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Plant growth promoting (PGP) bacterial strains possess different mechanisms to improve plant development under common environmental stresses, and are therefore often used as inoculants in soil phytoremediation processes. The aims of the present work were to study the effects of a collection of plant growth promoting bacterial strains on plant development, antioxidant enzyme activities and nutritional status of Cytisus striatus and/or Lupinus luteus plants a) growing in perlite under non-stress conditions and b) growing in diesel-contaminated soil. For this, two greenhouse experiments were designed. Firstly, C. striatus and L. luteus plants were grown from seeds in perlite, and periodically inoculated with 6 PGP strains, either individually or in pairs. Secondly, L. luteus seedlings were grown in soil samples of the A and B horizons of a Cambisol contaminated with 1.25% (w/w) of diesel and inoculated with best PGP inoculant selected from the first experiment. The results indicated that the PGP strains tested in perlite significantly improved plant growth. Combination treatments provoked better growth of L. luteus than the respective individual strains, while individual inoculation treatments were more effective for C. striatus . L. luteus growth in diesel-contaminated soil was significantly improved in the presence of PGP strains, presenting a 2-fold or higher increase in plant biomass. Inoculants did not provoke significant changes in plant nutritional status, with the exception of a subset of siderophore-producing and P-solubilising bacterial strains that resulted in significantly modification of Fe or P concentrations in leaf tissues. Inoculants did not cause significant changes in enzyme activities in perlite experiments, however they significantly reduced oxidative stress in contaminated soils suggesting an improvement in plant tolerance to diesel. Some strains were applied to non-host plants, indicating a non-specific performance of their plant growth promotion. The use of PGP strains in phytoremediation may help plants to overcome contaminant and other soil stresses, increasing phytoremediation efficiency. [ABSTRACT FROM AUTHOR]

• Polluted areas can persist in soils after land reclamation. • We investigated earthworm behaviour in response to different pollution scenarios. • Earthworms showed avoidance and pollutants affected burrowing and casting activity. • Earthworm responses varied with species and large polluted zones had greater effects. • Heterogeneous pollution could impact earthworms and soil functions in the ecosystem. Large amounts of chemicals are released into soil environments due to human activities, thereby harming organisms and their habitats. Earthworms are important bioindicators for chemical risk assessments of soil ecosystems, but the question remains of how they respond to different patterns of pollution. Laboratory experiments in soil mesocosms were carried out to observe changes in the burrowing behaviour of two earthworm species (Aporrectodea caliginosa and Lumbricus terrestris) with regard to different spatial distributions of benzo(a)pyrene, cadmium and copper, using X-ray computed microtomography (μCT). This is, to our knowledge, the first contribution of μCT to study earthworm behaviour in heterogeneously multi-polluted soils. Columns (14 × 30 cm) were filled with a silt loam soil and packed to represent two scenarios of heterogeneous pollution: a) 'binary', containing polluted soil in the low half part and unpolluted soil on top; and b) 'patches', containing small portions of polluted soil randomly placed into an unpolluted matrix. Earthworms were introduced individually in the columns and left in climatic chambers for 7 weeks. μCT images revealed that earthworms avoided the polluted zones differently (e.g. A. caliginosa > L. terrestris , 'binary' configuration). Individuals of L. terrestris that entered polluted areas showed a lower burrowing activity (volume and length of the burrow system) than those that did not enter. The presence of 'patches' of polluted soil did not cause a reduction of the burrowing activity such as that found in the 'binary' columns, although earthworms showed avoidance. Our results indicate that pollutants had a greater effect on earthworms' burrowing behaviour in the presence of larger portions of contaminated soil compared with small random ones. The more evident avoidance behaviour of A. caliginosa pinpoints its usefulness in toxicity bioassays based on avoidance patterns, but questions its use in assays based on bioaccumulation of pollutants in tissues, for which L. terrestris may perform better. [ABSTRACT FROM AUTHOR]

Plants and their associated microbiota can have a significant impact on the behaviour of soil contaminants. Particularly, root exudation is one of the most important plant-associated processes in this respect, as it may have a substantial effect on the bioavailability of soil contaminants, specially of hydrophobic contaminants strongly sorbed by soil. The aim of the present study was to evaluate the effect of root exudates (natural and artificial) on the desorption of α-, β-, δ- and γ-isomers of hexachlorocyclohexane (HCH) from contaminated soil, using batch experiments. Natural root exudates were obtained from Holcus lanatus plants growing in the same (contaminated) area. Fifteen compounds (mainly organic acids and phenolic compounds) usually found in root exudates were also tested, individually or as mixtures (1 and 10 mM). Both natural and artificial exudates favoured the mobilization of sorbed HCH in soil. The effect was highly significant for α-, β- and γ-HCH isomers, for which the desorption rates increased by 23.0, 26.8 and 15.5% in the presence of natural root exudates and by 40.1, 25.9 and 25.6% in the presence of the artificial mixture (at 10 mM). The δ-HCH desorption rates increased by less than 10%. The effect of individual exudate components was very variable, but increased with the carbon content, reflecting the significance of hydrophobic interactions between the exudates and HCH molecules in the desorption of these last from soil. These findings indicate that plants may significantly influence the bioavailability of persistent contaminants, with major implications for improving phyto- and bioremediation procedures. • HCH desorption from aged soil generally increased in the presence of root exudates. • The effect was more significant for the most hydrophobic isomers (α, β and γ-HCH). • The desorption effect increased with the carbon content of exudate components. • Root exudation may be crucial to increase HCH bioavailability in the rhizosphere. [ABSTRACT FROM AUTHOR]

Assessing the ecological risk of combined pollution, especially from a holistic perspective with the consideration of the overarching functions of soil ecosystem, is crucial and beneficial to the improvement of ecological risk assessment (ERA) framework. In this study, four soils with similar physicochemical properties but contrasting heavy metals contamination levels were selected to explore changes in the integrated functional sensitivity (MSI), resistance (MRS) and resilience (MRL) of soil microbial communities subjected to herbicide siduron, based on which the ecological risk of the accumulation of siduron in the four studied soils were evaluated. The results suggested that the microbial biomass carbon, activity of denitrification enzyme and nitrogenase were indicative of MSI and MRS, and the same three parameters plus soil basal respiration were indicative of MRL. Significant dose-effect relationships between siduron residues in soils and MSI, MRS and MRL under combined pollution were observed. Heavy metal polluted soils showed higher sensitivity and lower resistance to the additional disturbance of herbicide siduron due to the lower microbial biomass, while the resilience of heavy metal polluted soils was much higher due to the pre-adaption to the chemical stresses. The quantifiable indicator microbial functional stability was incorporated in the framework of ERA and the results showed that the accumulation of siduron in the studied soils could exhibit potential harm to the integrated functional stability of soil microbial community. Thus, this work provides insights into the application of integrated function of soil microbial community into the framework of ERA. Image 1 • The integrated index method is efficient at assessing combined effects of contaminant mixtures. • Heavy metal polluted soils show lower resistance but higher resilience ability to siduron stress. • The accumulation of siduron in soil exhibits potential harm to microbial functional stability. • MSI, MRS, MRL are applicable endpoints to assess the toxic effect of pollutants at community level. [ABSTRACT FROM AUTHOR]

• The loss of biodiversity caused by heavy metals did not affect microbial functional stability. • The decrease of the GP/GN ratio trigged by heavy metals increased microbial functional stability. • Soil organic matter, Cd, C/N were crucial factors governing microbial functional stability. • Microbial functional stability can be a feasible indicator in ecological risk assessment. The functional stability (FS) of soil microbial communities contributes in a fundamental way to maintaining soil ecosystem functioning and services, when environmental disturbances, e.g. contamination arise. While, the mechanisms enabling soil microbial communities to remain stable under stresses are not clear. Thus, this work aims at investigating changes in integrated microbial FS under long-term stress of heavy metals (FS HM) and subsequent herbicide siduron (FS sid). A second objective is to determine which factors affect the FS of microbial communities. The results show that the overall function, alpha diversity, and composition of microbial communities are affected significantly by the presence of heavy metals and herbicide siduron. The loss of biodiversity induced by heavy metal does not appear to impair the functions of microbial communities or their functional stability in response to the additional stress of siduron. Whereas, the change of microbial community composition, e.g., the decrease of the gram positive/gram negative ratio triggered by the adaption and tolerance process to the pre-disturbance of heavy metals, is the central factor governing FS. Soil organic matter content, the concentration of heavy metals and the C/N value are the dominant abiotic factors affecting FS through direct effect on microbial activities involved in functional processes and indirect effect on microbial community composition. Our study highlights that the structural composition of soil microbial community is more important than the diversity of microbial species to explain the functional stability of microbial communities under chemical stresses, and that a number of soil properties and the disturbance history also play a decisive role. Thus, the functional stability of soil microbial community can be used as a feasible and pragmatic indicator to assess the overarching responses to environmental stresses at ecological system level. [ABSTRACT FROM AUTHOR]