Co-composting of digestate and garden waste with biochar: effect on greenhouse gas production and fertilizer value of the matured compost.

Biogas digestate is a nitrogen (N) rich waste product that has potential for application to soil as a fertilizer. Composting of digestate is recognized as an effective step to reduce potentially negative consequences of digestate application to soils. However, the structure of the digestate and the high N content can hinder effective composting. Biochar, which can be produced through the pyrolysis of waste biomass, has shown the potential to improve compost structure and increase N retention in soils. We studied how a high-temperature wood biochar affects the composting process, including greenhouse gas emissions, and the fertilizer value of the compost product including nutrient content, leachability and plant growth. The high Biochar dose (17% w/w) had a significantly positive effect on the maximum temperature (5°C increase vs. no biochar) and appeared to improve temperature stability during composting with less variability between replicates. Biochar addition reduced cumulative N2O emission by 65-70%, but had no significant effect on CO2 and CH4 emission. Biochar did not contribute to greater retention of nitrogen (N) contained in the digestate, but had a dilution effect on both N content and mineral nutrients. Fertilization with compost enhanced plant growth and nutrient retention in soil compared to mineral fertilization (NPK), but biochar had no additional effects on these parameters. Our results show that biochar improves the composting of digestate with no subsequent negative effects on plants.

Moving forward in microplastic research: A Norwegian perspective.

Given the increasing attention on the occurrence of microplastics in the environment, and the potential environmental threats they pose, there is a need for researchers to move quickly from basic understanding to applied science that supports decision makers in finding feasible mitigation measures and solutions. At the same time, they must provide sufficient, accurate and clear information to the media, public and other relevant groups (e.g., NGOs). Key requirements include systematic and coordinated research efforts to enable evidence-based decision making and to develop efficient policy measures on all scales (national, regional and global). To achieve this, collaboration between key actors is essential and should include researchers from multiple disciplines, policymakers, authorities, civil and industry organizations, and the public. This further requires clear and informative communication processes, and open and continuous dialogues between all actors. Cross-discipline dialogues between researchers should focus on scientific quality and harmonization, defining and accurately communicating the state of knowledge, and prioritization of topics that are critical for both research and policy, with the common goal to establish and update action plans for holistic benefit. In Norway, cross-sectoral collaboration has been fundamental in supporting the national strategy to address plastic pollution. Researchers, stakeholders and the environmental authorities have come together to exchange knowledge, identify knowledge gaps, and set targeted and feasible measures to tackle one of the most challenging aspects of plastic pollution: microplastic. In this article, we present a Norwegian perspective on the state of knowledge on microplastic research efforts. Norway's involvement in international efforts to combat plastic pollution aims at serving as an example of how key actors can collaborate synergistically to share knowledge, address shortcomings, and outline ways forward to address environmental challenges.

Route of exposure has a major impact on uptake of silver nanoparticles in Atlantic salmon (Salmo salar).

The potential impact of silver nanoparticles (Ag NPs) on aquatic organisms is to a large extent determined by their bioavailability through different routes of exposure. In the present study juvenile Atlantic salmon (Salmo salar) were exposed to different sources of radiolabeled Ag (radiolabeled 110m Ag NPs and 110m AgNO3 ). After 48 h of waterborne exposure to 3 µg/L citrate stabilized 110m Ag NPs or 110m AgNO3 , or a dietary exposure to 0.6 mg Ag/kg fish (given as citrate stabilized or uncoated 110m Ag NPs, or 110m AgNO3 ), Ag had been taken up in fish regardless of route of exposure or source of Ag (Ag NPs or AgNO3 ). Waterborne exposure led to high Ag concentrations on the gills, and dietary exposure led to high concentrations in the gastrointestinal tract. Silver distribution to the target organs was similar for both dietary and waterborne exposure, with the liver as the main target organ. The accumulation level of Ag was 2 to 3 times higher for AgNO3 than for Ag NPs when exposure was through water, whereas no significant differences were seen after dietary exposure. The transfer (Bq/g liver/g food or water) from exposure through water was 4 orders of magnitude higher than from feed using the smallest, citrate-stabilized Ag NPs (4 nm). The smallest NPs had a 5 times higher bioavailability in food compared with the larger and uncoated Ag NPs (20 nm). Despite the relatively low transfer of Ag from diet to fish, the short lifetime of Ag NPs in water and their transfer to sediment, feed, or sediment-dwelling food sources such as larvae and worms could make diet a significant long-term exposure route. Environ Toxicol Chem 2018;37:2895-2903. © 2018 SETAC.

Suppression of the activity of arbuscular mycorrhizal fungi by the soil microbiota.

Arbuscular mycorrhizal fungi (AMF) colonise roots of most plants; their extra-radical mycelium (ERM) extends into the soil and acquires nutrients for the plant. The ERM coexists with soil microbial communities and it is unresolved whether these communities stimulate or suppress the ERM activity. This work studied the prevalence of suppressed ERM activity and identified main components behind the suppression. ERM activity was determined by quantifying ERM-mediated P uptake from radioisotope-labelled unsterile soil into plants, and compared to soil physicochemical characteristics and soil microbiome composition. ERM activity varied considerably and was greatly suppressed in 4 of 21 soils. Suppression was mitigated by soil pasteurisation and had a dominating biotic component. AMF-suppressive soils had high abundances of Acidobacteria, and other bacterial taxa being putative fungal antagonists. Suppression was also associated with low soil pH, but this effect was likely indirect, as the relative abundance of, e.g., Acidobacteria decreased after liming. Suppression could not be transferred by adding small amounts of suppressive soil to conducive soil, and thus appeared to involve the common action of several taxa. The presence of AMF antagonists resembles the phenomenon of disease-suppressive soils and implies that ecosystem services of AMF will depend strongly on the specific soil microbiome.

Assessment of addition of biochar to filtering mixtures for potential water pollutant removal.

Green roofs are used increasingly to alleviate peaks of water discharge into the sewage systems in urban areas. Surface runoff from roofs contain pollutants from dry and wet deposition, and green roofs offer a possibility to reduce the amounts of pollutants in the water discharged from roofs by degradation and filtering. These pollutants would otherwise enter wastewater treatments plants and ultimately end up in sewage sludge that is spread on agricultural soils. The most common substrates used in green roofs have limited capacity for filtration and sorption. Also, more sustainable alternatives are sought, due to the high carbon footprint of these materials. Biochar is a carbon-rich material produced by pyrolysis of biomass, and several types of biochar have been described as good sorbents and filter materials. Biochar is also a light and carbon negative material, which may fulfill other desired criteria for new green roof substrates. We here report on an experiment where two types of biochar, produced from olive husks at 450 °C or from forest waste at 850 ° C were mixed with volcanic rock or peat, and tested for retention capacity of phenanthrene and six heavy metals in a column experiment with unsaturated gravimetric water flow lasting for 3 weeks. The results suggest that biochar as a component in green roof substrates perform better than traditional materials, concerning retention of the tested pollutants, and that different types of biochar have different properties in this respect.

Zero-valent iron particles for PCB degradation and an evaluation of their effects on bacteria, plants, and soil organisms.

Two types of nano-scale zero-valent iron (nZVI-B prepared by borohydride reduction and nZVI-T produced by thermal reduction of iron oxide nanoparticles in H2) and a micro-scale ZVI (mZVI) were compared for PCB degradation efficiency in water and soil. In addition, the ecotoxicity of nZVI-B and nZVI-T particles in treated water and soil was evaluated on bacteria, plants, earthworms, and ostracods. All types of nZVI and mZVI were highly efficient in degradation of PCBs in water, but had little degradation effect on PCBs in soil. Although nZVI-B had a significant negative impact on the organisms tested, treatment with nZVI-T showed no negative effect, probably due to surface passivation through controlled oxidation of the nanoparticles.

Ecotoxicity testing and environmental risk assessment of iron nanomaterials for sub-surface remediation - Recommendations from the FP7 project NanoRem.

Nanoremediation with iron (Fe) nanomaterials opens new doors for treating contaminated soil and groundwater, but is also accompanied by new potential risks as large quantities of engineered nanomaterials are introduced into the environment. In this study, we have assessed the ecotoxicity of four engineered Fe nanomaterials, specifically, Nano-Goethite, Trap-Ox Fe-zeolites, Carbo-Iron® and FerMEG12, developed within the European FP7 project NanoRem for sub-surface remediation towards a test battery consisting of eight ecotoxicity tests on bacteria (V. fisheri, E. coli), algae (P. subcapitata, Chlamydomonas sp.), crustaceans (D. magna), worms (E. fetida, L. variegatus) and plants (R. sativus, L. multiflorum). The tested materials are commercially available and include Fe oxide and nanoscale zero valent iron (nZVI), but also hybrid products with Fe loaded into a matrix. All but one material, a ball milled nZVI (FerMEG12), showed no toxicity in the test battery when tested in concentrations up to 100 mg/L, which is the cutoff for hazard labeling in chemicals regulation in Europe. However it should be noted that Fe nanomaterials proved challenging to test adequately due to their turbidity, aggregation and sedimentation behavior in aqueous media. This paper provides a number of recommendations concerning future testing of Fe nanomaterials and discusses environmental risk assessment considerations related to these.

Uptake and elimination kinetics of the biocide triclosan and the synthetic musks galaxolide and tonalide in the earthworm Dendrobaena veneta when exposed to sewage sludge.

Sewage sludge is an important amendment that enriches soils with organic matter and provides plants with nutrients such as nitrogen and phosphorus. However, knowledge on the fate and effects of organic pollutants present in the sludge on soil organisms is limited. In the present study, the uptake of triclosan, galaxolide, and tonalide in the earthworm Dendrobaena veneta was measured 1 wk after amendment of agricultural soil with sewage sludge, while elimination kinetics were assessed over a 21-d period after transferring worms to clean soil. After 1-wk exposure, earthworms had accumulated 2.6 ± 0.6 µg g-1 galaxolide, 0.04 ± 0.02 µg g-1 tonalide, and 0.6 ± 0.2 µg g-1 triclosan. Both synthetic musks were efficiently excreted and below the limit of quantification after 3 and 14 d of depuration for tonalide and galaxolide, respectively. Triclosan concentrations, on the other hand, did not decrease significantly over the depuration period, which may lead to the transfer of triclosan in the food web. Environ Toxicol Chem 2017;36:2068-2073. © 2017 SETAC.

Bioavailability of CeO2 and SnO2 nanoparticles evaluated by dietary uptake in the earthworm Eisenia fetida and sequential extraction of soil and feed.

The growing number of nanotechnology products on the market will inevitably lead to the release of engineered nanomaterials with potential risk to humans and environment. This study set out to investigate the exposure of soil biota to engineered nanoparticles (NPs). Cerium dioxide (CeO2 NPs) and tin dioxide nanoparticles (SnO2 NPs) were radiolabelled using neutron activation, and employed to assess the uptake and excretion kinetics in the earthworm Eisenia fetida. Through sequential extraction, NPs bioavailability in two contrasting soils and in earthworm feed was also investigated. Neither CeO2 NPs nor SnO2 NPs bioaccumulated in earthworms, and both were rapidly excreted when worms were transferred to clean soil. Low bioavailability was also indicated by low amounts of NPs recovered during extraction with non-stringent extractants. CeO2 NPs showed increasing mobility in organic soil over time (28 days), indicating that organic matter has a strong influence on the fate of CeO2 NPs in soil.

DDT degradation efficiency and ecotoxicological effects of two types of nano-sized zero-valent iron (nZVI) in water and soil.

Nano-scale zero-valent iron (nZVI) has been conceived for cost-efficient degradation of chlorinated pollutants in soil as an alternative to e.g permeable reactive barriers or excavation. Little is however known about its efficiency in degradation of the ubiquitous environmental pollutant DDT and its secondary effects on organisms. Here, two types of nZVI (type B made using precipitation with borohydride, and type T produced by gas phase reduction of iron oxides under H2) were compared for efficiency in degradation of DDT in water and in a historically (>45 years) contaminated soil (24 mg kg(-1) DDT). Further, the ecotoxicity of soil and water was tested on plants (barley and flax), earthworms (Eisenia fetida), ostracods (Heterocypris incongruens), and bacteria (Escherichia coli). Both types of nZVI effectively degraded DDT in water, but showed lower degradation of aged DDT in soil. Both types of nZVI had negative impact on the tested organisms, with nZVI-T giving least adverse effects. Negative effects were mostly due to oxidation of nZVI, resulting in O2 consumption and excess Fe(II) in water and soil.

Responses of earthworms to repeated exposure to three biocides applied singly and as a mixture in an agricultural field.

The study aimed at investigating effects of three differently acting biocides; the insecticide esfenvalerate, the fungicide picoxystrobin and the bactericide triclosan, applied individually and as a mixture, on an earthworm community in the field. A concentration-response design was chosen and results were analyzed using univariate and multivariate approaches. Effects on juvenile proportions were less pronounced and more variable than effects on abundance, but effects in general were species- and chemical-specific, and temporal variations distinct. Esfenvalerate and picoxystrobin appeared to elicit stronger effects than triclosan at laboratory-based ECx values, which is in accordance with our previous laboratory study on Eisenia fetida. The mixture affected abundance and juvenile proportions, but the latter only at high mixture concentrations. Esfenvalerate and picoxystrobin appeared to be the main drivers for the mixture's toxicity. Species-specific toxicity patterns question the reliability of mixture toxicity predictions derived on E. fetida for field earthworms. Biocide concentrations equaling EC50s (reproduction) for E. fetida provoked effects on the field earthworms mainly exceeding 50%, indicating effect intensification from the laboratory to field as well as the influence of indirect effects produced by species interactions. The differing results of the present field study and the previous laboratory study imply that lower- and higher-tier studies may not be mutually exclusive, but to be used in complementary.

Effects of nano-sized zero-valent iron (nZVI) on DDT degradation in soil and its toxicity to collembola and ostracods.

Nano-sized zero valent iron (nZVI) has been studied for in situ remediation of contaminated soil and ground water. However, little is known about its effects on organisms in soil and aquatic ecosystems. In this study, the effect of nZVI on degradation of DDT and its ecotoxicological effects on collembola (Folsomia candida) and ostracods (Heterocypris incongruens) were investigated. Two soils were used in suspension incubation experiments lasting for 7 and 30 d; a spiked (20 mg DDT kg(-1)) sandy soil and an aged (>50 years) DDT-polluted soil (24 mg DDT kg(-1)). These were incubated with 1 or 10 g nZVI kg(-1), and residual toxicity in soil and the aqueous phase tested using ecotoxicological tests with collembola or ostracods. Generally, addition of either concentration of nZVI to soil led to about 50% degradation of DDT in spiked soil at the end of 7 and 30 d incubation, while the degradation of DDT was less in aged DDT-polluted soil (24%). Severe negative effects of nZVI were observed on both test organisms after 7 d incubation, but prolonged incubation led to oxidation of nZVI which reduced its toxic effects on the tested organisms. On the other hand, DDT had significant negative effects on collembolan reproduction and ostracod development. We conclude that 1 g nZVI kg(-1) was efficient for significant DDT degradation in spiked soil, while a higher concentration was necessary for treating aged pollutants in soil. The adverse effects of nZVI on tested organisms seem temporary and reduced after oxidation.

Ecotoxicological effects on earthworms of fresh and aged nano-sized zero-valent iron (nZVI) in soil.

Although nano-sized zero-valent iron (nZVI) has been used for several years for remediation of contaminated soils and aquifers, only a limited number of studies have investigated secondary environmental effects and ecotoxicity of nZVI to soil organisms. In this study we therefore measured the ecotoxicological effects of nZVI coated with carboxymethyl cellulose on two species of earthworms, Eisenia fetida and Lumbricus rubellus, using standard OECD methods with sandy loam and artificial OECD soil. Earthworms were exposed to nZVI concentrations ranging from 0 to 2000 mg nZVI kg soil(-1) added freshly to soil or aged in non-saturated soil for 30 d prior to exposure. Regarding avoidance, weight changes and mortality, both earthworm species were significantly affected by nZVI concentrations ≥500 mg kg(-1)soil. Reproduction was affected also at 100 mg nZVI kg(-1). Toxicity effects of nZVI were reduced after aging with larger differences between soils compared to non-aged soils. We conclude that doses ≥500 mg nZVI kg(-1) are likely to give acute adverse effects on soil organisms, and that effects on reproduction may occur at significantly lower concentrations.

Impact of Fe and Ag nanoparticles on seed germination and differences in bioavailability during exposure in aqueous suspension and soil.

The potential environmental toxicity of zero-valent iron nanoparticles (nZVI) and three types of nanosilver differing in average particle size from 1 to 20 nm was evaluated using seed germination tests with ryegrass, barley, and flax exposed to 0-5000 mg L(-1) nZVI or 0-100 mg L(-1) Ag. For nZVI, germination tests were conducted both in water and in two contrasting soils to test the impact of assumed differences in bioavailability of nanoparticles. Inhibitory effects were observed in aqueous suspensions at 250 mg L(-1) for nZVI and 10 mg L(-1) for Ag. Reduction in shoot growth was a more sensitive endpoint than germination percentage. Complete inhibition of germination was observed at 1000-2000 mg L(-1) for nZVI. For Ag, complete inhibition was not achieved. The presence of soil had a modest influence on toxicity, and inhibitory effects were observed at 300 mg nZVI L(-1) water in soil (equivalent to 1000 mg nZVI kg(-1) soil). Complete inhibition was observed at 750 and 1500 mg L(-1) in sandy soil for flax and ryegrass, respectively, while for barley 13% germination still occurred at 1500 mg L(-1) . In clay soil, inhibition was less pronounced. Our results indicate that nZVI at low concentrations can be used without detrimental effects on plants and thus be suitable for combined remediation where plants are involved. Silver nanoparticles inhibited seed germination at lower concentrations, but showed no clear size-dependent effects, and never completely impeded germination. Thus, seed germination tests seem less suited for estimation of environmental impact of

Bioavailability of cobalt and silver nanoparticles to the earthworm Eisenia fetida.

Due to difficulties in tracing engineered nanoparticles (ENPs) in complex media, there are few data on the exposure of soil biota to ENPs. This study used neutron activated cobalt (Co NPs) and silver (Ag NPs) nanoparticles, as well as soluble cobalt and silver salts, to assess the uptake, excretion and biodistribution in the earthworm Eisenia fetida. Concentrations of cobalt in worms after four weeks exposure reached 88% and 69% of the Co ions and Co NPs concentrations in food, respectively, while corresponding values for Ag ions and Ag NPs were 2.3% and 0.4%. Both Ag ions and Ag NPs in earthworms were excreted rapidly, while only 32% of the cobalt accumulated from Co ions and Co NPs were excreted within four months. High accumulation of cobalt was found in blood and in the digestive tract. Metal characterization in the exposure medium was assessed by sequential extraction and ultrafiltration. The Co NPs showed significant dissolution and release of ions, while Ag ions and particularly Ag NPs were more inert.

Ecotoxicological assessment of TiO2 byproducts on the earthworm Eisenia fetida.

The increasing production of nanomaterials will in turn increase the release of nanosized byproducts to the environment. The aim of this study was to evaluate the behaviour, uptake and ecotoxicity of TiO(2) byproducts in the earthworm Eisenia fetida. Worms were exposed to suspensions containing 0.1, 1 and 10 mg/L of byproducts for 24 h. Size of TiO(2) byproducts showed aggregation of particles up to 700 µm with laser diffraction. Only worms exposed at 10 mg/L showed bioaccumulation of titanium (ICP-AES), increasing expression of metallothionein and superoxide dismutase mRNA (Real-time PCR) and induction of apoptotic activity (Apostain and TUNEL). TiO(2) byproducts did not induce cytotoxicity on cœlomocytes, but a significant decrease of phagocytosis was observed starting from 0.1 mg/L. In conclusion, bioaccumulation of byproducts and their production of reactive oxygen species could be responsible for the alteration of the antioxidant system in worms.

Oxidative stress induced in microorganisms by zero-valent iron nanoparticles.

Nanoscale zero-valent iron particles (nZVI), with sizes smaller than 100 nm, are promising for environmental remediation of polluted water, soil and sediments. nZVI particles have high potential for migration in the environment and are likely to interact not only with pollutant chemicals but also with living organisms. For these reasons, an environmental concern is rising with respect to unintended effects that need to be weighed against the benefits of remediation. The nZVI particles have a tendency to release electrons and Fe(2+). The Fe(2+) can convert less reactive hydrogen peroxide to more reactive oxygen species, particularly hydroxyl radicals, via the Fenton reaction. Hydroxyl radicals show strong biochemical activity and can react directly with membrane lipids, proteins and DNA. Reactive oxygen species are normally scavenged by antioxidants and various enzymes; however, elevated concentrations of ROS in microbial cells can result in oxidative stress. Cells under severe oxidative stress show various dysfunctions of membrane lipids, proteins and DNA. This review focuses on the processes resulting in oxidative stress and on up-to-date studies of nZVI-induced intracellular changes leading to such stress in microorganisms.

Ecotoxicological effects of an aged TiO2 nanocomposite measured as apoptosis in the anecic earthworm Lumbricus terrestris after exposure through water, food and soil.

Titanium dioxide nanoparticles seem to have a low toxicity to terrestrial organisms, though few studies are published in this area. TiO(2) used in sunscreens are nanocomposites where TiO(2) has been coated with magnesium, silica or alumina, as well as amphiphilic organics like polydimethyl siloxane (PDMS), and these coatings are modified by ageing. We assessed the ecotoxicity and propensity for bioaccumulation of an aged TiO(2) nanocomposite used in sunscreen cosmetics, and its potential effect on the frequency of apoptosis in different earthworm tissues. The earthworm Lumbricus terrestris was exposed to the TiO(2) nanocomposite for 7 days in water or 2-8 weeks in soil with the nanocomposite mixed either into food or soil at concentrations ranging from 0 to 100 mg kg(-1). Apoptosis was then measured by immunohistochemistry and Ti localized by XRF microscopy. Results showed no mortality, but an enhanced apoptotic frequency which was higher in the cuticule, intestinal epithelium and chloragogenous tissue than in the longitudinal and circular musculature. TiO(2) nanoparticles did not seem to cross the intestinal epithelium/chloragogenous matrix barrier to enter the coelomic liquid, or the cuticule barrier to reach the muscular layers. No bioaccumulation of TiO(2) nanocomposites could thus be observed.

Ectomycorrhizas impede phytoremediation of polycyclic aromatic hydrocarbons (PAHs) both within and beyond the rhizosphere.

Exploitation of mycorrhizas to enhance phytoremediation of organic pollutants has received attention recently due to their positive effects on establishment of plants in polluted soils. Some evidence exist that ectomycorrhizas enhance the degradation of pollutants of low recalcitrance, while less easily degradable polyaromatic molecules have been degraded only by some of these fungi in vitro. Natural polyaromatic (humic) substances are degraded more slowly in soil where ectomycorrhizal fungi are present, thus phytoremediation of recalcitrant pollutants may not benefit from the presence of these fungi. Using a soil spiked with three polycyclic aromatic hydrocarbons (PAHs) and an industrially polluted soil (1 g kg(-1) of summation operator12 PAHs), we show that the ectomycorrhizal fungus Suillus bovinus, forming hydrophobic mycelium in soil that would easily enter into contact with hydrophobic pollutants, impedes rather than promotes PAH degradation. This result is likely to be a nutrient depletion effect caused by fungal scavenging of mineral nutrients.

Utilising the synergy between plants and rhizosphere microorganisms to enhance breakdown of organic pollutants in the environment.

BACKGROUND: Phytoremediation is a promising technology for the cleanup of polluted environments. The technology has so far been used mainly to remove toxic heavy metals from contaminated soil, but there is a growing interest in broadening its applications to remove/degrade organic pollutants in the environment. Both plants and soil microorganisms have certain limitations with respect to their individual abilities to remove/breakdown organic compounds. A synergistic action by both rhizosphere microorganisms that leads to increased availability of hydrophobic compounds, and plants that leads to their removal and/or degradation, may overcome many of the limitations, and thus provide a useful basis for enhancing remediation of contaminated environments. MAIN FEATURES: The review of literature presented in this article provides an insight to the nature of plant-microbial interactions in the rhizosphere, with a focus on those processes that are relevant to the breakdown and/or removal of organic pollutants. Due consideration has been given to identify opportunities for utilising the plant-microbial synergy in the rhizosphere to enhance remediation of contaminated environments, RESULTS AND DISCUSSION: The literature review has highlighted the existence of a synergistic interaction between plants and microbial communities in the rhizosphere. This interaction benefits both microorganisms through provision of nutrients by root exudates, and plants through enhanced nutrient uptake and reduced toxicity of soil contaminants. The ability of the plant-microbial interaction to tackle some of the most recalcitrant organic chemicals is of particular interest with regard to enhancing and extending the scope of remediation technologies. CONCLUSIONS: Plant-microbial interactions in the rhizosphere offer very useful means for remediating environments contaminated with recalcitrant organic compounds. OUTLOOK: A better knowledge of plant-microbial interactions will provide a basis for improving the efficacy of biological remediations. Further research is, however, needed to investigate different feedback mechanisms that select and regulate microbial activity in the rhizosphere.