Suppression of soil organic matter decomposition by gasoline and diesel as assessed by 13C natural abundance

Petroleum products are common contaminants in soils due to human activities. They are toxic for microorganisms and threat their functions, including decomposition of soil organic matter (SOM). The direct estimation of altered SOM decomposition – based on the CO2 efflux – is impossible after petroleum contamination because petroleum decomposition also contributes to these CO2 fluxes. We used the natural differences in the isotopic signature (δ13C) of SOM and of petroleum products to partition the total CO2 for both sources and to analyse the suppression of SOM decomposition. The dynamics of 13C fractionation during the mineralization of gasoline and diesel was measured during 42 days. The 13C fractionation varied between −8.8‰ and +3.6‰ within the first 10 days, and stabilized thereafter at about −5.3‰ for gasoline and +3.2‰ for diesel. These 13C fractionations and δ13C values of CO2 emitted from the soil were used to partition the total CO2. Contamination with gasoline reduced the CO2 efflux from SOM by a factor of 25 (from 151 to 6.1 mg C-CO2 kg−1 soil during 42 days). The negative effect of diesel was much lower: the CO2 efflux from SOM was decreased by less than a factor of 2. The strong effect of gasoline versus diesel reflects the lower absorption of gasoline to mineral particles and the development of a thin film on water surfaces, leading to toxicity for microorganisms. We conclude that the soil contamination by gasoline and diesel strongly decreased microbial functions and so, the degradation of native SOM.