Microbial Activity and ¹³C/¹²C Ratio as Evidence of N-Hexadecane and N-Hexadecanoic Acid Biodegradation in Agricultural and Forest Soils

Abstract

The dynamics of microbial degradation of exogenous contaminants, n-hexadecane and its primary microbial oxidized metabolite, n-hexadecanoic (palmitic) acid, was studied for topsoils, under agricultural management and beech forest on the basis the changes in O₂ uptake, CO₂ evolution and its associated carbon isotopic signature, the respiratory quotient (RQ) and the priming effect (PE) of substrates. Soil microbial communities in agricultural soil responded to the n-hexadecane addition more rapidly compared to those of forest soil, with lag-periods of about 23 ± 10 and 68 ± 13 hours, respectively. Insignificant difference in the lag-period duration was detected for agricultural (t_lag = 30 ± 13 h) and forest (t_lag = 30 ± 14 h) soils treated with n-hexadecanoic (palmitic) acid. These results demonstrate that the soil microbiota has different metabolic activities for using n-hexadecane as a reductive hydrocarbon and n-hexadecanoic acid as a partly oxidized hydrocarbon. The corresponding δ¹³C of respired CO₂ after the addition of the hydrocarbon contaminants to soils indicates a shift in microbial activity towards the consumption of exogenous substrates with a more complete degradation of n-hexadecane in the agricultural soil, for which some initial contents of hydrocarbons are inherent. It is supposed that the observed deviation of RQ from theoretically calculated value under microbial substrate mineralization is determined by difference in the time (Δt_i) of registration of CO₂ production and O₂ consumption. Positive priming effect (PE) of n-hexadecane and negative PE of n-hexadecanoic (palmitic) acid were detected in agricultural and forest soils. It is suggested that positive PE of n-hexadecane is conditioned by the induction of microbial enzymes that perform hydroxylation/oxygenation of stable SOM compounds mineralized by soil microbiota to CO₂. The microbial metabolism coupled with oxidative decarboxylation of n-hexadecanoic acid is considered as one of the most probable causes of the revealed negative PE value.

Keywords:

• microbial activity
• O₂ consumption and CO₂ evolution
• respiratory quotient
• hydrocarbon contamination
• respired CO₂-C isotopic signature
• priming effect

Acknowledgments

These studies were financially supported by DFG-RUS 17/41/106, the Ministry for Science, Research and Culture (MWFK) of the Brandenburg state, EFRE and SENSOR and the Ministry of Education and Science of Russian Federation by Grant RNP No 2.1.1/4341. The completion of this work was supported through the Cluster of Excellence “Climate System Analysis and Prediction—CliSAP (EXC177), University of Hamburg, funded through the German Science Foundation (DFG). The authors thank Maria Koon for carefully editing the linguistic style of this article.