summer).  Taken together, our results provide some novel information about functional and structural responses of microbes in these soils and the impact of decomposition events in terrestrial ecosystems.  This work can inform best practices associated with managing and interpreting vertebrate mortality decomposition for forensics or dead livestock disposal.