So far there has been only limited data on the typeability of human DNA from blood traces in soil samples. In a homicide investigation in spring 2023 involving significant blood loss, a blood trace distribution pattern was detected four weeks later on a field path using LumiScene. From soil samples collected at the crime scene, a DNA profile of the victim was generated. Following the case, questions arose about the durability of DNA in soil, prompting studies on the stability, detectability, and typeability of blood in different soil environments.

In this study, human blood from three individuals was applied in triplicates to three different natural soil types. Sampling occurred at nine-time intervals over three months, at two depths (0-1 cm and 4-5 cm). The experiments were conducted once during the winter months with moist soil and once during the summer months with dry soil. Additionally, samples from the same soil types were mixed with blood from the same three individuals under controlled laboratory conditions: "winter" (incubation at 4°C) and "summer" (incubation at 25°C). Before extracting and quantifying DNA from soil samples, a luminol test for blood was performed, followed by a PCR- and CE-based STR analysis of 16 autosomal STR markers using the Investigator ESSplex SE QS Kit from Qiagen.

In the winter trials in natural environment, blood was detected with luminol up to one week, but no DNA results could be obtained. In contrast, summer trials yielded detectable blood with complete STR profiles until the last sampling after three months. Laboratory conditions confirmed existing differences between dry/warm vs. wet/cold soil, although results where not as distinct.

Blood is a significant biological trace material, highly relevant for DNA analysis at crime scenes. Moisture greatly impacts DNA stability, especially in soil matrices. This study highlights that season, along with moisture and temperature, is crucial in natural environments. Notably, after one day, significantly less DNA was detectable in moist soil compared to dry soil. However, in natural environment there was no significant correlation between DNA degradation or typeability and time. Laboratory findings showed significant differences between soil types, suggesting that other, untested factors – such as humic acids or microbial activity – may also have a significant impact on DNA degradation and inhibition. Overall, our results indicate that if human blood is once deposited on dry soil and remains dry, significant degradation of DNA does not occur, preserving its typeability.

Authors

• Valentina Leonie Birne (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)
• Kerstin Jacobs (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)
• Romy Weber (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)
• Sarah C. Kölzer (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)
• Peter Jan Chabiera (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)
• Marcel A. Verhoff (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)
• Richard Zehner (Goethe-University Frankfurt, University Hospital, Institute of Legal Medicine, Germany)