Plant diversity enhances the reclamation of degraded lands by stimulating plant–soil feedbacks

A synthetic bacterial consortium improved the phytoremediation efficiency of ryegrass on polymetallic contaminated soil

Polymetallic contamination of soils caused by mining activities seriously threatens soil fertility, biodiversity and human health. Bioremediation is thought to be of low cost and has minimal environmental risk but its effectiveness needs to be improved. This study aimed to identify the combined effect of plant growth and microbial strains with different functions on the enhancement of bioremediation of polymetallic contaminated soil. The microbiological mechanism of bioremediation was explored by amplicon sequencing and gene prediction. Soil was collected from polymetallic mine wastelands and a non-contaminated site for use in a pot experiment. Remediation efficiency of this method was evaluated by planting ryegrass and applying a mixed bacterial consortium comprising P-solubilizing, N-fixing and SO4-reducing bacteria. The plant-microbe joint remediation method significantly enhanced the above-ground biomass of ryegrass and soil nutrient contents, and at the same time reduced the content of heavy metals in the plant shoots and soil. The application of the composite bacterial inoculum significantly affected the structure of soil bacterial communities and increased the bacterial diversity and complexity, and the stability of co-occurrence networks. The relative abundance of the multifunctional genera to which the strains belonged showed a significant positive correlation with the soil nutrient content. Genera related to carbon (C), nitrogen (N), phosphorus (P), and sulphur (S) cycling and heavy metal resistance showed an up-regulation trend in heavy metal-contaminated soils after the application of the mixed bacterial consortium. Also, bacterial strains with specific functions in the mixed consortium regulated the expression of genes involved in soil nutrient cycling, and thus assisted in making the soil self-sustainable after remediation. These results suggested that the remediation of heavy metal-contaminated soil needs to give priority to the use of multifunctional bacterial agents.

Microbial perspective on restoration of degraded urban soil using ornamental plants

The urban soil where abandoned buildings are demolished is barren and structurally poor, and this degraded soil requires restoration. Ornamental plants enhance the urban environment, increase biodiversity, and affect soil physicochemical properties, microbial diversity; however, their effects remain unclear. Thus, in this study, a mixed-planting meadow consisting of 14 perennial ornamental flower species, including Iris tectorum, Iris lacteal, and Patrinia scabiosaefolia, etc. Was planted at a demolition site with sewage-contaminated soil in Beijing. Simultaneously, a single-planting lawn of I. tectorum was established in a nearby park. We aimed to examine soil physicochemical properties, sequence soil bacterial 16S rRNA and fungal ITS amplicons, and analyze soil microbial diversity and community structure at both sites at five time points in the year after planting, To explore the effect of herbaceous ornamental plants on degraded urban soil, we used FAPROTAX and FUNGuild to predict bacterial and fungal functions, the bin-based null model to evaluate the soil microbial community, and random matrix theory to construct soil microbial molecular networks. The mixed-planting meadow produced a visually appealing landscape and dynamic seasonal enrichment, significantly increasing soil total nitrogen (TN) and organic matter (SOM) contents by 1.99 and 1.21 times, respectively. TN had a positive correlation with soil microbial α diversity and community structure. Dominant phyla at both sites included Proteobacteria, Actinobacteria, and Ascomycota. Although soil microorganisms were primarily influenced by stochastic processes, stochasticity was notably higher in the mixed-planting meadow than in the single-planting lawn. The mixed-planting meadow significantly increased the relative abundance of beneficial microorganisms, improving nitrification and aerobic ammonium oxidation of soil bacteria, as well as symbiotroph of fungi. No significant changes were observed in the single-planting lawn. The mixed-planting meadow established a complex soil microbial molecular network, enhancing the correlation between bacteria and fungi and increasing the number of key microorganisms. Our findings suggest the potential of mixed-planting meadow in restoring degraded urban soils by influencing the soil microbial community and enhancing the ecological service function. Our study provides theoretical support for applying mixed-planting meadow communities to improve the soil environment of urban green spaces.

Land abandonment as driver of woody vegetation dynamics in Tamaulipan thornscrub at Northeastern Mexico

Background

Vegetation structure is defined as the temporal and spatial distribution of plant species in a particular site. Vegetation structure includes vertical and horizontal distribution and has been widely used as an indicator of successional changes. Ecological succession plays an essential role in the determination of the mechanisms that structure plant communities under anthropogenic disturbances. After an anthropogenic disturbance, such as grazing, forests follow changes in the original composition and vegetation structure, which eventually could restore some of their attributes to become mature forests again. To know how the time of abandonment affects woody plant communities, we ask the following questions: (1) How does the species richness, diversity, and vertical structure (A index) change concerning the time of abandonment? (2) Are species similarities among woody vegetation communities determined by land abandonment? (3) Which woody species have the highest ecological importance in each successional stage?

Methods

We explored how successional stages after land abandonment mediated the species richness, species diversity (alpha and beta), and ecological importance value index on four areas of Tamaulipan thornscrub. We selected four areas that differed in time of abandonment: 10, 20, 30, and >30 years. The first three areas were used for cattle grazing, whereas the >30-year area was selected as a control since it does not have a record of disturbance by cattle grazing or agriculture. During the summer of 2012, we randomly established four square plots (40 m × 40 m) in each area, separated at least 200 m from each other. In each plot, we recorded all woody individuals per species with a basal diameter ≥1 cm at 10 cm above ground level. We estimated species richness indices, species diversity (alpha and beta), and ecological importance value index.

Results

We recorded 27 woody species belonging to 23 genera and 15 families. Fabaceae accounted for 40% of the species. Acacia farnesiana was the most important and abundant species in the first three successional stages. We suggested that older successional stages of Tamaulipan thornscrub promote woody plant communities, characterized by a higher complex structure than younger communities. We observed the highest species similarity between the sites with a closer time of abandonment, while the lowest similarity was shown between the sites with extreme time of abandonment. We conclude that Tamaulipan thornscrub shows a similar trend of ecological succession to other dry forests and the time of abandonment has a high mediation on plant dynamics in the Tamaulipan thornscrub. Also, we stand out the importance of secondary forests for Tamaulipan thornscrub woody plant communities. Finally, we recommended future studies include aspects of regeneration speed, the proximity of mature vegetation, and the interactions of plants with their seed dispersers.

Alleviation of heavy metal stress and enhanced plant complex functional restoration in abandoned Pb–Zn mining areas by the nurse plant Coriaria nepalensis

Heavy metal pollution caused by mining has been a topic of concern globally because it threatens ecological functions and human health. Nearly all current remediation strategies take into account only such short-term issues as how to reduce or stabilize the content of heavy metals in soil, how to reduce the toxicity of heavy metals, and how to preserve water, soil and nutrients. However, little attention is paid to such long-term issues as whether plants can survive, whether communities can be stabilized, and whether ecosystem functions can be restored. Therefore, improving plant diversity and community stability are key aspects of improved mine restoration. To explore the possibility of reconstructing plant complexes in mining areas, the local nurse plant Coriaria nepalensis was selected as the research object for a study in the Huize Pb–Zn mining area of southwest China. C. nepalensis could increase the contents of nutrient elements (C, N, and P), reduce the contents of heavy metals (Mn, Cu, Zn, Cd, and Pb), and strengthen the plant complex functions (diversity, functional traits, and complex biomass) in its root zone. In general, C. nepalensis can form fertility islands (survival islands) in mining areas, which facilitate the colonization and success of additional less stress-resistant species. We propose C. nepalensis as a key species for use in restoration based on its ability to restore ecosystem functions under extremely stressful conditions. We encourage combination of C. nepalensis with other nurse plants to reinforce the rehabilitation of ecosystem functions.

Microbial community composition in the rhizosphere of Pteris vittata and its effects on arsenic phytoremediation under a natural arsenic contamination gradient

Arsenic contamination causes numerous health problems for humans and wildlife via bioaccumulation in the food chain. Phytoremediation of arsenic-contaminated soils with the model arsenic hyperaccumulator Pteris vittata provides a promising way to reduce the risk, in which the growth and arsenic absorption ability of plants and the biotransformation of soil arsenic may be greatly affected by rhizosphere microorganisms. However, the microbial community composition in the rhizosphere of P. vittata and its functional role in arsenic phytoremediation are still poorly understood. To bridge this knowledge gap, we carried out a field investigation and pot experiment to explore the composition and functional implications of microbial communities in the rhizosphere of four P. vittata populations with a natural arsenic contamination gradient. Arsenic pollution significantly reduced bacterial and fungal diversity in the rhizosphere of P. vittata (p < 0.05) and played an important role in shaping the microbial community structure. The suitability of soil microbes for the growth of P. vittata gradually decreased following increased soil arsenic levels, as indicated by the increased abundance of pathogenic fungi and parasitic bacteria and the decrease in symbiotic fungi. The analysis of arsenic-related functional gene abundance with AsChip revealed the gradual enrichment of the microbial genes involved in As(III) oxidation, As(V) reduction, and arsenic methylation and demethylation in the rhizosphere of P. vittata following increased arsenic levels (p < 0.05). The regulation of indigenous soil microbes through the field application of fungicide, but not bactericide, significantly reduced the remediation efficiency of P. vittata grown under an arsenic contamination gradient, indicating the important role of indigenous fungal groups in the remediation of arsenic-contaminated soil. This study has important implications for the functional role and application prospects of soil microorganisms in the phytoremediation of arsenic-polluted soil.

Heavy metal pollution in coastal wetlands: A systematic review of studies globally over the past three decades

Coastal wetlands are ecosystems lying between land and ocean and are subject to inputs of heavy metals (HMs) from terrestrial, oceanic and atmospheric sources. Although the study on HM pollution in coastal wetlands has been rapidly developing over the past three decades, systematic reviews are still unavailable. Here, by analyzing 3343 articles published between 1990 and 2019, we provided the first holistic systematic review of studies on HM pollution in coastal wetlands globally. The results showed a trend of rapid increases in publications in this field globally, especially over the past ten years. Trends varied greatly among coastal countries, and global trends were primarily driven by the US before 2000, and in China after 2010. We also found that mercury (Hg), cadmium (Cd), and copper (Cu) were the most widely studied HM elements globally, but patterns differed geographically, with Hg being most widely examined in the Americas, Cd in China and India, and lead (Pb) in the western Europe and Australia, respectively. Among different types of coastal wetlands, salt marshes, mangrove forests, and estuaries were the most widely studied, in contrast to seagrass beds and tidal flats. As for ecosystem components, soils/sediments and plants were most extensively investigated, while algae, microbes, and animals were much less examined. Our analysis further revealed rapid emergence of topics on anthropogenic sources, interactions with other anthropogenic environmental changes (climate change in particular), and control and remediation methodology in the literature in the recent ten years. Moving forward, we highlight that future studies are needed to i) better understand the impacts of HM pollution in less studied coastal wetland systems and species, ii) deepen current understanding of the biogeochemical behaviors of HMs under anthropogenic activities, iii) examine interactions with other anthropogenic environmental changes, iv) conceive ecological remediation (i.e., "ecoremediation" as compared to traditional physiochemical remediation and bioremediation) strategies, and v) develop advanced analysis instruments and methods. The perspectives we brought forward can help stimulate many new advances in this field.

Plant-Soil Feedbacks for the Restoration of Degraded Mine Lands: A Review

Much effort has been made to remediate the degraded mine lands that bring severe impacts to the natural environments. However, it remains unclear what drives the recovery of biodiversity and ecosystem functions, making the restoration of these fragile ecosystems a big challenge. The interactions among plant species, soil communities, and abiotic conditions, i.e., plant-soil feedbacks (PSFs), significantly influence vegetation development, plant community structure, and ultimately regulate the recovery of ecosystem multi-functionality. Here, we present a conceptual framework concerning PSFs patterns and potential mechanisms in degraded mine lands. Different from healthy ecosystems, mine lands are generally featured with harsh physical and chemical properties, which may have different PSFs and should be considered during the restoration. Usually, pioneer plants colonized in the mine lands can adapt to the stressful environment by forming tolerant functional traits and gathering specific soil microbial communities. Understanding the mechanisms of PSFs would enhance our ability to predict and alter both the composition of above- and below-ground communities, and improve the recovery of ecosystem functions in degraded mine lands. Finally, we put forward some challenges of the current PSFs study and discuss avenues for further research in the ecological restoration of degraded mine lands.

Ammoides pusilla Essential Oil: A Potent Inhibitor of the Growth of Fusarium avenaceum and Its Enniatin Production

Enniatins are mycotoxins produced by Fusarium species contaminating cereals and various agricultural commodities. The co-occurrence of these mycotoxins in large quantities with other mycotoxins such as trichothecenes and the possible synergies in toxicity could lead to serious food safety problems. Using the agar dilution method, Ammoides pusilla was selected among eight Tunisian plants for the antifungal potential of its essential oil (EO) on Fusarium avenaceum mycelial growth and its production of enniatins. Two EO batches were produced and analyzed by GC/MS-MS. Their activities were measured using both contact assays and fumigant tests (estimated IC50 were 0.1 µL·mL⁻¹ and 7.6 µL·L⁻¹, respectively). The A. pusilla EOs and their volatiles inhibited the germination of spores and the mycelial growth, showing a fungistatic but not fungicidal activity. The accumulation of enniatins was also significantly reduced (estimated IC50 were 0.05 µL·mL⁻¹ for the contact assays and 4.2 µL·L⁻¹ for the fumigation assays). The most active batch of EO was richer in thymol, the main volatile compound found. Thymol used as fumigant showed a potent fungistatic activity but not a significant antimycotoxigenic activity. Overall, our data demonstrated the bioactivity of A. pusilla EO and its high potential to control F. avenaceum and its enniatins production in agricultural commodities.

Revegetation approach and plant identity unequally affect structure, ecological network and function of soil microbial community in a highly acidified mine tailings pond

Owing to its sustainability and low cost, direct revegetation (DR) has been considered a promising alternative to capped revegetation (CR) for dealing with the serious environmental problem derived from various types of mine wastelands that are widespread in the world. However, a direct comparison of the performance of these two revegetation approaches for reclamation of extremely acidic mine wastelands and the underlying mechanisms is still lacking. To bridge this critical knowledge gap, we established 5000 m² of vegetation on a highly acidified (pH < 3) Pb/Zn mine tailings pond employing both CR and DR schemes (2500 m² for each scheme). We then profiled the structure, ecological network and function of soil microbial communities associated with two dominant plant species of the vegetations via high-throughput sequencing. Our results showed that CR and DR achieved a vegetation coverage of 59.7% and 90.5% within two years, respectively. This pattern was accompanied by higher concentrations of plant nutrients and lower acidification potentials in topsoils of the rhizospheres of the vegetation established by DR compared to those of CR. Revegetation approach, rather than plant identity, mostly affected the structure, ecological network and function of soil microbial community in the mine tailings pond. Rhizosphere soils of the vegetation established by DR generally had higher microbial diversity, higher relative abundances of dominant microbial phyla (e.g. Nitrospirae) that can aid plant uptake of nutrients, more complicated microbial interactive networks and more microbial genes responsible for nutrient cycling than those by CR. As the first report on a direct comparison of CR and DR schemes for reclamation of an extremely acidic mine wasteland, our study has important implications for not only the understanding of microbial ecology in revegetated mine wastelands but also the further development of sustainable revegetation schemes.