Microbial Nitrification Research Articles

Cadmium immobilization during nitrate-reducing Fe(II) oxidation by Acidovorax sp. BoFeN1: Contribution of bacterial cells and secondary minerals

Microbially nitrate-reducing Fe(II) oxidation (NRFO) is widespread in anoxic environments at circumneutral pH, in which Fe(II) oxidation is considered to be mainly catalyzed by nitrite via chemodenitrification after microbial nitrate reduction. Cadmium (Cd) is toxic to organisms, and its availability is strongly affected by iron (oxyhydr)oxides and/or bacteria. However, it remains unclear how NRFO affects the fate of Cd with environmentally relevant concentrations, and the relative contributions of microbes and secondary minerals to Cd immobilization during NRFO. Here, Acidovorax sp. BoFeN1, a typical NRFO bacterium, was used to investigate the Cd immobilization during NRFO at pH 7.0 with 0.1, 0.5, and 1.0 mg/L Cd, respectively. Higher initial Cd concentration resulted in a higher amount of Cd immobilized in precipitates, but showed stronger inhibition on nitrate reduction, acetate metabolism, and cell growth. The presence of Cd barely influenced the Fe(II) oxidation during NRFO or chemodenitrification, but retarded the mineral transformation to secondary crystalline minerals, i.e., lepidocrocite and goethite. The extracellular polymeric substances deposited in the bacteria-mineral precipitates were rich in tyrosine-like and tryptophan-like proteins with negatively charged amino and carboxyl groups. The STEM-HAADF images with EDX mapping demonstrated that Cd was strongly co-localized with Fe on the cell surface and in the periplasm. Cadmium immobilization experiments with bacterial cells or during chemodenitrification indicated that the bacterial cells and secondary minerals formed during chemodenitrification are comparably important for Cd immobilization in the Cd0.1 treatment, while the secondary minerals played a predominant role in Cd immobilization in the Cd0.5 and Cd1.0 treatments. These results suggested that Cd was likely associated with the secondary iron minerals, which precipitated with the proteins on the cell surface and in the periplasm. Our findings provide a comprehensive understanding of the roles of Cd, bacterial cells, and secondary minerals formed via chemodenitrification, as well as their interactions during the complicated NRFO process at circumneutral pH.

Illuminating plant–microbe interaction: How photoperiod affects rhizosphere and pollutant removal in constructed wetland?

Rhizosphere is a crucial area in comprehending the interaction between plants and microorganisms in constructed wetlands (CWs). However, influence of photoperiod, a key factor that regulates photosynthesis and rhizosphere microbial activity, remains largely unknown. This study investigated the effect of photoperiod (9, 12, 15 h/day) on pollutant removal and underlying mechanisms. Results showed that 15-hour photoperiod treatment exhibited the highest removal efficiencies for COD (87.26%), TN (63.32%), and NO3–-N (97.79%). This treatment enhanced photosynthetic pigmentation and root activity, which increased transport of oxygen and soluble organic carbon to rhizosphere, thus promoting microbial nitrification and denitrification. Microbial community analysis revealed a more stable co-occurrence network due to increased complexity and aggregation in the 15-hour photoperiod treatment. Phaselicystis was identified as a key connector, which was responsible for transferring necessary carbon sources, ATP, and electron donors that supported and optimized nitrogen metabolism in the CWs. Structural equation model analysis emphasized the importance of plant–microbe interactions in pollutant removal through increased substance, information, and energy exchange. These findings offer valuable insights for CWs design and operation in various latitudes and rural areas for small-scale decentralized systems.

Metagenomic insights into the alteration of soil N‐cycling‐related microbiome and functions under long‐term conversion of cropland toMiscanthus

AbstractMiscanthusspp. show excellent application prospects due to its bioenergy potential and multiple ecological services. Annual N export with biomass harvest fromMiscanthus, even without fertilizer supplement, do not reduce soil N levels. The question arises regarding howMiscanthuscan maintain stable soil N levels. Metagenomic strategies were used to reveal soil N‐cycling‐related microbiome and their functional contributions to processes of soil N‐cycling based on the comparison among the bare land, cropland, 10‐yearMiscanthus × giganteus, and 15‐yearMiscanthus sacchariflorusfields. The results showed that, after long‐term cropland‐to‐Miscanthusconversion (LCMC), 16 of 21 bacterial phyla and all the archaeal phyla exhibited significant changes. Soil microbial denitrification and nitrification functions were significantly weakened, and N fixation (NF) was significantly enhanced. The biosynthesis of amino acids, especially alanine, aspartate, and glutamate metabolism, in soil N‐cycling‐related microbiome was dramatically promoted. The genus Anaeromyxobacter contributed largely to the NF process after LCMC. Variations in the soil available potassium, available N, organic C, and total N contents drove a functional shift of soil microbiome from cropland toMiscanthuspattern. We conclude thatMiscanthuscan recruit Anaeromyxobacter communities to enhance NF benefiting its biomass sustainability and soil N balance.

Shelterbelts increased soil inorganic carbon but decreased nitrate nitrogen, total phosphorus, and bulk density relative to neighbor farmlands depending on tree growth, geoclimate, and soil microbes in the Northeast China Plain

Black soils constitute the food basket of many countries, and the improvement of their properties has received increasing attention. We collected 750 samples at 0 to 1 m depth from poplar shelterbelts and farmlands of 15 regions within the NE China Plain. We measured soil organic carbon (SOC), inorganic carbon (SIC), nitrate nitrogen (NO3−-N), total N, available N, ammonia N (NH4+-N), total phosphorus (TP), available P, bulk density (BD), porosity, specific gravity, electrical conductivity (EC), water content (WC), pH, total Glomalin-related soil protein (TG) and easily extracted glomalin (EEG), soil microbial characteristics, and forest characteristics.The results showed that: 1) Pooled data collected from poplar shelterbelts decreased BD, TP and NO3−-N by 7%, 11% and 7%, and increased SIC by 16% (p < 0.05), respectively, in the 0–40 cm soil layer, and sharper NO3−-N decreases have been observed in the 40–100 cm soil layer soils. 2) Compared to farmlands, poplar shelterbelts decreased microbial α-diversity, and the undefined saprotroph-fungi abundance and microbial nitrification and denitrification by 9.25% to 11.67%, but increased ectomycorrhizal fungi dominance, complexity, and stability of microbial network compared with farmlands. 3) The partial least squares path model revealed that NO3−-N, TP, and BD changes were achieved by the direct effects of poplar growth and indirectly through soil microbial modification. At the same time, geo-climatic differences contributed to SIC changes. Poplar shelterbelts significantly affected soil properties in moist and warm areas, and the effects of shelterbelts on soil properties were more evident in the 0–40 cm layer than in the 40–100 cm layer.Our results highlight that, besides windbreaking functions, poplar shelterbelt's benefits for crop productivity should also include their effects on multiple soil properties into l m deep soils, soil microbial alteration, and associated with geoclimatic conditions.

Coastal embankments impact seasonal plant-soil nitrogen dynamics in a temperate intertidal Phragmites australis salt marsh

Coastal embankments are extensively employed in coastal salt marshes; however, knowledge is still lacking regarding the seasonal effects of embankments on nitrogen (N) storage in Phragmites australis salt marshes. Therefore, we surveyed live and dead aboveground N (AGN) storage, and belowground N (BGN) storage of plant system. Further, the soil total, recalcitrant, labile, and dissolved organic N (STON, SRON, SLON and SDON, respectively), ammonia (NH4+-N), nitrate (NO3−-N), and microbial biomass N (MBN) in soil system of embanked and neighbouring natural P. australis salt marshes were examined during the initial stage and vigorous stage of plant growth. Additionally, the soil physicochemical properties were quantified, as well as the microbial ammonization (RA) and nitrification (RN) rates. The embankment significantly strengthened the BGN storage during the initial (582.6%) and vigorous stage of plant growth (228.1%), and markedly decreased the dead AGN storage during the initial (70.3%) and vigorous stage of plant growth (65.5%). It also significantly amplified STON (51.9%) and SLON (58.1%) during the initial stage of plant growth, and STON (51.9%) and SRON (55.9%) during the vigorous stage of plant growth. However, the embankment had negligible effects on soil soluble N (e.g., SDON, NH4+-N and NO3−-N). Moreover, the microbial capacity of N immobilization (e.g., MBN) and mineralization (e.g., RN) were remarkably stimulated during the vigorous stage of plant growth due to the embankment. Our results implied that changes in physicochemical properties (e.g., soil moisture, electrical conductivity) due to the embankment were likely more suitable for the growth and survivability of P. australis during both the initial and vigorous stage of plant growth. Decreased N inputs from aboveground P. australis residues were offset by the increased N inputs from its immense complement of belowground tissues; thus, soil organic N storage was enhanced due to the embankment. Furthermore, soil soluble organic N was primarily derived from the AGN inputs of P. australis residues, which resisted decomposition, and the inorganic N that was mostly derived from soluble organic N. This meant that the soil soluble N was stabilized and not influenced by the embankment. Finally, due to weakening N demands by vegetation and additional soil N that microorganisms could utilize during the vigorous stage of plant growth, the microbial N immobilization and mineralization capacity strengthened. This study provides further comprehensive knowledge to elucidate the effects of coastal embankments on the N pool in P. australis salt marshes.

Early spring snowmelt and summer droughts strongly impair the resilience of bacterial community and N cycling functions in a subalpine grassland ecosystem

Subalpine grasslands support biodiversity, agriculture, and tourism but their resilience to extreme climatic events is challenged accelerating their vulnerability to tipping points. Microbial communities, central in ecosystem functioning, are usually considered more resistant and highly resilient to extreme events albeit their functional redundancy and strong selection by local harsh climatic conditions. This study explored the soil microbial responses upon recurrent spring‐summer droughts associated with early snowmelt in subalpine grasslands mesocosms set‐up at the Lautaret Pass (French Alps). Potential soil microbial respiration, nitrification and denitrification activities were monitored over a period of two growing seasons along with quantification of related gene abundances. Impacts of simulated spring‐summer drought and early snowmelt were quantified to assess their resistance and recovery. Results revealed that droughts had a low and short‐term adverse impact on bacterial total respiration supporting their hypothesized high resilience, i.e. resistance and ability to recover. Nitrification and abundances of the corresponding functional guilds showed relatively strong resistance to summer droughts but declined in response to early snowmelt. This resistance of nitrification was paralleled by the recovery of denitrification and abundances of denitrifying communities from all climatic extremes, except from the summer droughts where nitrifiers were collapsed. Denitrification and respective functional groups faced high impact of applied stresses with strong reduction in abundance and activity. Although, consequently lower denitrifiers' competition for nitrate may be positive for plant biomass production, warnings exist when considering the potential nitrate leaching as well as risks of greenhouses gases emission such as N2O from these ecosystems.

Hot spring distribution and survival mechanisms of thermophilic comammox Nitrospira

The recent discovery of Nitrospira species capable of complete ammonia oxidation (comammox) in non-marine natural and engineered ecosystems under mesothermal conditions has changed our understanding of microbial nitrification. However, little is known about the occurrence of comammox bacteria or their ability to survive in moderately thermal and/or hyperthermal habitats. Here, we report the wide distribution of comammox Nitrospira in five terrestrial hot springs at temperatures ranging from 36 to 80°C and provide metagenome-assembled genomes of 11 new comammox strains. Interestingly, the identification of dissimilatory nitrate reduction to ammonium (DNRA) in thermophilic comammox Nitrospira lineages suggests that they have versatile ecological functions as both sinks and sources of ammonia, in contrast to the described mesophilic comammox lineages, which lack the DNRA pathway. Furthermore, the in situ expression of key genes associated with nitrogen metabolism, thermal adaptation, and oxidative stress confirmed their ability to survive in the studied hot springs and their contribution to nitrification in these environments. Additionally, the smaller genome size and higher GC content, less polar and more charged amino acids in usage profiles, and the expression of a large number of heat shock proteins compared to mesophilic comammox strains presumably confer tolerance to thermal stress. These novel insights into the occurrence, metabolic activity, and adaptation of comammox Nitrospira in thermal habitats further expand our understanding of the global distribution of comammox Nitrospira and have significant implications for how these unique microorganisms have evolved thermal tolerance strategies.

Influence of carbon-based cathodes on biofilm composition and electrochemical performance in soil microbial fuel cells

Increasing energy demands and environmental pollution concerns press for sustainable and environmentally friendly technologies. Soil microbial fuel cell (SMFC) technology has great potential for carbon-neutral bioenergy generation and self-powered electrochemical bioremediation. In this study, an in-depth assessment on the effect of several carbon-based cathode materials on the electrochemical performance of SMFCs is provided for the first time. An innovative carbon nanofibers electrode doped with Fe (CNFFe) is used as cathode material in membrane-less SMFCs, and the performance of the resulting device is compared with SMFCs implementing either Pt-doped carbon cloth (PtC), carbon cloth, or graphite felt (GF) as the cathode. Electrochemical analyses are integrated with microbial analyses to assess the impact on both electrogenesis and microbial composition of the anodic and cathodic biofilm. The results show that CNFFe and PtC generate very stable performances, with a peak power density (with respect to the cathode geometric area) of 25.5 and 30.4 mW m−2, respectively. The best electrochemical performance was obtained with GF, with a peak power density of 87.3 mW m−2. Taxonomic profiling of the microbial communities revealed differences between anodic and cathodic communities. The anodes were predominantly enriched with Geobacter and Pseudomonas species, while cathodic communities were dominated by hydrogen-producing and hydrogenotrophic bacteria, indicating H2 cycling as a possible electron transfer mechanism. The presence of nitrate-reducing bacteria, combined with the results of cyclic voltammograms, suggests microbial nitrate reduction occurred on GF cathodes. The results of this study can contribute to the development of effective SMFC design strategies for field implementation.

Short Review of Self-Powered Nitrogen Removal via Abiotic Electrochemical Catalysis

Microbial nitrification and denitrification are efficient technologies for the treatment of nitrogen-containing wastewater. However, these biotic technologies are inapplicable for the treatment of toxic substances such as heavy metals, polyaromatic hydrocarbons, adsorbable organic halogens, and polychlorinated biphenyls, which have an inhibitory effect on microbial metabolism. It is therefore necessary to develop abiotic nitrogen removal technology with comparable cost efficiency. Nitrogen contaminants are promising indirect fuel sources. The integration of electrocatalysis energy conversion with nitrogen contaminants could drive an entire electrochemical system to obtain nitrogen removal in a self-powered fashion. Research advances in the development of fuel cells have corroborated their promising application for nitrogen removal. This work aims to review the most recent advances in the utilization of ammonia and nitrate as fuels for self-powered nitrogen removal and demonstrate how close this technology is to integration with future applications. The mechanism of ammonia–oxygen fuel cells is first summarized, followed by an overview of recent research on self-powered systems based on various noble-metal-free catalysts. We then introduce different harvesting and conversion methods using nitrate with a desired power output and nitrogen removal efficiency. The final section demonstrates the shortcomings of research and future innovative perspectives for self-powered wastewater treatment.

Petrography, mineralogy, and geochemistry of nitrogenous shale in the Western Desert of Egypt: implications for paleoenvironment

Besides nitrate deposits located in the Atacama Desert of Chile and the Mojave Desert of California, the present work documents, for the first time, the occurrence of potassium nitrate as oval-suboval-shaped aggregations associated with the Late Maastrichtian-Early Paleocene Dakhla Shale and the Paleocene-Early Eocene Esna Shale encountered in the northeastern part of the Kharga Oasis, particularly at G. Um El-Ghanayem and G. Ghaneima. Consequently, integrated petrographic, mineralogical, and geochemical investigations were carried out for shale deposits and nitrate salts to reconstruct the paleoenvironmental conditions of shale, reveal the extent to which nitrate salts are genetically related to the paleoenvironment of shale deposits, and build up a complete scenario about the source and the formation mechanism of nitrate salts. The overall results showed that the studied shale deposits were sourced from mafic igneous and quartzose sedimentary provenances where humid climatic conditions were dominant; the transported detrital particles were then settled down under oxidizing bottom water and shallow depositional conditions. Moreover, the nitrate salts are of an epigenetic origin and sourced from the microbial nitrification of organic matter and the wet atmospheric deposition that is believed to be triggered by the active volcanic eruptions during the Late Eocene/Early Oligocene transition where warm climatic conditions prevailed.

Activation and tolerance of Siegesbeckia Orientalis L. rhizosphere to Cd stress.

This experiment investigated the changes of rhizosphere soil microenvironment for hyperaccumulation-soil system under Cd stress in order to reveal the mechanism of hyperaccumulation and tolerance. Thus, Cd fractions, chemical compositions, and biochemical characteristics in rhizosphere soil of Siegesbeckia orientalis L. under Cd stress conditions of 0, 5, 10, 25, 50, 100, and 150 mg kg-1 were investigated through a root bag experiment, respectively. As a result, Cd induced the acidification of S. orientalis rhizosphere soil, and promoted the accumulation of dissolved organic carbon (DOC) and readily oxidizable organic carbon (ROC), which increased by 28.39% and 6.98% at the maximum compared with control. The percentage of labile Cd (acid-soluble and reducible Cd) in soil solution increased significantly (P < 0.05) from 31.87% to 64.60% and from 26.00% to 34.49%, respectively. In addition, rhizosphere microenvironment can alleviate the inhibition of Cd on soil microorganisms and enzymes compare with bulk soils. Under medium and low concentrations of Cd, the rhizosphere soil microbial biomass carbon (MBC), basal respiration, ammonification and nitrification were significantly increased (P < 0.05), and the activities of key enzymes were not significantly inhibited. This suggests that pH reduction and organic carbon (DOC and ROC) accumulation increase the bioavailability of Cd and may have contributed to Cd accumulation in S. orientalis. Moreover, microorganisms and enzymes in rhizosphere soils can enhance S. orientalis tolerance to Cd, alleviating the nutrient imbalance and toxicity caused by Cd pollution. This study revealed the changes of physicochemical and biochemical properties of rhizosphere soil under Cd stress. Rhizosphere soil acidification and organic carbon accumulation are key factors promoting Cd activation, and microorganisms and enzymes are the responses of Cd tolerance.

Microbial nitrate reduction in propane- or butane-based membrane biofilm reactors under oxygen-limiting conditions

Nitrate contamination has been commonly detected in water environments and poses serious hazards to human health. Previously methane was proposed as a promising electron donor to remove nitrate from contaminated water. Compared with pure methane, natural gas, which not only contains methane but also other short chain gaseous alkanes (SCGAs), is less expensive and more widely available, representing a more attractive electron source for removing oxidized contaminants. However, it remains unknown if these SCGAs can be utilized as electron donors for nitrate reduction. Here, two lab-scale membrane biofilm reactors (MBfRs) separately supplied with propane and butane were operated under oxygen-limiting conditions to test its feasibility of microbial nitrate reduction. Long-term performance suggested nitrate could be continuously removed at a rate of ∼40–50 mg N/L/d using propane/butane as electron donors. In the absence of propane/butane, nitrate removal rates significantly decreased both in the long-term operation (∼2–10 and ∼4–9 mg N/L/d for propane- and butane-based MBfRs, respectively) and batch tests, indicating nitrate bio-reduction was driven by propane/butane. The consumption rates of nitrate and propane/butane dramatically decreased under anaerobic conditions, but recovered after resupplying limited oxygen, suggesting oxygen was an essential triggering factor for propane/butane-based nitrate reduction. High-throughput sequencing targeting 16S rRNA, bmoX and narG genes indicated Mycobacterium/Rhodococcus/Thauera were the potential microorganisms oxidizing propane/butane, while various denitrifiers (e.g. Dechloromonas, Denitratisoma, Zoogloea, Acidovorax, Variovorax, Pseudogulbenkiania and Rhodanobacter) might perform nitrate reduction in the biofilms. Our findings provide evidence to link SCGA oxidation with nitrate reduction under oxygen-limiting conditions and may ultimately facilitate the design of cost-effective techniques for ex-situ groundwater remediation using natural gas.

Co-inoculation of antagonistic Bacillus velezensis FH-1 and Brevundimonas diminuta NYM3 promotes rice growth by regulating the structure and nitrification function of rhizosphere microbiome.

Microbial inoculation with plant growth-promoting microorganisms (PGPMs) is one of the most promising technologies to solve the current global challenges. Co-inoculants is more efficient and stable than mono-inoculants. However, the growth promoting mechanism of co-inoculants in complex soil system is still poorly understood. In this study, the effects on rice, soil and the microbiome of the mono-inoculant Bacillus velezensis FH-1 (F) and Brevundimonas diminuta NYM3 (N) and the co-inoculant FN obtained in previous works were compared. Correlation analysis and PLS-PM were used to explore the primary mechanism of different inoculants promoting rice growth. We hypothesized that inoculants promoted plant growth (i) by themselves, (ii) by improving soil nutrient availability or (iii) by regulating the rhizosphere microbiome in complex soil system. We also assumed that different inoculants had different ways of promoting plant growth. The results showed that FN significantly promoted rice growth and nitrogen absorption and slightly increased soil total nitrogen and microbial network complexity compared with F, N and the control (CK). B. velezensis FH-1 and B. diminuta NYM3 interfered with each other's colonization in FN. FN increased the complexity of the microbial network compared to F and N. The bacterial community of FN was quite different from CK and N, while the fungal community was not significantly different from other treatments. The species and functions enriched or inhibited by FN are part of F. The correlation analysis and PLS-PM results showed that inoculants (F/N/FN) promoted the growth of rice mainly by regulating the rhizosphere microbiome rather than by themselves or by improving soil nutrient availability. Co-inoculant FN promotes rice growth specifically by enhancing microbial nitrification function through enriching related species compared with F or N. This may provide theoretical guidance for the construction and application of co-inoculants in the future.

Scattered wild pistachio trees profoundly modify soil quality in semi-arid woodlands

Scattered wild pistachio trees (Pistacia atlantica Dest.) support various ecological functions and services, including soil conservation, in semi-arid woodlands of the Irano-Turanian zone, but over-exploitation puts the populations at risk. The current paucity of information precludes a thorough assessment of the effect of wild pistachio trees on soil quality. Therefore, we quantified their role in soil quality of semi-arid woodlands dominated by a Pistachio-Amygdalus community. At three sites in the southern Irano-Turanian zone, we contrasted moisture, pH, electrical conductivity (EC), organic carbon (SOC), total nitrogen (TN) and nutrient concentrations of bare soil with below-crown soil. In addition, we examined soil microbial respiration (Rmic), substrate-induced respiration (SIR), microbial biomass carbon (Cmic) and nitrification potential to evaluate the influence of pistachio trees on soil microbial activity. Soil pH and C/N ratio were similar between bare and below-crown soils but significantly greater soil moisture (+41 %), EC (+13 %), SOC (+82 %) and TN (+75 %) were observed below pistachio crowns. Most soil nutrients were more abundant beneath tree crowns, especially zinc and manganese. Soil microbial activity was greater below crowns compared to bare soil as evidenced by significantly elevated Rmic (+11 %), SIR (+86 %), Cmic (+61 %) and nitrification potential (+49 %). SOC-normalization of microbial variables revealed that a SOC-driven increase in Cmic was largely responsible for the enhanced soil microbial activity below crowns. Microbial communities in bare soil showed a higher metabolic quotient (Rmic/Cmic) than in below-crown soil, suggesting low microbial efficiency and higher metabolic costs. Our findings indicate that the soils below wild pistachio crowns represent fertility islands, where soil quality and microbial activity are supported by an increased supply of organic matter and more favourable soil moisture conditions. The few woodlands that have remained relatively intact in this area urgently require conservation action while restoration planting of wild pistachio trees in degraded areas may help restore soil quality.

Reactive and microbial inhibitory mechanisms depicting the panoramic view of pH stress effect on common biological nitrification

pH is a crucial factor of microbial nitrification, which often combines with high-strength ammonium to influence nitrogen removal pathway in wastewater treatment. However, the detailed inhibitory mechanisms of pH stress are not sufficiently disclosed yet. In this study, the pH stress effect on nitrification was comprehensively studied by a set of experiments which identified the reactivity of nitrification processes and activity of nitrifiers, the time dependence of inhibition effect and the hybrid pH stress effect with ammonium. The results revealed two distinct inhibitory mechanisms dominating in alkaline and acid ranges. In alkaline range (pH > 8), pH stress causes physiological damages on microorganisms which is named as microbial inhibition. It has the features of less recoverability of nitrifiers, time-dependent inhibition effect and low pH-tolerance of nitrite oxidation bacteria. Free ammonia enhanced microbial inhibition and greatly promoted nitrite accumulation. A novel reactive inhibition mechanism dominated in acid range (pH < 7) was disclosed. It only impedes ammonia oxidation process (AOP) but not impair microbial activity obviously and the effect is time-independent. The mechanism was clarified from H+ transport because AOP involved H+ production. The H+ transport was impeded under acid stress owing to the decrease of pH gradient across cell membrane. The two mechanisms formed a panoramic view of pH stress effect on nitrification advancing the understanding of nitrifier adaptability and nitritation regulation in wastewater treatment processes.

Mitigating nitrate leaching in cropland by enhancing microbial nitrate transformation through the addition of liquid biogas slurry

Nitrate leaching in croplands with excessive chemical fertilizer application is a serious issue for sustainable agriculture. Microbes are the primary drivers of nitrate transformation in the soil and are considered a potential solution for nitrogen leaching. However, practical approaches are lacking. In this study, the effects of the addition of biogas slurry on nitrogen leaching and soil microbial communities were investigated. Column incubation experiments showed that the addition of biogas slurry at a depth of 20 cm significantly decreased nitrate leaching by 30.97% and shaped a bacterial community rich in taxa involved in nitrate ammonification and denitrification. Using a mechanized application method, the field experiment showed that biogas slurry application significantly increased the abundance of nirK-, nirS-, nosZ-, and nrfA-harboring microbes and decreased the risk of nitrate leaching without reducing crop yield and the soil nitrogen pool. These results showed that the appropriate application of biogas slurry effectively mitigates nitrogen leaching by enhancing soil microbial nitrate transformation and confirmed the feasibility of solving environmental problems by regulating the microbial community and function.

Biogeochemical Permeable Barrier Based on Zeolite and Expanded Clay for Immobilization of Metals in Groundwater

Groundwater samples contaminated with potentially toxic elements (PTE), including metals and nitrate ions, were collected at a depth of 8–10 m from the Siberian Chemical Plant multicomponent waste storage. The possibility of developing a permeable biogeochemical barrier with zeolite and lightweight expanded clay aggregate (LECA) was investigated. The mass fraction and properties of several metals (Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg and Pb) were determined to investigate their fixation on the chosen materials at the given experimental conditions. It was established that metals in sulfide or phosphate forms can be effectively immobilized via biomineralization on LECA, whereas metals from the non-chalcogen group are primarily retained in the form of phosphates. The formation of biogenic deposits of iron sulfide, which serve as a sorption–precipitation phase during the immobilization of the majority of metals, is an important aspect of the LECA loading process. The use of LECA and zeolite in the form of a two-component barrier is feasible based on the data obtained. It is assumed that metal immobilization processes occur due to sorption mechanisms in the zone of zeolite loading. Microbial nitrate removal and the formation of iron sulfide phases under reducing conditions, which form a geochemical barrier for metals, are expected in the LECA zone.

Effects of microplastics on nitrogen and phosphorus cycles and microbial communities in sediments

Sediments are the long-term sinks of microplastics (MPs) and nutrients in freshwater ecosystems. Therefore, understanding the effect of MPs on sediment nutrients is crucial. However, few studies have discussed the effects of MPs on nitrogen and phosphorus cycles in freshwater sediments. Herein, 0.5% (w/w) polyvinyl chloride (PVC), polylactic acid (PLA), and polypropylene (PP) MPs were added to freshwater sediments to evaluate their effects on microbial communities and nitrogen and phosphorus release. The potential biochemical functions of the bacterial communities in the sediments were predicted and assessed via 16S rRNA gene sequencing. The results showed that MPs significantly affected the microbial community composition and nutrient cycling in the sediments. PVC and PP MPs can promote microbial nitrification and nitrite oxidation, while PP can significantly promote alkaline phosphatase (ALP) activity and the abundance of the phosphorus-regulation (phoR) gene. PLA MPs had the potential to promote the abundance of microbial phosphorus transporter (ugpB), nitrogen fixation (nifD, nifH, and nifX), and denitrification (nirS, napA, and norB) genes and inhibit nitrification, resulting in massive accumulation and release of ammonia nitrogen. Although PLA MPs inhibited the activity of ALP and the abundance of the organophosphorus mineralization (phoD) gene, it could enhance dissimilatory iron and sulfite reduction, which may promote the release of sedimentary phosphorus. Our findings may help understand the mechanisms of nitrogen and phosphorus cycles and microbial communities driven by MPs in sediments and provide a basis for future assessments of the environmental behavior of MPs in freshwater ecosystems.

Characteristics and influencing factors of microbial function in soils around a typical mining smelter

Soil microorganisms were sensitive to heavy metal pollution, whose ecological effect on soil microbial community was impacted by the interaction of contaminated stresses and environmental factors. To explore the dominant factors governing those effects in heavy metal contaminated soil, field investigation was conducted for soil from different land use types in an area surrounding a typical mining smelter in Hunan Province. Soil microbial function parameters including microbial biomass carbon (MBC), basal respiration (BR), substrate-induced respiration (SIR) and nitrification potential (PNR) were used as measure endpoints for ecological effect to reflect soil carbon and nitrogen cycling. The results showed that the effect of land use on MBC, BR, and SIR was insignificant. The dominant impacting factors on microbial functions included CaCl2 extracted Pb (CaCl2-Pb) and soil organic matter (SOM) content. Results of multiple regression analysis showed that soil CaCl2-Pb and SOM together explained 39.8%-58.3% of the total variations of BR, SIR and PNR in soil, when CaCl2-Pb and SOM ranged in 0.004-13.14 mg·kg-1 and 0.24%-4.34%, respectively. Significantly quantitative exposure-effect equations were developed between the responses of soil BR, SIR and PNR and soil CaCl2-Pb and SOM when soil samples with medium contents (namely, SOM 1.70%～2.36% and CaCl2-Pb 0.004-12.98 mg·kg-1), which meant they could be used to quantitatively assess the ecological effect of heavy metals on microbial community function as measure endpoints.

Spatiotemporal variability and control factors of NO3− in a polluted karst water system of an agricultural wetland in South China

Nitrate (NO3−) pollution in karst water is an important environmental issue in intensive agricultural regions worldwide. The integrated understanding of the spatiotemporal variability and control factors of NO3− pollution in karst water is imperative for controlling the diffuse pollution caused by agricultural activities. In this study, 49 water samples were collected from surface water (SW) and groundwater (GW) in the Huixian karst wetland (HKW) and analyzed using hydrogeochemical and isotopic data (δ18O–NO3–, δ15N–NO3– and δ13CDIC) in combination with a Bayesian mixing model to investigate the spatiotemporal distribution and control factors in NO3−-polluted karst water. The results showed that approximately 40.82% of the karst water samples exceeded the natural threshold value of 3 mg/L for NO3−-N, and 32.14% of the GW samples exceeded the permissible limit for drinking water established by WHO (10 mg/L as NO3−-N), indicating that high levels of NO3− were mainly found in GW samples from the agricultural core area, especially in the dry season. The NH4+-synthetic fertilizer (NHF) and soil organic nitrogen (SON) were the dominant factors controlling pollution sources in the HKW, accounting for 36.13% ± 4.66% and 28.68% ± 4.75% of the karst GW NO3− concentration, respectively. However, the seasonal differences in NO3− pollution sources were not significant in GW. Microbial nitrification was the main process affecting the NO3− levels in GW, whereas the occurrence of denitrification did not significantly affect NO3− concentration in the HKW due to the relatively low rate. Moreover, the HNO3 produced from NH4+ via microbial nitrification facilitated carbonate weathering, thereby controlling NO3− enrichment in karst GW. Our results suggest that NHF should be controlled to prevent further GW pollution in the HKW. Our study also provides a scientific basis for understanding the factors controlling the NO3− concentrations in karst water systems.