Characterization of glutathione reductase and catalase in the fronds of two Pteris ferns upon arsenic exposure

To better understand the mechanisms of plant tolerance to high concentration of arsenic, we characterized two antioxidant enzymes, glutathione reductase (GR) and catalase (CAT), in the fronds of Pteris vittata, an arsenic-hyperaccumulating fern, and Pteris ensiformis, an arsenic-sensitive fern. The induction, activation and apparent kinetics of GR and CAT in the plants upon arsenic exposure were investigated. Under arsenic exposure (sodium arsenate), CAT activity in P. vittata was increased by 1.5-fold, but GR activity was unchanged. Further, GR was not inhibited or activated by the arsenic in assays. No significant differences in K_m and V_max values of GR or CAT were observed between the two ferns. However, CAT activity in P. vittata was activated by 200 μM arsenate up to 300% compared to the control. Similar but much smaller increases were observed for P. ensiformis and purified bovine liver catalase (133% and 120%, respectively). This research reports, for the first time, the activation of CAT by arsenic in P. vittata. The increased CAT activities may allow P. vittata to more efficiently mediate arsenic-induced stress by preparing the fern for the impeding production of reactive oxygen species resulting from arsenate reduction to arsenite in the fronds.     

Development of suitable hydroponics system for phytoremediation of arsenic-contaminated water using an arsenic hyperaccumulator plant Pteris vittata

In this study, we found that high-performance hydroponics of arsenic hyperaccumulator fern Pteris vittata is possible without any mechanical aeration system, if rhizomes of the ferns are kept over the water surface level. It was also found that very low-nutrition condition is better for root elongation of P. vittata that is an important factor of the arsenic removal from contaminated water. By the non-aeration and low-nutrition hydroponics for four months, roots of P. vittata were elongated more than 500 mm. The results of arsenate phytofiltration experiments showed that arsenic concentrations in water declined from the initial concentrations (50?μg/L, 500?μg/L, and 1000?μg/L) to lower than the detection limit (0.1?μg/L) and about 80% of arsenic removed was accumulated in the fern fronds. The improved hydroponics method for P. vittata developed in this study enables low-cost phytoremediation of arsenic-contaminated water and high-affinity removal of arsenic from water.     

An arsenic hyperaccumulating fern,Pteris vittata L. (Pteridaceae) broadly affects terrestrial invertebrate abundance

1. The Chinese brake fern (Pteris vittata L.; Pteridaceae) can accumulate up to 27 000 mg kg^?1 dry wt. of arsenic (As) from the soil into its above‐ground biomass. They may use this As to deter invertebrate threats. 2. This study explored how As concentrations [As] in the fern, and in soil associated with the fern, influenced the abundance and composition of various invertebrates. 3. Populations of P. vittata were identified in the field. Soils from the base of the fern and from 3 m away of each plant were collected and pitfall traps were installed. Soil and fern arsenic concentrations ([As]) were measured via inductively coupled plasma mass spectrometry and invertebrates were identified to order and classified by feeding guild. 4. Increased [As] did not affect all feeding guilds and orders equally. For example, individual herbivore abundance did not decrease as [As] increased, but predator abundance did. In many cases, the impact of soil [As] on invertebrates depended on the distance from the fern. Fern [As] also influenced components of the community, but only at 3 m away from the fern. Furthermore, the abundances of many invertebrate groups were higher beneath the fern, where [As] was higher. 5. These results suggest that hyperaccumulated As can impact the invertebrate community, but the defensive benefits of hyperaccumulation are more complex than have been previously described. The authors advocate that future studies examining the potential defensive benefits of hyperaccumulation should do so in a natural setting that incorporates this complexity and invertebrate richness.     

Chelator-assisted phytoextraction of arsenic,cadmium and lead by Pteris vittata L. and soil microbial community structure response

Abstract

Using biodegradable chelators to assist in phytoextraction may be an effective approach to enhance the heavy-metal remediation efficiencies of plants. A pot experiment was conducted to investigate the effects of ethylenediamine disuccinic acid (EDDS), citric acid (CA), and oxalic acid (OA) on the growth of the arsenic (As) hyperaccumulator Pteris vittata L., its arsenic (As), cadmium (Cd), and lead (Pb) uptake and accumulation, and soil microbial responses in multi-metal(loid)-contaminated soil. The addition of 2.5-mmol kg^?1 OA (OA-2.5) produced 26.7 and 14.9% more rhizoid and shoot biomass, respectively compared with the control, while EDDS and CA treatments significantly inhibited plant growth. The As accumulation in plants after the OA-2.5 treatment increased by 44.2% and the Cd and Pb accumulation in plants after a 1-mmol kg^?1 EDDS treatment increased by 24.5 and 19.6%, respectively. Soil urease enzyme activities in OA-2.5 treatment were significantly greater than those in the control and other chelator treatments (p?<?0.05). A PCR–denatured gradient gel electrophoresis analysis revealed that with the addition of EDDS, CA and OA enhanced soil microbial diversity. It was concluded that the addition of OA-2.5 was suitable for facilitating phytoremediation of soil As and did not have negative effects on the microbial community.     

Arsenic resistance in Pteris vittata L.: identification of a cytosolic triosephosphate isomerase based on cDNA expression cloning in Escherichia coli

Arsenic hyperaccumulator Pteris vittata L. (Chinese brake fern) grows well in arsenic-contaminated media, with an extraordinary ability to tolerate high levels of arsenic. An expression cloning strategy was employed to identify cDNAs for the genes involved in arsenic resistance in P. vittata. Excised plasmids from the cDNA library of P. vittata fronds were introduced into Escherichia coli XL-1 Blue and plated on medium containing 4 mM of arsenate, a common form of arsenic in the environment. The deduced amino acid sequence of an arsenate-resistant clone, PV4-8, had cDNA highly homologous to plant cytosolic triosephosphate isomerases (cTPI). Cell-free extracts of PV4-8 had 3-fold higher level of triosephosphate isomerase (TPI) specific activities than that found in E. coli XL-1 Blue and had a 42 kD fusion protein immunoreactive to polyclonal antibodies raised against recombinant Solanum chacoense cTPI. The PV4-8 cDNA complemented a TPI-deficient E. coli mutant. PV4-8 expression improved arsenate resistance in E. coli WC3110, a strain deficient in arsenate reductase but not in AW3110 deficient for the whole ars operon. This is consistent with the hypothesis that PV4-8 TPI increased arsenate resistance in E. coli by directly or indirectly functioning as an arsenate reductase. When E. coli tpi gene was expressed in the same vector, bacterial arsenate resistance was not altered, indicating that arsenate tolerance was specific to P. vittata TPI. Paradoxically, P. vittata TPI activity was not more resistant to inhibition by arsenate in vitro than its bacterial counterpart suggesting that arsenate resistance of conventional TPI reaction was not the basis for the cellular arsenate resistance. P. vittata TPI activity was inhibited by incubation with reduced glutathione while bacterial TPI was unaffected. Consistent with cTPI’s role in arsenate reduction, bacterial cells expressing fern TPI had significantly greater per cent of cellular arsenic as arsenite compared to cells expressing E. coli TPI. Excised frond tissue infiltrated with arsenate reduced arsenate significantly more under light than dark. This research highlights a novel role for P. vittata cTPI in arsenate reduction.     

Hydathode structure and localization in Pteris vittata fronds and evidence for their involvement in arsenic leaching

Pteris vittata, a fern able to hyperaccumulate arsenic (As) in its fronds, has been object of a number of studies aimed to understand the mechanisms involved in As absorption and tolerance. This study has focused on a new mechanism, As leaching, already observed in P. vittata, but not explained, based on the possible involvement of hydathodes, not yet described in this fern; moreover, the results contained in this article will provide information on a more detailed frond anatomy of P. vittata. A combination of light and electron microscopic techniques (transmission electron microscopy and environmental scanning electron microscopy with energy dispersive X-ray) and chemical analyses (inductively coupled plasma atomic emission spectroscopy) was used. The results suggest that in phytoremediation processes under field conditions there could be the need to know the atmospheric conditions before harvesting plants, to avoid the loss of As in the environment.     

A Critical Review of the Arsenic Uptake Mechanisms and Phytoremediation Potential of Pteris vittata

The discovery of the arsenic hyperaccumulator, Pteris vittata (Chinese brake fern), has contributed to the promotion of its application as a means of phytoremediation for arsenic removal from contaminated soils and water. Understanding the mechanisms involved in arsenic tolerance and accumulation of this plant provides valuable tools to improve the phytoremediation efficiency. In this review, the current knowledge about the physiological and molecular mechanisms of arsenic tolerance and accumulation in P. vittata is summarized, and an attempt has been made to clarify some of the unresolved questions related to these mechanisms. In addition, the capacity of P. vittata for remediation of arsenic-contaminated soils is evaluated under field conditions for the first time, and possible solutions to improve the remediation capacity of Pteris vittata are also discussed.     

Higher plants as bioindicators of metal contamination from Shangdong abandoned karst bauxite,southwestern China

• Bauxite mining on karst generates numerous ecological and environmental problems, including metal pollution, water and soil erosion and destruction of vegetation. Among these, the most important environmental problem is soil metal pollution. Higher plants have a great ability to adsorb metals and can be used as biological indicators. However, the study of bioindicators for soil contamination in karst bauxite is not clear.
• Plants and their soil were collected from an abandoned karst bauxite area at Shangdong, Guizhou Province, southwestern China. Plants were collected and identified as Pteris vittata, Pinus massoniana, Miscanthus floridulus, Coriaria nepalensis, Artemisia argyi and Senecio scandens.
• The content of metals in plant roots were in the order: P. vittata > M. floridulus > C. nepalensis, other plants roots had no consistent pattern. Concentrations of metal in plants (P. vittata and M. floridulus) and soil were: soil > root > leaf > stem. Levels of metals in soil samples easily exceeded background values, indicating that soil had been contaminated. Al and Fe were highest in soil samples of P. vittata, with a good correlation.
• Results show that the metal content determined in plants is relatively high, particularly in P. vittata. Data also suggest that P. vittata colonies were able to tolerate and accumulate high levels of metal elements, which evidences their suitability for use as bioindicatord of soil metal contamination caused by mining activities.

     

Arsenic Uptake and Translocation by Plants in Pot and Field Experiments

A work undertaken by pot and field experiments to assess the suitability of poplars and ferns for the in-situ, phytoextraction, of a dumping site with residues from the roasting process of arseno-pyrite is reported. The main characteristic of this site is the high content of both the As metalloid and heavy metals (e.g., Al, Fe, Cu, Co, Cr, Pb). Two poplar clones (Populus deltoides ‘Dvina’ and Populus x canadensis ‘Orion’) and Pteris vittata (Chinese brake fern) were planted in the contaminated soil both ex situ in pots and in situ. Plant survival, As accumulation in plant tissues, leaf content of pigments, soluble proteins, activity of catalase and SH-groups in both roots and leaves were evaluated during a 24-month study period. Both poplar and fern plants exhibited an increase in the activity of catalase and SH group contents when grown in the presence of pyrite ashes. The results showed that the co-planting system (arsenic-hyperaccumulator fern Pteris vittata and Populus clones) was suitable for phytoextraction of multi-contaminated dumping sites. Agronomic measures such as irrigation, soil tillage and amendments also seem to be necessary for the successful establishment of poplar trees and ferns in contaminated soils in order to enhance plant growth through the improvement of soil conditions.     

Copper uptake by Pteris melanocaulon Fée from a Copper-Gold mine in Surigao del Norte,Philippines

The ability of some plants to take up metal contaminants in the soil has been of increasing interest as an environmental approach to pollution clean-up. This study aimed to assess the ability of Pteris melanocaulon for copper(Cu) uptake by determining the Cu levels in the fern vis-à-vis surrounding soil and the location of Cu accumulation within its biomass. It also aimed to add information to existing literature as P. melanocaulon are found to be less documented compared to other fern metal accumulators, such as P. vittata. The P. melanocaulon found in the Suyoc Pit of a Copper-Gold mine in Placer, Surigao del Norte, Philippines exhibited a high Bioaccumulation Factor(BF) of 4.04 and a low Translocation Factor(TF) of 0.01, suggesting more Cu accumulation in the roots (4590.22 ± 385.66µg g^?1 Cu). Noteworthy was the Cu concentration in the rhizome which was also high (3539.44 ± 1696.35µg g^?1 Cu). SEM/EDX analyses of the Cu content in the roots indicated high elemental %Cu in the xylem (6.95%) than in the cortex (2.68%). The high Cu content in the roots and rhizomes and the localization of Cu in the xylem manifested a potential utilization of the fern as a metallophyte for rhizofiltration and phytostabilization.     

Root exudates and arsenic accumulation in arsenic hyperaccumulating Pteris vittata and non-hyperaccumulating Nephrolepis exaltata

This study compared the roles of root exudates collected from two fern species, the As hyperaccumulating Chinese Brake fern (Pteris vittata L.) and the As-sensitive Boston fern (Nephrolepis exaltata L.), on As-mobilization of two As minerals (aluminum arsenate and iron arsenate) and a CCA (chromated copper arsenate)-contaminated soil as well as plant As accumulation. Chinese Brake fern exuded 2 times more dissolved organic carbon (DOC) than Boston fern and the difference was more pronounced under As stress. The composition of organic acids in the root exudates for both ferns consisted mainly of phytic acid and oxalic acid. However, Chinese Brake fern produced 0.46 to 1.06 times more phytic acid than Boston fern under As stress, and exuded 3–5 times more oxalic acid than Boston fern in all treatments. Consequently, root exudates from Chinese Brake fern mobilized more As from aluminum arsenate (3–4 times), iron arsenate (4–6 times) and CCA-contaminated soil (6–18 times) than Boston fern. Chinese Brake fern took up more As and translocated more As to the fronds than Boston fern. The molar ratio of P/As in the roots of Chinese Brake fern was greater than in the fronds whereas the reverse was observed in Boston fern. These results suggested that As-mobilization from the soil by the root exudates (enhancing plant uptake), coupled with efficient As translocation to the fronds (keeping a high molar ratio of P/As in the roots), are both important for As hyperaccumulation by Chinese Brake fern.     

Construction of IAA-Deficient Mutants of Pseudomonas moraviensis and Their Comparative Effects with Wild Type Strains as Bio-inoculant on Wheat in Saline Sodic Soil

Tryptophan role in microbial biosynthesis of Indole Acetic Acid (IAA) is very distinct. In present study IAA producing bacteria Pseudomonas moraviensis was applied on wheat for improving growth and physiology; in the presence or absence of L-tryptophan in saline sodic field. Aqueous solution of tryptophan was added to the rhizosphere soil at 10?mg L^?1 with irrigated water. The survival efficiency of P. moraviensis measured in the presence of NaCl and mixture of salts. P. moraviensis increased P, NO₃–N and K contents in soil by 18–35% and further 12–15% increase was recorded in the presence of tryptophan. There were 40–80% increases in indole acetic acid (IAA), abscisic acid (ABA) and gibberellic acid (GA) contents of rhizosphere soil, and 40–45% increase in leaves when tryptophan was added with P. moraviensis. In the second phase, IAA deficient mutants of P. moraviensis were constructed and tested for the conversion of tryptophan to IAA. In transposon mutagenesis, 1800 trans-conjugants were generated and tested for tryptophan conversion. Among these, 11 mutants were selected and inoculated into wheat to compare their growth responses to the wild type. P. moraviensis wild type served as PGPR under salinity, but IAA- deficient mutants of P. moraviensis were unable to produce IAA and halted plant growth.     

Microbial Communities and Functional Genes Associated with Soil Arsenic Contamination and the Rhizosphere of the Arsenic-Hyperaccumulating Plant Pteris vittata L.

To understand how microbial communities and functional genes respond to arsenic contamination in the rhizosphere of Pteris vittata, five soil samples with different arsenic contamination levels were collected from the rhizosphere of P. vittata and nonrhizosphere areas and investigated by Biolog, geochemical, and functional gene microarray (GeoChip 3.0) analyses. Biolog analysis revealed that the uncontaminated soil harbored the greatest diversity of sole-carbon utilization abilities and that arsenic contamination decreased the metabolic diversity, while rhizosphere soils had higher metabolic diversities than did the nonrhizosphere soils. GeoChip 3.0 analysis showed low proportions of overlapping genes across the five soil samples (16.52% to 45.75%). The uncontaminated soil had a higher heterogeneity and more unique genes (48.09%) than did the arsenic-contaminated soils. Arsenic resistance, sulfur reduction, phosphorus utilization, and denitrification genes were remarkably distinct between P. vittata rhizosphere and nonrhizosphere soils, which provides evidence for a strong linkage among the level of arsenic contamination, the rhizosphere, and the functional gene distribution. Canonical correspondence analysis (CCA) revealed that arsenic is the main driver in reducing the soil functional gene diversity; however, organic matter and phosphorus also have significant effects on the soil microbial community structure. The results implied that rhizobacteria play an important role during soil arsenic uptake and hyperaccumulation processes of P. vittata.Arsenic (As) is an abundant and widespread trace metalloid element present in virtually all environmental media and is well known to be carcinogenic even at low levels (24). Arsenic contaminations in soil and groundwater have been reported in many parts of the world (2, 29, 34). Recently, in parts of Asia, including China, chronic drinking of arsenic-contaminated groundwater has caused endemic arsenicosis, which has become a major threat to public health (36). Soil arsenic contamination also affects the physiology, growth, and grain quality of crops. For example, high arsenic concentrations were found in rice seeds from Chenzhou, Hunan province, which exceeded the maximal permissible limit of 0.5 mg/kg (dry weight) (21). Hence, remediation of arsenic-contaminated soil and water is one of the major challenges in environmental science and public health. Low-cost, efficient, and environmentally friendly remediation technologies to remove arsenic from contaminated soil and water are urgently needed.Phytoremediation, the use of plants to restore contaminated soil, has attracted great attention recently. A pivotal step toward the phytoremediation of arsenic-contaminated soils is the discovery of the arsenic hyperaccumulator Pteris vittata L. (Chinese brake fern), which possesses high arsenic tolerance and produces a large biomass. This plant species holds great promise for the phytoremediation of arsenic-contaminated soils. It was shown previously that the leaflets of P. vittata were able to accumulate about 100-fold of arsenic from soils (22). Plant arsenic uptake depends mainly on the arsenic source and bioavailability (25). P. vittata remediates arsenic contamination mainly by taking up arsenate [As(V)] via phosphate transport systems, whereas arsenite [As(III)] is very slowly taken up by P. vittata, at 1/10 of the rate of that for arsenate in the absence of phosphate (41). However, the uptake mechanisms still remain largely unknown.Microorganisms play a crucial role in arsenic geochemical cycling through microbial transformation processes, including reduction, oxidation, and methylation (2, 11, 31, 33, 40). Although the impacts of microbial metabolisms were previously reported to be associated with arsenic cycling of soil and water (7, 29), little is known about how rhizobacterial communities of P. vittata respond to arsenic. Recently, we found that inoculating arsenic resistance bacteria increased the arsenic accumulation efficiency of P. vittata by 13 to 110% (46). Therefore, rhizobacteria may play an important role during arsenic uptake and accumulation processes by P. vittata. Thus, it is important to elucidate the microbially diverse populations and functional genes associated with arsenic mobility and transport in the P. vittata rhizosphere. However, to fully understand the ecology of such complex rhizosphere-contaminated soils, it is necessary to analyze different microbial populations simultaneously.Our hypothesis is that the arsenic-hyperaccumulating ability of P. vittata is due to the interactions among plants, rhizobacteria, and arsenic. A study of microbial communities present in the plant rhizosphere is important to illustrate the mechanisms of arsenic hyperaccumulation in P. vittata. Thus, the objectives of this research were to understand how microbial metabolic diversities, communities, and functional genes/relative abundances were affected by soil arsenic contamination and the P. vittata rhizosphere environment. To determine the soil microbial metabolic diversity, the Biolog system (Biolog, Carlsbad, CA) was used to analyze the sole-carbon-source-utilizing capabilities of the soil microbial communities. For functional gene analysis, a high-density, sensitive, oligonucleotide-based microarray (GeoChip 3.0) was used. GeoChip-based technologies have revealed the structure, metabolic activity, and dynamics of microbial communities from complex environments, such as soil, sediments, and groundwater (10, 38, 39, 45, 48). Our results provide evidence that changes of microbial community structure, functional gene distribution, and microbial metabolic diversity are associated with the soil arsenic level and the rhizosphere effect of P. vittata and suggest that plant phytoremediation is an interactive process among plants, microorganisms, and soil contaminants.     

Effects of afforestation on phosphorus dynamics and biological properties in a New Zealand grassland soil

Selected chemical, biochemical and biological properties of mineral soil (0–30 cm) were measured under a 19 year old forest stand (mixture of Pinus ponderosa and Pinus nigra) and adjacent unimproved grassland at a site in South Island, New Zealand. The effects of afforestation on soil properties were confined to the 0–10 cm layer, which reflected the distribution of fine roots (< 2 mm) in the soil profile. Concentrations of organic C, total N and P and all organic forms of P were lower under the forest stand, while concentrations of inorganic P were higher under forest compared with grassland, supporting the previously described suggestion that afforestation may promote mineralisation of soil organic matter and organic P. On the other hand, microbial biomass C and P, soil respiration and phosphatase enzyme activity were currently all lower and the metabolic quotient was higher in soil under forest compared with grassland, which is inconsistent with increased mineralisation in the forest soil. Reduced biological fertility by afforestation may be mainly attributed to changes in the quantity, quality and distribution of organic matter, and reduction in pH of the forest soil compared with the grassland soil. We hypothesize that the lower levels of C, N and organic P found in soil under forest are due to enhanced microbial and phosphatase activity during the earlier stages of forest development. Forest floor material (L and F layer) contained large amounts of C, N and P, together with high levels of microbial and phosphatase enzyme activity. Thus, the forest floor may be an important source of nutrients for plant growth and balance the apparent reduction in C, N and P in mineral soil through mineralisation and plant uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Arsenic alters uptake and distribution of sulphur in Pteris vittata

Low‐molecular‐weight thiol (LMWT) synthesis has been reported to be directly induced by arsenic (As) in Pteris vittata, an As hyperaccumulator. Sulphur (S) is a critical component of LMWTs. Here, the effect of As treatment on the uptake and distribution of S in P. vittata was investigated. In P. vittata grown under low S conditions, the presence of As in the growth medium enhanced the uptake of SO₄^2?, which was used for LMWT synthesis in fronds. In contrast, As application did not affect SO₄^2? uptake in Nephrolepis exaltata, an As non‐hyperaccumulator. Moreover, the isotope microscope system revealed that S absorbed with As accumulated locally in a vacuole‐like organelle in epidermal cells, whereas S absorbed alone was distributed uniformly. These results suggest that S is involved in As transport and/or accumulation in P. vittata. X‐ray absorption near‐edge structure analysis revealed that the major As species in the fronds and roots of P. vittata were inorganic As(III) and As(V), respectively, and that As–LMWT complexes occurred as a minor species. Consequently, in case of As accumulation in P. vittata, S possibly acts as a temporary ligand for As in the form of LMWTs in intercellular and/or intracellular transport (e.g. vacuolar sequestration).     

Combined Application of Rice Straw and Fungus Penicillium Chrysogenum to Remediate Heavy-Metal-Contaminated Soil

The search for cheap and environmentally friendly materials is essential for remediation of heavy-metal-contaminated agricultural soils. A pot experiment was undertaken to evaluate the application of rice straw and filamentous fungus Penicillium chrysogenum (P. chrysogenum) on the fractionation of copper (Cu) and cadmium (Cd), soil microbial properties, and Cu and Cd uptake by romaine lettuce (Lactuca sativa) in a contaminated agricultural soil. Rice straw was applied at three rates (0, 7.8, and 11.7 g kg^?1 soil), and in combinations with P. chrysogenum (1.0 × 10⁶ spores g^?1 soil). It was found that the combined treatment of rice straw and P. chrysogenum significantly decreased the acid-extractable Cu and Cd by 15.4–25.1% and 20.2–27.3%, and increased the oxidizable Cu and Cd by 16.1–18.0% and 72.1–98.4%, respectively. Soil microbial biomass and fresh weight of lettuce were also remarkably enhanced after rice straw plus P. chrysogenum addition. Rice straw combined with P. chrysogenum was more effective in reducing Cu and Cd uptake by lettuce than rice straw alone. The joint application of rice straw and P. chrysogenum remarkably reduced Cu and Cd concentrations in lettuce shoots by 13.6–21.9% and 32.9–41.7%, respectively. These results indicate that the combined application of P. chrysogenum and rice straw is a promising method to alleviate the bioavailability of metals, and to improve soil microbial properties and plant yield in heavy-metal-polluted agricultural soils.     

Root microbiome assembly of As-hyperaccumulator Pteris vittata and its efficacy in arsenic requisition

The assemblage of root-associated microorganisms plays important roles in improving their capability to adapt to environmental stress. Metal(loid) hyperaccumulators exhibit disparate adaptive capability compared to that of non-hyperaccumulators when faced with elevated contents of metal(loid)s. However, knowledge of the assemblage of root microbes of hyperaccumulators and their ecological roles in plant growth is still scarce. The present study used Pteris vittata as a model plant to study the microbial assemblage and its beneficial role in plant growth. We demonstrated that the assemblage of microbes from the associated bulk soil to the root compartment was based on their lifestyles. We used metagenomic analysis and identified that the assembled microbes were primarily involved in root–microbe interactions in P. vittata root. Notably, we identified that the assembled root microbiome played an important role in As requisition, which promoted the fitness and growth of P. vittata. This study provides new insights into the root microbiome and potential valuable knowledge to understand how the root microbiome contributes to the fitness of its host.     

Metal accumulation and arbuscular mycorrhizal status in metallicolous and nonmetallicolous populations of <Emphasis Type="Italic">Pteris vittata</Emphasis> L. and <Emphasis Type="Italic">Sedum alfredii</Emphasis> Hance

Although Pteris vittata L. and Sedum alfredii Hance have been identified as an As hyperaccumulator and a Zn/Cd hyperaccumulator, respectively, for a few years, variations in metal accumulation among populations and their arbuscular mycorrhizal (AM) status have not been fully explored. Six populations of P. vittata and four populations of S. alfredii from southeast China were investigated. Up to 1,373 As, 680 Pb, 376 Zn, 4.8 Cd, 169 Cu mg kg⁻¹ in fronds of P. vittata and 358 As, 2,290 Pb, 23,403 Zn, 708 Cd, 342 Cu mg kg⁻¹ in shoots of S. alfredii were detected. Constitutive properties of As and Zn hyperaccumulation in metallicolous populations of P. vittata and S. alfredii, respectively, were confirmed. However, Cd hyperaccumulation in S. alfredii varied among populations. The two hyperaccumulators varied in efficiency in taking up other heavy metals. Different metal tolerance strategies adopted by the two hyperaccumulators varied among plant species and metal species. Low to moderate levels of AM colonization in P. vittata (4.2–12.8%) and S. alfredii (8.5–45.8%) were observed at uncontaminated and metal-contaminated sites. The relationship between metal concentrations and AM colonization in the two hyperacumulators was also examined. The abundance of AM fungal spores ranged from 16 to 190 spores per 25 g soil. Glomus microaggregatum, Glomus mosseae, Glomus brohultii and Glomus geosporum were the most common species associated with both P. vittata and S. alfredii. To our knowledge, this is the first report of AM fungal status in rhizosphere of P. vittata and S. alfredii.     

Controls on microbial CO2 production: a comparison of surface and subsurface soil horizons

Although a significant amount of the organic C stored in soil resides in subsurface horizons, the dynamics of subsurface C stores are not well understood. The objective of this study was to determine if changes in soil moisture, temperature, and nutrient levels have similar effects on the mineralization of surface (0–25 cm) and subsurface (below 25 cm) C stores. Samples were collected from a 2 m deep unsaturated mollisol profile located near Santa Barbara, CA, USA. In a series of experiments, we measured the influence of nutrient additions (N and P), soil temperature (10–35°C), and soil water potential (?0.5 to ?10 MPa) on the microbial mineralization of native soil organic C. Surface and subsurface soils were slightly different with respect to the effects of water potential on microbial CO₂ production; C mineralization rates in surface soils were more affected by conditions of moderate drought than rates in subsurface soils. With respect to the effects of soil temperature and nutrient levels on C mineralization rates, subsurface horizons were significantly more sensitive to increases in temperature or nutrient availability than surface horizons. The mean Q₁₀ value for C mineralization rates was 3.0 in surface horizons and 3.9 in subsurface horizons. The addition of either N or P had negligible effects on microbial CO₂ production in surface soil layers; in the subsurface horizons, the addition of either N or P increased CO₂ production by up to 450% relative to the control. The results of these experiments suggest that alterations of the soil environment may have different effects on CO₂ production through the profile and that the mineralization of subsurface C stores may be particularly susceptible to increases in temperature or nutrient inputs to soil.     

Effects of Revegetation on Soil Microbial Biomass,Enzyme Activities,and Nutrient Cycling on the Loess Plateau in China

Revegetation is a traditional practice widely used for soil and water conservation on the Loess Plateau in China. However, there has been a lack of reports on soil microbial–biochemical indices required for a comprehensive evaluation of the success of revegetation systems. In this study, we examined the effects of revegetation on major soil nutrients and microbial–biochemical properties in an artificial alfalfa grassland, an enclosed natural grassland, and an artificial shrubland (Caragana korshinskii), with an abandoned cropland as control. Results showed that at 0–5, 5–20, and 20–40 cm depths, soil organic carbon, alkaline extractable nitrogen and available potassium were higher in natural grassland and artificial shrubland compared with artificial grassland and abandoned cropland. Soil microbial biomass C (Cmic) and phosphorous (Pmic) substantially decreased with depth at all sites, and in abandoned cropland was significantly lower than those of natural grassland, artificial grassland, and artificial shrubland at the depth of 0–5 cm. Soil microbial biomass N (Nmic) was higher in artificial shrubland and abandoned cropland compared with that in natural and artificial grasslands. Both Cmic and Pmic were significantly different between the 23‐year‐old and the 13‐year‐old artificial shrublands at the 0–5 cm depth. The activities of soil invertase, urease, and alkaline phosphatase in natural grassland and artificial shrubland were higher than those in artificial grassland and abandoned cropland. This study demonstrated that the regeneration of both natural grassland and artificial shrubland effectively preserved and enhanced soil microbial biomass and major nutrient cycling, thus is an ecologically beneficial practice for recovery of degraded soils on the Loess Plateau.