Mechanisms of nitrite accumulation occurring in soilnitrification

Formation of brominated and nitrated byproducts during unactivated peroxymonosulfate oxidation of phenol

Brominated and nitrated byproducts generated from bromide (Br-) and nitrite (NO2-), respectively, by sulfate radical (SO4•-) oxidation have raised increasing concern. However, little is known about the concurrent generation of brominated and nitrated byproducts in the unactivated peroxymonosulfate (PMS) oxidation process. This study revealed that Br- can facilitate the transformation of NO2- to nitrated byproducts during unactivated PMS oxidation of phenol. In the co-existence of 0.1 mM Br- and 0.5 mM NO2-, the total yield of identified nitrated byproducts reached 2.316 μM in 20 min, while none was found with NO2- alone. Nitryl bromide (BrNO2) as the primary nitrating agent was formed via the reaction of NO2- with free bromine in situ generated through the oxidation of Br- by PMS. BrNO2 rapidly reacted with phenol or bromophenols, generating highly toxic nitrophenols or nitrated bromophenols, respectively. Increasing NO2- concentration led to more nitrated byproducts but less brominated byproducts. This study advances our understanding of the transformation of Br- and NO2- in the unactivated PMS oxidation process. It also provides important insights into the potentially underestimated environmental risks when PMS is applied to degrade organic contaminants under realistic environments, particularly when Br- and NO2- co-exist.

Comparison of nitrification performance in SBR and SBBR with response to NaCl salinity shock: Microbial structure and functional genes

Ammonia removal by nitrifiers at the extremely high salinity poses a great challenge for saline wastewater treatment. Sequencing batch reactor (SBR) was conducted with a stepwise increase of salinity from 10 to 40 g-NaCl·L-1, while sequencing batch biofilm reactor (SBBR) with one-step salinity enhancement, their nitrification performance, microbial structure and interaction were evaluated. Both SBR and SBBR can achieve high-efficiency nitrification (98% ammonia removal) at 40 g-NaCl·L-1. However, SBBR showed more stable nitrification performance than SBR at 40 g-NaCl·L-1 after a shorter adaptation period of 4-15 d compared to previous studies. High-throughput sequencing and metagenomic analysis demonstrated that the abundance and capability of conventional ammonia-oxidizing bacteria (Nitrosomonas) were suppressed in SBBR relative to SBR. Gelidibacter, Anaerolineales were the predominant genus in SBBR, which were not found in SBR. NorB and nosZ responsible for reducing NO to N2O and reducing N2O to N2 respectively had s strong synergistic effect in SBBR. This study will provide a valuable reference for the startup of nitrification process within a short period of time under the extremely high NaCl salinity.

Urea amendment decouples nitrification in hydrocarbon contaminated Antarctic soil

Hydrocarbon contaminated soils resulting from human activities pose a risk to the natural environment, including in the Arctic and Antarctic. Engineered biopiles constructed at Casey Station, Antarctica, have proven to be an effective strategy for remediating hydrocarbon contaminated soils, with active ex-situ remediation resulting in significant reductions in hydrocarbons, even in the extreme Antarctic climate. However, the use of urea-based fertilisers, whilst providing a nitrogen source for bioremediation, has also altered the natural soil chemistry leading to increases in pH, ammonium and nitrite. Monitoring of the urea amended biopiles identified rising levels of nitrite to be of particular interest, which misaligns with the long term goal of reducing contaminant levels and returning soil communities to a 'healthy' state. Here, we combine amplicon sequencing, microfluidic qPCR on field samples and laboratory soil microcosms to assess the impact of persistent nitrite accumulation (up to 60 months) on nitrifier abundances observed within the Antarctic biopiles. Differential inhibition of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) Nitrobacter and Nitrospira in the cold, urea treated, alkaline soils (pH 8.1) was associated with extensive nitrite accumulation (76 ± 57 mg N/kg at 60 months). When the ratio of Nitrospira:AOB dropped below ∼1:1, Nitrobacter was completely inhibited or absent from the biopiles, and nitrite accumulated. Laboratory soil microcosms (incubated at 7 °C and 15 °C for 9 weeks) reproduced the pattern of nitrite accumulation in urea fertilized soil at the lower temperature, consistent with our longer-term observations from the Antarctic biopiles, and with other temperature-controlled microcosm studies. Diammonium phosphate amended soil did not exhibit nitrite accumulation, and could be a suitable alternative biostimulant to avoid excessive nitrite build-up.

Genome-wide identification and comparative analysis of the Amino Acid Transporter (AAT) gene family and their roles during Phaseolus vulgaris symbioses

Amino acid transporters (AATs) are essential integral membrane proteins that serve multiple roles, such as facilitating the transport of amino acids across cell membranes. They play a crucial role in the growth and development of plants. Phaseolus vulgaris, a significant legume crop, serves as a valuable model for studying root symbiosis. In this study, we have conducted an exploration of the AAT gene family in P. vulgaris. In this research, we identified 84 AAT genes within the P. vulgaris genome sequence and categorized them into 12 subfamilies based on their similarity and phylogenetic relationships with AATs found in Arabidopsis and rice. Interestingly, these AAT genes were not evenly distributed across the chromosomes of P. vulgaris . Instead, there was an unusual concentration of these genes located toward the outer edges of chromosomal arms. Upon conducting motif analysis and gene structural analysis, we observed a consistent presence of similar motifs and an intron-exon distribution pattern among the subfamilies. When we analyzed the expression profiles of PvAAT genes, we noted tissue-specific expression patterns. Furthermore, our investigation into AAT gene expression under rhizobial and mycorrhizal symbiotic conditions revealed that certain genes exhibited high levels of expression. Specifically, ATLa5 and LHT2 was notably upregulated under both symbiotic conditions. These findings point towards a potential role of AATs in the context of rhizobial and mycorrhizal symbiosis in P. vulgaris, in addition to their well-established regulatory functions.

Incubation study on remediation of nitrate-contaminated soil by Chroococcus sp

The possibility of using the non-nitrogen-fixing cyanobacterium (Chroococcus sp.) for the reduction of soil nitrate contamination was tested through Petri dish experiments. The application of 0.03, 0.05 and 0.08 mg/cm2 Chroococcus sp. efficiently removed NO3--N from the soil through assimilation of nitrate nutrient and promotion of soil denitrification. At the optimal application dose of 0.05 mg/cm2, 44.06%, 36.89% and 36.17% of NO3--N were removed at initial NO3--N concentrations of 60, 90 and 120 mg/kg, respectively. The polysaccharides released by Chroococcus sp. acted as carbon sources for bacterial denitrification and facilitated the reduction of soil salinity, which significantly (p < 0.05) stimulated the growth of denitrifying bacteria (Hyphomicrobium denitrificans and Hyphomicrobium sp.) as well as significantly (p < 0.05) elevated the activities of nitrate reductase and nitrite reductase by 1.07-1.23 and 1.15-1.22 times, respectively. The application of Chroococcus sp. promoted the dominance of Nocardioides maradonensis in soil microbial community, which resulted in elevated phosphatase activity and increased available phosphorus content. The application of Chroococcus sp. positively regulated the growth of soil bacteria belonging to the genera Chitinophaga, Prevotella and Tumebacillus, which may contribute to increased soil fertility through the production of beneficial enzymes such as invertase, urease and catalase. To date, this is the first study verifying the remediation effect of non-nitrogen-fixing cyanobacteria on nitrate-contaminated soil.

Nitrification in acidic and alkaline environments

Aerobic nitrification is a key process in the global nitrogen cycle mediated by microorganisms. While nitrification has primarily been studied in near-neutral environments, this process occurs at a wide range of pH values, spanning ecosystems from acidic soils to soda lakes. Aerobic nitrification primarily occurs through the activities of ammonia-oxidising bacteria and archaea, nitrite-oxidising bacteria, and complete ammonia-oxidising (comammox) bacteria adapted to these environments. Here, we review the literature and identify knowledge gaps on the metabolic diversity, ecological distribution, and physiological adaptations of nitrifying microorganisms in acidic and alkaline environments. We emphasise that nitrifying microorganisms depend on a suite of physiological adaptations to maintain pH homeostasis, acquire energy and carbon sources, detoxify reactive nitrogen species, and generate a membrane potential at pH extremes. We also recognize the broader implications of their activities primarily in acidic environments, with a focus on agricultural productivity and nitrous oxide emissions, as well as promising applications in treating municipal wastewater.

Mechanism of nitro-byproducts formation during persulfate-based electrokinetic in situ oxidation for remediation of anthracene contaminated soil

Persulfate-based electrokinetic (EK) chemical oxidation appears to be a novel and viable strategy for the in situ remediation of polycyclic aromatic hydrocarbons (PAHs) polluted soil; however, the possible toxic byproducts of PAHs have been overlooked. In this study, we systematically investigated the formation mechanism of the nitro-byproducts of anthracene (ANT) during the EK process. Electrochemical experiments revealed that NH₄⁺ and NO₂^- originating from nitrate electrolyte or soil substrates were oxidized to NO₂^• and NO^• in the presence of SO₄^•-. Liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS/MS) analysis with ¹⁵N labeling revealed the formation of nitro-byproducts (14 kinds), including 1-hydroxy-4-nitro-anthraquinone and its similar derivatives, 4-nitrophenol, and 2,4-dinitrophenol. The nitration pathways of ANT have been proposed and described, mainly including the formation of hydroxyl-anthraquinone-oxygen and phenoxy radicals and the subsequent addition of NO₂^• and NO^•. ANT-based formation of nitro-byproducts during EK, which is usually underestimated, should be further investigated due to their enhanced acute toxicity, mutagenic effects, and potential threat to the ecosystem.

Different types of land use influence soil physiochemical properties, the abundance of nitrifying bacteria, and microbial interactions in tropical urban soil

Anthropogenic activities have led to unexpected changes in microbial community composition and structure, resulting in an interruption of soil ecological roles in urban environments. We questioned the impact of the different land use (e.g., agricultural, industrial, recreational, coastal, and residential areas) on the distribution of nitrifying bacteria and microbial interaction in tropical soil. The dominant nitrifying bacteria were ammonia-oxidizing archaea (AOA) in tropical soils up to 10⁷ copies/g of soil, while the abundance of ammonia-oxidizing bacteria (AOB) was significantly higher in agricultural soil only. Comammox (CMX) was ubiquitous up to 10⁵ copies/g of tropical soil, indicating that CMX might share ecological niches with AOA and considerably contribute to nitrification in urban areas. The most abundant phylum is Actinobacteria, accounting for 27-34 % relative abundance among most land-use types, but Proteobacteria was observed as the most prevalent phylum in agricultural soil. The physicochemical properties (e.g., soil pH and nutrient contents) of different types of land use influenced microbial richness and diversities associated with nitrogen cycling. Multivariate analysis disclosed that agricultural soils were distinct from other land uses because of the concentrations of nutrients and heavy metals and the abundance of microorganisms associated with nitrogen cycles. Also, the microbial co-occurrence network revealed that agricultural soils were a highly interconnected network of the microbial community. In this study, C: N ratio might have a significant impact on ecological networks and the abundance of nitrogen-related taxa, which could influence microbial interactions and complexity in tropical soils. Thus, the impact of anthropogenic land use induced changes in microbial composition and diversity, co-occurrence network, and nitrifying bacteria, leading to potential transformation in ecological services of tropical soils and nitrogen cycling in urban environments.

New Insights into the Role of Nitrite in the Degradation of Tetrabromobisphenol S by Sulfate Radical Oxidation

Tetrabromobisphenol S (TBBPS) is a brominated flame retardant and a contaminant of emerging concern. Several studies found that sulfate radical (SO₄^•-) oxidation is effective to degrade TBBPS. Here, we demonstrate that the presence of nitrite (NO₂^-) at environmentally relevant levels causes dramatic changes in the kinetics and pathways of TBBPS degradation by SO₄^•-. Initially, NO₂^- suppresses the reaction by competing with TBBPS for SO₄^•-. At the same time, SO₄^•- oxidizes NO₂^- to form nitrogen dioxide radicals (NO₂^•), which actively react with some key TBBPS degradation intermediates, thus greatly altering the transformation pathway. As a result, 2,6-dibromo-4-nitrophenol (DBNP) becomes the primary TBBPS product. As TBBPS undergoes degradation, the released bromide (Br^-) is oxidized by SO₄^•- to form bromine radicals and free bromine. These reactive bromine species immediately combine with NO₂^• or NO₂^- to form nitryl bromide (BrNO₂) that in turn attacks the parent TBBPS, resulting in its accelerated degradation and increased formation of toxic nitrophenolic byproducts. These results show that nitryl halides (e.g., BrNO₂ or ClNO₂) are likely formed yet inadequately recognized when SO₄^•- is applied to remediate halogenated pollutants in the subsurface environment where NO₂^- is ubiquitously found. These insights further underscore the potential risks of the application of SO₄^•- oxidation for the remediation of halogenated compounds in realistic environmental conditions.

Differentiation of Pathways of Nitrated Byproduct Formation from Ammonium and Nitrite During Sulfate Radical Oxidation

Recent studies found that both nitrite (NO₂^-) and ammonium (NH₄⁺) lead to nitrophenolic byproducts in SO₄^•- oxidation processes, during which NO₂^• generated through the oxidation of the inorganic nitrogen by SO₄^•- is the key nitrating agent. This study demonstrates that the formation of phenoxy radicals to which NO₂^• can be incorporated immediately is another governing factor. Two types of sites having distinct reactivities in natural organic matter (NOM) molecules can be transformed to phenoxy radicals upon SO₄^•- oxidation. Fast sites associated with phenolic functionalities are primarily targeted in the reaction sequence involving NO₂^-, because both are preferentially oxidized. Following the depletion of NO₂^-, NH₄⁺ becomes the main precursor of NO₂^• that interacts with slow sites associated with the carboxylic functionalities. Experimental data show that the formation of total organic nitrogen in 24 h reached 6.28 μM during SO₄^•- oxidation of NOM (4.96 mg/L organic carbon) in the presence of both NO₂^- (0.1 mM) and NH₄⁺ (1.0 mM), while the sum of those formed in the presence of each alone was only 3.52 μM. Results of this study provide further insights into the mechanisms of nitrated byproduct formation when SO₄^•- is applied for environmental remediation.

Role of chemical reactions in the nitrogenous trace gas emissions and nitrogen retention: A meta-analysis

Increasing evidence has been found that chemical reactions affect significantly the terrestrial nitrogen (N) cycle, which was previously assumed to be mainly dominated by biological processes. Due to the limitation of knowledge and analytical techniques, it is currently challenging to discern the contribution of biotic and abiotic processes to the terrestrial N cycle for geobiologists and biogeochemists alike. To better understand the role of abiotic reactions in the terrestrial N cycle, it is necessary to comprehend the chemical controls on nitrogenous trace gas emissions and N retention in soil under various environmental conditions. In this manuscript, we assess the role of abiotic reactions in nitrous oxide (N₂O) and nitric oxide (NO) emissions as well as N retention through a meta-analysis using all related peer-reviewed publications before August 2020. Results show that abiotic reactions contributed 29.3-37.7% and 44.0-57.0% to the total N₂O emission and N retention, representing 3.7-4.7 and 4.0-6.0 Tg year^-1 of global terrestrial N₂O emission and N retention, respectively. Much higher NO production was observed in sterilized soils than that in unsterilized treatments indicating the major contribution of chemical reactions to NO emission and rapid microbial reduction of NO to N₂O and N₂. Chemical hydroxylamine oxidation accounts for the largest abiotic contribution to N₂O emission, while chemical nitrite reduction and fixation represent for the largest contribution to abiotic NO production and soil N retention, respectively. Factors influencing the abiotic processes include pH, total organic carbon (TOC), total nitrogen (TN), the ratio of carbon to nitrogen (C/N), and transition metals. These results broadened our knowledge about the mechanisms involved in chemical N reactions and provided a simplified estimation about their contribution to nitrogenous trace gas emission and N retention, which is meaningful to further study interactions of biologically and chemically mediated reactions in biogeochemical N cycle.

Pollution in the interflow from a simple landfill in a mountainous and hilly area in Southwest China

Simple landfills lack pollution prevention measures and therefore continuously release pollutants into the surrounding environment. There are a large number of simple landfills in the mountainous and hilly areas in China, and the interflow accounts for a large proportion of runoff. However, the pollution in the interflow stemming from the simple landfill has not been extensively studied. Here, the pollution of the interflow caused by the simple landfill near the Yunxi Town Landfill in the mountainous and hilly region in Southwest China was studied. The composition and pollution release potential of aged refuse in the landfill were determined, and the water quality of interflow around the landfill was monitored for five months. Seasonal changes in water quality of the interflow were observed, and the concentration of pollutants in the interflow around the simple landfill greatly exceeded the water quality standard for the local water function zoning throughout the sampling period. Specifically, the chemical oxygen demand concentrations of the interflow were 247.90 ± 81.57 mg/L, and more than 50 types of refractory organics were detected, with as many as 10 types of polycyclic aromatic hydrocarbons and Environmental Protection Agency priority pollutants. The total nitrogen concentration of the interflow was 132.45 ± 108.68 mg/L; organic nitrogen (53.27%) was the main component, followed by nitrate nitrogen (32.28%) and nitrous nitrogen and ammonia nitrogen (14.45%). The results highlight the need for the remediation of interflow around simple landfills in mountainous and hilly areas. Generally, the basic data could be used to aid the development of remediation technology.

Impact of Urea and Ammoniacal Nitrogen Wastewaters on Soil: Field Study in a Fertilizer Industry (Bahía Blanca, Argentina)

Nitrogen compounds in industrial effluents are considered a serious threat to the environment. The aim of this work is to identify the effect produced by nitrogen-rich wastewater on alkaline soils from industrial land. Two plots were irrigated with wastewater as ammoniacal nitrogen (31 to 53 g N m^-2) and urea (167-301 g N m^-2) sources named P1 and P2, respectively. Inorganic nitrogen (N) concentrations (N-NH₃ + N-NH₄, N-NO₂, N-NO₃), soil pH, and N-NH₃ volatilization were monitored during a 2-year period. Variations in the fate of N compounds were distinguished according to the quantity and source of N applied to the soil. A higher N input in the form of urea was related to a greater concentration of nitrates and soil acidification in the topsoil (0-30 cm). Otherwise, ammoniacal N wastewater showed greater relative ammonia losses due to volatilization. Ammonia losses were estimated as 24.2% and 7.43% of the total N applied in P1 and P2, respectively. Besides, in P1 ammoniacal N predominated over nitrate, unlike results obtained in P2. The correct management of nitrogen-rich wastewaters in fertilizer industries could greatly reduce soil and groundwater degradation.

Persistence and spread of tetracycline resistance genes and microbial community variations in the soil of animal corrals in a semi-arid planted forest

At the spring, goat and sheep herds are transferred to planted forests, in a semi-arid region in the northern Negev Desert, Israel, to reduce herbaceous biomass and, fire risk. The herds are held overnight in corrals for about 4 months, enriching the soil with organic matter and nitrogen. This research examined the effect of these enrichments on soil bacterial community structure (BCS) and the abundance of tetracycline resistance genes (TRGs) in active and abandoned corrals (1-10-years-old). Based on 16S rRNA gene sequences, the Proteobacteria and Actinobacteria phyla dominated the soil of all corrals. The Actinobacteria were less abundant in the active and 1-year-old corrals (23-26%) than in the other corrals and the control (33-38%). A principal component analysis showed that, the BCS in the active and the 1-year-old abandoned corrals was significantly different from that in the older corrals and the control. The Firmicutes phylum constituted 28% of the BCS in the active corrals, 12.5% in the 1-year-old corrals and 2% in the older corrals and the control. In contrast, the Acidobacteria phylum was hardly detected in the active and 1-year-old abandoned corrals and constituted 10% of the BCS in the older corrals. Genes conferring resistance to tetracycline were detected in high numbers. The tetG and tetW genes were detected in the active and abandoned corrals (1-10 years). The tetQ gene was detected only in the active and 1-year-old abandoned corrals. None of the genes were detected in the control soil. The three genes were detected outside an active corral, in the downstream section of an ephemeral tributary. The results prove that abandoned and unobserved periodic animal corrals are an environmental reservoir for TRGs.

Linking meta-omics to the kinetics of denitrification intermediates reveals pH-dependent causes of N2O emissions and nitrite accumulation in soil

Soil pH is a key controller of denitrification. We analysed the metagenomics/transcriptomics and phenomics of two soils from a long-term liming experiment, SoilN (pH 6.8) and un-limed SoilA (pH 3.8). SoilA had severely delayed N₂O reduction despite early transcription of nosZ (mainly clade I), encoding N₂O reductase, by diverse denitrifiers. This shows that post-transcriptionally hampered maturation of the NosZ apo-protein at low pH is a generic phenomenon. Identification of transcript reads of several accessory genes in the nos cluster indicated that enzymes for NosZ maturation were present across a range of organisms, eliminating their absence as an explanation for the failure to produce a functional enzyme. nir transcript abundances (for NO₂⁻ reductase) in SoilA suggest that low NO₂⁻ concentrations in acidic soils, often ascribed to abiotic degradation, are primarily due to biological activity. The accumulation of NO₂⁻ in neutral soil was ascribed to high nar expression (nitrate reductase). The -omics results revealed dominance of nirK over nirS in both soils while qPCR showed the opposite, demonstrating that standard primer pairs only capture a fraction of the nirK pool. qnor encoding NO reductase was strongly expressed in SoilA, implying an important role in controlling NO. Production of HONO, for which some studies claim higher, others lower, emissions from NO₂⁻ accumulating soil, was estimated to be ten times higher from SoilA than from SoilN. The study extends our understanding of denitrification-driven gas emissions and the diversity of bacteria involved and demonstrates that gene and transcript quantifications cannot always reliably predict community phenotypes.

Effects of the nitrification inhibitor nitrapyrin and mulch on N2O emission and fertilizer use efficiency using 15N tracing techniques

Nitrous oxide (N₂O), is a potent greenhouse gas (GHG) that shares 7% of global warming around the world. Among different sources, agricultural systems account for approx. 60% of global anthropogenic N₂O emissions. These N₂O emissions are associated with the activity of nitrifiers and denitrifiers that contribute to >4 Tg (teragrams) N₂O-N emission per year. Application of nitrogen (N) fertilizers and manures in agricultural fields plays an imperative role in this regard. On the other hand nitrification inhibitors are an effective approach to minimize N₂O-N emissions from agricultural fields. Here we examined the effects of applying urea with a nitrification inhibitor (Ni) nitrapyrin and mulch (Mu) on urea transformation, nitrous oxide (N₂O) emissions, grain yield and nitrogen (N) uptake efficiency. The treatments include a control (zero N), urea (U) applied at 200 kg N ha^-1, U + Ni (Ni applied at 700 g ha^-1), U+ Mu (Mu applied at 4 t ha^-1) and U + Ni + Mu. The N₂O emission factor (EF) was 66% and 75% when U and Mu were applied, respectively. Yield-scaled N₂O emissions were lower in U and Mu by 45% and 55%, respectively. The Ni coupled with Mu enhanced urea-¹⁵N recovery by 58% and wheat grain yield by 23% and total N uptake by 30% compared with U alone. In conclusion, Ni usage is an effective strategy to mitigate N₂O emissions under field conditions.

DNRA: A short-circuit in biological N-cycling to conserve nitrogen in terrestrial ecosystems

This paper reviews dissimilatory nitrate reduction to ammonium (DNRA) in soils - a newly appreciated pathway of nitrogen (N) cycling in the terrestrial ecosystems. The reduction of NO₃^- occurs in two steps; in the first step, NO₃^- is reduced to NO₂^-; and in the second, unlike denitrification, NO₂^- is reduced to NH₄⁺ without intermediates. There are two sets of NO₃^-/NO₂^- reductase enzymes, i.e., Nap/Nrf and Nar/Nir; the former occurs on the periplasmic-membrane and energy conservation is respiratory via electron-transport-chain, whereas the latter is cytoplasmic and energy conservation is both respiratory and fermentative (Nir, substrate-phosphorylation). Since, Nir catalyzes both assimilatory- and dissimilatory-nitrate reduction, the nrfA gene, which transcribes the NrfA protein, is treated as a molecular-marker of DNRA; and a high nrfA/nosZ (N₂O-reductase) ratio favours DNRA. Recently, several crystal structures of NrfA have been presumed to producee N₂O as a byproduct of DNRA via the NO (nitric-oxide) pathway. Meta-analyses of about 200 publications have revealed that DNRA is regulated by oxidation state of soils and sediments, carbon (C)/N and NO₂^-/NO₃^- ratio, and concentrations of ferrous iron (Fe²⁺) and sulfide (S^2-). Under low-redox conditions, a high C/NO₃^- ratio selects for DNRA while a low ratio selects for denitrification. When the proportion of both C and NO₃^- are equal, the NO₂^-/NO₃^- ratio modulates partitioning of NO₃^-, and a high NO₂^-/NO₃^- ratio favours DNRA. A high S^2-/NO₃^- ratio also promotes DNRA in coastal-ecosystems and saline sediments. Soil pH, temperature, and fine soil particles are other factors known to influence DNRA. Since, DNRA reduces NO₃^- to NH₄⁺, it is essential for protecting NO₃^- from leaching and gaseous (N₂O) losses and enriches soils with readily available NH₄⁺-N to primary producers and heterotrophic microorganisms. Therefore, DNRA may be treated as a tool to reduce ground-water NO₃^- pollution, enhance soil health and improve environmental quality.

Genetic variation in eggplant for Nitrogen Use Efficiency under contrasting NO3 - supply

Eggplant (Solanum melongena L.) yield is highly sensitive to N fertilization, the excessive use of which is responsible for environmental and human health damage. Lowering N input together with the selection of improved Nitrogen-Use-Efficiency (NUE) genotypes, more able to uptake, utilize, and remobilize N available in soils, can be challenging to maintain high crop yields in a sustainable agriculture. The aim of this study was to explore the natural variation among eggplant accessions from different origins, in response to Low (LN) and High (HN) Nitrate (NO₃ ^- ) supply, to identify NUE-contrasting genotypes and their NUE-related traits, in hydroponic and greenhouse pot experiments. Two eggplants, AM222 and AM22, were identified as N-use efficient and inefficient, respectively, in hydroponic, and these results were confirmed in a pot experiment, when crop yield was also evaluated. Overall, our results indicated the key role of N-utilization component (NUtE) to confer high NUE. The remobilization of N from leaves to fruits may be a strategy to enhance NUtE, suggesting glutamate synthase as a key enzyme. Further, omics technologies will be used for focusing on C-N metabolism interacting networks. The availability of RILs from two other selected NUE-contrasting genotypes will allow us to detect major genes/quantitative trait loci related to NUE.

Nitrous oxide production by ammonia oxidizers: Physiological diversity, niche differentiation and potential mitigation strategies

Oxidation of ammonia to nitrite by bacteria and archaea is responsible for global emissions of nitrous oxide directly and indirectly through provision of nitrite and, after further oxidation, nitrate to denitrifiers. Their contributions to increasing N₂ O emissions are greatest in terrestrial environments, due to the dramatic and continuing increases in use of ammonia-based fertilizers, which have been driven by requirement for increased food production, but which also provide a source of energy for ammonia oxidizers (AO), leading to an imbalance in the terrestrial nitrogen cycle. Direct N₂ O production by AO results from several metabolic processes, sometimes combined with abiotic reactions. Physiological characteristics, including mechanisms for N₂ O production, vary within and between ammonia-oxidizing archaea (AOA) and bacteria (AOB) and comammox bacteria and N₂ O yield of AOB is higher than in the other two groups. There is also strong evidence for niche differentiation between AOA and AOB with respect to environmental conditions in natural and engineered environments. In particular, AOA are favored by low soil pH and AOA and AOB are, respectively, favored by low rates of ammonium supply, equivalent to application of slow-release fertilizer, or high rates of supply, equivalent to addition of high concentrations of inorganic ammonium or urea. These differences between AOA and AOB provide the potential for better fertilization strategies that could both increase fertilizer use efficiency and reduce N₂ O emissions from agricultural soils. This article reviews research on the biochemistry, physiology and ecology of AO and discusses the consequences for AO communities subjected to different agricultural practices and the ways in which this knowledge, coupled with improved methods for characterizing communities, might lead to improved fertilizer use efficiency and mitigation of N₂ O emissions.

The coupling interaction of NO2- with NH4+ or NO3- as an important source of N2O emission from agricultural soil in the North China Plain

NO₂^- plays a crucial role in regulating N₂O formation from the soil, while how it affects the production of soil N₂O is still not well understood. In this study, N₂O and NO emissions from an agricultural field of the North China Plain (NCP) were comparatively investigated under five different fertilizer treatments (NH₄⁺, NO₃^-, NO₂^-, NH₄⁺ + NO₂^- and NO₃^- + NO₂^-). Additionally, soil NH₄⁺, NO₂^- and NO₃^- concentrations and the abundance of functional genes associated with nitrogen cycling were also analyzed in the incubation experiment. The results showed that the N₂O average fluxes from the complex treatments of NO₂^- + NO₃^- were 1.4-2.4 times the sum of those from the separate treatments of NO₂^- and NO₃^- whereas from the complex treatments of NO₂^- + NH₄⁺ were a factor of 1-1.4 larger than those from the separate treatments of NO₂^- and NH₄⁺, indicating the coupling interaction of NO₂^- with NH₄⁺ or NO₃^- makes a remarkable contribution to N₂O emission from the soil. Significant reduction of the activity of N₂O reductase was found in the soil with the addition of NO₂^-, which favored the accumulation of N₂O formed through nitrification of NH₄⁺ and denitrification of NO₂^-, resulting in relatively high N₂O emissions from the complex treatments. As the intermediate product of nitrification and denitrification, NO₂^- produced is also expected to interact with NH₄⁺ or NO₃^- to promote N₂O emission from the soil, especially during fertilization events when NO₂^- is easily accumulated due to the acceleration of the nitrification and denitrification processes.

Intense rainfalls trigger nitrite leaching in agricultural soils depleted in organic matter

Nitrate and ammonium are common inorganic contaminants of anthropogenic origin in many shallow aquifers around the world, while nitrite is less common, but it is most harmful than nitrate and ammonium due to its high reactivity. This paper presents evidence of nitrite accumulation after intense rainfalls in soil samples collected in an agricultural field characterized by organic matter chronic depletion. Moreover, an intact core from the same site was also collected to perform an unsaturated column experiment (60 cm long and 20 cm outer diameter) mimicking heavy rainfalls (230 mm in 2 days). Results from the field site showed nitrite accumulation (up to 0.45 mmol/kg) at 50-70 cm depth, just below the plough layer. The column experiment showed very high initial concentrations of nitrate and nitrite in the leachate and a progressive decrease of nitrate due to denitrification. The numerical flow model was calibrated versus the observed volumetric water contents and leachate flow rates. The numerical reactive transport model was calibrated versus the leachate concentrations of six dissolved species (ammonium, nitrate, nitrite, dissolved organic carbon, chloride and bromide). The optimized model resulted to be robustly calibrated providing insights on the kinetic rates driving the production, accumulation and leakage of nitrite, showing that incomplete denitrification is the source of nitrite. As far as the authors are aware, this is the first study reporting a clear link between high nitrite leaching rates and extreme rainfall events in lowland agricultural soils depleted in organic matter. The proposed methodology could be applied to quantify nitrite cycling processes in many other agricultural areas of the world affected by extreme rainfall events.

Ecological and physiological implications of nitrogen oxide reduction pathways on greenhouse gas emissions in agroecosystems

Microbial reductive pathways of nitrogen (N) oxides are highly relevant to net emissions of greenhouse gases (GHG) from agroecosystems. Several biotic and abiotic N-oxide reductive pathways influence the N budget and net GHG production in soil. This review summarizes the recent findings of N-oxide reduction pathways and their implications to GHG emissions in agroecosystems and proposes several mitigation strategies. Denitrification is the primary N-oxide reductive pathway that results in direct N2O emissions and fixed N losses, which add to the net carbon footprint. We highlight how dissimilatory nitrate reduction to ammonium (DNRA), an alternative N-oxide reduction pathway, may be used to reduce N2O production and N losses via denitrification. Implications of nosZ abundance and diversity and expressed N2O reductase activity to soil N2O emissions are reviewed with focus on the role of the N2O-reducers as an important N2O sink. Non-prokaryotic N2O sources, e.g. fungal denitrification, codenitrification and chemodenitrification, are also summarized to emphasize their potential significance as modulators of soil N2O emissions. Through the extensive review of these recent scientific advancements, this study posits opportunities for GHG mitigation through manipulation of microbial N-oxide reductive pathways in soil.

Cobalt-dependent inhibition of nitrite oxidation in Nitrobacter winogradskyi

Nitrobacter winogradskyi is an abundant, intensively studied autotrophic nitrite-oxidizing bacterium, which is frequently used as a model strain in the two-step nitrification of ammonia (NH₃) to nitrate (NO₃^-) via nitrite (NO₂^-), either in activated sludge, agricultural field studies or more recently in artificial microbial consortia for organic hydroponics. We observed a hitherto unknown cobalt ion-dependent inhibition of cell growth and NO₂^- oxidation activity of N. winogradskyi in a mineral medium, which strongly depended on accompanying Ca²⁺ and Mg²⁺ concentrations. This inhibition was bacteriostatic, but susceptible to natural chelators. l-Histidine effectively restored cell growth and NO₂^- oxidation activity of N. winogradskyi in mineral media containing Co²⁺ with >90% recovery. Our results suggest that Co²⁺ competed with alkaline earth metals during uptake and that its toxicity was significantly reduced by complexation.

Nitrogen Cycle Dynamics Revealed Through δ18O-NO3− Analysis in California Groundwater

Nitrate is a significant water-quality issue in California, the United States as a whole, and the world. Critical to addressing nitrate contamination is understanding the presence and extent of denitrification, and further refining the techniques used to identify nitrate sources. The use and understanding of nitrate isotopic signatures to identify nitrate sources have advanced tremendously; however, knowledge gaps remain concerning specific fractionation pathways and the role of denitrification in altering source values. Using a large unique database of California groundwater nitrate isotopic compositions, we explored the utility of nitrate–oxygen isotope ratios in determining specific nitrate origins. Lawrence Livermore National Lab (LLNL) samples were supplemented by United States Geological Society (USGS) data to create a dataset of over 1200 dual-isotope results. Methods used at LLNL allowed for the determination of δ15N-NO3−, δ18O-NO3−, δ18O-H2O, δ2H-H2O, excess air, major dissolved gases, and excess N2. Results were examined for the degree to which δ18O-NO3− conforms to the model of nitrification in which two atoms of oxygen are sourced from ambient water and one from the atmosphere. Almost 80% of the results fall within one standard deviation of predicted values. However, 19% of samples had significantly higher values, suggesting the preservation of a synthetic nitrate source signature, mixing of sources, or widespread denitrification. Results were examined with respect to general land-use classifications and, while nitrate concentrations followed the expected pattern of being higher in agricultural settings, δ18O-NO3−patterns are complicated by application of N-fertilizer in various forms, and subsequent N cycling in the soil zone. We found that the current understanding of oxygen isotope-fractionation mechanisms cannot yet explain the prevalence of oxygen-isotope compositions with higher than predicted δ18O values, but when paired with related data such as land use and indicators of denitrification, oxygen-isotope compositions of nitrate can help to assess nitrogen cycle dynamics.

Dicyandiamide has more inhibitory activities on nitrification than thiosulfate

Dicyandiamide (DCD) and thiosulfates are two type of nitrification inhibitors (NIs) that have been widely used in agriculture to improve nitrogen (N) fertilizer use efficiency and mitigate negative effect of N on environment. Little information is available concerning the comparison of the efficacy of DCD and thiosulfate on N transformations in soil. The aim of this study was to compare the effects of DCD and thiosulfate (K2S2O3) on changes of NH4+-N, nitrification inhibition and N recovery in a latosolic red soil. An incubation experiment was conducted with four treatments of control (CK), N, N+DCD, and N+K2S2O3. Soil samples were collected periodically over 50 d to determine concentrations of mineral N, and the amoA gene abundance of ammonia monooxygenase (AMO) for ammonia-oxidizing bacteria (AOB) was estimated by qPCR after 10 d incubation. In the N treatment, 67.8% of the applied N as NH4+-N disappeared from the mineral N pool and only 2.7% and 30.8% of the applied N was accumulated as NO2--N and NO3--N, respectively. Addition of DCD and thiosulfate to the soil prevented NH4+-N disappearance by 63.0% and 13.6%, respectively. DCD suppressed the production of NO2--N by 97.41%, whereas thiosulfate increased accumulation of NO2--N by 14.6%. Application of N along with DCD and thiosulfate inhibited nitrification, respectively, by 72.6% and 33.1%, resulting in the delay of the nitrification process for 30 days and 10 days, respectively. Apparent N recovery in N treatment was 66.2%, which increased by 55.2% and 4.8% by DCD and thiosulfate, respectively. Numbers of AOB amoA gene copy was significantly inhibited by both DCD and thiosulfate, and the stronger inhibition induced by DCD than thiosulfate was recorded. Results indicated that both DCD and thiosulfate were effective inhibitors for NH4+-N oxidation, NO3--N production, mineral N losses and AOB growth. DCD showed a more pronounced effect on nitrification inhibition than thiosulfate.

Chinese Milk Vetch Improves Plant Growth, Development and 15N Recovery in the Rice-Based Rotation System of South China

Chinese milk vetch (CMV) is vital for agriculture and environment in China. A pot experiment combined with ¹⁵N labeling (including three treatments: control, no fertilizer N and CMV; ¹⁵N-labeled urea alone, ¹⁵NU; substituting partial ¹⁵NU with CMV, ¹⁵NU-M) was conducted to evaluate the impact of CMV on plant growth, development and ¹⁵NU recovery in rice-based rotation system. The ¹⁵NU-M mitigated oxidative damage by increasing antioxidant enzymes activities and chlorophyll content while decreased malondialdehyde content in rice root and shoot, increased the biomass, total N and ¹⁵N uptake of plant shoots by 8%, 12% and 39% respectively, thus inducing a noticeable increase of annual ¹⁵N recovery by 77% versus ¹⁵NU alone. Remarkable increases in soil NH₄⁺ and populations of bacteria, actinomycetes and azotobacter were obtained in legume-rice rotation system while an adverse result was observed in soil NO₃⁻ content versus fallow-rice. CMV as green manure significantly increased the fungal population which was decreased with cultivating CMV as cover crop. Therefore, including legume cover crop in rice-based rotation system improves plant growth and development, annual N conservation and recovery probably by altering soil nitrogen forms plus ameliorating soil microbial communities and antioxidant system which alleviates oxidative damages in plants.

Nitrifying Kinetics and the Persistence of Nitrite in the Seine River, France

Although a higher oxidation rate for nitrite than for ammonia generally prevents nitrite accumulation in oxic waters, nitrite concentrations in the Seine River (1-31 μM) exceed European norms. We investigated the kinetics of in situ ammonia- and nitrite-oxidizing communities in river water and wastewater treatment plant (WWTP) effluents to determine the role of pelagic nitrification in the origin and persistence of nitrite downstream of Paris. The main source of nitrite is the major Parisian WWTP, and its persistence, up to tens of kilometers downstream of the plant, is explained by low ammonia and nitrite oxidation rates and high river flow. Furthermore, similar nitrite and ammonia oxidation rates preclude a rapid consumption of both preexisting nitrite and nitrite produced by ammonia oxidation. Maximum ammonia oxidation rates are two to three times higher downstream than upstream of the WWTP, indicating the input of ammonia oxidizers and ammonia from the WWTP. In both river water and WWTP effluents, nitrite oxidizers were unable to oxidize all available nitrite. In the human-impacted Seine River, this phenomenon might be due to mixotrophy. This study highlights the low resilience of the river to nitrite contamination as well as the importance of managing nitrite, nitrifiers, and organic matter concentrations in WWTP effluents to avoid nitrite persistence in rivers.

Interactions between nitrate and ammonium in their uptake, allocation, assimilation, and signaling in plants

Nitrogen (N) availability is a major factor determining plant growth and productivity. Plants acquire inorganic N from the soil, mainly in the form of nitrate and ammonium. To date, researchers have focused on these N sources, and demonstrated that plants exhibit elaborate responses at both physiological and morphological levels. Mixtures of nitrate and ammonium are beneficial in terms of plant growth, as compared to nitrate or ammonium alone, and therefore synergistic responses to both N sources are predicted at different steps ranging from acquisition to assimilation. In this review, we summarize interactions between nitrate and ammonium with respect to uptake, allocation, assimilation, and signaling. Given that cultivated land often contains both nitrate and ammonium, a better understanding of the synergism between these N sources should help to identify targets with the potential to improve crop productivity.

Arabidopsis Root-Type Ferredoxin:NADP(H) Oxidoreductase 2 is Involved in Detoxification of Nitrite in Roots

Ferredoxin:NADP(H) oxidoreductase (FNR) plays a key role in redox metabolism in plastids. Whereas leaf FNR (LFNR) is required for photosynthesis, root FNR (RFNR) is believed to provide electrons to ferredoxin (Fd)-dependent enzymes, including nitrite reductase (NiR) and Fd-glutamine-oxoglutarate aminotransferase (Fd-GOGAT) in non-photosynthetic conditions. In some herbal species, however, most nitrate reductase activity is located in photosynthetic organs, and ammonium in roots is assimilated mainly by Fd-independent NADH-GOGAT. Therefore, RFNR might have a limited impact on N assimilation in roots grown with nitrate or ammonium nitrogen sources. AtRFNR genes are rapidly induced by application of toxic nitrite. Thus, we tested the hypothesis that RFNR could contribute to nitrite reduction in roots by comparing Arabidopsis thaliana seedlings of the wild type with loss-of-function mutants of RFNR2 When these seedlings were grown under nitrate, nitrite or ammonium, only nitrite nutrition caused impaired growth and nitrite accumulation in roots of rfnr2 Supplementation of nitrite with nitrate or ammonium as N sources did not restore the root growth in rfnr2 Also, a scavenger for nitric oxide (NO) could not effectively rescue the growth impairment. Thus, nitrite toxicity, rather than N depletion or nitrite-dependent NO production, probably causes the rfnr2 root growth defect. Our results strongly suggest that RFNR2 has a major role in reduction of toxic nitrite in roots. A specific set of genes related to nitrite reduction and the supply of reducing power responded to nitrite concomitantly, suggesting that the products of these genes act co-operatively with RFNR2 to reduce nitrite in roots.

Nitrite-Oxidizing Bacterium Nitrobacter winogradskyi Produces N-Acyl-Homoserine Lactone Autoinducers

Nitrobacter winogradskyi is a chemolithotrophic bacterium that plays a role in the nitrogen cycle by oxidizing nitrite to nitrate. Here, we demonstrate a functional N-acyl-homoserine lactone (acyl-HSL) synthase in this bacterium. The N. winogradskyi genome contains genes encoding a putative acyl-HSL autoinducer synthase (nwi0626, nwiI) and a putative acyl-HSL autoinducer receptor (nwi0627, nwiR) with amino acid sequences 38 to 78% identical to those in Rhodopseudomonas palustris and other Rhizobiales. Expression of nwiI and nwiR correlated with acyl-HSL production during culture. N. winogradskyi produces two distinct acyl-HSLs, N-decanoyl-l-homoserine lactone (C10-HSL) and a monounsaturated acyl-HSL (C10:1-HSL), in a cell-density- and growth phase-dependent manner, during batch and chemostat culture. The acyl-HSLs were detected by bioassay and identified by ultraperformance liquid chromatography with information-dependent acquisition mass spectrometry (UPLC-IDA-MS). The C=C bond in C10:1-HSL was confirmed by conversion into bromohydrin and detection by UPLC-IDA-MS.

Nitrite Control over Dissimilatory Nitrate/Nitrite Reduction Pathways in Shewanella loihica Strain PV-4

Shewanella loihica strain PV-4 harbors both a functional denitrification (NO3 (-)→N2) and a respiratory ammonification (NO3 (-)→NH4 (+)) pathway. Batch and chemostat experiments revealed that NO2 (-) affects pathway selection and the formation of reduced products. Strain PV-4 cells grown with NO2 (-) as the sole electron acceptor produced exclusively NH4 (+). With NO3 (-) as the electron acceptor, denitrification predominated and N2O accounted for ∼90% of reduced products in the presence of acetylene. Chemostat experiments demonstrated that the NO2 (-):NO3 (-) ratio affected the distribution of reduced products, and respiratory ammonification dominated at high NO2 (-):NO3 (-) ratios, whereas low NO2 (-):NO3 (-) ratios favored denitrification. The NO2 (-):NO3 (-) ratios affected nirK transcript abundance, a measure of denitrification activity, in the chemostat experiments, and cells grown at a NO2 (-):NO3 (-) ratio of 3 had ∼37-fold fewer nirK transcripts per cell than cells grown with NO3 (-) as the sole electron acceptor. In contrast, the transcription of nrfA, implicated in NO2 (-)-to-NH4 (+) reduction, remained statistically unchanged under continuous cultivation conditions at NO2 (-):NO3 (-) ratios below 3. At NO2 (-):NO3 (-) ratios above 3, both nirK and nrfA transcript numbers decreased and the chemostat culture washed out, presumably due to NO2 (-) toxicity. These findings implicate NO2 (-) as a relevant modulator of NO3 (-) fate in S. loihica strain PV-4, and, by extension, suggest that NO2 (-) is a relevant determinant for N retention (i.e., ammonification) versus N loss and greenhouse gas emission (i.e., denitrification).

Effects of combined application of organic and inorganic fertilizers plus nitrification inhibitor DMPP on nitrogen runoff loss in vegetable soils

The application of nitrogen fertilizers leads to various ecological problems such as large amounts of nitrogen runoff loss causing water body eutrophication. The proposal that nitrification inhibitors could be used as nitrogen runoff loss retardants has been suggested in many countries. In this study, simulated artificial rainfall was used to illustrate the effect of the nitrification inhibitor DMPP (3,4-dimethyl pyrazole phosphate) on nitrogen loss from vegetable fields under combined organic and inorganic nitrogen fertilizer application. The results showed that during the three-time simulated artificial rainfall period, the ammonium nitrogen content in the surface runoff water collected from the DMPP application treatment increased by 1.05, 1.13, and 1.10 times compared to regular organic and inorganic combined fertilization treatment, respectively. In the organic and inorganic combined fertilization with DMPP addition treatment, the nitrate nitrogen content decreased by 38.8, 43.0, and 30.1% in the three simulated artificial rainfall runoff water, respectively. Besides, the nitrite nitrogen content decreased by 95.4, 96.7, and 94.1% in the three-time simulated artificial rainfall runoff water, respectively. A robust decline in the nitrate and nitrite nitrogen surface runoff loss could be observed in the treatments after the DMPP addition. The nitrite nitrogen in DMPP addition treatment exhibited a significant low level, which is near to the no fertilizer application treatment. Compared to only organic and inorganic combined fertilizer treatment, the total inorganic nitrogen runoff loss declined by 22.0 to 45.3% in the organic and inorganic combined fertilizers with DMPP addition treatment. Therefore, DMPP could be used as an effective nitrification inhibitor to control the soil ammonium oxidation in agriculture and decline the nitrogen runoff loss, minimizing the nitrogen transformation risk to the water body and being beneficial for the ecological environment.

Degradation and bound-residue formation of nonylphenol in red soil and the effects of ammonium

Fate of nonylphenol (NP) in soils and the effects of nitrogen fertilizers are unclear. Using (14)C-tracer, we studied the aerobic and anaerobic degradation of 4-NP111 in a paddy red soil amended without and with ammonium chloride. Under oxic conditions, 4-NP111 had a half-life of 16.1 ± 1.6 days and minor mineralization (3.84 ± 0.02%), forming no extractable metabolite but abundant bound residues (60.9 ± 1.7%, mostly bound to humin) after 49 days of incubation. The ammonium amendment (8 mmol/kg soil) significantly inhibited the degradation (half-life of 68.0 ± 7.7 days), mineralization (2.0 ± 1.1%), and bound-residue formation (23.7 ± 0.2%). Under anoxic conditions, 4-NP111 did not degrade during 49 days of incubation and the ammonium amendment (40 mmol/kg soil) did not affect its persistence. Our results demonstrate that bound-residue formation was a major mechanism for NP dissipation in the red soil under oxic conditions and that chemical nitrogen fertilizer at average field application rate may already considerably increase NP recalcitrance in agricultural soils.

Nitrogen mineralization from sludge in an alkaline, saline coal gasification ash environment

Rehabilitating coal gasification ash dumps by amendment with waste-activated sludge has been shown to improve the physical and chemical properties of ash and to facilitate the establishment of vegetation. However, mineralization of organic N from sludge in such an alkaline and saline medium and the effect that ash weathering has on the process are poorly understood and need to be ascertained to make decisions regarding the suitability of this rehabilitation option. This study investigated the rate and pattern of N mineralization from sludge in a coal gasification ash medium to determine the prevalent inorganic N form in the system and assess the effect of ash weathering on N mineralization. An incubation experiment was performed in which fresh ash, weathered ash, and soil were amended with the equivalent of 90 Mg ha sludge, and N mineralization was evaluated over 63 d. More N (24%) was mineralized in fresh ash than in weathered ash and soil, both of which mineralized 15% of the initial organic N in sludge. More nitrification occurred in soil, and most of the N mineralized in ash was in the form of ammonium, indicating an inhibition of nitrifying organisms in the ash medium and suggesting that, at least initially, plants used for rehabilitation of coal gasification ash dumps will take up N mostly as ammonium.

Nitrogen removal and simultaneous nitrification and denitrification in a fluidized bed step-feed process

For urban wastewater treatment, we conducted a novel four-stage step-feed wastewater treatment system combined with a fluidized bed laboratory experiment to investigate chemical oxygen demand (COD), NH(4+)-N, and total nitrogen (TN) removal performance. The removal rates of COD, NH(4+)-N and TN were 88.2%, 95.7%, and 86.4% with effluent concentrations of COD, NH(4+)-N and TN less than 50, 8, and 10 mg/L, respectively. Biomass and bacterial activities were also measured, with results showing more nitrobacteria in the activated sludge than in the biofilm; however, bacterial activity of the biofilm biomass and the activated sludge were similar. Nitrogen concentrations during the process were also detected, with simultaneous nitrification and denitrification found to be obvious.

Enzymology and ecology of the nitrogen cycle

The nitrogen cycle describes the processes through which nitrogen is converted between its various chemical forms. These transformations involve both biological and abiotic redox processes. The principal processes involved in the nitrogen cycle are nitrogen fixation, nitrification, nitrate assimilation, respiratory reduction of nitrate to ammonia, anaerobic ammonia oxidation (anammox) and denitrification. All of these are carried out by micro-organisms, including bacteria, archaea and some specialized fungi. In the present article, we provide a brief introduction to both the biochemical and ecological aspects of these processes and consider how human activity over the last 100 years has changed the historic balance of the global nitrogen cycle.

Optimization of pig slurry application to heavy metal polluted soils monitoring nitrification processes

Nitrification is often negatively affected by heavy metal pollution in soils, this limiting land revegetation. Thus, the potential use of pig slurry as a nitrogen-rich organic amendment in different heavy metal contaminated soils has been evaluated; this also being a way of recycling this waste. In order to identify the factors affecting nitrification processes in heavy metal polluted soils (soil pH, heavy metal solubility and the N source), incubation experiments were run using two polluted soils with different pH values (5.0 and 7.1) and a non-contaminated soil (pH 8.2). Ammonium was added as pig slurry or as ammonium sulphate for comparison (both added at 150 mg NH(4)(+)-N kg(-1) of soil). Pig slurry provoked higher nitrification rates and N-immobilisation than ammonium sulphate, especially in the neutral-polluted soil, reflecting an improvement of the microbial activity in the soil. The microbial immobilisation of N led to an inverse relationship between the amount of N added and nitrate conversion in the neutral-polluted soil and in the non-contaminated soil amended with different pig slurry dosages (75, 150 and 225mg NH(4)(+)-N kg(-1) of soil). Low rates of nitrification and N-immobilisation were found in the acidic soil. Pig slurry addition to metal polluted soils enhanced soil nitrification, especially when metals were in low-solubility forms.

Nitrogen transformation in acid soils subjected to pH value changes

The aim of this investigation was to determine which application of fertilizer and lime material does not form toxic quantities of nitrite nitrogen and when the losses by denitrification are the lowest in the examined acid soils. Investigations were performed on pseudogley soils of different acidity. Changes of available nitrogen forms were examined by the method of short-term incubation experiments. Experimental treatments were without the use of mineral fertilizers and with application of (NH4)2SO4 (100 ppm of NO3-N) and KNO3 (100 ppm of NO3-N); with and without addition of Ca(OH)2 (50% of full neutralization and full neutralization). When (NH4)2SO4 was used, nitrites occurred in both examined soils as a result of decelerated nitrification and when KNO3 was added as a result of chemical denitrification. Application of Ca(OH)2 caused the intensification of mineralization, nitrification and biological denitrification processes. When a higher dose of lime material was used (full neutralization), nitrites occurred in larger quantities as a result of the strengthening of nitrification and denitrification processes. Application of a lower lime dose caused nitrite occurrence in smaller quantities. Therefore, in these soils as well as in soils of similar chemical properties, application of lower doses of lime material can be recommended (<50% of full neutralization) as well as the application of ammonium fertilizer, bearing in mind that in such conditions losses of added fertilizer in the denitrification process are reduced and the occurrence of nitrites as an intermediate product of this process is prevented.

NO3-/NO2- assimilation in halophilic archaea: physiological analysis, nasA and nasD expressions

The haloarchaeon Haloferax mediterranei is able to assimilate nitrate or nitrite using the assimilatory nitrate pathway. An assimilatory nitrate reductase (Nas) and an assimilatory nitrite reductase (NiR) catalyze the first and second reactions, respectively. The genes involved in this process are transcribed as two messengers, one polycistronic (nasABC; nasA encodes Nas) and one monocistronic (nasD; codes for NiR). Here we report the Hfx mediterranei growth as well as the Nas and NiR activities in presence of high nitrate, nitrite and salt concentrations, using different approaches such as physiological experiments and enzymatic activities assays. The nasA and nasD expression profiles are also analysed by real-time quantitative PCR. The results presented reveal that the assimilatory nitrate/nitrite pathway in Hfx mediterranei takes place even if the salt concentration is higher than those usually present in the environments where this microorganism inhabits. This haloarchaeon grows in presence of 2 M nitrate or 50 mM nitrite, which are the highest nitrate and nitrite concentrations described from a prokaryotic microorganism. Therefore, it could be attractive for bioremediation applications in sewage plants where high salt, nitrate and nitrite concentrations are detected in wastewaters and brines.

Specific transformations of mineral forms of nitrogen in acid soils

Investigations were performed on soils of different acidity, ranging in the pH interval 4.65-5.80 (in water). Changes of the mineral nitrogen forms in the examined soils were studied by applying short-term incubation experiments performed under aerobic conditions, with a humidity of 30 % and a temperature of 20?C, both with and without the addition of 100 and 300 ppm NH4-N. The results of the incubation experiments showed that retarded nitrification was present in all the examined soils. Increased and toxic quantities of nitrites (35.7 ppm) were formed during the incubation, which remained in the soil solution for several days, and even weeks, in spite of favorable conditions of moisture, aeration and temperature for the development of the process of chemoautotrophic nitrification. Decelerated chemoautotrophic nitrification was the source of the occurrence of nitrite in the examined less acid soil (soil 1), while in soils of higher acidity (soils 2 and 3) after addition of 100 and 300 ppm NH4-N, nitrite occurred due to chemical denitrification (chemodenitrification). Nitrites formed in the process of chemodenitrification underwent spontaneous chemical oxidation resulting in nitrate formation (chemical nitrification). The content of mineral nitrogen (NH4 + NO3 + NO2-N) decreased during the incubation period, proving gaseous losses from the examined soils. Application of lower doses of nitrogen fertilizers could decrease nitrogen losses by denitrification as well as the occurrence of nitrite in toxic quantities in the investigated pseudogley soil.

Nitrification potentials of Chinese tea orchard soils and their adjacent wasteland and forest soils

To investigate the nitrifying activities of different soil types, soil samples collected from 8-, 50- and 90-year old tea orchards, the adjacent wasteland, and 90-year old forest were measured for their nitrification potentials using the conventional soil incubation and the liquid incubation method. Among different soil types, the nitrification potential of soil in tea orchards was higher than that of wasteland and forest soils. The slurry shaken liquid incubation method was confirmed to be more accurate and have reliable results than the soil incubation. Interestingly, experimental result revealed that the generally applied pH value of 7.2 for the liquid media was not the optimal pH for these acid soils with a strong buffer capacity. This suggested that tea orchard soils may have nitrifiers requiring pH-neutral condition for the best activity. Our data also showed that treatment with the commonly used nitrogen fertilizer urea significantly improved nitrification potential of the soils; such enhancement effect was stronger on all of three tea orchard soils than on wasteland and forest soils, and also stronger on the younger (8- and 50-year old) tea orchard soils than on the older one (90-year old).

Effect of nitrification inhibitor DMPP on nitrogen leaching, nitrifying organisms, and enzyme activities in a rice-oilseed rape cropping system

DMPP (3,4-dimethylpyrazole phosphate) has been used to reduce nitrogen (N) loss from leaching or denitrification and to improve N supply in agricultural land. However, its impact on soil nitrifying organisms and enzyme activities involved in N cycling is largely unknown. Therefore, an on-farm experiment, for two years, has been conducted, to elucidate the effects of DMPP on mineral N (NH4(+)-N and NO3(-)-N) leaching, nitrifying organisms, and denitrifying enzymes in a rice-oilseed rape cropping system. Three treatments including urea alone (UA), urea + 1% DMPP (DP), and no fertilizer (CK), have been carried out. The results showed that DP enhanced the mean NH4(+)-N concentrations by 19.1%--24.3%, but reduced the mean NO3(-)-N concentrations by 44.9%--56.6% in the leachate, under a two-year rice-rape rotation, compared to the UA treatment. The population of ammonia oxidizing bacteria, the activity of nitrate reductase, and nitrite reductase in the DP treatment decreased about 24.5%--30.9%, 14.9%--43.5%, and 14.7%--31.6%, respectively, as compared to the UA treatment. However, nitrite oxidizing bacteria and hydroxylamine reductase remained almost unaffected by DMPP. It is proposed that DMPP has the potential to either reduce NO3(-)-N leaching by inhibiting ammonia oxidization or N losses from denitrification, which is in favor of the N conversations in the rice-oilseed rape cropping system.

Influences of nitrification inhibitor 3,4-dimethyl pyrazole phosphate on nitrogen and soil salt-ion leaching

An undisturbed heavy clay soil column experiment was conducted to examine the influence of the new nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP), on nitrogen and soil salt-ion leaching. Regular urea was selected as the nitrogen source in the soil. The results showed that the cumulative leaching losses of soil nitrate-N under the treatment of urea with DMPP were from 57.5% to 63.3% lower than those of the treatment of urea without DMPP. The use of nitrification inhibitors as nitrate leaching retardants may be a proposal in regulations to prevent groundwater contaminant. However, there were no great difference between urea and urea with DMPP treatments on ammonium-N leaching. Moreover, the soil salt-ion leaching losses of Ca2+, Mg2+, K+, and Na+ were reduced from 26.6% to 28.8%, 21.3% to 27.8%, 33.3% to 35.5%, and 21.7% to 32.1%, respectively. So, the leaching losses of soil salt-ion were declined for nitrification inhibitor DMPP addition, being beneficial to shallow groundwater protection and growth of crop. These results indicated the possibility of ammonium or ammonium producing compounds using nitrification inhibitor DMPP to control the nitrate and nutrient cation leaching losses, minimizing the risk of nitrate pollution in shallow groundwater.

Influence of the DMPP (3,4-dimethyl pyrazole phosphate) on nitrogen transformation and leaching in multi-layer soil columns

The application of nitrogen fertilizers leads to various ecological problems such as nitrate leaching. The use of nitrification inhibitors as nitrate leaching retardants is a proposal that has been suggested for inclusion in regulations in many countries. In this study, using a multi-layer soil column device, the influence of new nitrification inhibitor DMPP (3,4-dimethyl pyrazole phosphate) was studied for understanding the nitrogen vertical transformation and lowering the nitrate leaching at different soil profile depths. The results indicated that, within 60 d of experiment, the regular urea added 1.0% DMPP can effectively inhibit the ammonium oxidation in the soil, and improve the ammonium concentration in soil solution over the 20cm depths of soil profile, while decline the concentrations of nitrate and nitrite. No obvious difference was found on ammonium concentrations in soil solution collected from deep profile under 20cm depths between regular urea and the urea added 1.0% DMPP. There was also no significant difference for the nitrate, ammonium and nitrite concentrations in the soil solution under 40cm depths of soil profile with the increasing nitrogen application level, among the treatments of urea added 1.0% DMPP within 60 d. It is proposed that DMPP could be used as an effective nitrification inhibitor in some region to control ammonium oxidation and decline the ion-nitrogen leaching, minimizing the shallow groundwater pollution risk and being beneficial for the ecological environment.

Evaluation of nitrification inhibitor 3,4-dimethyl pyrazole phosphate on nitrogen leaching in undisturbed soil columns

The application of nitrogen fertilizers leads to various ecological problems such as nitrate leaching. The use of nitrification inhibitors (NI) as nitrate leaching retardants is a proposal that has been suggested for inclusion in regulations in many countries. In this study, the efficacy of the new NI, 3,4-dimethyl pyrazole phosphate (DMPP), was tested under simulated high-risk leaching situations in two types of undisturbed soil columns. The results showed that the accumulative leaching losses of soil nitrate under treatment of urea with 1.0% DMPP, from columns of silt loam soil and heavy clay soil, were 66.8% and 69.5% lower than those soil columns tested with regular urea application within the 60 days observation, respectively. However, the losses of ammonium leaching were reversely increased 9.7% and 6.7% under the former treatment than the latter one. Application of regular urea with 1.0% DMPP addition can reduce about 59.3%-63.1% of total losses of inorganic nitrogen via leaching. The application of DMPP to urea had stimulated the inhibition effects of DMPP on the ammonium nitrification process in the soil up to 60 days. It is proposed that the DMPP could be used as an effective NI to control inorganic N leaching losses, minimizing the risk of nitrate pollution in shallow groundwater.