Abstract
Decomposing vertebrates, including humans, result in pronounced changes in surrounding soil biogeochemistry, particularly nitrogen (N) and carbon (C) availability, and alter soil micro- and macrofauna. However, the impacts of subsurface human decomposition, where oxygen becomes limited and microbial biomass is generally lower, are far less understood. The goals of this study were to evaluate the impact of human decomposition in a multi-individual, shallow (~70 cm depth) grave on soil biogeochemistry and soil microbial and nematode communities. Three individuals were interred and allowed to decay for four years. Soils were collected from two depths (0‒5 and 30‒35 cm) along linear transects radiating from the grave as well as from within and below (85‒90 cm depth) the grave during excavation to assess how decomposition affects soil properties. Along radiating surface transects, several extracellular enzymes rates and nematode richness increased with increasing distance from the grave, and likely reflect physical site disruption due to grave excavation and infill. There was no evidence of carcass-sourced C and N lateral migration from the grave, at least at 30‒35 cm depth. Within the grave, soils exhibited significant N-enrichment (e.g., ammonium, dissolved organic N), elevated electrical conductivity, and elevated respiration rates with depth. Soil biogeochemistry within the grave, particularly in the middle (30‒35 cm) and base (70‒75 cm depth), was significantly altered by human decomposition. Mean microbial gene abundances changed with depth in the grave, demonstrating increased microbial presence in response to ongoing decomposition. Human-associated Bacteroides were only detected at the base of the grave where anoxic conditions prevailed. Nematode community abundance and richness were reduced at 70‒75 cm and not detectable below 85‒90 cm. Further, we identified certain Plectus spp. as potential indicators of enrichment due to decomposition. Here we demonstrate that human decomposition influences soil biogeochemistry, microbes, and microfauna up to four years after burial.
Highlights
Decomposing vertebrates transform their surrounding environment by releasing compounds, carbon (C) and nitrogen (N), [1,2,3,4] that stimulate invertebrate [5,6] and vertebrate scavengers [7], and alter surrounding physiochemistry and biological communities [2,8,9,10,11]
In soils radiating laterally from the grave, gravimetric moisture was significantly elevated in surface samples (0–5 cm) compared to their depth equivalent (30–35 cm) along the transects (p < 0.001; F = 36.600), and gravimetric moisture significantly increased with distance from the grave (p < 0.001; F = 12.480)
Table), and there were no significant differences in pH with proximity to the grave (Table 2)
Summary
Decomposing vertebrates transform their surrounding environment by releasing compounds, carbon (C) and nitrogen (N), [1,2,3,4] that stimulate invertebrate [5,6] and vertebrate scavengers [7], and alter surrounding physiochemistry and biological communities [2,8,9,10,11]. Numerous insect and arthropod taxa change in response to carcass decomposition, with changes in community membership diagnostic of specific decay stages [20,21,22] Soil microbial communities, both bacteria and fungi, change during decay [9,10,23,24,25]: microbial activity peaks during active decomposition, and communities shift towards a higher proportion of anaerobic taxa such as Bacteroidetes and Firmicutes [10,24,26]. Both bacteria and fungi, change during decay [9,10,23,24,25]: microbial activity peaks during active decomposition, and communities shift towards a higher proportion of anaerobic taxa such as Bacteroidetes and Firmicutes [10,24,26] Such changes generally coincide with an overall decrease in diversity over time [27]
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