Nitrate Reduction and Nitrogen Cycle Partitioning: Denitrification, DNRA, and Assimilatory Utilization Mechanisms

Nitrate reduction is a key reaction in the biological nitrogen cycle that determines the fate of NO₃⁻. After NO₃⁻ is reduced to NO₂⁻, it can enter denitrification, DNRA, or assimilatory utilization pathways, which respectively affect nitrogen removal, nitrogen retention, and biomass nitrogen formation.

Keywords: nitrate reduction; nitrate reductase; denitrification; DNRA; assimilatory utilization; nitrite; nitrous oxide; nitrogen cycle

1 Partitioning Position of Nitrate Reduction in the Nitrogen Cycle

1.1 Entry Role of NO₃⁻ to NO₂⁻ Conversion

Nitrate (NO₃⁻) is a highly mobile inorganic nitrogen form in the nitrogen cycle and mainly originates from nitrification, nitrogen fertilizer input, atmospheric deposition, and the continuous transformation following organic nitrogen mineralization. Nitrate reduction converts NO₃⁻ to NO₂⁻, allowing nitrogen in a high oxidation state to enter subsequent reduction pathways. This step itself does not directly determine the final product, but it determines whether nitrate can enter assimilatory utilization, denitrification, or DNRA.

(1) Connection with assimilatory utilization

Plants, algae, fungi, and many microorganisms can use nitrate as a nitrogen source. NO₃⁻ is first reduced to NO₂⁻, then further reduced to NH₄⁺, and finally incorporated into amino acids, nucleotides, and other nitrogen-containing compounds. The core function of this pathway is to convert inorganic nitrogen into biomass nitrogen.

(2) Connection with anaerobic respiration

In low-oxygen or anoxic environments, some microorganisms can use NO₃⁻ as an electron acceptor for anaerobic respiration. In this context, nitrate reduction is directly related to energy metabolism and can support microbial oxidation of electron donors such as organic carbon, sulfide, and ferrous iron under oxygen-limited conditions.

(3) Determination of nitrogen partitioning

NO₂⁻ is the key branching node after nitrate reduction. It can continue into the denitrification pathway to form NO, N₂O, and N₂; it can also enter DNRA to generate NH₄⁺; or it can enter the cellular nitrogen pool through assimilatory systems. Different partitioning pathways determine whether nitrogen is removed, retained, or incorporated into organisms.

1.2 Major Transformation Pathways

In the nitrogen cycle, nitrate reduction mainly connects three types of pathways: denitrification, DNRA, and assimilatory utilization. All three start with the reduction of NO₃⁻ to NO₂⁻, but their metabolic purposes and ecological outcomes differ: denitrification tends toward gaseous nitrogen release, DNRA tends toward ammonium nitrogen retention, and assimilatory utilization tends toward biomass nitrogen formation.

Table 1 Major nitrate reduction pathways and ecological significance

Pathway	Main products	Representative enzymes/genes	Metabolic attribute	Nitrogen cycle outcome
Denitrification	NO, N₂O, N₂	Nar/Nap, NirK/NirS, Nor, NosZ	Anaerobic respiration	Nitrogen removal; may release N₂O
DNRA	NH₄⁺	Nar/Nap, NrfA/NrfH, some NirBD, etc.	Dissimilatory reduction	Retains nitrate as ammonium nitrogen
Assimilatory nitrate reduction	Cellular organic nitrogen	NR/Nas, NirA, GS-GOGAT	Nitrogen source assimilation	Increases biomass nitrogen
Nitrate respiration	NO₂⁻ or downstream reduced products	Nar, Nap	Energy metabolism	Supports microbial growth in low-oxygen environments

2 Nitrate Reductase Systems

2.1 Assimilatory Nitrate Reductase

Assimilatory nitrate reductase participates in nitrate uptake and utilization as a nitrogen source and catalyzes the reduction of NO₃⁻ to NO₂⁻. In plants, this enzyme is usually located in the cytosol and is associated with molybdenum cofactor, FAD, heme, and NADH/NADPH electron donors. In microorganisms, the structure and localization of assimilatory nitrate reductase systems vary among species, but their function is directed toward the biosynthesis of reduced nitrogen.

(1) Metabolic role

Assimilatory nitrate reductase determines the entry flux of nitrate into the cellular nitrogen assimilation system. Increased activity usually indicates enhanced nitrate uptake and utilization potential. However, if downstream NO₂⁻ reduction or NH₄⁺ assimilation is limited, enhanced nitrate reduction may also be accompanied by nitrite accumulation.

(2) Regulatory factors

This enzyme system is jointly regulated by nitrate induction, ammonium feedback, carbon skeleton supply, reducing power, and cellular growth status. In plants, light can promote nitrate assimilation by providing reducing power, ATP, and carbon skeletons. When ammonium nitrogen is sufficient, the demand for nitrate assimilation usually decreases.

(3) Experimental interpretation

Increased expression or activity of assimilatory nitrate reductase alone cannot prove that nitrate has entered the organic nitrogen pool. A more complete interpretation should combine NO₂⁻ levels, NH₄⁺ assimilation, GS-GOGAT-related indicators, amino acid content, and changes in biomass nitrogen.

2.2 Membrane-Bound Nitrate Reductase

Membrane-bound nitrate reductase mainly participates in dissimilatory nitrate reduction and nitrate respiration and is often coupled to the cell membrane electron transport chain. In anoxic environments, this system can use NO₃⁻ as an electron acceptor, reduce it to NO₂⁻, and participate in energy conservation.

(1) Nar system

The Nar system is usually closely linked to membrane-associated electron transport and is more oriented toward anaerobic respiration and energy metabolism. Genes such as narG, narH, and narI are commonly used to characterize membrane-bound nitrate reduction potential.

(2) Pathway boundary

The Nar system can occur in denitrifying, DNRA, or other nitrate-respiring microorganisms. Therefore, increased narG abundance or expression only indicates enhanced nitrate reduction potential and cannot directly determine whether the final product is N₂, N₂O, or NH₄⁺.

2.3 Periplasmic Nitrate Reductase

Periplasmic nitrate reductase is located in the periplasmic space and often functions under low-oxygen, low-nitrate, or fluctuating redox conditions. Compared with membrane-bound systems, periplasmic nitrate reductase may contribute less to energy conservation, but it is more suitable for redox balance regulation and low-oxygen adaptation.

(1) Nap system

The Nap system can help cells maintain electron flow balance under fluctuating oxygen and nitrate conditions. Genes such as napA and napB are commonly used to characterize periplasmic nitrate reduction capacity.

(2) Downstream interpretation

Enhanced napA expression needs to be interpreted together with downstream genes and products. If nirK/nirS are simultaneously enhanced and N₂O or N₂ increases, NO₂⁻ is more likely to enter denitrification. If nrfA increases together with NH₄⁺ accumulation, this suggests an increased DNRA contribution.

Table 2 Nitrate reductase types and experimental interpretation

Enzyme system	Main localization	Representative genes	Main function	Interpretation focus
Assimilatory nitrate reductase	Cytosol or intracellular assimilation system	NR, nasA, etc.	Reduces NO₃⁻ to NO₂⁻ and enters nitrogen assimilation	Should be combined with NO₂⁻ reduction, NH₄⁺ assimilation, and biomass nitrogen
Membrane-bound nitrate reductase	Cell membrane-associated	narG, narH, narI	Nitrate respiration and dissimilatory reduction	Indicates reduction potential, but cannot alone determine final products
Periplasmic nitrate reductase	Periplasmic space	napA, napB	Nitrate reduction under low oxygen or redox fluctuation	Requires downstream denitrification and DNRA genes to determine direction

3 Nitrate Reduction in the Denitrification Pathway

3.1 Denitrification Process

Denitrification is an important nitrogen removal process in anoxic environments, with the typical pathway NO₃⁻ → NO₂⁻ → NO → N₂O → N₂. This process can reduce nitrate loads in soil, sediments, wetlands, and wastewater treatment systems, but incomplete reduction may lead to N₂O accumulation.

(1) NO₃⁻ to NO₂⁻

The Nar or Nap system catalyzes the reduction of NO₃⁻ to NO₂⁻, providing the entry substrate for denitrification. Enhancement of this stage accelerates nitrate consumption, but if downstream Nir, Nor, or NosZ is limited, NO₂⁻ or N₂O may accumulate.

(2) NO₂⁻ to gaseous nitrogen

NO₂⁻ can be reduced to NO by NirK or NirS, then converted to N₂O by Nor, and finally reduced to N₂ by NosZ. Whether denitrification is complete depends on whether these steps operate continuously and whether electron donor, pH, oxygen, and metal cofactor conditions are suitable.

3.2 N₂O Generation and Reduction

N₂O accumulation usually reflects a mismatch in rates among denitrification steps, rather than simply indicating enhanced nitrate reduction. Low pH, oxygen fluctuation, copper limitation, insufficient electron donors, or inadequate nosZ expression can all weaken N₂O reduction capacity. If NO₂⁻ and N₂O increase simultaneously, downstream denitrification reduction may be limited. If N₂ generation increases without obvious N₂O accumulation, this indicates more complete denitrification.

Table 3 Denitrification steps and key interpretation indicators

Step	Main reaction	Key genes/enzymes	Common detection indicators	Interpretation focus
Nitrate reduction	NO₃⁻ → NO₂⁻	narG, napA	NO₃⁻ decrease, NO₂⁻ change	Determines denitrification entry potential
Nitrite reduction	NO₂⁻ → NO	nirK, nirS	NO₂⁻ consumption, NO generation	Determines whether the gaseous nitrogen pathway is entered
Nitric oxide reduction	NO → N₂O	norB, etc.	N₂O generation	Assesses intermediate accumulation risk
Nitrous oxide reduction	N₂O → N₂	nosZ	N₂O/N₂ ratio	Determines complete denitrification capacity

4 Nitrate Reduction in the DNRA Pathway

4.1 Reaction Characteristics

DNRA is a dissimilatory process that reduces NO₃⁻ or NO₂⁻ to NH₄⁺. Compared with assimilatory reduction, the NH₄⁺ generated by DNRA does not necessarily enter biomass directly. Compared with denitrification, DNRA does not convert nitrogen to N₂ and remove it from the system; instead, it retains nitrate nitrogen as ammonium nitrogen.

(1) NO₂⁻ to NH₄⁺

The key step of DNRA is the further reduction of NO₂⁻ to NH₄⁺. NrfA is commonly used as an important marker of DNRA potential. If NO₃⁻ consumption is accompanied by increased NH₄⁺ and increased nrfA abundance or expression, this suggests enhanced DNRA contribution.

(2) Distinction from organic nitrogen mineralization

NH₄⁺ increase may also originate from organic nitrogen mineralization, so DNRA cannot be determined based only on increased ammonium nitrogen. A more reliable interpretation should combine NO₃⁻ consumption, NO₂⁻ dynamics, nrfA, ¹⁵N-nitrate tracing, and nitrogen mass balance.

4.2 Competition Between DNRA and Denitrification

DNRA and denitrification often compete for nitrate and electron donors in the same anoxic environment. Their relative contributions determine whether nitrogen is retained or lost.

(1) Carbon-to-nitrate ratio

A high C/NO₃⁻ ratio usually favors DNRA because sufficient electron donors can support deeper reduction of nitrate to ammonium nitrogen. Denitrification also requires a carbon source, but it often has a competitive advantage when nitrate supply is relatively sufficient and redox conditions are moderate.

(2) Reduction intensity

Strongly reducing environments, organic-rich sediments, and sulfide supply often promote DNRA. In wetlands, estuarine sediments, and anaerobic mud layers, DNRA can significantly affect NH₄⁺ resupply and nitrogen retention in the system.

(3) Ecological consequences

Denitrification contributes to nitrogen removal but may generate N₂O emissions. DNRA helps retain nitrogen but may provide NH₄⁺ substrate for subsequent nitrification and algal growth. The ecological evaluation of these two pathways should be based on management objectives.

Table 4 Functional differences between denitrification and DNRA

Comparison dimension	Denitrification	DNRA
Main final products	N₂, N₂O	NH₄⁺
Nitrogen cycle outcome	Nitrogen is removed from the system or emitted as N₂O	Nitrogen remains in the system
Common favorable conditions	Anoxia, sufficient nitrate, moderate organic carbon	Strongly reducing conditions, high C/NO₃⁻ ratio, organic-rich conditions, sulfide presence
Key genes	nirK/nirS, nor, nosZ	nrfA, some nirBD
Environmental risk	N₂O emission	Ammonium accumulation and subsequent re-nitrification
Ecological value	Water nitrogen removal and nitrate load reduction	Maintenance of soil or sediment nitrogen pools

5 Nitrate Reduction in Assimilatory Utilization

5.1 From Inorganic Nitrogen to Biomass Nitrogen

Assimilatory nitrate reduction usually includes three steps: NO₃⁻ reduction, NO₂⁻ reduction, and NH₄⁺ assimilation. After NO₃⁻ is reduced to NO₂⁻, it must be further reduced to NH₄⁺ and incorporated into glutamine, glutamate, and other nitrogen-containing metabolites through systems such as GS-GOGAT. The key interpretation of this pathway is not whether nitrate decreases, but whether nitrate nitrogen truly enters the organic nitrogen pool.

(1) Risk of NO₂⁻ accumulation

When assimilatory nitrate reduction is enhanced, NO₂⁻ may accumulate if nitrite reduction capacity is insufficient. Because NO₂⁻ is potentially toxic to cells, plants and microorganisms usually need to coordinate nitrate reductase and nitrite reductase activity.

(2) Coordination of carbon and nitrogen metabolism

Nitrate assimilation requires support from carbon skeletons and reducing power. When carbon metabolism is insufficient, even if nitrate reductase activity is high, downstream NH₄⁺ assimilation and amino acid synthesis may still be limited.

5.2 Experimental Interpretation

The evidence chain for assimilatory utilization should include NO₃⁻ consumption, no abnormal NO₂⁻ accumulation, enhanced NH₄⁺ assimilation, increased amino acid or protein synthesis, and increased biomass nitrogen. When conditions allow, ¹⁵N-nitrate tracing can directly confirm the incorporation of nitrate nitrogen into the cellular nitrogen pool.

6 Regulation of Nitrate Reduction Direction by Environmental Factors

6.1 Oxygen and Redox Status

Oxygen is an important factor controlling dissimilatory nitrate reduction. When oxygen is sufficient, microorganisms usually preferentially use O₂ as the electron acceptor, and denitrification and DNRA are inhibited. Under anoxic conditions, nitrate can replace oxygen in electron transfer. Periodic redox fluctuations may also cause NO₂⁻ or N₂O accumulation, especially during rainfall, irrigation, drainage, and wetland water-level changes in agricultural systems.

6.2 Organic Carbon and Electron Donors

Organic carbon provides electron donors for nitrate reduction. Carbon source deficiency limits NO₃⁻ reduction and subsequent steps. When high organic carbon is accompanied by a strongly reducing environment, the relative advantage of DNRA may increase. Labile organic carbon usually drives dissimilatory nitrate reduction rapidly, whereas recalcitrant organic matter affects process rates more slowly.

6.3 pH and Temperature

pH affects the activity of nitrate reductase, nitrite reductase, and N₂O reductase. Under low-pH conditions, the reduction of N₂O to N₂ is easily inhibited, increasing the risk of N₂O emission. Higher temperature usually increases microbial metabolic rates, but the actual effect also depends on oxygen diffusion, organic carbon supply, and changes in community structure.

6.4 Sulfur, Iron, and Other Redox-Active Components

Sulfide, ferrous iron, and other reducing components can serve as electron donors or alter environmental redox status. In wetlands and sediments, sulfur cycling, iron cycling, and nitrogen cycling are often coupled. In the presence of sulfide, nitrate reduction can be linked to sulfur oxidation and may promote DNRA or nitrate-dependent sulfur oxidation processes.

Table 5 Effects of environmental factors on nitrate reduction pathways

Factor	Effect on nitrate reduction	Effect on denitrification	Effect on DNRA
Low oxygen/anoxia	Promotes dissimilatory nitrate reduction	Enhances denitrification potential	Enhances DNRA potential
High organic carbon	Provides electron donors	Can promote denitrification	More favorable under high C/NO₃⁻ ratio
High nitrate	Provides electron acceptors	Often promotes denitrification	May reduce the relative advantage of DNRA
Strongly reducing environment	Promotes deep reduction	Depends on downstream enzyme systems	Usually favors DNRA
Low pH	Affects enzyme activity	May increase N₂O accumulation	Affects microbial community and enzyme activity
Sulfide	Provides reducing power and changes Eh	System-dependent	Often promotes DNRA

7 Functional Significance in Ecosystems

7.1 Agricultural Soil

In agricultural soils, nitrate originates from nitrogen fertilizers, mineralization, and nitrification. Nitrate reduction determines whether part of the nitrate enters crop uptake, denitrification loss, or DNRA retention. Enhanced denitrification can reduce nitrate accumulation, but if N₂O reduction is insufficient, greenhouse gas emission risk increases. Enhanced DNRA can convert nitrate to NH₄⁺ and retain it in soil, but it may also provide substrate for subsequent nitrification.

7.2 Wetlands and Sediments

Wetlands and sediments have clear redox gradients and are important environments where denitrification and DNRA coexist. Surface oxidized zones can generate NO₃⁻ through nitrification, while nitrate reduction occurs in deeper anoxic zones. Denitrification contributes to nitrogen removal from water bodies, whereas DNRA enhances nitrogen retention. Their relative proportions determine whether a wetland functions more like a nitrogen removal system or a nitrogen regeneration system.

7.3 Control of Water Eutrophication

In rivers, lakes, and estuarine systems, denitrification can reduce nitrate loads and is important for eutrophication control. If DNRA dominates, nitrate is converted to NH₄⁺ and can still be used by algae or microorganisms. Therefore, in water remediation, it is necessary to distinguish nitrate concentration decline from true total nitrogen removal.

7.4 Nitrogen Removal in Wastewater Treatment

In wastewater treatment systems, denitrification is one of the core processes of biological nitrogen removal. In anoxic tanks, nitrate reduction and nitrite reduction require suitable carbon sources, pH, dissolved oxygen, and sludge retention time. If process control is inadequate, NO₂⁻ or N₂O may accumulate, affecting total nitrogen removal efficiency and greenhouse gas emission levels.

8 Experimental Detection and Result Interpretation

8.1 Nitrogen Species Dynamics

Nitrate reduction studies should not only detect NO₃⁻ decline. NO₃⁻ reduction may result from assimilation, denitrification, DNRA, leaching, dilution, or adsorption. A more reasonable detection combination includes NO₃⁻, NO₂⁻, NH₄⁺, N₂O, N₂, and total nitrogen, with microbial biomass nitrogen or biomass ¹⁵N included when necessary.

(1) NO₂⁻ accumulation

NO₂⁻ accumulation suggests a mismatch between nitrate reduction and downstream reduction. Common causes include insufficient nitrite reductase activity, electron donor limitation, unsuitable pH, or oxygen interference.

(2) NH₄⁺ increase

When NO₃⁻ decline is accompanied by NH₄⁺ increase, DNRA should be considered, but the contribution from organic nitrogen mineralization must be excluded. ¹⁵N-nitrate tracing can confirm whether NH₄⁺ is produced from nitrate reduction.

(3) N₂O accumulation

N₂O accumulation suggests that denitrification may be incomplete. If nosZ abundance or expression is low, or environmental conditions inhibit N₂O reduction, N₂O may become the main gaseous product.

8.2 Functional Genes and Transcription Levels

Functional gene detection can reflect nitrate reduction and downstream partitioning potential. narG and napA indicate nitrate reduction capacity; nirK and nirS indicate denitrifying nitrite reduction capacity; nosZ indicates N₂O reduction potential; nrfA indicates DNRA potential. Gene abundance represents potential, while transcription level is closer to process activity, but both need to be interpreted together with nitrogen species and gaseous products.

8.3 Isotope Tracing

¹⁵N-nitrate tracing can distinguish the fate of nitrogen after nitrate reduction. By detecting ¹⁵NH₄⁺, ¹⁵N₂O, ¹⁵N₂, and biomass ¹⁵N, the contributions of DNRA, denitrification, and assimilation can be evaluated separately. For complex systems such as soil, sediments, and sludge, isotope tracing is more suitable than concentration changes alone for resolving true fluxes.

Table 6 Detection combinations in nitrate reduction research

Research objective	Recommended detection indicators	Key interpretation
Determine whether nitrate is reduced	NO₃⁻, NO₂⁻, narG, napA	Only indicates nitrate reduction trend or potential
Determine denitrification contribution	NO₂⁻, N₂O, N₂, nirK/nirS, nosZ	Distinguish complete from incomplete denitrification
Determine DNRA contribution	NH₄⁺, nrfA, ¹⁵NH₄⁺	Exclude organic nitrogen mineralization sources
Determine assimilation contribution	Biomass nitrogen, cellular nitrogen, biomass ¹⁵N	Interpret together with growth status and nitrogen source demand
Determine N₂O risk	N₂O, nosZ, pH, oxygen, organic carbon	Focus on imbalance between N₂O generation and reduction
Determine environmental drivers	C/NO₃⁻, Eh, pH, temperature, sulfide	Used to explain partitioning between denitrification and DNRA

9 Related Reagent and Material Selection

Table 7 Products for nitrate reduction and nitrogen cycle research

Application module	Cat. No.	Product Name	Grade/Specification	Application Positioning
Nitrite reductase activity detection	N1515945	Nitrite Reductase (NiR) Activity Assay Kit (NO₂⁻, Micro Method)	BioReagent	Detects NiR activity; suitable for assimilatory reduction or denitrifying nitrite reduction research
Nitrite reductase activity detection	N1515946	Nitrite Reductase (NiR) Activity Assay Kit (NO₂⁻, Colorimetric Method)	BioReagent	Colorimetric detection of NiR activity; suitable for routine sample throughput analysis
Soil nitrite reductase detection	S1515947	Soil Nitrite Reductase (S-NiR) Activity Assay Kit (NO₂⁻, Micro Method)	BioReagent	Soil sample NiR activity detection, suitable for denitrification potential analysis
Soil nitrite reductase detection	S1515948	Soil Nitrite Reductase (S-NiR) Activity Assay Kit (NO₂⁻, Colorimetric Method)	BioReagent	Colorimetric detection of soil NiR activity, suitable for farmland, wetland, and sediment samples
Soil nitrate reductase detection	S1508233	Soil Nitrate Reductase (S-NR) Activity Assay Kit (Naphthylamine, Micro Method)	BioReagent	Soil NR activity detection for evaluating NO₃⁻ → NO₂⁻ reduction potential
Nitrate reductase sample processing	N1518233	Nitrate Reductase (NR) Extraction Reagent	BioReagent,Suitable for plant cell and tissue extracts	NR extraction from plant samples, suitable for assimilatory nitrate reduction research
Nitrate reductase activity detection	N1521792	Nitrate Reductase (NR) Activity Assay Kit (in vivo Micro-Method)	BioReagent	In vivo NR activity detection, suitable for plant nitrate assimilation analysis
Nitrate reductase activity detection	N1521793	Nitrate Reductase (NR) Activity Assay Kit (in vivo Colorimetric Method)	BioReagent	In vivo colorimetric detection of NR activity, suitable for routine plant sample analysis
Nitrate reductase activity detection	N1521790	Nitrate Reductase (NR) Activity Assay Kit (in vitro Micro-Method)	BioReagent	In vitro NR activity detection, suitable for enzyme activity conditions and treatment group comparison
Nitrate reductase activity detection	N1521791	Nitrate Reductase (NR) Activity Assay Kit (in vitro Colorimetric Method)	BioReagent	In vitro colorimetric detection of NR activity, suitable for routine experimental platforms
Nitrate reductase activity detection	N1515816	Nitrate Reductase (NR) Activity Assay Kit (Naphthylamine, Micro Method)	BioReagent	Micro-method detection of NR activity, suitable for plant or microbial samples
Nitrate reductase	N1522975	Nitrate Reductase (NAD[P]H)	Bioactive, ActiBioPure™, Native, High Performance, EnzymoPure™, from Aspergillus niger; ≥50 U/mg enzyme powder	In vitro NO₃⁻ reduction reaction, NR enzyme activity method validation, and cofactor dependence research
Water and soil nitrite detection	N1508420	Water and Soil Nitrite Content Assay Kit (NED, Micro Method)	BioReagent,for environmental analysis, Colorimetry	NO₂⁻ accumulation detection in soil, water, and sediment
Water and soil nitrite detection	N1508208	Water and Soil Nitrite Content Assay Kit (NED, Micro Method)		Micro-method NO₂⁻ detection, suitable for environmental sample analysis
Water and soil nitrite detection	N1521785	Nitrite Content in Water and Soil Assay Kit (Naphazoline hydrochloride, Colorimetric Method)	BioReagent	Colorimetric NO₂⁻ detection, suitable for analyzing mismatch between nitrate reduction and downstream reduction
Ammonium ion standard	A301468	Standard material for analysis of Ammonium ion in water	100μg/ml ±2% (20℃)	NH₄⁺ standard curve, supporting DNRA product detection
Ammonium ion standard	A117451	NH4+ in Water	1000μg/ml ±1% (20℃)	High-concentration NH₄⁺ standard solution, suitable for method calibration and dilution preparation
Ammonium ion standard	I141284	Ammonium ion standard solution	1000μg/ml in Water (20℃)	NH₄⁺ quantitative standard, suitable for water samples and extracts
Ammonium ion electrode standard	I123811	Ammonium Ion Selective Electrode Solutions	0.1M Standard	Calibration of ammonium ion-selective electrodes
Ammonium ion electrode standard	I123827	Ammonium Ion Selective Electrode Solutions	1000ppm Standard	Calibration for ISE-based NH₄⁺ detection
Ammonium ion electrode auxiliary solution	I123792	Ammonium Ion Selective Electrode Solutions	ISA	Ionic strength control in ammonium ion electrode detection
Ammonium ion electrode auxiliary solution	I123774	Ammonium Ion Selective Electrode Solutions	Fill Solution	Ammonium ion-selective electrode maintenance and detection system support
Cation mixed standard	BWW292898	Four canions mixed standard (Calcium, Magnesium，Sodium，Ammonium)	1000μg/mL in H2O	NH₄⁺ calibration in ion chromatography or multi-cation analysis
Cation mixed standard	BWW292895	Four canions mixed standard (Calcium, Magnesium，Sodium，Ammonium)	100μg/mL in H2O	Low-concentration multi-cation standard, suitable for environmental water sample analysis
Cation mixed standard	BWW292931	Five canions mixed standard ( Sodium,Ammonium,Potassium, Magnesium,Calcium )	1000μg/mL in H2O，Uncertainty：1%	Multi-ion calibration standard containing NH₄⁺
Cation mixed standard	BWW292924	Five canions mixed standard ( Sodium,Ammonium,Potassium, Magnesium,Calcium )	100μg/mL in H2O，Uncertainty：2%	Low-concentration multi-ion standard, suitable for environmental sample NH₄⁺ quantification
Cation mixed standard	BWW292934	Six canions mixed standard ( Sodium,Ammonium,Potassium, Magnesium,Calcium,Lithium )	100μg/mL in H2O，Uncertainty：2%	Calibration for multi-cation detection methods
Cation mixed standard	BWW292944	Six canions mixed standard ( Sodium,Ammonium,Potassium, Magnesium,Calcium,Lithium )	1000μg/mL in H2O，Uncertainty：1%	High-concentration multi-cation standard, suitable for dilution preparation
Total nitrogen standard	N742313	Total Nitrogen solution	100μg/ml in water	Total nitrogen standard curve and nitrogen budget analysis
Total nitrogen standard	N117445	Total Nitrogen solution	analytical standard, 1000μg/ml in water	Total nitrogen detection method calibration
Total nitrogen standard	N119804	Standard solution of total nitrogen in water	500mg/L	Standard for total nitrogen detection in water samples
Total nitrogen standard	N117895	Water quality total nitrogen standard	analytical standard, 1.21-3.58(mg/L)	Water quality total nitrogen quality control and method validation
¹⁵N nitrite tracing	P1491399	Potassium nitrite-¹⁵N		NO₂⁻ downstream partitioning tracing, applicable to intermediate tracing in denitrification or DNRA
¹⁵N nitrate tracing	P117721	Potassium nitrate -¹⁵N	≥99 atom%,≥99%	Traces the fate of NO₃⁻ entering N₂O, N₂, NH₄⁺, or biomass

Nitrate reduction determines whether NO₃⁻ enters denitrification, DNRA, or assimilatory utilization. When studying this process, nitrogen species, enzyme activity assays, functional genes, isotope tracing, and environmental conditions should be combined to determine nitrogen fate.

For more related articles, please see below:

[1] Determination of nitrate reductase activity

[2] In vivo assay of nitrate reductase activity in objects

[3] Experimental determination of nitrate reductase activity in plants by the in vitro method

[4] Nitrate reductase activity assay (in vivo method)