Nitrite Reductase Activity Research Articles

Combined application of asparagine and thiourea improves tolerance to lead stress in wheat by modulating AsA-GSH cycle, lead detoxification and nitrogen metabolism

Lead (Pb), like other heavy metals, is not essentially required for optimal plant growth; however, plants uptake it from the soil, which poses an adverse effect on growth and yield. Asparagine (Asp) and thiourea (Thi) are known to assuage the negative impacts of heavy metal pollution on plant growth; however, combined application of Asp and Thi has rarely been tested to discern if it could improve wheat yield under Pb stress. Thus, this experimentation tested the role of individual and combined applications of Asp (40 mM) and Thi (400 mg/L) in improving wheat growth under lead (Pb as PbCl2, 0.1 mM) stress. Lead stress significantly reduced plant growth, chlorophyll contents and photosystem system II (PSII) efficiency, whereas it increased Pb accumulation in the leaves and roots, leaf proline contents, phytochelatins, and oxidative stress related attributes. The sole or combined application of Asp and Thi increased the vital antioxidant biomolecules/enzymes, including reduced glutathione (GSH), ascorbic acid (AsA), ascorbate peroxsidase (APX), catalase (CAT), superoxide dismutase (SOD), glutathione S-transferase (GST), dehydroascorbate reductase (DHAR), and glutathione reductase (GR). Furthermore, the sole or the combined application of Asp and Thi modulated nitrogen metabolism by stimulating the activities of nitrate and nitrite reductase, glutamate synthase (GOGAT) and glutamine synthetase (GS). Asp and Thi together led to improve plant growth and vital physiological processes, but lowered down Pb accumulation compared to those by their sole application. The results suggest that Asp and Thi synergistically can improve wheat growth under Pb-toxicity.

Effective mechanisms of water purification for nitrogen-modified attapulgite, volcanic rock, and combined exogenous microorganisms.

The study tested the water purification mechanism of the combination of microorganisms and purification materials via characteristic, enzymatic, and metagenomics methods. At 48 h, the removal rates of total nitrogen, total phosphorous, and Mn chemical oxygen demand in the combination group were 46.91, 50.93, and 65.08%, respectively. The alkaline phosphatase (AKP) activity increased during all times tested in the volcanic rock, Al@TCAP, and exogenous microorganism groups, while the organophosphorus hydrolase (OPH), dehydrogenase (DHO), and microbial nitrite reductase (NAR) activities increased at 36-48, 6-24, and 36-48 h, respectively. However, the tested activities only increased in the combination groups at 48 h. Al@TCAP exhibits a weak microbial loading capacity, and the Al@TCAP removal is primarily attributed to adsorption. The volcanic rock has a sufficient ability to load microorganisms, and the organisms primarily perform the removal for improved water quality. The predominant genera Pirellulaceae and Polynucleobacter served as the sensitive biomarkers for the treatment at 24, 36-48 h. Al@TCAP increased the expression of Planctomycetes and Actinobacteria, while volcanic rock increased and decreased the expression of Planctomycetes and Proteobacteria. The growth of Planctomycetes and the denitrification reaction were promoted by Al@TCAP and the exogenous microorganisms. The purification material addition group decreased the expression of Hyaloraphidium, Chytridiomycetes (especially Hyaloraphidium), and Monoblepharidomycetes and increased at 36-48 h, respectively. Ascomycota, Basidiomycota, and Kickxellomycota increased in group E, which enhanced the nitrogen cycle through microbial enzyme activities, and the growth of the genus Aspergillus enhanced the phosphorous purification effect.

Biological nitrogen removal and metabolic characteristics of a novel cold-resistant heterotrophic nitrification and aerobic denitrification Rhizobium sp. WS7

For improving the poor de-nitrogen efficiency and effluent quality faced by wastewater treatment plants in winter, a novel cold-resistant strain, Rhizobium sp. WS7 was isolated. Strain WS7 presented dramatic de-nitrogen efficiencies including 98.73 % of NH4+-N, 99.98 % of NO3–-N, 100 % of NO2–-N and approximately 100 % of mixed nitrogen (NH4+-N and NO3–-N) at 15 °C. Optimum parameters of WS7 for aerobic denitrification were determined. Additionally, functional genes (amoA, napA, nirK, norB, and nosZ) and key enzymes (nitrate reductase and nitrite reductase) activities were determined. Nitrogen balance analysis suggested that assimilation played a dominant role in de-nitrogen by WS7, the NH4+-N metabolic pathway was deduced as NH4+-N → NH2OH → NO → N2O → N2, and the NO3–-N/NO2–-N metabolic pathway was deduced as NO3–-N → NO2–-N → NO → N2O → N2. The cold-resistant Rhizobium sp. WS7 has great application feasibility in cold sewage treatment.

Stimulation of Akt Phosphorylation and Glucose Transport by Metalloporphyrins with Peroxynitrite Decomposition Catalytic Activity.

Iron porphyrin molecules such as hemin and iron(III) 4,4',4″,4‴-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) (FeTBAP) have previously been shown to influence insulin signaling and glucose metabolism. We undertook this study to determine whether a catalytic action of iron porphyrin compounds would be related to their stimulation of insulin signaling and glucose uptake in C2C12 myotubes. FeTBAP did not display nitrite reductase activity or alter protein S-nitrosylation in myotubes, eliminating this as a candidate mode by which FeTBAP could act. FeTBAP displayed peroxynitrite decomposition catalytic activity in vitro. Additionally, in myotubes FeTBAP decreased protein nitration. The peroxynitrite decomposition catalyst Fe(III)5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato chloride (FeTPPS) also decreased protein nitration in myotubes, but the iron porphyrin Fe(III)tetrakis(1-methyl-4-pyridyl)porphyrin pentachlorideporphyrin pentachloride (FeTMPyP) did not. FeTBAP and FeTPPS, but not FeTMPyP, showed in vitro peroxidase activity. Further, FeTBAP and FeTPPs, but not FeTMPyP, increased Akt phosphorylation and stimulated glucose uptake in myotubes. These findings suggest that iron porphyrin compounds with both peroxynitrite decomposition activity and peroxidase activity can stimulate insulin signaling and glucose transport in skeletal muscle cells.

Response mechanism of microorganisms to the inhibition of endogenous pollution release by calcium peroxide

To further explore the response mechanism of microorganisms to the synchronous control of nitrogen and phosphorus release from sediments by CaO2, the spatiotemporal changes in the physical, chemical and biological indicators of the overlying water, interstitial water and sediments in each reactor were measured in the experiment. The experiment results showed that CaO2 could increase the ammonia monooxygenase activity, nitrite oxidase activity and Nitrospira abundance in the sediment near its dosing position, and enhanced the activities of nitrate reductase and nitrite reductase at a certain distance from the dosing position, thereby promoting nitrogen removal in sediments through the alternating process of nitrification and denitrification. At the same time, the increase of alkaline phosphatase activity and Saccharimonadales abundance in the test groups accelerated the hydrolysis of organic phosphorus, and the P immobilization in sediments was realized through the subsequent precipitation reaction of Ca2+ and PO43− under alkaline conditions. In addition, the enhanced activities of dehydrogenase and catalase ensured that CaO2 would not cause great killing effect on microorganisms when improving the hypoxic conditions and inhibiting endogenous release. As a result, the dissolved product of CaO2 such as Ca(OH)2 and H2O2 reduced the nutrients concentration and killed the algae, which kept the algae density and chlorophyll a concentration at a low level throughout the test groups. Therefore, this study systematically clarified the microbial mechanism of CaO2 synchronously controlling the release of nitrogen and phosphorus from sediments, which provided a new idea for the remediation of endogenous pollution in the water system.

Scrutinizes the Sustainable Role of Halophilic Microbial Strains on Oxygen-Evolving Complex, Specific Energy Fluxes, Energy Flow and Nitrogen Assimilation of Sunflower Cultivars in a Suboptimal Environment.

Environmental extremes such as hypersaline conditions are significant threats to agricultural productivity. The sustainable use of halophilic microbial strains was evaluated in plant in a salt stress environment. Oxygen-evolving complex (OEC), energy compartmentalization, harvesting efficiencies (LHE), specific energy fluxes (SEF), and nitrogen assimilation of oilseed crops (Sunflower cultivars) in a suboptimal environment was examined. Plants were grown in a plastic pot (15 ×18 cm2) containing sterilized (autoclaved at 120°C for 1 h) soil. Twenty-five ml suspension (107 CFU/ml) each of Bacillus cereus strain KUB-15 and KUB-27 (accession number NR 074540.1) and Bacillus licheniformis strain AAB9 (accession number MW362506), were applied via drenching method. Month-old plants were subjected to salt stress via gradual increment method. The energy compartmentalization of microbial inoculated plants exposed to salt stress revealed higher photosystem II (PSII) activity at the donor side, lesser photo-inhibition, and increased performance of oxygen-evolving complex compared to control. High potassium (K+) and low sodium (Na+) ions in treated leaves with the activated barricade of the antioxidant system stimulated by Bacillus strains favored enhanced photochemical efficiency, smooth electron transport, and lesser energy dissipation in the stressed plants. Moreover, the results reveal the increased activity of nitrite reductase (NiR) and nitrate reductase (NR) by microbial inoculation that elevated the nitrogen availability in the salt-stressed plant. The current research concludes that the application of bio-inoculants that reside in the hyper-saline environment offers substantial potential to enhance salt tolerance in sunflowers by modulating their water uptake, chlorophyll, nitrogen metabolism, and better photochemical yield.

Self-assembled binary structures of Fe(III) and metal-free porphyrins for solar light-assisted catalytic hydrogenation of 4-nitrophenol

Eco-sustainable reduction reactions using nontoxic and non-noble metal-based materials can be inspired on biomimetic catalysis although their development remains a challenge. In this work, nanostructured materials carrying Fe(III) porphyrins have been prepared by ionic self-assembly in water and tested in the mimics of the six electron reduction activity of nitrite reductase, in the presence of NaBH4. A structuring effect is observed in solar light-assisted conditions. For materials built with two oppositely charged Fe(III) porphyrins, the increase in catalytic activity was temperature-dependent, whereas in a material built with a metal-free porphyrin and a Fe(III) porphyrin, the activity was dependent on visible-light irradiation. This latter material shows the highest catalytic activity after an induction time, with K1 = 388 min−1·g−1 and a good performance after reuse studies.

Cloning of Nitrate Reductase and Nitrite Reductase Genes and Their Functional Analysis in Regulating Cr(VI) Reduction in Ectomycorrhizal Fungus Pisolithus sp.1.

Assimilatory-type nitrate reductase (NR) and nitrite reductase (NiR) are the key enzymes that involve in nitrate assimilation and nitrogen cycling in microorganisms. NR and NiR with NADH or NADPH and FMN or FAD domains could be coupled to the reduction process of hexavalent chromium [Cr(VI)] in microorganisms. A new assimilatory-type NR gene (named niaD) and a new assimilatory-type NiR gene (named niiA) are cloned, identified, and functionally characterized by 5′ and 3′ RACE, alignment, annotation, phylogenetic tree, and yeast mutant complementation analyses from Pisolithus sp.1, a dominant symbiotic ectomycorrhizal fungi (EMF) that can assist in phytoremediation. Assimilatory-type niaD and niiA were 2,754 bp and 3,468 bp and encode a polypeptide with 917 and 1,155 amino acid residues, respectively. The isoelectric points of NR (Pisolithus sp.1 NR) and NiR (Pisolithus sp.1 NiR) of Pisolithus sp.1 are 6.07 and 6.38, respectively. The calculated molecular mass of Pisolithus sp.1 NR and Pisolithus sp.1 NiR is 102.065 and 126.914 kDa, respectively. Yeast mutant complementation analysis, protein purification, and activities of NR and NiR under Cr treatment suggest that Pisolithus sp.1 NR is a functional NR that mediates Cr(VI) tolerance and reduction. The multiple alignment demonstrates that Pisolithus sp.1 NR is potentially a nicotinamide adenine dinucleotide phosphate-dependent flavin mononucleotide reductase and also Class II chromate reductase. Our results suggest that Pisolithus sp.1 NR plays a key role in Cr(VI) reduction in the EMF Pisolithus sp.1.

Synchronised interaction of carbon and nitrogen provides drought tolerance in Cyamopsis tetragonoloba

Drought is a major adverse environmental factor that limits plant productivity. Concomitant assimilation of carbon (C) and nitrogen (N) in illuminated leaves requires the regulated partitioning of reductant and photosynthate to sustain the demands of amino acid and carbohydrate biosynthesis. The short-term exposure to drought and its responses of photosynthesis and photosynthate partitioning to N enrichment was studied in Cyamopsis tetragonoloba (L.) Taub (guar). Four contrasting varieties of guar, HG-563, RGC-986 (drought-tolerant) and RGC-471, Varsha (drought-sensitive) were subjected to 15 days water stress and leaf tissues were studied in order to contemplate the counter strategy engaged under drought conditions. In guar plants, sensors trigger a signalling cascade that activates numerous proteins to induce the expression of certain genes involved in carbon and nitrogen metabolism.The levels of glucose, starch, and Glutamate dehydrogenase (GDH), Aspartate aminotransferase (AspAT), isocitrate dehydrogenase (ICDH), Nitrate Reductase (NR), and Nitrite reductase (NiR) activities were determined biochemically during 15 days drought treatment, at three stages. Further, the carbon (oxidative phosphorylation and TCA cycle) - nitrogen metabolic pathway interaction was studied at the molecular level. The respiratory pathway genes were upregulated, while the nitrogen metabolic pathway genes were downregulated under all stages of drought in guar plants. The physiological, biochemical and molecular data suggests that the high rates of dark respiration was insufficient to provide extra carbon required to assist amino acid synthesis but rather sufficed by a rapid metamorphosis in the photosynthetic carbon partitioning to amino acid metabolism via the nitrogen metabolism. • Drought causes NADH DEHYDROGENASE gene to escalate dark respiration. • Nitrogen assimilation reduces under drought in leguminous Cyamopsis tetragonoloba. • Drought triggers protein kinase cascade signalling in guar plants. • Drought regulates nitrogen metabolic genes in guar. • Drought triggers increase in sugar content at the cost of amino acids.

THE IMPACT OF MOLYBDENUM ON XANTHINE OXIDASE AND ITS ASSOCIATED ACTIVITIES IN OVINE MILK

"Xanthine oxidase is a molybdenum and iron containing flavoprotein, catalyzing the final oxidation stage of purines and oxidative transformation of pterins and some aliphatic and aromatic aldehydes. The results show that in fresh sheep milk xanthine oxidase does not contain molybdenum. Apparently, XO located in the inner membrane of the fat globule micelles (MFGM) is not available for exogenous molybdenum. During heat treatment at 80 °C, the milk globules are destroyed, then the molecule is denatured. As a result, the access of molybdenum to the MPT (‘molybdopterin' or metal-binding Pterin ene-1,2-dithiolate)-containing active center increases. Our and other numerous studies have shown that MPT is extremely sensitive to oxygen. Despite the importance of this enzyme, the distribution of xanthine oxidase in traditional household animal tissues is unknown. Formerly, we have found most of the xanthine oxidase molecules in animal milk are inactive because of lack of molybdenum. Ovine milk was processed by inserting in vivo molybdenum in drinking water. Heating the milk of animals at 80 °C for 5 minutes in the presence of molybdenum and cysteine led to a sharp increase of xanthine oxidase and it’s associated - nitrate reductase and nitrite reductase activities."

Application of low-intensity ultrasound to enhance simultaneous nitrification/iron-based autotrophic denitrification.

Intermittent ultrasound with an intensity of 0.2W/ml was applied during simultaneous nitrification/iron-based autotrophic denitrification to evaluate its impacts on total nitrogen (TN) removal efficiency and microbial characteristics during low carbon/nitrogen ratio (C/N) wastewater treatment. At an optimal dissolved oxygen (DO) concentration of 1.2mg/L, the TN removal rate was 91 ± 4%, and the TN concentration in the effluent decreased by 31% owing to the ultrasound treatment. In addition, the number of iron-compounds that formed in the sludge and wastewater increased by 55% and 37%, respectively. Low-intensity ultrasound caused a substantial increase in ammonia monooxygenase activity. Moreover, when the DO concentration increased to 1.2mg/L, the activities of nitrate reductase and nitrite reductase, both of which are associated with denitrification, were effectively maintained. High-throughput sequencing indicated that low-intensity ultrasound enriched ammonium oxidising bacteria (Nitrosomonas) and suppressed the growth of heterotrophic denitrifying bacteria (Zoogloea and Simplicispira). These changes benefited simultaneous nitrification and autotrophic denitrification. Thus, low-intensity ultrasound promoted the simultaneous nitrification/iron-based autotrophic denitrification process during low C/N wastewater treatment.

Magnetic Treatment Improves the Seedling Growth, Nitrogen Metabolism, and Mineral Nutrient Contents in Populus × euramericana ‘Neva’ under Cadmium Stress

This pot experiment was carried out to investigate the mechanism underlying nutrient metabolism and seedling growth responses to magnetic treatment following exposure to cadmium (Cd) stress. A magnetic device of 300 Gs was applied during Cd(NO3)2 solution treatment at 0 and 100 mM·L−1. One-year-old seedlings of Populus × euramericana ‘Neva’ were treated with different Cd(NO3)2 solutions in the presence or absence of magnetic treatment for 30 days. Seedling growth and physiological–biochemical indexes were measured under Cd stress. The contents of ammonium (NH4+–N), nitrate (NO3––N), and total nitrogen (TN) in leaves, as well as NH4+–N and TN in roots, were increased by magnetic treatment combined with Cd stress, although the NO3––N content was decreased. The activities of nitrate reductase (NR), nitrite reductase (NiR), glutathione reductase (GR), and glutamate synthase (GOGAT) in leaves and the activities of NR, glutamine synthetase (GS), and GOGAT in roots were stimulated by magnetic treatment; conversely, the NiR activity in roots was inhibited by magnetic effects. Magnetic treatment improved the synthesis of cysteine (Cys) and glutamine (Gln) in leaves and reduced the contents of glutamic acid (Glu) and glycine (Gly), while the contents of Cys, Glu, Gln, and Gly were increased in roots. The contents of Ca, Mg, Fe, Mn, Zn, and Cu in leaves were increased by magnetic treatment under Cd stress, whereas the content of K was reduced. In roots, the contents of K, Ca, and Fe were increased by magnetic treatment under Cd stress, but the contents of Na, Mg, Mn, Zn, and Cu were decreased. Magnetization could regulate the uptake of mineral nutrients by roots and translocation from the roots to the aboveground parts by affecting root morphology. Magnetic treatment could also improve nitrogen assimilation and the synthesis of free amino acids by stimulating the activities of key enzymes.

The Effects of 3,5-Dimethylpyrazole on Soil Nitrification and Related Enzyme Activities in Brown Soil

Heterocyclic nitrogen compounds containing two adjacent nitrogen atoms generally have a significant effect on soil nitrification inhibition, and 3,5-dimethylpyrazole (DMP) is a typical representative of this structure. However, the inhibitory effect and the regulatory mechanism of DMP on soil N transformation are unclear. In this study, a microcosm with different concentrations of DMP was carried out in brown soil to detect the dynamic changes of soil NH4+–N, NO3–N and related soil enzyme activities. Results showed that DMP inhibited soil nitrification effectively and decreased soil nitrate reductase activity, while increasing nitrite reductase and dehydrogenase activities. The inhibition effects were dose dependent, and DMP at the rate of 0.025 g kg−1 dry soil showed the strongest inhibitory effect on N transformation in brown soil. The soil dehydrogenase activity was increased with an increasing DMP application rate. The changes in the soil’s chemical and biological properties caused by DMP application provided a new idea for systematically explaining how DMP participated in the soil N transformation process. This study further implied that DMP would play positive roles in alleviating environmental pressure by delaying nitrate-N formation and decreasing the activity of nitrate reductase.

Nickel Toxicity Interferes with NO3-/NH4+ Uptake and Nitrogen Metabolic Enzyme Activity in Rice (Oryza sativa L.).

The excessive use of nickel (Ni) in manufacturing and various industries has made Ni a serious pollutant in the past few decades. As a micronutrient, Ni is crucial for plant growth at low concentrations, but at higher concentrations, it can hamper growth. We evaluated the effects of Ni concentrations on nitrate (NO3−) and ammonium (NH4+) concentrations, and nitrogen metabolism enzyme activity in rice seedlings grown in hydroponic systems, using different Ni concentrations. A Ni concentration of 200 μM significantly decreased the NO3− concentration in rice leaves, as well as the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthetase (GOGAT), respectively, when compared to the control. By contrast, the NH4+ concentration and glutamate dehydrogenase (GDH) activity both increased markedly by 48% and 46%, respectively, compared with the control. Furthermore, the activity of most active aminotransferases, including glutamic pyruvic transaminase (GPT) and glutamic oxaloacetic transaminase (GOT), was inhibited by 48% and 36%, respectively, in comparison with the control. The results indicate that Ni toxicity causes the enzymes involved in N assimilation to desynchronize, ultimately negatively impacting the overall plant growth.

Effects of biochar and biogas residue amendments on N2O emission, enzyme activities and functional genes related with nitrification and denitrification during rice straw composting

This study was carried out to determine the response characteristics of N2O emission, enzyme activities, and functional gene abundances involved in nitrification/denitirification process with biochar and biogas residue amendments during rice straw composting. The results revealed that N2O release mainly occurred during the second fermentation phase. Biogas residue amendment promoted N2O emission, while biochar addition decreased its emission by 33.6%. The nirK gene was abundant through composting process. Biogas residues increased the abundance of denitrification genes, resulting in further release of N2O. Biochar enhanced nosZ gene abundance and accelerated the reduction of N2O. Nitrate reductase (NR), nitrite reductase (NiR), N2O reductase (N2OR), and ammonia monooxygenase (AMO) activities were greatly stimulated by biochar or biogas residue rather than their combined addition. Pearson regression analysis indicated that N2O emission negatively correlated with ammonium and positively correlated with nosZ, nirK, 18S rDNA, total nitrogen, and nitrate (P < 0.05).

The effects of fullerene on photosynthetic apparatus, chloroplast-encoded gene expression, and nitrogen assimilation in Zea mays under cobalt stress.

Carbon nanostructures, such as the water-soluble fullerene (FLN) derivatives, are considered perspective agents for agriculture. FLN can be a novel nano-agent modulating plant response against stress conditions. However, the mechanism underlying the impacts of FLN on plants in agroecosystems remains unclear. Zea mays was exposed to exogenous C60 -FLN applications (FLN1: 100; FLN2: 250; and FLN3: 500 mg L-1 ) with/without cobalt stress (Co, 300 μM) for 3 days (d). In the maize chloroplasts, Co stress disrupted the photosynthetic efficiency and the expression of genes related to the photosystems (psaA and psbA). FLNs effectively improved the efficiency and photochemical reaction of photosystems. Co stress induced the accumulation of reactive oxygen species (ROS) as confirmed by ROS-specific fluorescence in guard cells. Co stress increased only chloroplastic superoxide dismutase (SOD) and peroxidase (POX). Stress triggered oxidative damages in maize chloroplasts, measured as an increase in TBARS content. In Co-stressed seedlings exposed to FLN1 and FLN2 exposures, the hydrogen peroxide (H2 O2 ) was scavenged through the nonenzymes/enzymes-related to the AsA-GSH cycle by preserving ascorbate (AsA) conversion, as well as GSH/GSSG and glutathione (GSH) redox state. Also, the alleviation effect of FLN3 against stress could be attributed to increased glutathione S-transferase (GST) activity and AsA regeneration. FLN applications reversed the inhibitory effects of Co stress on nitrogen assimilation. In maize chloroplasts, FLN increased the activities of nitrate reductase (NR), glutamate dehydrogenase (GDH), nitrite reductase (NiR), and glutamine synthetase (GS), which provided conversion of inorganic nitrogen (N) into organic N. The ammonium (NH4 + ) toxicity was removed via GS and GDH but not glutamate synthase (GOGAT). The increased NAD-GDH (deaminating) and NADH-GDH (aminating) activities indicated that GDH was needed more for NH4 + detoxification. Therefore, FLN exposure to Co-stressed maize plants might play a role in N metabolism regarding the partitioning of N assimilates. Exogenous FLN conceivably removed Co toxicity by improving the expressions of genes related to reaction center proteins of photosystems, increasing the level of enzymes related to the defense system, and improving the N assimilation in maize chloroplasts.

Inorganic Nitrogen Enhances the Drought Tolerance of Evergreen Broad-Leaved Tree Species in the Short-Term, but May Aggravate Their Water Shortage in the Mid-Term.

With global climate change, atmospheric nitrogen (N) deposition and drought have been well documented to cause substantial challenges for tropical and subtropical evergreen broad-leaved forests. Here, we conducted an experiment that measured the physiological responses of the seedlings of three dominant tree species (Tabebuia chrysantha, Elaeocarpus sylvestris, and Bischofia javanica) of the evergreen broad-leaved forests in South China under control (CT), drought stress (D), N addition (N), and drought stress plus N addition (N+D). We found that N addition significantly decreased malondialdehyde (MDA) content, abscisic acid (ABA) content, total antioxidant capacity (T-AOC), but significantly increased the content of proline (PRO), and the activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco), nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) in the three species under D. Meanwhile, we also found that under drought conditions, N addition promoted the leaf transpiration rate (E), stomatal conductance (gs), and light-saturated net photosynthetic rate (Amax) of the three species. These results indicate that N addition can enhance the drought tolerance of the three species by osmotic adjustment and protecting the photosystem. However, the enhancement in Amax and E will cause plants to face more severe drought conditions, especially B. javanica (large tree species). This study helps to explain why the evergreen broad-leaved forests in South China are gradually degrading to shrublands in recent decades.

Low irrigation water minimizes the nitrate nitrogen losses without compromising the soil fertility, enzymatic activities and maize growth

Nitrate nitrogen (NO3−_N) leaching increased with nitrogen (N) fertilization under high water supply to the field negatively affected the maize growth and performance. This study aimed to understand the mechanisms of NO3−_N leaching on a biochemical basis and its relationship with plant performance with 5 different doses (0, 200, 250, 300, 350 kg N ha− 1) of N fertilizers under low (60%; LW) and high (80%; HW) water holding capacity. Soil and plant enzymes were observed at different growth stages (V9, R1, R3, and R6) of the maize, whereas the leachates were collected at 10-days intervals from the sowing date. The LW had 10.15% lower NO3−_N leachate than HW, with correspondence increases in grain yield (25.57%), shoot (17.57%) and root (28.67%) dry matter. Irrespective of the irrigation water, RubisCo, glutamine synthase (GS), nitrate reductase (NR), nitrite reductase (NiR), and glutamate synthase (GOGAT) activities increased with increasing N fertilizer up to the V9 growth stage and decreased with approaching the maturity stage (R6) in maize. In HW irrigation, soil total N, GOGAT, soil nitrate (NO3−_N), leached nitrate (LNO3−_N), root N (RN), leaf N (LN) were positively correlated with N factors suggesting the higher losses of N through leaching (11.3%) compared to LW irrigation. However, the malondialdehyde (MDA), hydrogen peroxide (H2O2), superoxide (O2−), and proline were negatively correlated with the other enzymatic activities both under LW and HW irrigation. Thus, minimizing the NO3−_N leaching is possibly correlated with the LW and N300 combination without compromising the yield benefit and improving enzyme activities.

Improved lakeshore sediment microenvironment and enhanced denitrification efficiency by natural solid carbon sources

At present, the high level of nitrate in tailwater of wastewater treatment plants (WWTPs) has continued to cause serious environmental problems. In current study, wood chips and bark, serving as natural solid carbon source (NSCS), were applied to enhance the denitrification capacity of lakeshore sediment to remove superabundant nitrate in tailwater. The results show that NSCS had a positive effect on denitrification process for lakeshore sediment and increased the total phosphorus removal capacity by 212%–601%. Comparatively, wood chips were better in enhancing denitrification ability of lakeshore sediment for a longer time. At the 70th day, the nitrate elimination (NEL) of the wood chip-sediment integrated system (WSIS) was 20% and 9% higher than the control system (CS) and bark-sediment integrated system (BSIS), respectively. For the WSIS, the microbial activity and the abundance of nicotinamide adenine dinucleotide (NADH) were significantly increased, and the sediment anaerobic zone was rapidly formed, which were conducive to the denitrification of the sediment. At the DNA level, both groups presented an inconsistency in the abundance of functional denitrification genes with the NEL. At the enzyme level, wood chips significantly increased the nitrate and nitrite reductase activities of the sediment, and promoted denitrification. From the perspective of microbial communities, wood chips highly promoted the abundance of norank_f_BIrii41 Proteobacteria and Ruminclostridum more than other denitrifying and potential denitrifying bacteria in the process of sediment denitrification enhancement.

Promotion of anammox process by different graphene-based materials: Roles of particle size and oxidation degree

Graphene oxide (GO) and reduced graphene oxide (RGO) have been applied in the anaerobic ammonium oxidation (anammox) process for nitrogen removal as electron shuttles. However, there is still controversy about their efficacy. In this study, nine graphene-based materials with a gradient of three particle sizes (large (l), medium (m) and small (s) sizes) and oxidation degrees, were used to compare their effects on the anammox process efficiency. The graphene-based materials include GO and its reduced products (RGO250 and RGO800) obtained at temperatures of 250 °C and 800 °C respectively. It was observed that their enhancements on the anammox process were in the order of GO > RGO800 > RGO250. In detail, at the dose of 100 mg/L, specific anammox activities (SAA) were promoted by 6.7% (l-GO), 4.9% (l-RGO800), 11.5% (m-GO), 7.3% (m-RGO800), 13.2% (s-GO) and 8.3% (s-RGO800) compared to the control respectively; while RGO250 with the same dose inhibited the process. In addition, the enhancement of the anammox process was increasing with the decreasing size of GO and RGO800. The nitrite reductase (NIR) activity was greatly increased by up to 24.9% with the presence of GO, which might be attributed to organized and specific electron transport with oxygen functional groups. The finding of hydroxyl on RGO and increasing content of oxygen determined after reaction detected by Fourier transform infrared spectroscopy and energy dispersive spectrometer respectively, indicated the essential condition for RGO's function on transferring electrons for key enzymes in annamox bacteria. Most importantly, O/C (Oxygen/Carbon) ratios of graphene-based materials had greater effects on the promotion of the anammox process than the particle size and electrical conductivity.