Soil microbial communities and biogeochemistry during human decomposition differs between seasons: evidence from year-long trials

Abstract

Background Human decomposition in terrestrial ecosystems is a dynamic process that creates localized hotspots of soil nutrient cycling and microbial activity, and the study of its progression is crucial to understanding the processes involved in recycling carcass-derived organic matter. However, longer-term (beyond a few months) impacts on microbial communities in these environments are poorly characterized and do not typically connect microbial community analysis to biogeochemistry, limiting our understanding of these ephemeral hotspots. We performed a pair of year-long human decomposition trials designed to characterize seasonal differences (summer and winter) with high temporal resolution. Microbial (16S and ITS rRNA) amplicon sequencing and quantification were integrated with biogeochemical measurements to identify key drivers of microbial successional patterns. Results In both seasonal trials we observed soil acidification, elevated microbial respiration, and concomitant reductions in soil oxygen concentrations. Soil oxygen concentration was a primary driver of microbial succession and nitrogen transformation patterns, while pH largely drove fungal diversity and abundance. Relative abundances of facultative anaerobic taxa (Firmicutes and Saccharomycetes) increased during the period of reduced soil oxygen. The magnitude and timing of the decomposition responses varied by season, with greater increases in the warmer season and longer pulse times in the cooler season, even when corrected for thermal inputs (accumulated degree days). After one year, several soil chemical parameters, microbial community structure, and fungal gene abundances remained altered, suggesting longer-term impacts on soil ecosystems. Conclusions Our work shows that decomposition patterns differ seasonally in terms of their impacts on soil biogeochemistry and microbial successional patterns, thus refining our understanding of decomposition dynamics. When considering the decomposition of human remains specifically, these collective biogeochemical and ecological shifts have potential to be harnessed as a forensic tool for refining postmortem interval (PMI) estimates.