Search

In 1991, at the launch of a national symposium devoted to soil structure, the Australian Society of Soil Science invited Professor John Letey to deliver a keynote address, which was later published in the society’s journal. In his lecture, he shared the outcome of his reflexion about what the assessment of soil structure should amount to, in order to produce useful insight into the functioning of soils. His viewpoint was that the focus should be put on the openings present in the structure, rather than on the chunks of material resulting from its mechanical dismantlement. In the present article, we provide some historical background for Letey’s analysis, and try to explain why it took a number of years for the paradigm shift that he advocated to begin to occur. Over the last decade, his perspective that soil structure needs to be characterised via non-destructive methods appears to have gained significant momentum, which is likely to increase further in the near future, as we take advantage of recent technological advances. Other valuable lessons that one can derive from Letey’s pioneering article relate to the extreme value for everyone, even neophytes, to constantly ask questions about where research on given topics is heading, what its goals are, and whether the methods that are used at a certain time are optimal.

The increasing use of petroleum-derived fuels over the last few decades has subsequently augmented the risk of spills in the environment. Soil pollution with petroleum hydrocarbons (principally caused by leaks in pipelines and underground storage tanks) is one of the major sources of soil degradation. Once in soil, fuel hydrocarbons suffer from a wide variety of multiphase processes including transport (advection, diffusion, and dispersion) among and within phases (aqueous and non-aqueous liquid, gas, and soil solids), mass transfer among phases (volatilization, sorption, and solution), and other natural attenuation processes, such as biodegradation and plant uptake and metabolism. This review identifies and describes the major processes occurring in soil that have a significant influence on the environmental fate of petroleum hydrocarbons. The definition of the processes involved in pollutant migration and distribution in soil and the formulation of adequate equations using accurate parameters (e.g., diffusion coefficients, velocity of advective flows, and mass transfer coefficients) will allow prediction of the final fate of soil pollutants. In addition to transport and mass transfer processes, which are more widely studied, the incorporation of attenuation mechanisms driven by microorganisms and plants is essential to predict the final concentration of the pollutants in the whole multiphase scenario. This work underlines the importance of the determination of accurate parameters through the performance of laboratory and/or field-scale experiments to develop precise pollutant migration models.

The association of plants and rhizospheric bacteria provides a successful strategy to clean up contaminated soils. The purpose of this work was to enhance diesel degradation in Lupinus luteusrhizosphere by inoculation with selected bacterial strains: a diesel degrader (D), plant growth–promoting (PGP) strains, or a combination (D+PGP). Plants were set up in pots with the A or B horizon of an umbric Cambisol (ACamband BCamb) spiked with diesel (1.25%, w/w). After 1 mo, the dissipation of diesel range organics (DRO) with respect to t= 0 (i.e., 1 wk after preparing the pots with the seedlings) concentration was significantly higher in inoculated than in noninoculated (NI) pots: The highest DRO losses were found in ACambD+PGP pots (close to 15–20% higher than NI) and in BCambD pots (close to 10% higher). The water‐extractable DRO fraction was significantly higher at t= 30 d (15–25%) compared with t= 0 (<5%), probably due to the effects of plant root exudates and biosurfactants produced by the degrader strain. The results of this experiment reflect the importance of the partnerships between plants and bacterial inoculants and demonstrate the relevance of the effect of bacterial biosurfactants and plant root exudates on contaminant bioavailability, a key factor for enhancing diesel rhizodegradation. The association of lupine with D and PGP strains resulted in a promising combination for application in the rhizoremediation of soils with moderate diesel contamination. Biostimulated rhizodegradation of diesel contaminated soils was applied.The synergy between plants and microorganisms (diesel‐degraders and PGP) was exploited.Biosurfactants and root exudation highly influenced contaminant bioavailability.

Recent research has demonstrated that chemotactic bacteria can disperse inside microsized pores while traveling toward favorable conditions. Microbe–microbe cotransport might enable nonmotile bacteria to be carried with motile partners to enhance their dispersion and reduce their deposition in porous systems. The aim of this study was to demonstrate the enhancement in the dispersion of nonmotile bacteria (Mycobacterium gilvumVM552, a polycyclic aromatic hydrocarbon-degrader, and Sphingobiumsp. D4, a hexachlorocyclohexane-degrader, through micrometer-sized pores near the exclusion-cell-size limit, in the presence of motile Pseudomonas putidaG7 cells. For this purpose, we used bioreactors equipped with two chambers that were separated with membrane filters with 3, 5, and 12 μm pore sizes and capillary polydimethylsiloxane (PDMS) microarrays (20 μm × 35 μm × 2.2 mm). The cotransport of nonmotile bacteria occurred exclusively in the presence of a chemoattractant concentration gradient, and therefore, a directed flow of motile cells. This cotransport was more intense in the presence of larger pores (12 μm) and strong chemoeffectors (γ-aminobutyric acid). The mechanism that governed cotransport at the cell scale involved mechanical pushing and hydrodynamic interactions. Chemotaxis-mediated cotransport of bacterial degraders and its implications in pore accessibility opens new avenues for the enhancement of bacterial dispersion in porous media and the biodegradation of heterogeneously contaminated scenarios.

Sorption of volatile compounds from fuel by soils affects the final environmental fate of these contaminants and strongly determines the efficiency of decontamination techniques. The headspace‐analysis approach used in this study indirectly relates the sorption exerted by soils with the contaminant analytical recovery, which will be different according to the matrix effect of each particular sample. The aim of this study was to assess the sorption of benzene, toluene, ethylbenzene, and xylene (BTEX) and fuel oxygenates by a wide selection of soil components and soil samples and to determine the influence of the physicochemical properties of the sample and the contaminant, the contaminant concentration, the incubation time, and the temperature on sorption. For this purpose, the samples were spiked with BTEX and fuel oxygenates in hermetically sealed vials and later analyzed by headspace‐gas chromatography–mass spectrometry under several contaminant concentration, incubation time, and temperature conditions. The results were then compared to assess the sorption exerted by each sample under each scenario. Significant differences were found between the recovery of BTEX and fuel oxygenates, mainly due to the different mobility, polarity, and sorption mechanisms involved while interacting with soil surface charges. Furthermore, these interactions determined the kinetic and strength of sorption and had a strong influence on the recovery of BTEX and fuel oxygenates at different temperatures. The headspace analysis approach resulted in a quick, easy, simple, automatable, and environmentally friendly technique to obtain important information for understanding the behavior of fuel volatile compounds in soil under very different conditions. In addition, it establishes a good starting point for developing more sophisticated adsorption and soil remediation studies.