Soil Biogeochemical Cycling and Stable Isotope Dynamics during Vertebrate Carcass Decomposition.

Soil biogeochemical cycling reflects the dynamic interplay between in situ pools of elements, such as carbon (C) and nitrogen (N), and exogenous and potentially ephemeral inputs via rainfall, dry deposition, and decomposing plants and animals. Compared to plant litter, little is known about N cycling during animal decay, and potential changes in soil stable isotopic composition is unexplored. The goals of this study were to assess C and N transformations and cycling in soil during animal carcass decomposition (beavers, Castor canadensis), and to determine how decay influences bulk soil δ¹³C and δ¹⁵N. Decomposition stages were divided into three main biogeochemical phases, characterized by significant shifts in C and N cycling. Initial and early decay was characterized by aerobic soils with low nitrate (<0.1 mg/L/g dry weight soil), elevated C:N (16.8), low NO₂^- (0.2 mg/gdw/day), and net uptake of N₂O. Once beavers reached active and advanced decay, a second biogeochemical phase began, characterized by anaerobic soils, peak CO₂ release (1500 ppm above background), peak ammonium (up to 130 mg/L/gdw), increased bulk N and decreased in C:N (12.5), and increased CH₄ and N₂O release. Additionally, bulk δ¹⁵N significantly increased from 1.5 to 2 ‰ in background soil to 6.2 to 8.2 ‰. Following fluid release from decomposing beavers and most soft tissue breakdown, soils returned to full oxygenation during early and late skeletal decay, after 1 and 4 months. This third phase was marked by a rapid decrease in CO₂ release, peak CH₄ and N₂O release, and peak NO₃^- (7.7 mg/L/gdw) and NO₂^- (8.6 mg/gdw/day). Low C:N ratios (~12.5) persisted and bulk δ¹⁵N increased to 9.3 ‰. Pulsed nutrients associated with vertebrate decay stimulated endogenous microbial communities, significantly altered soil chemistry on timescales ranging from days to months, and resulted in biogeochemically distinct phases of C and N cycling.