The Forensic Science Toolbox: Examining Changes in Soil Biodiversity
Lois Stacy Taylor
National Institute of Justice Graduate Research Fellow in STEM
PhD Candidate, University of Tennessee, Department of Soil Science
Jennifer DeBruyn, PhD
University of Tennessee, Department of Soil Science
Ernest Bernard, PhD
University of Tennessee, Department of Entomology and Plant Pathology
Dead animals, or carrion, are an undeniable part of every ecosystem. Carcasses are a source of high-quality food for scavengers and insects, and as they decompose, they create local patches of nutrient–rich “hotspots” within the soil that are areas of increased microbial activity. These patches help create heterogeneity in the landscape, providing more niches and habitats for organisms, and ultimately contribute to biodiversity in an ecosystem. Vertebrate decomposition research, or taphonomy, also has an important place in the medico-legal field of forensic science. In forensic taphonomy, the focus is on documenting the manner and progression of human decomposition, usually with the specific intent of estimating time since death, or the post-mortem interval (PMI). The biological changes associated with carrion ecology are turned into tools designed to associate particular windows of time with the chemical and biological changes that occur during decomposition progression. Forensic entomology, which uses the development of fly larvae in a decomposing body to mark the passage of time, has been successfully applied to estimate PMI in numerous cases. Following that concept, forensic scientists have begun to investigate chemical and biological changes during decomposition as potential markers for PMI.1-3 Researchers have begun to explore the contributions of soil biota—particularly those from nematodes for this type of study, especially given their partitioned food requirements and reproductive characteristics!4
Unfortunately, forensic taphonomy studies can be notoriously tricky to perform. Decomposition is a dynamic process, dependent upon an array of environmental variables: temperature, rainfall, soil type, topography, humidity, scavenging, seasonality, ground cover, and in some cases clothing constraints. Under ideal circumstances, human decomposition studies require using human subjects, because humans decompose slightly differently compared to other vertebrates.5 On the other hand, there are challenges in using human donors: human physical variability and composition is quite considerable, not to mention the rather complex medical histories and pharmaceutical uses that are likely to impact decomposition. In addition, access to human donors is a challenge: there are only few facilities in the world that can perform human decomposition research. As a result, animals (carrion) have often been used as human proxies in order to provide good statistical power, and they are frequently employed in proof-of-concept studies.
Our study, funded by the Department of Justice, was designed to look at the long-term temporal impacts of decomposition and its derived products upon soil chemistry and biota. This study consisted of two parts. The first was a one-year-long proof-of-concept experiment examining nematode successional patterns and identifying possible indicator taxa found in soil during animal decomposition (we used beaver carcasses!). This was followed by a pair of human decomposition experiments performed at the University of Tennessee Anthropology Research Facility, each of which lasted a year in duration, and took a high-resolution look at the complex interactions between soil biogeochemistry, microbial ecology (with a focus on mycology), and nematode ecology under conditions that would explore the seasonal differences between both progression sequences.
Our animal study showed that a very distinct change in nematode communities occurred in both surface and deeper soil layers. We identified key taxa at the genus level that might be important in gauging the progression of decomposition through time. We also discovered that after one year, nematode communities had largely returned to their original state. This project has also contributed to our understanding of nematode biology: for many taxa, life histories and environmental sensitivities are not well known and we have been able to show which taxa are most resilient and most sensitive to environmental disturbance.
Our human study yielded slightly different results. We saw that seasonality exerts considerable influence over the temporal progression and patterns associated with decomposition. We also found, much to our surprise, that soils and soil biodiversity remained strongly impacted well after a years’ time. The successional patterns in nematode communities we observed have been extremely useful in cross-informing results from biogeochemical and microbial data, as well as suggesting new environmental factors that might play a role in decomposition. This holds great potential for refining and creating PMI models. You can read more about this experiment here.
References:
1. Keenan, S. W., A. L. Emmons, L. S. Taylor, G. Phillips, A. R. Mason, A. Z. Mundorff, E. C. Bernard, J. Davoren and J. M. DeBruyn (2018). "Spatial impacts of a multi-individual grave on microbial and microfaunal communities and soil biogeochemistry." PloS one 13(12): e0208845-e0208845.
2. Cobaugh, K. L., S. M. Schaeffer and J. M. DeBruyn (2015). "Functional and Structural Succession of Soil Microbial Communities below Decomposing Human Cadavers." Plos One 10(6): 20.
3. Keenan, S. W., S. M. Schaeffer, V. L. Jin and J. M. DeBruyn (2018). "Mortality hotspots: Nitrogen cycling in forest soils during vertebrate decomposition." Soil Biology & Biochemistry 121: 165-176.
4. Szelecz, I., F. Sorge, C. V. W. Seppey, M. Mulot, H. Steel, R. Neilson, B. S. Griffiths, J. Amendt and E. A. D. Mitchell (2016). "Effects of decomposing cadavers on soil nematode communities over a one-year period." Soil Biology and Biochemistry 103: 405-416.
5. Dautartas, A., M. W. Kenyhercz, G. M. Vidoli, L. M. Jantz, A. Mundorff and D. W. Steadman (2018). "Differential Decomposition Among Pig, Rabbit, and Human Remains." Journal of Forensic Sciences 63(6): 1673-1683.