Nitrous oxide, methane and carbon dioxide patterns and dynamics from an experimental pig mass grave

The objective of the three-year study was to examine spatial and temporal patterns of fluxes and soil pore air concentrations of methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) from an experimental mass grave located in a temperate environment. The mass grave (5×10m) contained twenty pig carcasses at a depth of approximately 1m was compared to a plot of the same dimensions containing only disturbed soil, as well as an undisturbed plot. Soil pore air CH₄ concentrations were sub-ambient (<1.8ppm) except at 75 and 100cm depths at the mass grave in years 1 and 2 but decreased in year 3. The consumption of CH₄ within the aerobic soil resulted in small negative fluxes at the soil surface. Soil pore air CO₂ concentration showed an increase with depth in all three plots, with the largest increase (>100,000ppm at 1m) in the mass grave, though there was a marked decrease from years 1 to 3. Surface fluxes of CO₂ showed strong seasonal variations, peaking in summer. Soil pore air N₂O concentration showed major increases in the mass grave, compared to the other two plots with the pattern maintained over the three years, resulting in larger surface fluxes of N₂O. To establish the role of the carcasses in N₂O dynamics, we incubated a soil sample containing carcass material which resulted in fast rates of N₂O production and consumption. The maintenance of elevated pore air concentration and surface flux of N₂O throughout the 3 years suggests that this is a long-term pattern and likely the best of the three gases to use to detect graves. Thus, we suggest that measurement of soil pore air concentrations, especially of N₂O, could be a simple and effective approach to help determine the location of clandestine graves.