Factors Affecting Decomposition Process in Soil

Several studies showed that the rates of decomposition of a cadaver in soil are slower than in open air. Some authors wrote that the rate was eight times reduced (Rodriguez and Bass 1985; Rodriguez 1997), mainly because of the soil environment but also because of the limitation of the wide spreading of decaying odours, and resctriction of the accessibility of the necrophagous fauna (vertebrates and invertebrates). Two types of preservation of soft tissues are generally observed on buried cadaver: mummification and adipocere formation (Forbes 2008). Decomposition depends on the nature of the deceased person (weight, size, nature of death and post mortem treatment, clothes, and coffin type), the environment (temperature, rain, wind, etc.), and the physical parameters of the burial site (depth, soil type, moisture, air content, temperature, etc.): so-called intrinsic and extrinsic factors. Studies too showed that cadavers in coffins decompose more rapidly than those directly buried in soil (Forbes 2008). Indeed, the bodies' dissolution is accelerated in coffins and does not allow adipocere formation.

The main taphonomic factors affecting the decay rate in soil are listed in Table 13.1 (after Haglund and Sorg 2002). Some of them will be discussed below.

Table 13.1 Main taphonomic factors affecting the decomposition in soil (after Haglund and Sorg 2002)

Grave characteristics

• Compaction Inclusions Temporal factors

• Duration of burial Season

Body characteristic

• State of decomposition

Presence of clothes, wrapping Soil characteristics

• Oxygen contents

• Compaction

• Coarseness and type of soil (clay, sand, silt) composition of soil (C/N etc. )

• Contaminants Proteolytic activity

Body assemblage characteristics

• Thickness and extent

Position relative to core and perimeter Other characteristics

• Temperature during pre-burial period

• Post-burial exposure of remains to atmosphere

• Disturbance, treatment (deposit of toxical product: acid, limestone, etc.) Insects, bacteria (aerobic and anaerobic), plants

13.3.2.1 Temperature

Because soil acts as a shield preventing solar radiation, temperatures are lower and fluctuation is less important. Such fall of temperature has a direct impact on the rate of decomposition by cooling the body, which remains thus during a longer period than in an exposed location. Rodriguez (1997) wrote that at depths greater than 2 ft (about 64 cm), fluctuation of temperature was quite non-existent. Below 4 ft (greater than 1.28 m), a cadaver was very well preserved over 1 year and skeletoni-sation took 2-3 years.

Studies showed that cadavers buried in the summer demonstrated a greater rate of decomposition than those buried in winter (Rodriguez and Bass 1985). Fresh temperatures tend to inhibit bacterial activity and proliferation. Putrefactive process is highly inhibited below 10°C and above 40°C. A temperature of 37°C is considered as an optimum for having significant impact on bacterial growth and decomposition

(Forbes 2008). Temperatures have a direct impact on insect biology. Activity of flight and oviposition of adults depend on temperature. The development rate of immature stages, from eggs to metamorphosis, is also temperature-dependent and specific (Kamal 1958; Marchenko 2001; Lefebvre and Pasquerault 2003). Donovan et al. (2006) proposed cold tolerance, introducing a hypothesis that there was within the same species of Calliphoridae (Calliphora vicina Robineau-Desvoidy) a bio-geographical variation in larval growth. Other authors also showed that during the post-embryonic development, influence of cooling may be totally different on the successive developmental stages of a Calliphoridae species, Protophormia ter-raenovae (Robineau-Desvoidy) (Myskowiak and Doums 2002).

13.3.2.2 Depth

Depth has a significant effect on the rate of decomposition. Rodriguez and Bass (1985) reported the following observations on buried human cadavers. Cadavers buried at 1.2 m (4 ft) for 1 year showed a good state of preservation with skeletoni-sation limited to the head, hand and feet. The body was quite totally covered in adipocere. Cadavers buried at 0.6 m (2 ft) for 6 months showed little decomposition. Bones of the hands and feet were skeletonised. The genital area was decomposed. The body was dark brown in colour and covered with white adipocere (chest and legs). Cadavers buried at 0.3 m (1 ft) for 3 months were decomposed. Skeletonisation of arms and legs was complete with disarticulation at the major long bone joints. The feet were mummified. Small adipocere was visible along the upper chest cavity. Small traces of mould and fungi were observed on trousers.

The same authors highlighted that the duration of burial influences the decomposition rate. Indeed, cadavers buried at 0.3 m (1 ft) for two and a half months exhibited moderate decomposition with muscle visible on hands and legs and adipo-cere covering slightly the legs. Body colouration was dark pinkish brown. Small patches of fungi on the exposed chest were observed. Cadavers buried at 0.3 m (1 ft) for 1 month showed slight decomposition. Hands and feet were preserved, and bloating was slight in the face and marked in the abdomen. Body colouration was mainly dark pink. Fungi were more visible on trousers.

13.3.2.3 Soil Composition

Studies showed that the type of soil has to be taken into account to interpret the decomposition process and thus insect activity in soil. The decomposition rate of organic matter (cadaveric material) varies according to the characteristics of soil (Carter and Tibett 2008). A soil mainly composed of clay retains moisture and induces covering of the body by adipocere (Turner and Wilshire 1999). Lime burial sites (highly alkaline) delay the decomposition by adipocere formation and by limiting the survival and thriving of destructive bacteria. Forbes (2008) considered a loamy soil as a common soil that can be considered as control for experimentation.

Table 13.2 Stages in the decomposition of buried pigs after Wilson et al. (2007). Moreland (overlying boulder clay and coarse weathered sediments of Millstone Grit), woodland (free-draining loam with degraded Millstone Grit inclusions overlying Millstone Grit), pasture (overlying a silty clay on coal measure)

Moreland Woodland Pasture Month 6 12 6 12 24

Table 13.2 Stages in the decomposition of buried pigs after Wilson et al. (2007). Moreland (overlying boulder clay and coarse weathered sediments of Millstone Grit), woodland (free-draining loam with degraded Millstone Grit inclusions overlying Millstone Grit), pasture (overlying a silty clay on coal measure)

Moreland Woodland Pasture Month 6 12 6 12 24

Fresh stage

Begun at death, includes rigor mortis, post mortem hypostasis and cooling Continues until bloating of the carcass is visible

Skin intact, hair firmly anchored

Primary bloat stage

Accumulation of gases within the body No disarticulation Hair and epidermis losses Soil-skin interface grey Strong odour

Secondary bloat stage

Body still bloated Disarticulation of limbs Purging

Soil-skin interface black Strong odour

Active decay stage

Deflation of the carcass Disarticulation of the limbs and head Flesh and skin still present Carcass very wet Strong odour

Advanced decay stage

Collapse of abdomen, rib cage Most or the flesh liquefied or gone Skin, bone, fat and cartilage may remain Carcass very wet Adipocere formation

Skeletonisation stage

Flesh, skin and cartilage disappear Some adipocere and ligaments may remain

Wilson et al. (2007) showed varying conditions after exhumation of pig carcasses buried for 6, 12 or 24 months in three contrasting fields (Table 13.2).

13.3.2.4 Oxygen Content

Fungi and aerobic bacteria need a minimum amount of oxygen to thrive (Forbes 2008). Thus, a poor presence of oxygen delays the decomposition process. In other words, preservation of tissue is better when oxygen is low.

13.3.2.5 Moisture

Moisture is present in soil located close to a water environment and in soil mainly composed of clay. Clay retains moisture and helps adipocere formation on the surface of the bodies, helping conversion of triglycerids to free fatty acids. In dried soil the body is rather mummified. The season can be also very important in such a process. Indeed, warm temperature but also very cold temperature (freeze-drying) makes the body desiccate and induce mummification. In a dry environment, adipocere formation may be due to the moisture included in the tissues.

Soil pH is important too because high acidity as well as high alkalinity of soil can inhibit bacterial growth and, consequently, the decomposition process. Corpses in sealed coffins with adipocere formation release odour of ammoniac, inducing an alkaline environment. On the contrary, a slight alkaline pH of soil is favourable for the thriving of destructive bacteria (Forbes 2008).

The presence of decaying bodies has two kinds of effect on soil pH, at first increasing alkalinity followed by a fall of pH (Carter and Tibett 2008). Rodriguez and Bass (1985) showed an increase of pH prior to burial and after exhumation whatever the depth. The higher increases were observed at 0.3 m deep after 3 months (+2.1), at 0.6 m after 6 months (+1.1) and at 1.2 m after 1 year (+0.5). However, at a shallow depth of 0.3 m (1 ft), below 2 months and a half, the difference was lower (+0.2 max). Wilson et al. (2007) also observed that 12 months after burial of pig carcasses (Sus scrofa) at 0.3 and 0.6 m deep, pH rose significantly at woodland sites (+2.1 and +2.6).

13.3.2.7 Microbial Degradation

Invertebrates, fungi and also saprophagous bacteria (aerobic) participate actively in the decay of organic matters. Indeed, the microbial community produces prote-olytic enzymes responsible for the breakdown of polymers in organic matter. Such a process is vital to the local ecosystem. Studies have shown that several parameters have a direct influence on bacteria proliferation and thus on biomass loss. Temperature has a major impact on it. An increase from 10°C to 30/35°C of temperature doubles the microbial activity. In other words, at lower temperature, the microbial activity is lower and hence organic matter is better preserved. On the other hand, Wilson et al. (2007) showed that burial of pigs' carcasses in soil led to higher microbial community. Carter and Tibett (2008) added that repeated burial of cadavers (or carcasses) in the same grave soil simulates microbial proliferation and enhances decomposition. Such information has to be taken into account because of potential impact on the insect fauna.

13.3.2.8 Fauna

Macrofauna and microfauna can be attracted by a decaying corpse causing damages and affecting the decomposition process. Contrary to carcasses in open air, a burial environment prevents attacks from scavengers (birds, rats, foxes, badgers, etc.). Holes of rodents were observed close to buried sheep carcasses at 10 cm depth 1 year after burial, probably because of presence of crevices in soils (Gaudry E, unpublished data). Such holes made by these animals very likely increase the accessibility of the entomological fauna to the cadaveric material and enhance decomposition.

Decaying carcasses and bodies (exposed or buried) provide, during a more or less long period (according to depth, season, etc.), an important quantity of nutritive substrate. Such necromasses can also be used by other organisms such as invertebrates for mating or as shelter. That is why the diversity of arthropods that can be found during collection is important, gathering necrophages, predators, parasites, omnivores and opportunists (Dadour and Harvey 2008). Collection of evidences at a burial death scene is particularly important and requires a specific protocol much more complex than in a crime scene where the body is exposed above the surface.

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