Differences in the response of soil dehydrogenase activity to Cd contamination are determined by the different substrates used for its determination

Investigating the Mechanism of Cadmium-Tolerant Bacterium Cellulosimicrobium and Ryegrass Combined Remediation of Cadmium-Contaminated Soil

Cadmium (Cd) pollution has been rapidly increasing due to the global rise in industries. Cd not only harms the ecological environment but also endangers human health through the food chain and drinking water. Therefore, the remediation of Cd-polluted soil is an imminent issue. In this work, ryegrass and a strain of Cd-tolerant bacterium were used to investigate the impact of inoculated bacteria on the physiology and biochemistry of ryegrass and the Cd enrichment of ryegrass in soil contaminated with different concentrations of Cd (4 and 20 mg/kg). The results showed that chlorophyll content increased by 24.7% and 41.0%, while peroxidase activity decreased by 56.7% and 3.9%. In addition, ascorbic acid content increased by 16.7% and 6.3%, whereas glutathione content decreased by 54.2% and 6.9%. The total Cd concentration in ryegrass increased by 21.5% and 10.3%, and the soil's residual Cd decreased by 86.0% and 44.1%. Thus, the inoculation of Cd-tolerant bacteria can improve the antioxidant stress ability of ryegrass in Cd-contaminated soil and change the soil's Cd form. As a result, the Cd enrichment in under-ground and above-ground parts of ryegrass, as well as the biomass of ryegrass, is increased, and the ability of ryegrass to remediate Cd-contaminated soil is significantly improved.

Comprehensive bioremediation effect of phosphorus-mineralized bacterium Enterobacter sp. PMB-5 on cadmium contaminated soil-crop system

Phosphate-mineralizing bacteria (PMBs) have been widely studied by inducing phosphate heavy metal precipitation, but current researches neglect to study their effects on soil-microbe-crop systems on cadmium (Cd) contaminated. Based on this, a strain PMB, Enterobacter sp. PMB-5, was inoculated into Cd contaminated pots to detect soil characteristics, Cd occurrence forms, soil biological activities, plant physiological and biochemical indicators. The results showed that the inoculation of strain PMB-5 significantly increased the available phosphorus content (85.97%-138.64%), Cd-residual fraction (11.04%-29.73%), soil enzyme activities (31.94%-304.63%), plant biomass (6.10%-59.81%), while decreased the state of Cd-HOAc (11.50%-31.17%) and plant bioconcentration factor (23.76%-44.24%). These findings indicated that strain PMB-5 could perform the function of phosphorus solubilization to realize the immobilization of Cd in the complex soil environment. Moreover, SEM-EDS, FTIR, XPS, and XRD analysis revealed that strain PMB-5 does not significantly alter the soil morphology, structure, elemental distribution, and chemical composition, which suggested that remediation of Cd contamination using strain PMB-5 would not further burden the soil. This research implies that PMB-5 could be a safe and effective bioinoculant for remediating Cd-contaminated soils, contributing to the sustainable management of soil health in contaminated environments.

Differences in the activities of six soil enzymes in response to cadmium contamination of paddy soils in high geological background areas

East Yunnan province in southwest China is a region with elevated natural abundance (high geological background levels) of Cd due to high metal (loid) contents in the soils. Enzyme activities are useful indicators of metal (loid) toxicity in contaminated soils and whether Cd inhibits enzyme activities in paddy soils in high geological background areas is of considerable public concern. A pot experiment combined with field investigation was conducted to assess the effects of Cd on six soil enzymes that are essential to the cycling of C, N, and P in soils. Inhibitory effects of Cd fractions on enzyme activities were assessed using ecological dose-response models. The impact of soil properties on the inhibition of sensitive soil enzymes by Cd were assessed using linear and structural equation models. Cadmium was enriched in the paddy soils with 72.2 % of soil samples from high geological background areas exceeding the Chinese threshold values (GB 15618-2018) of Cd. Enzyme responses to Cd contamination varied markedly with a negative response by catalase but a positive response by invertase. Urease, β-glucosidase, and alkaline phosphatase activities were stimulated at low Cd concentrations and inhibited at high concentrations. The average inhibition ratios of β-glucosidase, urease, and catalase in high Cd levels were 19.9, 38.9, and 51.9%, respectively. Ecological dose-response models indicate that catalase and urease were the most Cd-sensitive of the enzymes studied and were suitable indicators of soil quality in high geological background areas. Structural equation modeling (SEM) indicates that soil properties influenced sensitive enzymes through various pathways, indicating that soil properties were factors determining Cd inhibition of enzyme activities. This suggests that Cd concentrations and soil physicochemical properties under a range of environmental conditions should be considered in addressing soil Cd pollution.

Synergistic effect of pyrene and heavy metals (Zn, Pb, and Cd) on phytoremediation potential of Medicago sativa L. (alfalfa) in multi-contaminated soil

The environment in India is contaminated with polycyclic aromatic hydrocarbons (PAHs) due to the occurrence of large anthropogenic activities, i.e., fuel combustion, mineral roasting, and biomass burning. Hence, 13 toxic PAHs were detected: phenanthrene, anthracene, fluoranthene, pyrene, and benz(a) anthracene, ben-zo; (b) fluoranthene, benzo(k) fluoranthene, benzo(a) pyrene, benzo(ghi)perylene, dibenz (ah) anthracene, indeno1,2,3-(cd) pyrene, coronene and coronene in the environment (i.e., ambient particulate matter, road dust, sludge, and sewage) of the most industrialized area. Pollutants such as heavy metals and polycyclic aromatic hydrocarbons co-contaminate the soil and pose a significant hazard to the ecosystem because these pollutants are harmful to both humans and the environment. Phytoremediation is an economical plant-based natural approach for soil clean-up that has no negative impact on ecosystems. The aim of this study was to investigate the effects of pyrene (500 mg kg-1), Zn (150 mg kg-1), Pb (150 mg kg-1), and Cd (150 mg kg-1) alone and in combination on the phytoextraction efficiency of Medicago sativa growing in contaminated soil. Plant biomass, biochemical activities, translocation factors, accumulation of heavy metals, and pyrene removal were determined. After 60 days of planting, compared with those of the control plants, the growth parameters, biomass, and chlorophyll content of the M. sativa plants were significantly lower, and the reactive oxygen species activity, such as proline and polyphenol content and metallothionein protein content, was markedly greater in the pyrene and heavy metal-polluted soils. Furthermore, the combined toxicity of pyrene and all three metals on M. sativa growth and biochemical parameters was significantly greater than that of pyrene, Zn, Pb, or Cd alone, indicating the synergistic effect of pyrene and heavy metals on cytotoxicity. Pyrene stress increased Cd accumulation in M. sativa. After pyrene exposure alone or in combination with Zn-pyrene, a greater pyrene removal rate (85.5-81.44%) was observed than that in Pb-pyrene, Cd-pyrene, and Zn-Pb-Cd-pyrene polluted soils (62.78-71.27%), indicating that zinc can enhance the removal of pyrene from contaminated soil. The resulting hypotheses demonstrated that Medicago sativa can be used as a promising phytoremediation agent for co-contaminated soil.

Residual hydrocarbons in long-term contaminated soils: implications to risk-based management

Petroleum hydrocarbon (PHC) contamination is a widespread and severe environmental issue affecting many countries' resource sectors. PHCs are mixtures of hydrocarbon compounds with varying molar masses that naturally attenuate at different rates. Lighter fractions attenuate first, followed by medium-molar-mass constituents, while larger molecules remain for longer periods. This results in significant regulatory challenges concerning residual hydrocarbons in long-term contaminated soils. This study examined the potential risks associated with residual PHC and its implications for risk-based management of heavily contaminated soils (23,000-26,000 mg PHC/kg). Ecotoxicological properties, such as seedling emergence and growth of two native plant species-small Flinders grass (Iseilema membranaceum) and ruby saltbush (Enchylaena tomentosa)-and earthworm survival tests in PHC-contaminated soils, were assessed. Additionally, the effects of aging on the attenuation of PHC in contaminated soils were evaluated. Toxicity responses of plant growth parameters were determined as no-observed-effect concentrations: 75%-100% for seedling emergence, < 25%-75% for plant shoot height, and 75%-100% for earthworm survival. After 42 weeks of aging, the total PHC levels in weathered soils decreased by 14% to 30% and by 67% in diesel-spiked soil due to natural attenuation. Dehydrogenase enzyme activity in soils increased during the initial aging period. Furthermore, a clear shift of bacterial communities was observed in the soils following aging, including enrichment of PHC-resistant and -utilizing bacteria-for example, Nocardia sp. This study underscores the potential of natural attenuation for eco-friendly and cost-effective soil management, underlining that its success depends on site-specific factors like water content and nutrient availability. Therefore, we recommend detailed soil assessments to evaluate these conditions prior to adopting a risk-based management approach.

Enhanced Cd activation by Coprinus comatus endophyte Bacillus thuringiensis and the molecular mechanism

Fungal endophytes not only tolerate and activate Cd in soil but also promote host growth, yet its Cd activation capacity and mechanism remain unrevealed. Our previous study isolated a robust endophyte Bacillus thuringiensis L1 from Coprinus comatus fruiting body with splendid Cd resistance and activation abilities under laboratory conditions. In this study, those peculiarities were investigated in the actual soil environment. L1 could significantly increase the soil bioavailable Cd content and effectively compensate for alkali-hydro nitrogen losses and microbial inhibition caused by Cd. Furthermore, L1 inoculation improved the soil's bacterial community structure and increased the relative abundance of Cd-resistant bacteria, such as Actinobacteria, Chloroflexi, Acidobacter, and Firmicutes, closely associated with the soil enzyme activity shift. The genome sequencing analysis revealed the presence of genes related to growth promotion, resistance to Cd stress, and Cd activation, which were significantly up-regulated under Cd stress. Notably, L1 mainly activates Cd in soil by secreting citric acid, succinic acid, siderophore, and soluble phosphorus substances to chelate with Cd or dissolve bounded Cd. Meanwhile, the metal-responsive transcription repressor (CadC) and the Cd-translocating protein P-type ATPase (CadA) can help the L1 to suppress the toxicity of Cd. Those results help to unveil the possible mechanism of L1 in Cd-contaminated soil remediation, providing a clear strategy for Cd bio-extraction from soil.

Enhanced lead phytoextraction and soil health restoration through exogenous supply of organic ligands: Geochemical modeling"

Phytoremediation of lead (Pb) contaminated soil is a green technology to reduce Pb exposure and root exudates-derived organic acids play a vital role in this treatment process. In this study, Pb hyperaccumulator Pelargonium hortorum was chosen to investigate root-induced organic acid secretions and their subsequent role in Pb phytoextraction. In the first step, root exudation of P. hortorum was investigated in hydroponic experiments (0.2X Hoagland solution) under control and Pb stress conditions. Possible chemical interactions between Pb and the observed root exudates were then analyzed using Visual MINTEQ modeling. In the next step, the effects of the exogenous application of organic acids on Pb phytoextraction and soil enzymatic activities were studied in a pot experimental setup. Results indicated significant exudation of malic acid > citric acid > oxalic acid > tartaric acid in root exudates of P. hortorum under 50 mg L-1 Pb. Visual MINTEQ modeling results revealed that organic acids directly affect Pb dissolution in the nutrient solution by modulation of solution pH. Experimental results revealed that malic acid and citric acid significantly increased available Pb contents (7.2- and 6.7-folds) in the soil with 1500 mg kg-1 Pb contamination. Whereas, in shoot and root, the highest increase in Pb concentration was observed with citric acid (2.01-fold) and malic (3.75-fold) supplements, respectively. Overall, Pb uptake was notably higher when malic acid was applied (2.8-fold) compared to other organic acids, followed by citric acid (2.7-fold). In the case of soil enzymatic activities, oxalic acid significantly improved dehydrogenase, alkaline phosphatase, and microbial biomass by 1.6-, 1.4- and 1.3-folds, respectively. The organic acids were successful in reviving enzyme activity in Pb-contaminated soil, and might thus be used for long-term soil regeneration.

Mercury distribution in plants and soils from the former mining area of Abbadia San Salvatore (Tuscany, Central Italy)

The distribution of heavy metals in plants (Castanea sativa, Sambucus nigra, Verbascum thapsus, Popolus spp., Salix spp., Acer pseudoplatanus, Robinia pseudoacacia) growing in soils from active and abandoned mining areas is of scientific significance as it allows to recognize their ability to survive in a hostile environment and provide useful indications for phytoremediation operations. In this work, soils from the former Hg-mining area of Abbadia San Salvatore (Tuscany, Central Italy) were analyzed for total, leached Hg, % of organic and inorganic-related Hg. The dehydrogenase enzyme activity (DHA) was also measured with the aim to evaluate the status of the soil, being characterized by high Hg contents (up to 1068 mg kg-1). Eventually, the concentration of Hg in the different parts of the plants growing on these soils was also determined. Most studied soils were dominated by inorganic Hg (up to 92%) while the DHA concentrations were < 151 µg TPF g-1 day-1, suggesting that the presence of Hg is not significantly affecting the enzymatic soil activity. This is also supported by the bioaccumulation factor (BF), being predominantly characterized by values < 1. Sambucus nigra and Verbascum thapsus had the highest Hg contents (39.42 and 54.54 mg kg-1, respectively). The plant leaves appear to be the main pathways of Hg uptake, as also observed in other mining areas, e.g., Almadèn (Spain), indicating that particulate-Hg and Hg0 are the main forms entering the plant system, the latter derived by the GEM emitted by both the edifices hosting the roasting furnaces and the soils themselves.

Pseudomonas aeruginosa improved the phytoremediation efficiency of ryegrass on nonylphenol-cadmium co-contaminated soil

The effect of Pseudomonas aeruginosa (P. aeruginosa) on the phytoremediation efficiency of ryegrass on soil contaminated with nonylphenol (NP) and cadmium (Cd) was investigated by pot experiments. Pseudomonas aeruginosa application stimulated the adsorption of Cd by ryegrass and facilitated the biodegradation of NP in the soil. Exogenous P. aeruginosa inoculation increased the activities of urease, dehydrogenase, and polyphenol oxidase in the soil of the T4 treatment by 38.5%, 50.0%, and 56.5% compared to that of the T2 treatment, respectively. There was a significant positive correlation between the activities of dehydrogenase and polyphenol oxidase and the NP removal rate (P < 0.001). The relative abundances of beneficial microorganisms (such as Sphingomonas, Lysobacter, Streptomyces, Chloroflexia, Deltaproteobacteria, and Alphaproteobacteria) were increased as a result of P. aeruginosa inoculation. These microorganisms play important roles in nutrient cycling, Cd adsorption, and NP degradation. Additionally, P. aeruginosa was not the dominate bacterial species at the end of the experiment. According to this study, P. aeruginosa application improved the phytoremediation efficiency of ryegrass on soil contaminated with NP and Cd, with a minimal risk of alien microbial invasion.

Enhancement and mechanisms of iron-assisted anammox process

Anaerobic ammonium oxidation (anammox) is a sustainable biological nitrogen removal technology that has limited large-scale applications owing to the low cell yield and high sensitivity of anammox bacteria (AnAOB). Fortunately, iron-assisted anammox, being a highly practical method could be an effective solution. This review focused on the iron-assisted anammox process, especially on its performance and mechanisms. In this review, the effects of iron in three different forms (ionic iron, zero-valent iron and iron-containing minerals) on the performance of the anammox process were systematically reviewed and summarized, and the strengthening effects of Fe (II) seem to be more prominent. Moreover, the detailed mechanisms of iron-assisted anammox in previous researches were discussed from macro to micro perspectives. Additionally, applicable iron-assisted methods and unified strengthening mechanisms for improving the stability of nitrogen removal and shortening the start-up time of the system in anammox processes were suggested to explore in future studies. This review was intended to provide helpful information for scientific research and engineering applications of iron-assisted anammox.

Climate and edaphic factors drive soil enzyme activity dynamics and tolerance to Cd toxicity after rewetting of dry soil

The intense drying-rewetting cycle due to climate change can affect soil microbial community composition and function, resulting in long-term consequences for belowground carbon and nutrient dynamics. However, how climatic and edaphic factors influence the responses of enzymes to rewetting and their responses to additional perturbation (e.g., heavy metal pollution) after the drying-rewetting history are not well understood. In this study, we collected 18 surface soils from farmlands across various climate zones in China. We chose dehydrogenase (DHA) and alkaline phosphomonoesterase (ALP) as representative intracellular and extracellular enzymes, respectively, and investigated their tolerance to additional perturbation by adding metal ions (i.e., Cd²⁺) upon rewetting. In all soils, rewetting increased DHA activities but did not affect ALP activities compared to air-dried soils. Rewetting increased the tolerances of DHA and ALP to Cd stress, suggesting that the drying-rewetting history may reduce the susceptibility of soil enzymes to additional disturbance. The results demonstrate that differentiating enzymes based on their location in the soil will improve our ability to assess the stress response of microbial communities to drastic fluctuations in soil moisture, thereby better predicting the legacy of climate change on microbial function in soils contaminated with heavy metals.

Artificially constructing mixed bacteria system for bioaugmentation of nitrogen removal from saline wastewater at low temperature

To alleviate the inhibition effects of multi-stresses, a multi-bacterial bioaugmentation based on stimulating cell-to-cell interactions was applied to improve the stress potential of salt-tolerant aerobic granular sludge (AGS). Results showed that the consortium formed by a combination of salt-tolerant ammonia-nitrogen utilizing bacteria, salt-tolerant nitrite-nitrogen utilizing bacteria and salt-tolerant nitrate-nitrogen utilizing bacteria with a whole biomass ratio of 1:2:1 achieved maximum nitrogen consumption rate (μNH₄⁺-N, μNO₂^--N and μNO₃^--N of 1.03, 0.57 and 11.62 mgN/L·h, respectively) at 35 gNaCl/L salinity and 15 °C. The flocculent consortium was aggregated by Aspergillus tubingensis mycelium pellet, which was made into a compound bacterial agent (CBA), and the comprehensive nitrogen consumption capability of CBA was further improved to 2.47-4.36-fold of single functional bacteria. 5% CBA (m/m) was introduced into the seafood processing wastewater in batches, in winter (12-16 °C), the removal efficiencies of NH₄⁺-N and total nitrogen increased from 66.89% to 52.77% of native AGS system to 79.02% and 69.97% of nascent bioaugmentation system, respectively. The analysis of key enzyme activities demonstrated that the ammonia monooxygenase and nitrate reductase activities of the bioaugmentation system were increased to 2.73-folds and 1.94-folds those of the native system. Moreover, due to an increase of 6.18 mg/gVSS and 0.11 in the secreted exopolysaccharide and tightly-bound/total extracellular polymeric substances, respectively, bioaugmentation boosted the cell bioflocculation ability by 13.53%, which enhanced the robustness. This work provided a detailed and feasible technical proposal for enhancing the biological treatment performance of saline wastewater in cold regions.

Adaptive regulation of activated sludge's core functional flora based on granular internal spatial microenvironment

A great deal of efforts has been put into studying the influence of the external macroenvironment for activated sludge to survive on microbial community succession, while granular internal spatial microenvironment should be given equal attention, because it is more directly involved in the information exchange and material transfer among microorganisms. This study systematically investigated the effects of granular microenvironment on spatial colonization and composition of sludge's core functional flora, and the corresponding difference of biological treatment performance. High content of extracellular-proteins (67.53 mg/gVSS) or extracellular-polysaccharide (65.02 mg/gVSS) stimulated the microbial flocculation and aggregation of 0.5-1.5 mm granules (GS) or 1.5-3.0 mm granules (GM), respectively, which was resulted from excellent cell hydrophobicity (59.26%) or viscosity (3.47 mPa s), therefore, constituted relatively dense porous frame. More hollow space existed in 3.0-5.0 mm granules (GL), which formed loose skeleton with 0.213 mL/g of total pore volume and 17.21 nm of average pore size. Combining scanning electron microscope images and fluorescent in-situ hybridization based microbiological analysis, aerobic nitrifiers were observed to wrap or surround anaerobic bacteria, or facultative/anaerobic bacteria were self-encapsulated, which created granule's unique microenvironment with alternating aerobic and anaerobic zones. GS has the most rich organic matter degrading bacteria and anaerobic heterotrophic denitrifiers, while GM and GL presented the greatest relative abundance of facultative and aerobic denitrifiers, respectively. The activity of dehydrogenase and nitrogen invertase of GM showed be 1.32-3.09 times higher than those of GS and GL, contributing to its higher carbon and nitrogen removal. These findings highlight the importance of granular microenvironment to adaptive regulation of activated sludge's core functional flora and corresponding pollutant removal performance.

Remediation potential of immobilized bacterial strain with biochar as carrier in petroleum hydrocarbon and Ni co-contaminated soil

The remediation of organic pollutant-heavy metal co-contaminated soil is a great challenge. Immobilized microorganism technology (IMT) is a potential approach to remediate co-contaminated soil. In this study, we evaluated the feasibility of IMT for the remediation of petroleum hydrocarbon-heavy metal nickel (Ni) co-contaminated soil. The Ni resistant and hydrocarbon-degrading bacteria strain Citrobacter sp. was added to co-contaminated soil by immobilizing on corncob biochar. The potential performance in biodegradation of petroleum hydrocarbon and changing the mobility and speciation of nickel (Ni) in soil were determined, with consideration of the influences of the soil properties and dehydrogenase activity. The results demonstrated that the degradation rate of petroleum hydrocarbons by immobilized microorganisms group (IM) was 45.52%, significantly higher than that of the free bacteria (30.15%), biochar (25.92%) and blank group (18.47%) (P<0.05). At the same time, IM was more effective in immobilizing Ni in the soil by transforming available Ni to a stable fraction with a maximum residual concentration increasing by 101.50 mg·kg^-1, and the carcinogenic nickel sulfide was not detected after remediation in IM. IM exhibited a higher level of soil dehydrogenase activity (0.3956 μg·mL^-1·h^-1·g^-1) than that of free bacteria (0.2878 μg·mL^-1·h^-1·g^-1). A linear correlation was found between the petroleum pollutants degradation rate and dehydrogenase activity (P<0.05). This study indicates the effectiveness and potential of IMT application in degrading petroleum hydrocarbon and immobilizing heavy metals in co-contaminated soil.

Insights into the inhibition effects of Cd on soil enzyme activities: From spatial microscale to macroscale

Cd is a heavy metal with high toxicity and adverse influence on the activities of soil enzymes. However, how Cd toxicity varies with the microenvironment or different regions needs further investigation. In this study, 17 soil samples were collected from different locations in China. Each soil sample was divided into two parts in accordance with aggregate size, macroaggregate fraction (> 250 µm) and microaggregate fraction (< 250 µm). The inhibitory effects of Cd on three soil enzymes were analyzed. Results showed that on the microscale, the inhibitory effect of Cd in macroaggregates was significantly stronger than that in microaggregates. Moreover, the inhibitory effect of Cd varied obviously across different regions, with the greatest effects being observed in soils from northeast China. This result indicates that the temperature and precipitation played a synergistic role in the Cd inhibition effects. The Cd inhibition ratios showed significantly negative correlations with clay content and positive correlations with pH value and organic carbon content, indicating that soil clay, pH and organic carbon were the main factors determining the Cd inhibition ratios on enzyme activities. This suggests that factors on the microscale and macroscale should be considered in addressing Cd pollution in soils.

Microbial community structure and metabolome profiling characteristics of soil contaminated by TNT, RDX, and HMX

This experiment was conducted to evaluate the ecotoxicity of typical explosives and their mechanisms in the soil microenvironment. Here, TNT (trinitrotoluene), RDX (cyclotrimethylene trinitramine), and HMX (cyclotetramethylene tetranitramine) were used to simulate the soil pollution of single explosives and their combination. The changes in soil enzyme activity and microbial community structure and function were analyzed in soil, and the effects of explosives exposure on the soil metabolic spectrum were revealed by non-targeted metabonomics. TNT, RDX, and HMX exposure significantly inhibited soil microbial respiration and urease and dehydrogenase activities. Explosives treatment reduced the diversity and richness of the soil microbial community structure, and the microorganisms able to degrade explosives began to occupy the soil niche, with the Sphingomonadaceae, Actinobacteria, and Gammaproteobacteria showing significantly increased relative abundances. Non-targeted metabonomics analysis showed that the main soil differential metabolites under explosives stress were lipids and lipid-like molecules, organic acids and derivatives, with the phosphotransferase system (PTS) pathway the most enriched pathway. The metabolic pathways for carbohydrates, lipids, and amino acids in soil were specifically inhibited. Therefore, residues of TNT, RDX, and HMX in the soil could inhibit soil metabolic processes and change the structure of the soil microbial community.

Ecological and Health Risk Assessments of an Abandoned Gold Mine (Remance, Panama): Complex Scenarios Need a Combination of Indices

The derelict Remance gold mine is a possible source of pollution with potentially toxic elements (PTEs). In the study area, diverse mine waste has been left behind and exposed to weather conditions, and poses risks for soil, plants and water bodies, and also for the health of local inhabitants. This study sought to perform an ecological and health risk assessment of derelict gold mining areas with incomplete remediation, including: (i) characterizing the geochemical distribution of PTEs; (ii) assessing ecological risk by estimating the pollution load index (PLI) and potential ecological risk index (RI); (iii) assessing soil health by dehydrogenase activity; and iv) establishing non-carcinogenic (HI) and carcinogenic risks (CR) for local inhabitants. Soil health seems to depend on not only PTE concentrations, but also on organic matter (OM). Both indexes (PLI and RI) ranged from high to extreme near mining and waste accumulation sites. As indicated by both the HI and CR results, the mining area poses a health risk for local inhabitants and particularly for children. For this reason, it will be necessary to set up environmental management programs in the areas that are most affected (tailings and surrounding areas) and accordingly establish the best remediation strategies to minimize risks for the local population.

Endophyte inoculation redistributed bioavailable Cd and nutrient in soil aggregates and enhanced Cd accumulation in Phytolacca acinosa

Plant growth and heavy metal (HM) accumulation is affected by heavy metal bioavailability and nutrient content in soil aggregates during endophyte-assisted phytoremediation. In this study, we evaluated the influence of endophytes inoculation on P. acinosa HM accumulation and soil aggregate physicochemical properties and explored the correlation among them. Endophyte inoculation increased the plant growth and Cd accumulation by 7.95-25.13% and 3.27-19.22%, respectively and the soil aggregate was redistributed with a decrease of 1.88-5.41% of the clay fraction. The available nitrogen, phosphorus and potassium, and organic matter in macro-aggregate and micro-aggregate were significantly improved with endophyte inoculation. In addition, compared to the no inoculation group, endophytes inoculation enhanced the bioavailability of Cd in macro-aggregates by 4.92-15.00% and in micro-aggregate by 0-9.37%. Both multiple linear regression analysis and the structural equation modeling (SEM) analysis showed that the Cd accumulation in P. acinosa was mainly depended on the Cd bioavailability in macro-aggregates and micro-aggregates. In general, this study helped to improve our understanding of soil aggregate HM bioavailability and nutrient content distribution characteristics under endophyte inoculation, which could further explain the mechanisms of endophytes in plant growth promoting and HM accumulation improving.

Optimization of integrated phytoremediation system (IPS) for enhanced lead removal and restoration of soil microbial activities

Improving phytoremediation efficiency in lead (Pb) contaminated soil through either bacterial or fungal inoculants have extensively been studied with different successes and limitations. In this study, co-application of bacteria and fungi have been investigated for development of an integrated phytoremediation system (IPS) for efficient Pb removal and restoration of soil microbial and enzymatic activities in degraded soil. For this purpose, Pb tolerant bacterial and fungal strains were firstly analyzed for antifungal and antibacterial activities through disc diffusion method. Afterwards, the co-inoculation studies were performed to investigate the effects on phytoavailability and uptake of Pb by Pelargonium hortorum through soil incubation and pot culture experiments, respectively. Results indicated significant (p < 0.05) antibacterial activity of Mucor spp. against bacterial species (Klebsiella variicola and K. quasipneumoniae). The highest significant increase in extractable Pb fraction (5.0-folds) was observed when soil was co-inoculated with Aspergillus flavus + Microbacterium paraoxydans compared to the control soil (un-inoculated soil) at 2000 mg Pb kg^-1 concentration. Similarly, uptake results also indicated significantly higher Pb uptake in plants inoculated with A. flavus + M. paraoxydans. Soil microbial results indicated significant decrease in microbial health indicators and enzymatic activities with increasing Pb concentration and exposure time, as compared to control soil. A relatively severe decline was observed in soil respiration and dehydrogenase (DEH) activities by 2.8- and 2.5-folds, respectively at 2000 mg Pb kg^-1 of soil. The optimized IPS was effective for restoring enzymatic activities in Pb contaminated soil and could be applied for sustainable restoration of Pb contaminated soil.

Soil chemical properties rather than the abundance of active and potentially active microorganisms control soil enzyme kinetics

Soil enzymes secreted by microorganisms play a critical role in nutrient cycling, soil structure maintenance, and crop production. However, understanding of the linkage between soil enzyme kinetics and microbial metabolism and active microbial communities is remarkably limited. In this study, we measured the kinetics of three hydrolase enzymes, active microbial abundance and substrate-induced respiration (SIR) from 21 farmlands differing in their fertilities collected from the Loess Plateau, China. Results showed the high fertility soils had higher total organic carbon (TOC) and nutrient contents, potential microbial activity, the colony-forming units (CFU) of actinomycetes, and values of enzyme V_max and K_m than those of low fertility soils. We also observed that the CFU of fungi and other bacterial groups did not change with soil fertility status. Soil chemical properties explained 74.0% of the variance in V_max and 28.3% of the variance in K_m, respectively. Whereas, the abundance of main microbial groups and fungi/bacteria ratio only explained 10.2% and 7% of the variance of V_max and K_m, respectively. The interactive effect of soil properties and microbial community could explain 20.2% of the variance in K_m. Our results suggest that the substrate availability would mainly drive enzyme kinetics compared to the abundance of active/potentially active microbes in the farmland soils.

Impacts and tolerance responses of Coprinus comatus and Pleurotus cornucopiae on cadmium contaminated soil

Large amounts of cadmium (Cd) have been discharged into soil with the rapid development of industry. In this study, we revealed the impacts of Coprinus comatus (C. comatus) and Pleurotus cornucopiae (P. cornucopiae) on soil and the tolerance responses of macrofungi in the presence of Cd by the analysis of soil biochemical properties and macrofungi growth indexes. Results showed that with the cultivation of C. comatus and P. cornucopiae, the HOAc-extractable Cd in soil individually reduced by 9.53% and 11.35%, the activities of soil urease, acid phosphatase, dehydrogenase, and Fluorescein diacetate (FDA) hydrolysis increased by 18.11-101.45%, 8.39-18.24%, 9.37-55.50% and 28.94-41.92%, respectively. Meanwhile, different soil bacterial communities were observed with various macrofungi cultivations. Also, Cd accumulation significantly enhanced the macrofungi antioxidant enzyme activities, which increased by 24.10-45.43%, 30.11-61.53% and 7.03-26.81% for catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) activities in the macrofungi, respectively. Moreover, the enhanced macrofungi endophytic bacterial diversities with Cd existence was firstly observed in the present experiment. These findings revealed the possible Cd resistance mechanisms in macrofungi, suggesting C. comatus and P. cornucopiae were promising ameliorators for Cd contaminated soil.

Compost application increases the ecological dose values in a non-calcareous agricultural soil contaminated with cadmium

Soil cadmium (Cd) pollution resulting from anthropogenic activities has become a major concern for microbial and biochemical functions that are critical for soil quality and ecosystem sustainability. Organic amendments can reduce Cd toxicity to the microbial community and enzymatic activity in Cd-polluted soils and thus would increase the ecological dose (ED) values. However, there has been less focus on the effect of organic amendments on microbial and biochemical responses to Cd toxicity in non-calcareous soils using the concept ED. The aim of this study was to assess the impact of compost application on microbial activity, microbial biomass, turnover rates of carbon and nitrogen, and enzymatic activities as the key ecological functions in a non-calcareous soil spiked with different Cd concentrations (0-200 mg kg^-1). Results showed that soil amendment with compost decreased Cd availability by 48-76%, depending on the total soil Cd content. The application of compost reduced the negative influence of Cd eco-toxicity on most soil microbial and biochemical functions by 20-122%, depending on the Cd level and the assay itself. The ED values, derived from the sigmoidal dose-response and kinetic models, were 1.10- to 2.24-fold higher in the compost-amended soils than the unamended control soils at all Cd levels. In conclusion, the potential risks associated with high levels of Cd pollution can be alleviated for microbial and biochemical indicators of soil quality/health with application of 2500 kg ha^-1 compost as a cost-effective source of organic matter to non-calcareous soils. The findings would have some useful implications for organic matter-limited non-calcareous soils polluted with Cd.

Effects of aluminum oxide nanoparticles on the performance, extracellular polymeric substances, microbial community and enzymatic activity of sequencing batch reactor

The performance, pollutant removal rate, microbial community and enzymatic activity of a sequencing batch reactor (SBR) were investigated under oxide nanoparticles (Al₂O₃ NPs) stress. Al₂O₃ NPs at 0-50 mg/L showed no evident impact on the COD and NH₄ ⁺ removals of SBR. The oxygen-uptake rate, nitrifying rate and nitrite-reducing rate slightly diminished with the increase of Al₂O₃ NPs concentration. Compared with 0 mg/L Al₂O₃ NPs, the dehydrogenase activity declined by 23.52% at 50 mg/L Al₂O₃ NPs. The activities of ammonia monooxygenase, nitrite oxidoreductase and nitrite reductase decreased with the increase of Al₂O₃ NPs concentration from 0 to 50 mg/L Al₂O₃ NPs. However, the nitrate reductase (NR) activity slightly increased at 5 and 15 mg/L Al₂O₃ NPs and declined at 30 and 50 mg/L Al₂O₃ NPs. The microbial reactive oxygen species (ROS) production and lactate dehydrogenase (LDH) release merely raised 14.80% and 20.72% at 50 mg/L Al₂O₃ NPs by contrast with 0 mg/L Al₂O₃ NPs, respectively. Al₂O₃ NPs enhanced the production, protein content and polysaccharide content of extracellular polymeric substances owing to preventing the microbes from Al₂O₃ NPs biotoxicity. The existence of Al₂O₃ NPs led to the variations of microbial richness and diversity in the SBR due to their biotoxicity.

Indicators for assessment of soil quality: a mini-review

Soil quality is the competence of soil to perform necessary functions that are able to maintain animal and plant productivity of the soil. Soil consists of various physical, chemical, and biological parameters, and all these parameters are involved in the critical functioning of soil. There is a need for continuous assessment of soil quality as soil is a complex and dynamic constituent of Earth's biosphere that is continuously changing by natural and anthropogenic disturbances. Any perturbations in the soil cause disturbances in the physical (soil texture, bulk density, etc.), chemical (pH, salinity, organic carbon, etc.), and biological (microbes and enzymes) parameters. These physical, chemical, and biological parameters can serve as indicators for soil quality assessment. However, soil quality assessment cannot be possible by evaluating only one parameter out of physical, chemical, or biological. So, there is an emergent need to establish a minimum dataset (MDS) which shall include physical, chemical, and biological parameters to assess the quality of the given soil. This review attempts to describe various physical, chemical, and biological parameters, combinations of which can be used in the establishment of MDS.

Effect of Adding Drinking Water Treatment Sludge on Excess Activated Sludge Digestion Process

Drinking water treatment sludge (DWTS) is a waste by-product from water treatment plants where aluminum and iron salts are the most commonly used coagulants. DWTS was reused to investigate the effects of DWTS on the digestion liquid quality and microorganism activity of excess activated sludge (EAS). DWTS with four suspended solid (SS) concentrations (0%, 2%, 5% and 10%) was added to EAS which was sampled during aerobic and anaerobic digestion processes, then batch tests were carried out which followed the coagulation-flocculation process. It was found that DWTS can improve total dissolved nitrogen (TDN) and dissolved phosphorus (DP) removal efficiencies for anaerobic EAS. The highest removal efficiency of TDN (29.97%) as well as DP (55.38%) was observed when DWTS dosage was SS = 10%. The release of dissolved organic matter (DOM) by DWTS could increase dissolved organic carbon (DOC) concentration and lead to the accumulation of non-biodegradable humic acid-like substance in aerobic and anaerobic digestion liquid. The dehydrogenase activity (DHA) values of anaerobic EAS were higher than aerobic EAS. DWTS could reduce DHA for both EAS. These results indicate that potential risk of release of DOM should be considered when reusing DWTS in future research.

Insights into the effects of single and combined divalent copper and humic acid on the performance, microbial community and enzymatic activity of activated sludge from sequencing batch reactor

The performance, microbial community and enzymatic activity of activated sludge from four identical sequencing batch reactors (SBRs) were compared by treating synthetic wastewater under the single and combined divalent copper (Cu²⁺) at 20 mg/L and humic acid (HA) at 20 mg/L. Compared with the absence of Cu²⁺ and HA, the single HA slightly enhanced the oxygen uptake rate (OUR), the nitrification and denitrification rates and the activities of dehydrogenase, nitrifying enzymes and denitrifying enzymes, whereas the single Cu²⁺ had the opposite results. The combined Cu²⁺ and HA inhibited the OUR, nitrogen removal rate and enzymatic activity of activated sludge almost the same as the single Cu²⁺. The single HA had no obvious effect on the balance between the microbial oxidative stress and antioxidant activity. However, the variations of microbial reactive oxygen species production, peroxidase activity, catalase activity, superoxide dismutase activity, and lactate dehydrogenase release demonstrated that the combined Cu²⁺ and HA and single Cu²⁺ produced obvious toxicity to microorganisms in activated sludge. The microbial richness and diversity had some obvious changes under the single and combined Cu²⁺ and HA. The relative abundances of Nitrosomonas, Nitrospira and some denitrifying genera (e.g. Zoogloea, Dokdonella, Denitratisoma, Flavobacterium and Thermomonas) under the combined Cu²⁺ and HA were less than those under the single Cu²⁺.

Single and combined effects of divalent copper and hexavalent chromium on the performance, microbial community and enzymatic activity of sequencing batch reactor

Divalent copper (Cu²⁺) and hexavalent chromium (Cr⁶⁺) are often encountered in industrial wastewater and municipal wastewater, the effect of combined Cu²⁺ and Cr⁶⁺ on biological wastewater treatment systems has cause wide concern. In the present research, the performance, microbial community and enzymatic activity of sequencing batch reactors (SBRs) were compared under the single and combined Cu²⁺ at 20 mg/L and Cr⁶⁺ at 10 mg/L. The chemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N) removal efficiencies under the combined Cu²⁺ and Cr⁶⁺ were less than those under the single Cu²⁺ and Cr⁶⁺. The combined Cu²⁺ and Cr⁶⁺ displayed more inhibition effects on the oxygen uptake rate, nitrification rate and denitrification rate of activated sludge than the single Cu²⁺ and Cr⁶⁺. The inhibitory effects of the combined Cu²⁺ and Cr⁶⁺ on the activities of dehydrogenase, ammonia monooxygenase, nitrite oxidoreductase, nitrite reductase and nitrate reductase showed significant increases by comparison with the single Cr⁶⁺. However, the combined Cu²⁺ and Cr⁶⁺ had a little more inhibitory effects on the enzymatic activities than the single Cu²⁺. The microbial richness and diversity displayed some obvious changes under the single and combined Cu²⁺ and Cr⁶⁺ by comparison the absence of Cu²⁺ and Cr⁶⁺. The relative abundances of nitrifying genera (e.g. Nitrosomonas and Nitrospira) under the combined Cu²⁺ and Cr⁶⁺ was less than those under the single Cu²⁺ and Cr⁶⁺. These findings will be helpful to better understand the combined effects of multiple heavy metals on biological wastewater treatment systems.

Influence of multi-walled carbon nanotubes on the microbial biomass, enzyme activity, and bacterial community structure in 2,4-dichlorophenol-contaminated sediment

The rise in manufacture and use of carbon nanotubes has aroused the concern about their potential risks associated with coexisting pollutants in the aquatic environment. 2,4-dichlorophenol (2,4-DCP), with a high toxicity to many aquatic organisms, is a widespread pollutant resulting from the extensive use of pesticides and preservatives. In this article, the adsorption of 2,4-DCP by riverine sediment and the responses of sediment microbial community to 2,4-DCP were studied in the presence of multi-walled carbon nanotubes (MWCNTs). Adding MWCNTs significantly increased the adsorption amount of sediment for 2,4-DCP from 0.541 to 1.44 mg/g as the MWCNT concentration increased from 0 to 15 mg/g. The responses of sediment microbial community were determined after one-month exposure to MWCNTs at different concentrations (0.05, 0.5, 5, and 50 mg/g). The microbial biomass carbon in the sediment contaminated with 2,4-DCP increased in the presence of 5 mg/g of MWCNTs (from 0.06 to 0.11 mg/g), but not significantly changed at other MWCNT concentrations. For the sediments contaminated with 2,4-DCP, the presence of MWCNTs made no difference to urease activity, while the dehydrogenase activity slightly increased with the addition of 5 mg/g of MWCNTs and decreased in the presence of 50 mg/g of MWCNTs. The changes of sediment bacterial communities were further determined by 16S rRNA gene sequencing. Based on the weighted UniFrac distance between communities, the clustering analysis suggested that the contamination of 2,4-DCP affected the bacterial community structure in a greater degree than that caused by MWCNTs at relatively low concentrations (≤5 mg/g). Bacteroidetes, Planctomycetes, and Nitrospirae were feature bacterial phyla to reflect the effects of MWCNTs and 2,4-DCP on sediment bacterial community. These results may contribute to the understanding of microbial community response to co-exposure of MWCNTs and 2,4-DCP and the assessment of associated ecological risks.

Soil enzyme kinetics indicate ecotoxicity of long-term arsenic pollution in the soil at field scale

Information on the kinetic characteristics of soil enzymes under long-term arsenic (As) pollution in field soils is scarce. We investigated Michaelis-Menten kinetic properties of four soil enzymes including β-glucosidase (BG), acid phosphatase (ACP), alkaline phosphatase (ALP), and dehydrogenase (DHA) in field soils contaminated by As resulting from long-term realgar mining activity. The kinetic parameters, namely the maximum reaction velocity (V_max), enzyme-substrate affinity (K_m) and catalytic efficiency (V_max/K_m) were calculated. Results revealed that the enzyme kinetic characteristics varied in soils and were significantly influenced by total nitrogen (N) and total As, which explained 31.8% and 30.7% of the variance in enzyme kinetics respectively. Enzyme pools (V_max) and catalytic efficiency (V_max/K_m) of BG, ACP and DHA decreased with elevated As pollution, while the enzyme affinity for substrate (K_m) was less affected. Redundancy analysis and stepwise regression suggested that the adverse influence of As on enzyme kinetics may offset or weakened by soil total N and soil organic matter (SOM). Concentration-response fitting revealed that the specific kinetic parameters expressed as the absolute enzyme kinetic parameters multiplied by normalized soil total N and SOM were more relevant than the absolute ones to soil total As. The arsenic ecological dose values that cause 10% decrease (ED₁₀) in the specific enzyme kinetics were 20-49 mg kg^-1, with a mean value of 35 mg kg^-1, indicating a practical range of threshold for As contamination at field level. This study concluded that soil enzymes exhibited functional adaptation to long-term As stress mainly through the reduction of enzyme pools (Vmax) or maintenance of enzyme-substrate affinity (K_m). Further, this study demonstrates that the specific enzyme kinetics are the better indicators of As ecotoxicity at field-scale compared with the absolute enzyme parameters.

Performance enhancement by adding ferrous to a combined modified University of Cape Town and post-anoxic/aerobic-membrane bioreactor

The removal of nutrients in a combined modified University of Cape Town and post-anoxic/aerobic-membrane bioreactor (UCT-A/MBR) was investigated. Denitrifying phosphorus removal (DPR) and nitrate-dependent anaerobic ferrous oxidation (NAFO) were applied to enhance the nutrient removal performances. The results showed that NAFO with the addition of Fe(II) and DPR could promote nitrogen and phosphorus removal. The total nitrogen removal efficiency gradually increased from 71.05 ± 2.00% to 73.84 ± 1.74% and 75.70 ± 1.47% with no Fe(II) addition, addition to the post-anoxic tank, and addition to the anoxic tank, and the total phosphorus removal efficiency increased from 89.37 ± 1.91% to 95.21 ± 0.85% and 96.01 ± 1.10%, respectively. Gene sequencing was conducted, and Saprospiraceae was determined to be the dominant DPR-related bacteria, with its abundance increasing from 16.31% to 22.45% after Fe(II) addition. Additionally, the proportion of the NAFO-related bacteria Azospira increased from 0.58% to 1.91% after Fe(II) addition. The microbial succession caused by the addition of Fe(II) may have resulted in the enhanced removal performance.

Time-Dependent Hormetic Response of Soil Alkaline Phosphatase Induced by Cd and the Association with Bacterial Community Composition

Hormetic dose-response that involved Cd in soils is increasingly paid attentions for risk assessment of Cd toxicity, but insufficient studies were conducted to define the temporary modification of soil enzyme and the potential microbial responses. The present study chooses soil alkaline phosphatase (ALP) as endpoint to uncover the time-dependent hormetic responses to low doses of Cd and its association with bacterial community composition. The results showed that addition of 0.01-3.0 mg kg^-1 Cd significantly increased ALP's activities with maximum stimulatory magnitude of 11.4-27.2%, indicating a typical hormesis. The response started at 12 h after Cd addition and maintained about 24 h. This demonstrated that the hormetic response is time-dependent and transient. Changes of soil bacterial community composition showed that, at 6 h, relative abundances (RAs) of Proteobacteria and Firmicutes at phylum and Pontibacter, Bacillaceae-Bacillus, Bacillaceae1-Bacillus, and Paenisporosarcina at genus significantly correlated with ALP's activities at 12-36 h (P < 0.05). This suggests that soil bacteria likely showed an earlier response to Cd and potentially contributes to the subsequent soil enzyme's hormesis. In addition, it was found that Gram-negative bacteria other than Gram-positive bacteria are prone to exhibiting a hormetic response under Cd stress. Our findings provide much insight into ecotoxicological risk assessment for soil Cd pollution.

Removal of Cd(II) by modified maifanite coated with Mg-layered double hydroxides in constructed rapid infiltration systems

To improve the adsorption performance of Cd(II) by maifanite in constructed rapid infiltration systems (CRIS), Mg-layered double hydroxides (MgAl-LDHs, MgFe-LDHs) are prepared by a co-precipitation method and in-situ coated on the surface of original maifanite. Characterization of the successful LDHs-coating modification is realized by the following: scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Brunauer Emmett Teller (BET). In the purification experiments, the average removal rates of Cd(II) were 97.66% for maifanite/MgAl-LDHs and 97.54% for maifanite/MgFe-LDHs, approximately 11% greater than for the original maifanite. Isothermal adsorption experiments and adsorption kinetic experiments were conducted to explore the Cd(II) adsorption mechanism. The modified maifanite demonstrated a higher Langmuir adsorption capacity and stronger surface bond energies compared to the original maifanite. The adsorption type of Cd(II) by maifanite/Mg-LDHs and original maifanite was monolayer adsorption based mainly on chemical adsorption. Furthermore, the extracellular polymeric substances and dehydrogenase activities of the microorganisms were measured and analyzed to study the effect of microorganisms on the removal of Cd(II) in the test columns. High-throughput sequencing technology was also applied to analyze the composition and diversity of bacterial communities. Based on a simple estimation, the synthesis cost of maifanite/MgAl-LDHs was only ¥ 0.33/Kg. In brief, maifanite/Mg-LDHs is an efficient and economical substrate for a CRIS for Cd(II) removal.

Insights into the effect of nickel (Ni(II)) on the performance, microbial enzymatic activity and extracellular polymeric substances of activated sludge

The performance, nitrogen removal rate, microbial enzymatic activity and extracellular polymeric substances (EPS) of activated sludge were assessed under nickel (Ni(II)) stress. The organic matter and NH₄⁺-N removal efficiencies were stable at less than 10 mg/L Ni(II) and subsequently decreased with the increment of Ni(II) concentration from 10 to 30 mg/L. The specific oxygen uptake rate and dehydrogenase activity kept stable at less than 5 mg/L Ni(II) and then declined at 5-30 mg/L Ni(II). Both specific ammonia-oxidizing rate (SAOR) and specific nitrite-oxidizing rate (SNOR) decreased with the increment of Ni(II) concentration. The changing trends of ammonia monooxygenase and nitrite oxidoreductase activities were matched those of SAOR and SNOR, respectively. The nitrite-reducing rate and nitrate-reducing rate illustrated a similar variation tendency to the nitrite reductase activity and nitrate reductase activity, respectively. Ni(II) impacted on the production, chemical composition and functional group of EPS. The relation between the sludge volume index and the EPS production exhibited a better linear function with a negative slope, demonstrating that Ni(II) improved the sludge settleability despite of the increase of EPS production.

Effects of tree-herb co-planting on the bacterial community composition and the relationship between specific microorganisms and enzymatic activities in metal(loid)-contaminated soil

Tree-herb co-planting is regarded as an ecologically sustainable approach for the remediation of metal(loid)-contaminated soil. In this study, two herb species, Pteris vittata L. and Arundo donax L., and two woody species, Morus alba L. and Broussonetia papyrifera L., were selected for the tree-herb co-planting, and their impacts on the changing of microbial community structure in metal(loid)-contaminated soil were studied by high-throughput sequencing. The results showed that the microbial diversity was stably maintained by the tree-herb interactions, while the composition of the microbial community was clearly affected in metal(loid)-contaminated soil. According to the Venn and flower diagrams, heat map and principal coordinate analysis, both plant monocultures and co-planting had specific microbial community structures, which suggested that the composition and abundance of bacterial communities varied between plant monoculture and tree-herb co-planting treatments. In particular, A. donax L. played a vital role in increasing the abundances of Cyanobacteria (>1%) in metal(loid)-contaminated soil when co-planted with woody plants. Furthermore, some specific microorganisms combined with plants played a key role in improving enzyme activity in the contaminated soil. Correspondingly, sucrase and acid phosphatase activities in monoculture and co-planting treatments significantly (p < 0.05) increased by 1.05-3.37 and 7.24-20.3 times. These results indicated that the rhizospheric interactions in the tree-herb co-planting system positively affected the soil microbes and had stronger impacts on the composition of soil microorganisms, which was closely related to the improvement of the biological quality in the metal(loid)-contaminated soil.

Impact of ampicillin on the nitrogen removal, microbial community and enzymatic activity of activated sludge

The performance, nitrogen removal rate, extracellular polymeric substances (EPS), microbial community and enzymatic activity of activated sludge have been assessed in a sequencing batch reactor under ampicillin stress. The chemical oxygen demand and ammonia removal kept relatively stable at 0-30 mg/L ampicillin. No obvious nitrite and nitrate accumulation was found in the effluent. However, the oxygen utilization rate, nitrification rate and denitrification rate declined with the increment of ampicillin concentration. The activities of dehydrogenase and microbial enzymes relating to nitrogen removal were inhibited under ampicillin stress. Ampicillin at 20 and 30 mg/L heightened the microbial lactate dehydrogenase release and reactive oxygen species production. Ampicillin promoted the production of EPS, loosely bound EPS and tightly bound EPS and affected their chemical composition. Additionally, the protein/polysaccharide ratios in the EPS and the sludge settleability reduced with the increment of ampicillin concentration. Ampicillin obviously affected the relative abundance of nitrifying- and denitrifying bacteria.

Soil mineral alters the effect of Cd on the alkaline phosphatase activity

The toxicity of heavy metals (HMs) to soil enzymes is directly influenced by the status of the enzyme (free vs. immobilized on minerals) and the duration of exposure. However, little information is available on the interaction effect of HMs, mineral, and exposure time on soil enzyme activities. We investigated the interaction mechanism of alkaline phosphatase (ALP) with minerals (montmorillonite and goethite) and the response of free and immobilized ALP to cadmium (Cd) toxicity under different exposure times. The adsorption isotherms of ALP on both minerals were L-type. The maximum adsorption capacity of goethite for ALP was 3.96 times than montmorillonite, although both had similar adsorption constant (K). Goethite showed a greater inhibitory effect on ALP activity than montmorillonite. The toxicity of Cd to free- and goethite-ALP was enhanced with increasing exposure time, indicating a time-dependent inhibition. However, Cd toxicity to montmorillonite-ALP was not affected by the exposure time. The inhibition of Cd to soil enzyme activity is influenced by the properties of mineral complexes and the duration of exposure. A further understanding of the time pattern of HMs toxicity is helpful for accurately assessing the hazards of HMs to soil enzyme activity.

Effect of Fe (II) in low-nitrogen sewage on the reactor performance and microbial community of an ANAMMOX biofilter

In this study, the effect of Fe (II) on Anaerobic Ammonium Oxidation (ANAMMOX) process was investigated by step-wise increasing the Fe (II) in influent from 1 to 50 mg L^-1. The nitrogen removal, biofilm property and the microbial community were analyzed in each phase. Results showed that, the anaerobic ammonia-oxidizing bacteria (AAOB) bioactivity and the nitrogen removal of ANAMMOX system were slightly improved to 0.58 from the initial 0.51 kg m^-3 d^-1 by Fe (II) in 1-5 mg L^-1. The nitrogen removal was suppressed and could recover to the initial level during the same period under 10-20 mg L^-1 Fe (II), while it did not recover to the initial level under 30 mg L^-1 Fe (II) and showed no recovery performance under 50 mg L^-1 Fe (II). The irreversible suppression threshold of Fe (II) was calculated as 50 mg L^-1. The iron content in ANAMMOX biofilm presented linear correlation with the influent Fe (II) in 1-20 mg L^-1, which then tended to be stable when Fe (II) was higher. Dehydrogenase activity (DHA) showed similar and faster response to Fe (II) than the microbial activity, and it was an effective pre-indicator for the nitrogen removal performance in the ANAMMOX system suffered Fe (II). The Fe (II) feeding firstly led to the relative abundance of AAOB decreased to 11.04% from the initial 35.46%, and finally picked up to 19.39% after the long-term acclimatization.

Soil properties influence kinetics of soil acid phosphatase in response to arsenic toxicity

Soil phosphatase, which plays an important role in phosphorus cycling, is strongly inhibited by Arsenic (As). However, the inhibition mechanism in kinetics is not adequately investigated. In this study, we investigated the kinetic characteristics of soil acid phosphatase (ACP) in 14 soils with varied properties, and also explored how kinetic properties of soil ACP changed with different spiked As concentrations. The results showed that the Michaelis constant (K_m) and maximum reaction velocity (V_max) values of soil ACP ranged from 1.18 to 3.77mM and 0.025-0.133mMh^-1 in uncontaminated soils. The kinetic parameters of soil ACP in different soils changed differently with As contamination. The K_m remained unchanged and V_max decreased with increase of As concentration in most acid and neutral soils, indicating a noncompetitive inhibition mechanism. However, in alkaline soils, the K_m increased linearly and V_max decreased with increase of As concentration, indicating a mixed inhibition mechanism that include competitive and noncompetitive. The competitive inhibition constant (K_ic) and noncompetitive inhibition constant (K_iu) varied among soils and ranged from 0.38 to 3.65mM and 0.84-7.43mM respectively. The inhibitory effect of As on soil ACP was mostly affected by soil organic matter and cation exchange capacity. Those factors influenced the combination of As with enzyme, which resulted in a difference of As toxicity to soil ACP. Catalytic efficiency (V_max/K_m) of soil ACP was a sensitive kinetic parameter to assess the ecological risks of soil As contamination.

Factors controlling cadmium and lead activities in different parent material-derived soils from the Pearl River Basin

Labile metals in agricultural soils are available to crops and thus pose a great health risk for human beings. Therefore, factors influencing heavy metal activity are of interest to researchers. In this study, a total of 142 soil samples representing 5 typical parent materials in the Pearl River Basin (PRB), China were collected to investigate factors impacting the distribution of labile Cd and Pb in the soils. The results showed that the labile fractions accounted for 0.03%-14.7% for Cd and 0.01%-0.39% for Pb of the total metals, and the labile fractions were linearly correlated to their corresponding total contents. The step regression analyses suggested that the key factors impacting labile Cd and Pb varied in different parent material soils. Pb activity was highly sensitive to pH in alkaline limestone soils. The quartz sand remained in granite-produced soils enhanced Cd activity. And dissolved organic matter (DOM) compositions considerably influenced Cd and Pb activities in sand shale, diluvium, and alluvium soils. Land use impacts heavy metal activities. The labile Cd and Pb in paddy soils were higher than those in non-paddy soils, although total metals in the soils were comparable. It could be ascribed to the long-term equilibrium of metals between the solution and solid phases of the paddy soils. The results provide a theoretical basis for preliminary prediction of heavy metal activity and provide a technical support for heavy metal activity management and pollution control based on soil parent materials.

Kinetics of soil dehydrogenase in response to exogenous Cd toxicity

Soil dehydrogenase plays a role in the biological oxidation of soil organic matter and can be considered a good measure of the change of microbial oxidative activity under environmental pollutions. However, the kinetic characteristic of soil dehydrogenase under heavy metal stresses has not been investigated thoroughly. In this study, we characterized the kinetic characteristic of soil dehydrogenase in 14 soil types, and investigated how kinetic parameters changed under spiked with different concentrations of cadmium (Cd). The results showed that the K_m and V_max values of soil dehydrogenase was among 1.4-7.3mM and 15.9-235.2μMh^-1 in uncontaminated soils, respectively. In latosolic red soil and brown soil, the inhibitory kinetic mechanism of Cd to soil dehydrogenase was anticompetitive inhibition with inhibition constants (K_i) of 12 and 4.7mM, respectively; in other soils belonged to linear mixed inhibition, the values of K_i were between 0.7-4.2mM. Soil total organic carbon and K_i were the major factors affecting the toxicity of Cd to dehydrogenase activity. In addition, the velocity constant (k) was more sensitive to Cd contamination compared to V_max and K_m, which was established as an early indicator of gross changes in soil microbial oxidative activity caused by Cd contamination.