Non-sterile Soil Research Articles

Improvement of growth of common bean in phosphorus-deficient soils by phosphate-solubilizing and phytohormone-producing bacteria

The use of phosphate fertilizers is generally required for normal growth and development of crops. The cost of manufactured phosphate fertilizers has increased drastically in recent years and these are becoming out of reach for low-income farmers. The objective of this research was to select phosphate-solubilizing and phytohormone-producing bacteria to improve the growth and P uptake by the common bean (IAC Alvorada) in Red-Yellow Oxisol without the addition of phosphate fertilizer. The experimental design was completely randomized with three replicates and the following treatments: non-inoculated bean plants and plants inoculated separately with seven bacterial isolates: K24, K36, K71, T30, T79, A24, and T22. The previously treated bean plants were grown in the presence and absence of phosphate fertilization. Only the plants inoculated with the isolates K36 and T79 showed higher values for the dry weight and the quantities of P and N uptake by the shoot when cultivated in the absence of phosphate fertilizer. These results demonstrate the potential use of the bacteria T79 and K36 as inoculants for bean plants grown in non-sterilized soil with low available P content, but also indicate that P solubilization may not be the only mechanism responsible for the positive growth response.

Latent Pathogenic Fungi in the Medicinal Plant Houttuynia cordata Thunb. Are Modulated by Secondary Metabolites and Colonizing Microbiota Originating from Soil.

Latent pathogenic fungi (LPFs) affect plant growth, but some of them may stably colonize plants. LPFs were isolated from healthy Houttuynia cordata rhizomes to reveal this mechanism and identified as Ilyonectria liriodendri, an unidentified fungal sp., and Penicillium citrinum. Sterile H. cordata seedlings were cultivated in sterile or non-sterile soils and inoculated with the LPFs, followed by the plants’ analysis. The in vitro antifungal activity of H. cordata rhizome crude extracts on LPF were determined. The effect of inoculation of sterile seedlings by LPFs on the concentrations of rhizome phenolics was evaluated. The rates of in vitro growth inhibition amongst LPFs were determined. The LPFs had a strong negative effect on H. cordata in sterile soil; microbiota in non-sterile soil eliminated such influence. There was an interactive inhibition among LPFs; the secondary metabolites also regulated their colonization in H. cordata rhizomes. LPFs changed the accumulation of phenolics in H. cordata. The results provide that colonization of LPFs in rhizomes was regulated by the colonizing microbiota of H. cordata, the secondary metabolites in the H. cordata rhizomes, and the mutual inhibition and competition between the different latent pathogens.

Novel Mechanism and Kinetics of Tetramethrin Degradation Using an Indigenous Gordonia cholesterolivorans A16.

Tetramethrin is a pyrethroid insecticide that is commonly used worldwide. The toxicity of this insecticide into the living system is an important concern. In this study, a novel tetramethrin-degrading bacterial strain named A16 was isolated from the activated sludge and identified as Gordonia cholesterolivorans. Strain A16 exhibited superior tetramethrin degradation activity, and utilized tetramethrin as the sole carbon source for growth in a mineral salt medium (MSM). High-performance liquid chromatography (HPLC) analysis revealed that the A16 strain was able to completely degrade 25 mg·L−1 of tetramethrin after 9 days of incubation. Strain A16 effectively degraded tetramethrin at temperature 20–40 °C, pH 5–9, and initial tetramethrin 25–800 mg·L−1. The maximum specific degradation rate (qmax), half-saturation constant (Ks), and inhibition constant (Ki) were determined to be 0.4561 day−1, 7.3 mg·L−1, and 75.2 mg·L−1, respectively. The Box–Behnken design was used to optimize degradation conditions, and maximum degradation was observed at pH 8.5 and a temperature of 38 °C. Five intermediate metabolites were identified after analyzing the degradation products through gas chromatography–mass spectrometry (GC-MS), which suggested that tetramethrin could be degraded first by cleavage of its carboxylester bond, followed by degradation of the five-carbon ring and its subsequent metabolism. This is the first report of a metabolic pathway of tetramethrin in a microorganism. Furthermore, bioaugmentation of tetramethrin-contaminated soils (50 mg·kg−1) with strain A16 (1.0 × 107 cells g−1 of soil) significantly accelerated the degradation rate of tetramethrin, and 74.1% and 82.9% of tetramethrin was removed from sterile and non-sterile soils within 11 days, respectively. The strain A16 was also capable of efficiently degrading a broad spectrum of synthetic pyrethroids including D-cyphenothrin, chlorempenthrin, prallethrin, and allethrin, with a degradation efficiency of 68.3%, 60.7%, 91.6%, and 94.7%, respectively, after being cultured under the same conditions for 11 days. The results of the present study confirmed the bioremediation potential of strain A16 from a contaminated environment.

Plant Growth Promoting Bacterial Consortium Induces Shifts in Indigenous Soil Bacterial Communities and Controls Listeria monocytogenes in Rhizospheres of Cajanus cajan and Festuca arundinacea.

The rhizosphere is a dynamic and complex interface between plant roots and microorganisms. Owing to exudates, a web of interactions establishes among the microbial members of this micro-environment. The present study explored the impact of a bacterial consortium (Azotobacter chroococcum, Bacillus megaterium and Pseudomonas fluorescens, ABP), on the fate of a human pathogen, Listeria monocytogenes EGD-e, in soil and in the rhizospheres of Cajanus cajan and Festuca arundinacea, in addition to its plant growth promoting effect. The study further assessed the impact these bioinoculants exert on the autochthonous soil bacterial communities. Experiments in sterilised soil inoculated with bioinoculants and L. monocytogenes revealed the inhibition of L. monocytogenes by approximately 80-fold compared to that without the consortium. Subsequently, experiments were conducted in non-sterile soil microcosms planted with C. cajan and F. arundinacea, and in bulk soil. The consortium led to a significant increase in plant growth in both plants and prevented growth of L. monocytogenes. However, the presence of resident soil bacterial communities overshadowed this inhibitory effect, and a sharp decline in L. monocytogenes populations (5-6 log reduction) was recorded under non-sterile soil conditions. A shift in the soil resident bacterial communities was observed upon amendment with the bioinoculants. A significant increase of potential Plant Growth Promoting Rhizobacteria (PGPR) and biocontrol agents was observed, while the abundance of potential phytopathogens dropped. The present study opens up new avenues for the application of such a consortium given their dual benefits of plant growth promotion and restricting phytopathogens as well as human pathogen.

Role of Rhizospheric Microbiota as a Bioremediation Tool for the Protection of Soil-Plant Systems from Microcystins Phytotoxicity and Mitigating Toxin-Related Health Risk.

Frequent toxic cyanoblooms in eutrophic freshwaters produce various cyanotoxins such as the monocyclic heptapeptides microcystins (MCs), known as deleterious compounds to plant growth and human health. Recently, MCs are a recurrent worldwide sanitary problem in irrigation waters and farmland soils due to their transfer and accumulation in the edible tissues of vegetable produce. In such cases, studies about the persistence and removal of MCs in soil are scarce and not fully investigated. In this study, we carried out a greenhouse trial on two crop species: faba bean (Vicia faba var. Alfia 321) and common wheat (Triticum aestivum var. Achtar) that were grown in sterile (microorganism-free soil) and non-sterile (microorganism-rich soil) soils and subjected to MC-induced stress at 100 µg equivalent MC-LR L−1. The experimentation aimed to assess the prominent role of native rhizospheric microbiota in mitigating the phytotoxic impact of MCs on plant growth and reducing their accumulation in both soils and plant tissues. Moreover, we attempted to evaluate the health risk related to the consumption of MC-polluted plants for humans and cattle by determining the estimated daily intake (EDI) and health risk quotient (RQ) of MCs in these plants. Biodegradation was liable to be the main removal pathway of the toxin in the soil; and therefore, bulk soil (unplanted soil), as well as rhizospheric soil (planted soil), were used in this experiment to evaluate the accumulation of MCs in the presence and absence of microorganisms (sterile and non-sterile soils). The data obtained in this study showed that MCs had no significant effects on growth indicators of faba bean and common wheat plants in non-sterile soil as compared to the control group. In contrast, plants grown in sterile soil showed a significant decrease in growth parameters as compared to the control. These results suggest that MCs were highly bioavailable to the plants, resulting in severe growth impairments in the absence of native rhizospheric microbiota. Likewise, MCs were more accumulated in sterile soil and more bioconcentrated in root and shoot tissues of plants grown within when compared to non-sterile soil. Thereby, the EDI of MCs in plants grown in sterile soil was more beyond the tolerable daily intake recommended for both humans and cattle. The risk level was more pronounced in plants from the sterile soil than those from the non-sterile one. These findings suggest that microbial activity, eventually MC-biodegradation, is a crucial bioremediation tool to remove and prevent MCs from entering the agricultural food chain.

Sulfamethoxazole biodegradation and impacts on soil microbial communities in a Bolivian arid high altitude catchment

The processes controlling antibiotics fate in ecosystems are poorly understood, yet their presence can inhibit bacterial growth and induce the development of bacterial resistance. Sulfamethoxazole (SMX) is one of the most frequently detected sulfonamides in natural environments due to its low metabolism and molecular properties. This work presents pioneering results on SMX biodegradation and impact in high altitude soils (Bolivian Altiplano), allowing a better understanding of the persistence, spread and impact of this antibiotic at the global watershed scale. Our results showed significant dissipation of SMX in relation to its adsorption, hydrolysis and biotransformation. However, biodegradation appears to be lower in these mountain soils than in lowland soils as widely described in the literature. The half-life of SMX ranges from 12 to 346 days in non-sterile soils. In one soil, no biotic degradation was observed, indicating a likely high persistence. Biodegradation was related to OC content and to proximity to urban activities. Regarding the study of the impacts of SMX, the DGGE results were less sensitive than the sequencing. In general, SMX strongly changes the structure and composition of the studied soils at high altitudes, which is comparable to the observations of other authors in lowland soils. The phylum Actinobacter showed high sensitivity to SMX. In contrast, the abundance of ɣ-proteobacteria remained almost unchanged. Soil contamination with SMX did not lead to the development of the studied resistance genes (sul1 and sul2) in soils where they were absent at the beginning of the experiment. Thus, the presence of SMX resistance genes seems to be related to irrigation with wastewater carrying the studied resistance genes.

Biological control of Western flower thrips, Frankliniella occidentalis using a self-sustaining granular fungal treatment.

Western flower thrips (WFT), Frankliniella occidentalis, is one of the most destructive pests of vegetables, fruits and ornamental crops worldwide, causing extensive damage by direct feeding of the crop and transmitting economically important viruses. Despite the successes of biocontrol agents to control WFT, more efficient and cost-effective ways must be found to encourage grower adoption of integrated pest management. A sustainable fungal treatment was developed to preserve fungal inoculum in potting soil and reduce thrips populations. Combining cooked, oven-dried millet with BotaniGard® (a commercial form of Beauveria bassiana strain GHA) to potting soil increased spore production and persistence of the fungus in the soil. In treated pots with millet, spore concentrations were 3-4 times greater after 30 days compared with spore yields at 10 days. The number of WFT adults was significantly lower in the marigold pots treated with GHA mix + millet than untreated controls, 12% and 10% in treated pots and 70% and 68% in untreated pots in sterile and non-sterile soil, respectively. Incorporation of millet in the potting mix enhanced the effect of the fungal treatments by providing a nutritive substrate on which the fungus could become established. This method is relatively inexpensive and easy for growers to use in greenhouses because granular formulations of B. bassiana are not commercially available.

Bacillus velezensis strain MBY2, a potential agent for the management of crown gall disease.

The reduction of the use chemical pesticides in agriculture is gaining importance as an objective of decision-makers in both politics and economics. Consequently, the development of technically efficient and economically affordable alternatives as, e.g., biological control agents or practices is highly solicited. Crown gall disease of dicotyledonous plants is caused by ubiquitous soil borne pathogenic bacteria of the Agrobacterium tumefaciens species complex, that comprises the species Agrobacterium fabrum and represents a globally relevant plant protection problem. Within the framework of a screening program for bacterial Agrobacterium antagonists a total of 14 strains were isolated from Tunisian soil samples and assayed for antagonistic activity against pathogenic agrobacteria. One particularly promising isolate, termed strain MBY2, was studied more in depth. Using a Multilocus Sequence Analysis (MLSA) approach, the isolate was assigned to the taxonomic species Bacillus velezensis. Strain MBY2 was shown to display antagonistic effects against the pathogenic A. fabrum strain C58 in vitro and to significantly decrease pathogen populations under sterile and non-sterile soil conditions as well as in the rhizosphere of maize and, to a lower extent, tomato plants. Moreover, the ability of B. velezensis MBY2 to reduce C58-induced gall development has been demonstrated in vivo on stems of tomato and almond plants. The present study describes B. velezensis MBY2 as a newly discovered strain holding potential as a biological agent for crown gall disease management.

Root and arbuscular mycorrhizal effects on soil nutrient loss are modulated by soil texture

Despite their importance, there is a lack of knowledge on the impact of forming arbuscular mycorrhizas (AM) on soil phosphorus (P) leaching in soils with different textures. Therefore, the objective of this study was to investigate the impacts of mycorrhizal and non-mycorrhizal roots on P leaching in two non-sterilised soils of contrasting texture. A mycorrhiza-defective tomato (Solanum lycopersicum L.) genotype (named rmc), and its wild-type progenitor that is able to form AM (named 76R), were used to investigate the effects of AM on soil P loss via leaching. Concentrations of reactive and un-reactive P in the leachate and soil were measured and related to plant growth, plant P uptake, soil water relations and leachate dissolved organic carbon (DOC) concentration. Soil texture affected mycorrhizal colonization, plant growth and plant P concentration, and influenced the concentration and chemical composition of P and the concentration of DOC leached. The chemical composition of P leached and P remaining in soil varied with soil texture, the presence or absence of roots, and their arbuscular mycorrhizal status. Mycorrhizal plants reduced P lost via leaching in the sandy soil substrate, where DOC leached was also high. The roots, regardless of mycorrhizal colonization, appeared to have the greatest impact on increasing P and DOC leached. Taken together, this study provides new insights into the role of AM on soil P loss via leaching in soils of contrasting texture.

Colonization of Listeria monocytogenes in potting soils as affected by bacterial community composition, storage temperature, and natural amendment.

The online version contains supplementary material available at 10.1007/s10068-021-00925-9.

Development of a multiplex PCR for the identification of Fusarium solani and F. oxysporum in a single step

Fusarium solani and F. oxysporum are plant pathogenic fungi that cause root rot and wilt, respectively, in many economically important crops. Identification of both species by morphological characteristics is complex and time-consuming, especially when a crop is host to several species, as in the case of strawberry and other many crops. The objective of this research was to develop a molecular method for the precise and sensitive identification of Fusarium solani and F. oxysporum by multiplex PCR. Specific PCR conditions were developed for the amplification of about 170-bp PCR product from the internal transcribed spacer of F. oxysporum and a PCR product of about 650 bp from the elongation factor 1-alpha gene of F. solani. The specificity of the multiplex PCR was tested using gDNA extracted from 232 strains of F. solani and F. oxysporum, 12 strains of other Fusarium spp. and 14 strains of other genera. The sensitivity of each primer set was tested individually and simultaneously with one strain of F. oxysporum and F. solani. This protocol was then used to identify both species in natural isolates from diseased plant and soil samples from strawberry nursery. Each primer set was validated using genomic DNA from non-sterile and sterile soils artificially inoculated with both species. Under these conditions, both Fusarium species were detected and distinguished over a range of concentrations (1 × 107 to 1 × 102 conidia g−1 soil) by successful amplification of corresponding PCR products for each species. This multiplex PCR technique is a fast tool to identify natural isolates of both species, F. solani and F. oxysporum, in a single PCR reaction.

Soil microbes and seed mucilage promote growth of the desert ephemeral plant Nepeta micrantha under different water conditions

Ephemeral plants, which can complete their life cycle in two to three months, play an important role in stabilizing sand dune surfaces from wind erosion in spring in the Gurbantunggut Desert, northwestern China. Soil microbes and seed mucilage could promote seedling growth of desert species via independent effects and / or interactions. However, it is uncertain whether both factors affect growth of adult ephemeral plants when exposed to different water regimes. Intact or demucilaged seeds of Nepeta micrantha were sown in sterile and non-sterile soil and exposed to three water regimes. The results indicated that plants under middle water treatment had higher dry mass accumulation as compared to that under low and high water treatments. Soil microbes and seed mucilage could significantly increase plant growth in all three water regimes. Their interactions showed positive effect on growth only under low water treatment. Nepeta micrantha plants had the highest biomass in the treatment of intact seeds in non-sterile soil and exposed to middle water regimes. Our study provides a better understanding of the growth regulation mechanism by which water, soil microbes and seed mucilage interactively promote growth of ephemeral plants in arid desert.

Rhizosphere Streptomyces formulas as the biological control agent of phytopathogenic fungi Fusarium oxysporum and plant growth promoter of soybean

Abstract. Sari M, Nawangsih AA, Wahyudi AT. 2021. Rhizosphere Streptomyces formulas as the biological control agent of phytopathogenic fungi Fusarium oxysporum and plant growth promoter of soybean. Biodiversitas 22: 3015-3023. Rhizosphere Streptomyces are considered as promising sources of plant growth-promoting rhizobacteria (PGPR) and biocontrol agents against pathogenic fungi, particularly Fusarium oxysporum causing root rot, cotyledon rot, hypocotyl rot, and stunted growth in soybean. Formulation of rhizosphere Streptomyces with appropriate carrier materials is necessary to facilitate storage and application in plants. This study aimed to develop a formulation of rhizosphere Streptomyces, apply the formula to control F. oxysporum, and promote soybean plant growth. Five Streptomyces isolates, i.e., Streptomyces panaciradicis ARK 13, Streptomyces tritolerans ARK 17, Streptomyces recifensis ARK 63, Streptomyces tendae ARK 91, and Streptomyces manipurensis ARK 94 were used in this study. All of the isolates could grow in potato broth, rice bran extract, and molasses as alternative media. The highest biomass produced from the molasses growth medium. All five isolates had antifungal activity against F. oxysporum with the inhibition percentage ranging from 41% to 76%, and all of them were detected to have the iaaM gene. Indole-3-acetic acid (IAA) hormone produced by these isolates were ranging from 8.99-15.14 mg L-1, with the phosphate solubilization index of 2.13-2.47. Five rhizosphere Streptomyces formulas with the main carrier of peat could maintain the viability with the population density of 108 CFU g-1 for 8 weeks of storage at room temperature. Two formulas, F17 and F94, were the best formulas to control disease caused by F. oxysporum with disease suppression of 74% in sterile soil and 80-85% in non-sterile soil. Formula F17 and F94 significantly increased soybean growth in sterile and non-sterile soils. Therefore, these formulas could be recommended as biocontrol and plant growth promoters of soybean.

Cordia africana but not Juniperus procera and Podocarpus falcatus respond positively to arbuscular mycorrhizal fungi at the early stages of seedling development

Abstract. Asmelash F, Bekele T, Belay Z, Kebede F. 2021. Cordia africana but not Juniperus procera and Podocarpus falcatus respond positively to arbuscular mycorrhizal fungi at the early stages of seedling development. Biodiversitas 22: 2971-2980. AMF (Arbuscular mycorrhizal fungi) inoculation could be an important technology to improve the growth and field survival of trees. Hence, we evaluated the mycorrhizal responsiveness of Cordia africana Lam., Juniperus procera (Hoechst. ex Endl.), and Podocarpus falcatus (Thumb.) Mirb. seedlings. Seedlings germinated on sterile sand were transplanted to 1-liter plastic pots filled with sterile and non-sterile degraded bulk soil. Rhizospheric soil from adult C. africana and J. procera were used as whole-soil AMF inocula. Cordia africana and J. procera received conspecific whole-soil AMF inocula while P. falcatus received J. procera inoculum. Hence, in the two-by-two factorial experiment, we also evaluated the growth effects of AMF inoculation, soil type, and their interaction. On the sterile potting soil, MRi (mycorrhizal responsiveness due to AMF inoculation) of C. africana was positive and significantly (p<0.05) greater than the MRi of J. procera and P. falcatus. However, on the non-sterile potting soil, it was significantly greater than the MRi of P. falcatus only. MRs (MR due to the existing potting soil inocula) and considering all growth variables were mostly positive for C. africana but negative for J. procera and P. falcatus. AMF inoculation significantly increased most growth variables of C. africana seedlings and no significant “inoculation” x “soil type” interaction effects were detected. Hence, AMF inoculation of C. africana seedlings could be merited and under wide range of field conditions. In the case of J. procera and P. falcatus, after-planting care could be more appropriate

Genome-wide gene expression changes of Pseudomonas veronii 1YdBTEX2 during bioaugmentation in polluted soils

BackgroundBioaugmentation aims to use the capacities of specific bacterial strains inoculated into sites to enhance pollutant biodegradation. Bioaugmentation results have been mixed, which has been attributed to poor inoculant growth and survival in the field, and, consequently, moderate catalytic performance. However, our understanding of biodegradation activity mostly comes from experiments conducted under laboratory conditions, and the processes occurring during adaptation and invasion of inoculants into complex environmental microbiomes remain poorly known. The main aim of this work was thus to study the specific and different cellular reactions of an inoculant for bioaugmentation during adaptation, growth and survival in natural clean and contaminated non-sterile soils, in order to better understand factors limiting bioaugmentation.ResultsAs inoculant we focused on the monoaromatic compound-degrading bacterium Pseudomonas veronii 1YdBTEX2. The strain proliferated in all but one soil types in presence and in absence of exogenously added toluene. RNAseq and differential genome-wide gene expression analysis illustrated both a range of common soil responses such as increased nutrient scavenging and recycling, expression of defense mechanisms, as well as environment-specific reactions, notably osmoprotection and metal homeostasis. The core metabolism of P. veronii remained remarkably constant during exponential growth irrespective of the environment, with slight changes in cofactor regeneration pathways, possibly needed for balancing defense reactions.ConclusionsP. veronii displayed a versatile global program, enabling it to adapt to a variety of soil environments in the presence and even in absence of its target pollutant toluene. Our results thus challenge the widely perceived dogma of poor survival and growth of exogenous inoculants in complex microbial ecosystems such as soil and provide a further basis to developing successful bioaugmentation strategies.

Bioremediation of lindane contaminated soil: Exploring the potential of actinobacterial strains

Lindane, an organochlorine pesticide, causes detrimental impacts on the environment and human health owing to its high toxicity, low degradation, and bioaccumulation. Its toxic nature can be overcome by biological and eco-friendly approaches involving its degradation and detoxification. The biodegradation of lindane was assessed using actinobacterial species Thermobifida cellulosilytica TB100 (T. cellulosilytica), Thermobifida halotolerans DSM 44931 (T. halotolerans) and Streptomyces coelicolor A3 (S. coelicolor). The degradation conditions of Lindane such as pH, temperature, inoculum volume, glucose concentration and number of days were optimized under broth conditions. Lindane degradation at different concentrations was studied in soil using reverse phase-high performance liquid chromatography over a 30 day period. A bioassay test was performed on seeds of Lactuca sativa (Lettuce) to assess the success of bioremediated soil. Maximum lindane degradation in soil was observed using T. cellulosilytica sp. The degradation trend for different concentrations of lindane using T. halotolerans in sterilized soil was 55 mg kg−1 (82%) ˃ 155 mg kg−1 (75%) ˃ 255 mg kg−1 (70%) after an incubation period of 30 days. Lindane degradation in soil followed the first order reaction kinetics. Phytotoxicity test on seeds of Lactuca sativa showed considerably good vigor index values for the bioremediated sterilized and non-sterilized soil by T. cellulosilytica, T. halotolerans and S. coelicolor in comparison to the contaminated soil without bacteria. This confirms that these actinobacterial species can be implemented in bioaugmentation of contaminated sites to efficiently remediate high lindane concentrations.

Indigenous Fungivorous Nematodes Affect the Biocontrol Efficacy of Trichoderma harzianum through Reducing the Hyphal Density.

Indigenous fungus-feeding nematodes may adversely affect the growth and activity of introduced biocontrol fungi. Alginate pellets of the biocontrol fungus Trichoderma harzianum ThzID1-M3 and sclerotia of the fungal plant pathogen Sclerotinia sclerotiorum were added to nonsterile soil at a soil water potential of -50 or -1,000 kPa. The biomass of ThzID1-M3, nematode populations, and extent of colonization of sclerotia by ThzID1-M3 were monitored over time. The presence of ThzID1-M3 increased the nematode population under both moisture regimes (p < 0.05), and fungivores comprised 69-75% of the nematode population. By day 5, the biomass of ThzID1-M3b and its colonization of sclerotia increased and were strongly correlated (R2 = 0.98), followed by a rapid reduction, under both regimes. At -50 kPa (the wetter of the two environments), fungal biomass and colonization by ThzID1-M3 were less, in the period from 5 to 20 days, while fungivores were more abundant. These results indicate that ThzID1-M3 stimulated the population growth of fungivorous nematodes, which in turn, reduced the biocontrol ability of the fungus to mycoparasitize sclerotia. However, colonization incidence reached 100% by day 5 and remained so for the experimental period under both regimes, although hyphal fragments disappeared by day 20. Our results suggest that indigenous fungivores are an important constraint for the biocontrol activity of introduced fungi, and sclerotia can provide spatial refuge for biocontrol fungi from the feeding activity of fungivorous nematodes.

The dynamic linkage between phytotoxicity and metabolites in Wedelia trilobata soils

AbstractThe phytotoxicity of soils under Wedelia trilobata has been described, but the dynamic between phytotoxic and metabolites in W. trilobata soil and the restriction posed by soil properties have been poorly addressed. In this study, the phytotoxic variation of W. trilobata soil in response to the changes in soil properties and soil metabolomics was determined at monthly intervals for 11 months using bioassays and metabolomic approaches. Strong variability in soil properties and obvious temporal variations in the soil phytotoxicity were observed over the 11‐month period. Among the soil properties that were analyzed, soil organic matter, electrical conductivity, total nitrogen, and soil nutrients presented the most contribution in differentiating soils according to time of sampling and strength of phytotoxicity. Nonsterilized soil exerted stronger percentage inhibition of seed germination (IG) of lettuce, whereas the percentage inhibition of shoot height (SH) and root length (RL) of lettuce was higher in sterilized soil. Decline in the overall phytotoxicity was clearly observed in nonsterilized soil, suggesting the role of soil microbes in mediating the soil phytotoxic effect of W. trilobata, albeit the negative correlation. Putative metabolites contributing to the discrimination of soils according to time of sampling and strong correlation with percentage inhibition (%) of IG, SH, and RL were identified and discussed. Overall, the results showed that the phytotoxicity of soils under W. trilobata varied throughout the 11 months of study and can be restricted by the changes in soil properties.

Promotion of polylactic acid biodegradation by a combined addition of PLA-degrading bacterium and nitrogen source under submerged and soil burial conditions

This article investigated an efficient process for accelerating polylactic acid (PLA) biodegradation. A combined use of bioaugmentation and biostimulation to faster PLA biodegradation under submerged and soil burial conditions was studied. Under submerged condition, PLA films in basal salt medium were inoculated with either Pseudomonas geniculata WS3 (PLA-degrading bacterium), manure extract, or wastewater sludge extract, with and without the addition of different nitrogen sources. The results revealed that soytone addition with the cultures of P. geniculata WS3, manure extract and wastewater sludge extract significantly increased PLA weight loss percentage and lactic acid content, indicating that soytone is an excellent nitrogen source for enhancing PLA biodegradation. These conditions were selected to evaluate PLA biodegradation under soil burial exposure. It was found that soytone with manure extract addition increased microbial activity and PLA biodegradation in soil. Moreover, the addition both soytone and P. geniculata WS3 speeded up biodegradation rate, resulting in almost 100 percent of PLA mass loss within 60 days in non-sterile soil burial. In conclusion, a combined use of P. geniculata WS3 with soytone is an efficient process for accelerating PLA biodegradation under both submerged and soil burial exposure conditions.

Degradation of the fungicide metalaxyl and its non-extractable residue formation in soil clay and silt fractions

The proportion of organic matter and mineral composition are important factors determining the formation and type of non-extractable residues (NERs) of pesticides in soil. In this study, we investigated the enantioselectivity in degradation and NER formation of the chiral fungicide metalaxyl in soil particle size fractions (silt and clay). Microbial and extracellular enzyme activities during these processes were monitored in incubation of silt and clay samples isolated from sterilized and non-sterilized soil samples collected from a long-term agricultural field experimental site in Ultuna, Sweden. The temporal influence on the fate of the fungicide was noted by short-term (10-d) and long-term (92-d) incubations. Besides the acquisition of quantitative data with gas chromatography-mass spectrometry (GC/MS), stereoselective analyses were performed with chiral GC/MS. Quantitative results pointed to a higher metabolism rate of the pesticide through microbial activity than through extracellular enzyme activity. This was also confirmed by the enantioselective depletion of R-metalaxyl and the subsequent formation of R-metalaxyl acid in microbially active samples from non-sterilized soil. The silt fraction containing a high amount of organic matter exhibited a significant hydrolyzable proportion of metalaxyl NERs that was releasable under alkaline conditions. On the contrary, the clay fraction showed an enhanced affinity for covalently bound residues. Based on our results, we recommend differentiating between reversibly and irreversibly bound proportions of pesticides in persistence and environmental risk assessment because the reversible fraction contained potentially bioavailable amounts of residues that may be released under natural conditions.