Resilience of microbial respiration, respiratory quotient and stable isotope characteristics to soil hydrocarbon addition

Abstract

On the basis of CO₂ evolution rate, O₂ uptake rate, and ¹³C isotopic signature of respired CO₂, the metabolic response to the addition of ¹³C labelled n-hexadecane and palmitic acid each with supplementary nitrogen was studied for two topsoils, one under continuous agricultural management and the other under beech forest. The CO₂ evolution rate was immediately stimulated in the agricultural soil and the respiratory quotient (RQ) decreased from 0.8 to 0.4 mol CO₂ evolution rate per mol O₂ uptake rate, which was below the theoretically expected value of 0.65 and 0.70 for the degradation of n-hexadecane and palmitic acid, respectively. The microbial response was delayed in the forest soil, but developed better than in the agricultural soil throughout the subsequent 2–4 weeks. Consequently, the respiration rate returned earlier to the initial level for the beech forest soil and the δ¹³C of respired CO₂ and RQ approached values before hydrocarbon addition. Based on the link among respiration rates, RQ and ¹³C–CO₂ value, the added oil-analogue compounds induced a more rapid response in the agricultural soil and were degraded more completely in the forest soil. We concluded that the resilience, which we defined here as the capacity of the soil microbiota to buffer perturbance and to reorganise in response to change resulting in a more desirable system, was higher in our forest soil than for the agricultural soil.

Introduction

Our global transport system and power generating industry is based on the extraction of fossil hydrocarbon fuels such as crude oil that may contribute to increased concentrations of organic pollutants in the atmosphere, hydrosphere, and geosphere (O’Donoghue and Broderick, 2009, Li and Boufadel, 2010). The combustion of fossil fuels has already led to significant changes in atmospheric carbon dioxide content and signature (Intergovernmental Panel on Climate Change, 2007). In addition, the pollution of soil poses the need for efficient monitoring and control of the fate of the anthropogenic hydrocarbons as they affect sustainability issues such as plant growth (Adam and Duncan, 2002, Adam and Duncan, 2003).

Microbial communities have the unique capability to adjust to unfavourable conditions and to use a broad spectrum of substrates. They have metabolic systems that allow them to utilise both natural and synthetic sources such as exogenous petroleum for energy and tissue constituents. In particular, the n-hexadecane and palmitic acid are constituents of fossil fuels. In natural systems, fatty acids such as palmitic acids are metabolised for NADPH and ATP generation. Fatty acids are esterified to a glycerol backbone to form a group of compounds known as mono-, di-, and tri-glycerides (neutral fats). Energy is released when fatty acids are degraded and also serve as precursors for a number of cell components that are structurally and physiologically essential.

Monitoring of soil hydrocarbon pollution and transformation activity can be done by estimating the concentration of the pollutant and the formation of the respective metabolites. One of the most ubiquitous and universal metabolites is carbon dioxide since respiration is the prominent pathway of biologically processed carbon. When hydrocarbons have carbon isotope characteristics that differ from soil organic components, the ¹³C–CO₂ characteristics can also be used to monitor their mineralisation (Zyakun et al., 2003). Since hydrocarbons require a high amount of oxygen for complete oxidation, the RQ should be affected as well (Dilly, 2001) as oxygen limitation may induce changes in cytochrome levels (Asperger et al., 1986).

The aim of this study is to evaluate the response of soil microbial community to addition of reduced substrates such as n-hexadecane (C₁₆H₃₄) and palmitic acid (C₁₆H₃₂O₂) using the CO₂ evolution rate, the RQ, and the ¹³C–CO₂ isotopic signature. Additionally, the degradation of palmitic acid was studied due to its primary functions for cell systems. After monitoring the respiratory characteristics, the residual hydrocarbons were estimated. This study should give information on the resilience of soil systems to buffer perturbations and change resulting in a more desirable (here non-polluted) system and also about the soil energetic characteristics, which can be defined as soil energomics (Dilly et al., 2010).