Influence of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on ammonia-oxidizing bacteria and archaea in rhizosphere and bulk soil

Assessing the effectiveness of 3, 4-dimethylpyrazole phosphate (DMPP) inhibitor in mitigating N2O emissions from contrasting Cd-contaminated soils

Improving nitrogen use efficiency of chemical fertilizers is essential to mitigate the negative environmental impacts of nitrogen. Nitrification, the conversion of ammonium to nitrate via nitrite by soil microbes, is a prominent source of nitrogen loss in soil systems. The effectiveness of nitrification inhibitors in reducing nitrogen loss through inhibition of nitrification is well-documented, however, their efficacy in heavy metals-contaminated soils needs thorough investigations. The current study assessed the efficacy of nitrification inhibitor 3, 4-dimethylpyrazole phosphate (DMPP) in reducing nitrous oxide (N2O) emissions in cadmium (Cd) contaminated paddy and red soils under lab-controlled environment. Obtained results indicated the substantial reduction in N2O emissions with DMPP in paddy and red soil by 48 and 35 %, respectively. However, Cd contamination resulted in reduced efficacy of DMPP, thus decreased the N2O emissions by 36 and 25 % in paddy and red soil, respectively. It was found that addition of DMPP had a significant effect on the abundance of ammonia oxidizing bacteria (AOB) and archaea (AOA). Notably, the reduction in N2O emissions by DMPP varied with the abundance of AOB. Moreover, Cd pollution resulted in a significant (P < 0.05) reduction in the abundance of archaeal and bacterial amoA genes, as well as bacterial nirK, nirS, and nosZ genes. The combined treatment of Cd and DMPP had a detrimental impact on denitrifiers, thereby influencing the overall efficiency of DMPP. These findings provide novel insights into the application of DMPP to mitigate nitrification and its potential role in reducing N2O emissions in contaminated soils.

Drug discovery-based approach identifies new nitrification inhibitors

Nitrogen (N) fertilization is crucial to sustain global food security, but fertilizer N production is energy-demanding and subsequent environmental N losses contribute to biodiversity loss and climate change. N losses can be mitigated be interfering with microbial nitrification, and therefore the use of nitrification inhibitors in enhanced efficiency fertilizers (EEFs) is an important N management strategy to increase N use efficiency and reduce N pollution. However, currently applied nitrification inhibitors have limitations and do not target all nitrifying microorganisms. Here, to identify broad-spectrum nitrification inhibitors, we adopted a drug discovery-based approach and screened 45,400 small molecules on different groups of nitrifying microorganisms. Although a high number of potential nitrification inhibitors were identified, none of them targeted all nitrifier groups. Moreover, a high number of new nitrification inhibitors were shown to be highly effective in culture but did not reduce ammonia consumption in soil. One archaea-targeting inhibitor was not only effective in soil, but even reduced - when co-applied with a bacteria-targeting inhibitor - ammonium consumption and greenhouse gas emissions beyond what is achieved with currently applied nitrification inhibitors. This advocates for combining different types of nitrification inhibitors in EEFs to optimize N management practices and make agriculture more sustainable.

Evaluation of a crop rotation with biological inhibition potential to avoid N2O emissions in comparison with synthetic nitrification inhibition

Agriculture has increased the release of reactive nitrogen to the environment due to crops' low nitrogen-use efficiency (NUE) after the application of nitrogen-fertilisers. Practices like the use of stabilized-fertilisers with nitrification inhibitors such as DMPP (3,4-dimethylpyrazole phosphate) have been adopted to reduce nitrogen losses. Otherwise, cover crops can be used in crop-rotation-strategies to reduce soil nitrogen pollution and benefit the following culture. Sorghum (Sorghum bicolor) could be a good candidate as it is drought tolerant and its culture can reduce nitrogen losses derived from nitrification because it exudates biological nitrification inhibitors (BNIs). This work aimed to evaluate the effect of fallow-wheat and sorghum cover crop-wheat rotations on N₂O emissions and the grain yield of winter wheat crop. In addition, the suitability of DMPP addition was also analyzed. The use of sorghum as a cover crop might not be a suitable option to mitigate nitrogen losses in the subsequent crop. Although sorghum-wheat rotation was able to reduce 22% the abundance of amoA, it presented an increment of 77% in cumulative N₂O emissions compared to fallow-wheat rotation, which was probably related to a greater abundance of heterotrophic-denitrification genes. On the other hand, the application of DMPP avoided the growth of ammonia-oxidizing bacteria and maintained the N₂O emissions at the levels of unfertilized-soils in both rotations. As a conclusion, the use of DMPP would be recommendable regardless of the rotation since it maintains NH₄⁺ in the soil for longer and mitigates the impact of the crop residues on nitrogen soil dynamics.

A Novel High Temperature Resistant and Multifunctional Nitrification Inhibitor: Synthesis, Characterization, and Application

The industrialized nitrification inhibitors are not suitable for compound fertilizer fabrication through high tower melt granulation process due to their poor resistance to high temperature. In this paper, a novel high temperature resistant and multifunctional nitrification inhibitor (HTRMFNI) was synthesized. The HTRMFNI is a polymer product with the complex of silicic acid and 3,4-dimethylpyrazole (DMPZ) wrapped inside the polymer and the effective content of DMPZ is 0.484 wt %. The HTRMFNI presents good nitrification inhibitory performance and rapid phosphate-solubilizing ability. The decomposition temperature of HTRMFNI is ∼212 °C, satisfying the temperature requirements for the high tower melt granulation process. The fabricated compound fertilizer presents good nitrogen immobilization performance but loses the phosphate-solubilizing ability, possibly due to the damages of carboxyl functional group on the wrapping polymer by the high melting temperature. Moreover, the addition of HTRMFNI did not affect the physicochemical properties and the overall performance of the compound fertilizer.

Quantification of 3,4-Dimethyl-1H-Pyrazole Using Ion-Pair LC-MS/MS on a Reversed-Phase Column

BACKGROUND

Few methods exist for the analysis of the soil nitrification inhibitor 3,4-dimethyl-1H-pyrazole (3,4-DMP), which is a pesticide with the ability to reduce the production of nitrogenous greenhouse gases in soils as a result of fertilizer application. Due to its small size and polar nature, 3,4-DMP can be difficult to retain on an LC column, which makes diversion of a co-extracted soil matrix away from the MS/MS impossible.

OBJECTIVE

The current study aims to better control the retention time (RT) of 3,4-DMP. Additionally, 3,4-DMP-15N2 was synthesized and used as an internal standard for the soil extraction of 3,4-DMP.

METHODS

Perfluoroalkanoic acids were used as ion-pair reagents and were compared for their abilities to improve peak shape and RT, to better separate 3,4-DMP from the soil matrix without the need for cleanup during soil extraction.

RESULTS

RTs increased with both the carbon number and the concentration of the perfluoroalkanoic acid, and this improved peak shape and height. Perfluorooctanoic acid performed best, and improved peak height (PH) and shape were obtained by increasing the flow rate, resulting in a better S/N than from formic acid. The method provided a 10-fold improvement limit of quantitation on the most sensitive existing method and the use of 3,4-DMP-15N2 as an internal standard resulted in recoveries of 101-107%.

CONCLUSION

Ion-pair reagents drastically increased the retention of 3,4-DMP and allowed the re-use of old LC columns that may otherwise be discarded. Improved separation of 3,4-DMP from the soil matrix allowed much of the matrix to be diverted from the MS/MS spray chamber.

HIGHLIGHTS

Greater control of 3,4-DMP retention by the LC column resulting in the ability to separate 3,4-DMP from the soil matrix. The inclusion of ion-pair reagents only in the aqueous phase reduced ionization suppression of the analytes in the MS source.

Arbuscular Mycorrhiza and Nitrification: Disentangling Processes and Players by Using Synthetic Nitrification Inhibitors

Both plants and their associated arbuscular mycorrhizal (AM) fungi require nitrogen (N) for their metabolism and growth. This can result in both positive and negative effects of AM symbiosis on plant N nutrition. Either way, the demand for and efficiency of uptake of mineral N from the soil by mycorrhizal plants are often higher than those of nonmycorrhizal plants. In consequence, the symbiosis of plants with AM fungi exerts important feedbacks on soil processes in general and N cycling in particular. Here, we investigated the role of the AM symbiosis in N uptake by Andropogon gerardii from an organic source (¹⁵N-labeled plant litter) that was provided beyond the direct reach of roots. In addition, we tested if pathways of ¹⁵N uptake from litter by mycorrhizal hyphae were affected by amendment with different synthetic nitrification inhibitors (dicyandiamide [DCD], nitrapyrin, or 3,4-dimethylpyrazole phosphate [DMPP]). We observed efficient acquisition of ¹⁵N by mycorrhizal plants through the mycorrhizal pathway, independent of nitrification inhibitors. These results were in stark contrast to ¹⁵N uptake by nonmycorrhizal plants, which generally took up much less ¹⁵N, and the uptake was further suppressed by nitrapyrin or DMPP amendments. Quantitative real-time PCR analyses showed that bacteria involved in the rate-limiting step of nitrification, ammonia oxidation, were suppressed similarly by the presence of AM fungi and by nitrapyrin or DMPP (but not DCD) amendments. On the other hand, abundances of ammonia-oxidizing archaea were not strongly affected by either the AM fungi or the nitrification inhibitors. IMPORTANCE Nitrogen is one of the most important elements for all life on Earth. In soil, N is present in various chemical forms and is fiercely competed for by various microorganisms as well as plants. Here, we address competition for reduced N (ammonia) between ammonia-oxidizing prokaryotes and arbuscular mycorrhizal fungi. These two functionally important groups of soil microorganisms, participating in nitrification and plant mineral nutrient acquisition, respectively, have often been studied in separation in the past. Here, we showed, using various biochemical and molecular approaches, that the fungi systematically suppress ammonia-oxidizing bacteria to an extent similar to that of some widely used synthetic nitrification inhibitors, whereas they have only a limited impact on abundance of ammonia-oxidizing archaea. Competition for free ammonium is a plausible explanation here, but it is also possible that the fungi produce some compounds acting as so-called biological nitrification inhibitors.

A meta-analysis to examine whether nitrification inhibitors work through selectively inhibiting ammonia-oxidizing bacteria

Nitrification inhibitor (NI) is often claimed to be efficient in mitigating nitrogen (N) losses from agricultural production systems by slowing down nitrification. Increasing evidence suggests that ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) have the genetic potential to produce nitrous oxide (N₂O) and perform the first step of nitrification, but their contribution to N₂O and nitrification remains unclear. Furthermore, both AOA and AOB are probably targets for NIs, but a quantitative synthesis is lacking to identify the “indicator microbe” as the best predictor of NI efficiency under different environmental conditions. In this present study, a meta-analysis to assess the response characteristics of AOB and AOA to NI application was conducted and the relationship between NI efficiency and the AOA and AOB amoA genes response under different conditions was evaluated. The dataset consisted of 48 papers (214 observations). This study showed that NIs on average reduced 58.1% of N₂O emissions and increased 71.4% of soil NH4+ concentrations, respectively. When 3, 4-dimethylpyrazole phosphate (DMPP) was applied with both organic and inorganic fertilizers in alkaline medium soils, it had higher efficacy of decreasing N₂O emissions than in acidic soils. The abundance of AOB amoA genes was dramatically reduced by about 50% with NI application in most soil types. Decrease in N₂O emissions with NI addition was significantly correlated with AOB changes (R² = 0.135, n = 110, P < 0.01) rather than changes in AOA, and there was an obvious correlation between the changes in NH4+ concentration and AOB amoA gene abundance after NI application (R² = 0.037, n = 136, P = 0.014). The results indicated the principal role of AOB in nitrification, furthermore, AOB would be the best predictor of NI efficiency.

Changes in Ammonia-Oxidizing Archaea and Bacterial Communities and Soil Nitrogen Dynamics in Response to Long-Term Nitrogen Fertilization

Ammonia oxidizing archaea (AOA) and bacteria (AOB) mediate a crucial step in nitrogen (N) metabolism. The effect of N fertilizer rates on AOA and AOB communities is less studied in the wheat-fallow system from semi-arid areas. Based on a 17-year wheat field experiment, we explored the effect of five N fertilizer rates (0, 52.5, 105, 157.5, and 210 kg ha^-1 yr^-1) on the AOA and AOB community composition. This study showed that the grain yield of wheat reached the maximum at 105 kg N ha^-1 (49% higher than control), and no further significant increase was observed at higher N rates. With the increase of N, AOA abundance decreased in a regular trend from 4.88 × 10⁷ to 1.05 × 10⁷ copies g^-1 dry soil, while AOB abundance increased from 3.63 × 10⁷ up to a maximum of 8.24 × 10⁷ copies g^-1 dry soil with the N105 treatment (105 kg N ha^-1 yr^-1). Application rates of N fertilizer had a more significant impact on the AOB diversity than on AOA diversity, and the highest AOB diversity was found under the N105 treatment in this weak alkaline soil. The predominant phyla of AOA and AOB were Thaumarchaeota and Proteobacteria, respectively, and higher N treatment (N210) resulted in a significant decrease in the relative abundance of genus Nitrosospira. In addition, AOA and AOB communities were significantly associated with grain yield of wheat, soil potential nitrification activity (PNA), and some soil physicochemical parameters such as pH, NH₄-N, and NO₃-N. Among them, soil moisture was the most influential edaphic factor for structuring the AOA community and NH₄-N for the AOB community. Overall, 105 kg N ha^-1 yr^-1 was optimum for the AOB community and wheat yield in the semi-arid area.

Dimethylpyrazole-based nitrification inhibitors have a dual role in N2O emissions mitigation in forage systems under Atlantic climate conditions

Nitrogen fertilization is the most important factor increasing nitrous oxide (N₂O) emissions from agriculture, which is a powerful greenhouse gas. These emissions are mainly produced by the soil microbial processes of nitrification and denitrification, and the application of nitrification inhibitors (NIs) together with an ammonium-based fertilizer has been proved as an efficient way to decrease them. In this work the NIs dimethylpyrazole phosphate (DMPP) and dimethylpyrazole succinic acid (DMPSA) were evaluated in a temperate grassland under environmental changing field conditions in terms of their efficiency reducing N₂O emissions and their effect on the amount of nitrifying and denitrifying bacterial populations responsible of these emissions. The stimulation of nitrifying bacteria induced by the application of ammonium sulphate as fertilizer was efficiently avoided by the application of both DMPP and DMPSA whatever the soil water content. The denitrifying bacteria population capable of reducing N₂O up to N₂ was also enhanced by both NIs provided that sufficiently high soil water conditions and low nitrate content were occurring. Therefore, both NIs showed the capacity to promote the denitrification process up to N₂ as a mechanism to mitigate N₂O emissions. DMPSA proved to be a promising NI, since it showed a more significant effect than DMPP in decreasing N₂O emissions and increasing ryegrass yield.

Fertilizer stabilizers reduce nitrous oxide emissions from agricultural soil by targeting microbial nitrogen transformations

Nitrous oxide (N₂O) is a pollutant released from agriculture soils following N fertilizer application. N stabilizers, such as N-(n-butyl) thiophosphoric triamide (NBPT) and 3,4-dimethylpyrazole phosphate (DMPP) could mitigate these N₂O emissions when applied with fertilizer. Here, field experiments were conducted to investigate the microbial mechanisms by which NBPT and DMPP mitigate N₂O emissions following urea application. We determined dynamic N₂O emissions and inorganic N concentrations for two wheat seasons and combined this with metagenomic sequencing. Application of NBPT, DMPP, and both NBPT and DMPP together with urea decreased mean N₂O accumulative emissions by 77.8, 91.4 and 90.7%, respectively, compared with urea application alone, mainly via repressing the increase in NO₂^- concentration after N fertilization. Sequencing results indicated that urea application enriched microorganisms that were positively correlated with N₂O production, whereas N stabilizers enriched microorganisms that were negatively correlated with N₂O production. Furthermore, compared to urea application alone, NBPT with urea reduced the abundances of genes related to denitrification, including napA/nasA, nirS/nirK, and norBC, resulting in a higher soil NO₃^- pool. Conversely, DMPP application, either alone or together with NBPT, decreased the abundance of genes involved in ammonia oxidation and denitrification, including amoCAB, hao, napA/nasA, nirS/nirK, and norBC, and maintained a greater soil NH₄⁺ pool. Both N stabilizers resulted in similar abundances of nirABD-which is related to NO₂^- reducers-as when no N fertilizer was applied, which could prevent NO₂^- accumulation, consequently mitigating N₂O emissions. These findings suggest that the high effectiveness of N stabilizers on mitigating N₂O emissions could be attributed to changes to soil microbial communities and N-cycling functional genes to control the by-product or intermediate products of microbial N-cycling processes in agricultural soils.

How nitrification-related N2O is associated with soil ammonia oxidizers in two contrasting soils in China?

As a key process contributing to N₂O emissions, nitrification is regulated by soil microbes and mainly affected by soil pH, NH₃ availability, temperature and O₂ availability. Current knowledge gaps include how nitrification-related N₂O is associated with soil microbes in different pH soils. In the current study, a microcosm incubation experiment was conducted with two contrasting soils of different pH (5.08, 8.30) under controlled conditions. The soils were amended with ammonium sulphate ((NH₄)₂SO₄, 50 mg N kg^-1) combined with or without nitrification inhibitors and incubated under 20 °C, 65% water hold capacity (WHC) for three weeks. N₂O fluxes, mineral nitrogen (N) concentrations and ammonia oxidizers populations were measured during the incubation to investigate the correlations of nitrification-related N₂O with ammonia oxidizers. The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) was used to inhibit nitrification albeit to various inhibition effects with different soils. Acetylene (0.1% v/v C₂H₂), an inhibitor of AOA and AOB ammonia monooxygenase (AMO), was used to distinguish N₂O emissions by nitrifiers and denitrifiers. 1-octyne (5 μM aqueous), a selective specific AOB inhibitor, was used to assess the relative contributions of AOA and AOB to N₂O emissions. The results showed that N₂O yield for AOA and AOB varied with soil pH. AOB was the key microbial player in alkaline soil, contributing about 85% of nitrification-related N₂O. Conversely, about 78% of nitrification-related N₂O was contributed by AOA in acidic soil. Furthermore, there was a significant and positive relationship between mineral N (NO₂^-, NO₃^-), AOA and AOB populations and nitrification-related N₂O in alkaline soil. However, in acidic soil, NO₃^- concentration and AOA had significantly positive relationships with nitrification-related N₂O. To conclude, soil pH was a key factor affecting the contribution of ammonia oxidizers to nitrification-related N₂O emissions. AOA-related N₂O production dominated at low pH (5.08), while AOB-related N₂O was favored in alkaline soil (pH 8.3).

Biological nitrification inhibition in the rhizosphere: determining interactions and impact on microbially mediated processes and potential applications

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3-), and in fertilized soils it can lead to substantial N losses via NO3- leaching or nitrous oxide (N2O) production. To limit such problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the 'where' and 'how' of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3- retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

Effects of boric acid on urea-N transformation and 3,4-dimethylpyrazole phosphate efficiency

BACKGROUND

3,4-Dimethylpyrazole phosphate (DMPP) is a nitrification inhibitor which can restrict nitrate (NO₃ ^- ) production. Boric acid is a substance which inhibits urease activity. However, few studies have focused on the inhibitory effect of boric acid on urea hydrolysis and the possible synergistic effect with DMPP. Thus, an incubation trial was conducted to determine the impact of boric acid and DMPP addition on urea-N transformation, and their synergistic effects, in chernozem soil (Che) and red soil (RS). Four treatments were set up in each soil: urea only (U); urea combined with DMPP (UD); urea combined with boric acid (UB); and urea combined with both DMPP and boric acid (UDB).

RESULTS

Compared to U, adding DMPP (UD) increased NH₃ emissions by 11% and 13% and decreased soil NO₃ ^- -N concentration by 38% and 13% in Che and RS, respectively. Boric acid addition (UB) effectively prolonged the half-life time of urea by 0.8 and 0.4 days, reduced NH₃ volatilizations by 11% and 16% and delayed the occurrence of NH₃ emission peaks for 3 and 4 days in contrast to U treatment in Che and RS, respectively. UDB treatment mitigated the NH₃ volatilizations caused by the addition of DMPP (UD) by 16% and 29% in Che and RS, respectively. Additionally, a better nitrification inhibition rate was found in the UDB treatment compared to other treatments in both soils.

CONCLUSIONS

There is potential to develop a new N transformation inhibition strategy with the use of a combination of boric acid and DMPP. © 2020 Society of Chemical Industry.

Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate Application During the Later Stage of Apple Fruit Expansion Regulates Soil Mineral Nitrogen and Tree Carbon-Nitrogen Nutrition, and Improves Fruit Quality

In order to solve the problems of nitrogen (N) losses and fruit quality degradation caused by excessive N fertilizer application, different dosages of the nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP) (0, 0.5, 1, 2, and 4 mg kg^-1 soil), were applied during the later stage of 'Red Fuji' apple (Malus domestica Borkh.) fruit expansion in 2017 and 2018. The effects of DMPP on soil N transformation, carbon (C)-N nutrition of tree, and fruit quality were investigated. Results revealed that DMPP decreased the abundance of ammonia-oxidizing bacteria (AOB) amoA gene, increased the retention of NH₄ ⁺-N, and decreased NO₃ ^--N concentration and its vertical migration in soil. DMPP reduced ¹⁵N loss rates and increased ¹⁵N residual and recovery rates compared to the control. ¹³C and ¹⁵N double isotope labeling results revealed that DMPP reduced the capacity of ¹⁵N absorption and regulation in fruits, decreased ¹⁵N accumulation in fruits and whole plant, and increased the distribution of ¹³C from vegetative organs to fruits. DMPP increased fruit anthocyanin and soluble sugar contents, and had no significant effect on fruit yield. The comprehensive analysis revealed that the application of 1 mg DMPP kg^-1 soil during the later stage of fruit expansion effectively reduced losses due to N and alleviated quality degradation caused by excessive N fertilizer application.

Detection and Quantification of Nitrifying Bacteria Using Real-Time PCR

Nitrification is the microbial-mediated transformation of ammonium (NH₄⁺) into nitrate (NO₃^-). Many plant species depend on the availability of NO₃^- as the main source of nitrogen (N). On the other hand, because NO₃^- is highly mobile in the soil profile, its excess concentration can cause environmental pollution. Nitrification can be estimated at the process level, but with the development of molecular techniques it is also possible to estimate the abundance of nitrifying bacteria in the soil. Hence, in this chapter we describe the procedure for detection and quantification of nitrifying bacteria in soil samples using real-time quantitative polymerase chain reaction (PCR).

Effect of the nitrification inhibitor (3, 4-dimethylpyrazole phosphate) on the activities and abundances of ammonia-oxidizers and denitrifiers in a phenanthrene polluted and waterlogged soil

Through a 60-day microcosm incubation, the effect of 3, 4-dimethylpyrazole phosphate (DMPP) on the activities and abundances of ammonia-oxidizers and denitrifiers in phenanthrene-polluted soil was investigated. Five treatments were conducted for clean soil (CK), phenanthrene added (P), phenanthrene and DMPP added (PD), phenanthrene and urea added (PU), and phenanthrene, urea, and DMPP added (PUD) soils. The results indicate that the potential nitrification rate (PNR) in the P treatment was significantly higher than that in the PD treatment only on day 7, whereas the PNR in the PU treatment was significantly higher than that in the PUD treatment on each sampling day. The abundance of soil ammonia-oxidizing bacteria (AOB) in the PU treatment was significantly higher than that in the PUD treatment on each sampling day. Moreover, the abundance of AOB but rather than the ammonia-oxidizing archaea (AOA) had significantly positive correlation with soil PNR (P < 0.05). DMPP showed no obvious effect on the soil denitrification enzyme activity (DEA), which could have inhibited the abundances of denitrification-related narG, nirS, and nirK genes. The results of this study should provide a deeper understanding of the interaction between soil polycyclic aromatic hydrocarbons (PAH) contamination, ammonia oxidization, and denitrification, and offer valuable information for assessing the potential contribution of denitrification for soil PAH elimination.

Multiple-year nitrous oxide emissions from a greenhouse vegetable field in China: Effects of nitrogen management

The greenhouse vegetable (GV) field is an important agricultural system in China. It may also be a hot spot of nitrous oxide (N₂O) emissions. However, knowledge on N₂O emission from GV fields and its mitigation are limited due to considerable variations of N₂O emissions. In this study, we performed a multi-year experiment at a GV field in Beijing, China, using the static opaque chamber method, to quantify N₂O emissions from GV fields and evaluated N₂O mitigation efficiency of alternative nitrogen (N) managements. The experiment period spanned three rotation periods and included seven vegetable growing seasons. We measured N₂O emissions under four treatments, including no N fertilizer use (CK), farmers' conventional fertilizer application (FP), reduced N fertilizer rate (R), and R combined with the nitrification inhibitor "dicyandiamide (DCD)" (R+DCD). The seasonal cumulative N₂O emissions ranged between 2.09 and 19.66, 1.13 and 11.33, 0.94 and 9.46, and 0.15 and 3.27kgNha^-1 for FP, R, R+DCD, and CK, respectively. The cumulative N₂O emissions of three rotational periods varied from 18.71 to 26.58 (FP), 9.58 to 15.96 (R), 7.11 to 13.42 (R+DCD), and 1.66 to 3.73kgNha^-1 (CK). The R and R+DCD treatments significantly (P<0.05) reduced the N₂O emissions under FP by 38.1% to 48.8% and 49.5% to 62.0%, across the three rotational periods, although their mitigation efficiencies were highly variable among different vegetable seasons. This study suggests that GV fields associated with intensive N application and frequent flooding irrigation may substantially contribute to the N₂O emissions and great N₂O mitigations can be achieved through reasonably reducing the N-fertilizer rate and/or applying a nitrification inhibitor. The large variations in the N₂O emission and mitigation across different vegetable growing seasons and rotational periods stress the necessity of multi-year observations for reliably quantifying and mitigating N₂O emissions for GV systems.

Numerical Relationships Between Archaeal and Bacterial amoA Genes Vary by Icelandic Andosol Classes

Bacterial amoA genes had not been detectable by qPCR in freshly sampled Icelandic Andosols thus far. Hence, a new primer set yielding shorter gene fragments has been designed to verify the absence of ammonia-oxidizing bacteria in different Icelandic Andosol classes. At the same time, a new primer set was also constructed for archaeal amoA genes that should improve the quality of PCR products. Although a large part of the soil samples were found to be amoA-negative, bacterial amoA genes were detectable with new as well as old primer sets. The same results were obtained for the archaeal amoA genes. The relative distribution of archaeal and bacterial amoA genes varied between Andosol classes. Archaeal amoA genes were significantly more abundant in Brown than in Histic Andosols, while the opposite was observed for bacterial amoA genes. The numbers of archaeal and bacterial amoA genes in Gleyic Andosols were not significantly different from those in Histic and Brown Andosols. The numbers of bacterial amoA genes, but not the numbers of archaeal amoA genes, correlated significantly and positively with potential ammonia oxidation activities. The presence of the bacterial nitrification inhibitor allylthiourea inhibited the potential ammonia oxidation activities during the first 12 h of incubation. Hence, it was concluded that ammonia-oxidizing bacteria profited most from the conditions during the measurements of potential ammonia oxidation activities.

Comparison of the Effects of Phenylhydrazine Hydrochloride and Dicyandiamide on Ammonia-Oxidizing Bacteria and Archaea in Andosols

Dicyandiamide, a routinely used commercial nitrification inhibitor (NI), inhibits ammonia oxidation catalyzed by ammonia monooxygenase (AMO). Phenylhydrazine hydrochloride has shown considerable potential for the development of next-generation NIs targeting hydroxylamine dehydrogenase (HAO). The effects of the AMO inhibitor and the HAO inhibitor on ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) present in agricultural soils have not been compared thus far. In the present study, the effects of the two inhibitors on soil nitrification and the abundance of AOA and AOB as well as their community structure were investigated in a soil microcosm using quantitative polymerase chain reaction and pyrosequencing. The net nitrification rates and the growth of AOA and AOB in this soil microcosm were inhibited by both NIs. Both NIs had limited effect on the community structure of AOB and no effect on that of AOA in this soil microcosm. The effects of phenylhydrazine hydrochloride were similar to those of dicyandiamide. These results indicated that organohydrazine-based NIs have potential for the development of next-generation NIs targeting HAO in the future.

Nitrogen nutrition in cotton and control strategies for greenhouse gas emissions: a review

Cotton (Gossypium hirustum L.) is grown globally as a major source of natural fiber. Nitrogen (N) management is cumbersome in cotton production systems; it has more impacts on yield, maturity, and lint quality of a cotton crop than other primary plant nutrient. Application and production of N fertilizers consume large amounts of energy, and excess application can cause environmental concerns, i.e., nitrate in ground water, and the production of nitrous oxide a highly potent greenhouse gas (GHG) to the atmosphere, which is a global concern. Therefore, improving nitrogen use efficiency (NUE) of cotton plant is critical in this context. Slow-release fertilizers (e.g., polymer-coated urea) have the potential to increase cotton yield and reduce environmental pollution due to more efficient use of nutrients. Limited literature is available on the mitigation of GHG emissions for cotton production. Therefore, this review focuses on the role of N fertilization, in cotton growth and GHG emission management strategies, and will assess, justify, and organize the researchable priorities. Nitrate and ammonium nitrogen are essential nutrients for successful crop production. Ammonia (NH₃) is a central intermediate in plant N metabolism. NH₃ is assimilated in cotton by the mediation of glutamine synthetase, glutamine (z-) oxoglutarate amino-transferase enzyme systems in two steps: the first step requires adenosine triphosphate (ATP) to add NH₃ to glutamate to form glutamine (Gln), and the second step transfers the NH₃ from glutamine (Gln) to α-ketoglutarate to form two glutamates. Once NH₃ has been incorporated into glutamate, it can be transferred to other carbon skeletons by various transaminases to form additional amino acids. The glutamate and glutamine formed can rapidly be used for the synthesis of low-molecular-weight organic N compounds (LMWONCs) such as amides, amino acids, ureides, amines, and peptides that are further synthesized into high-molecular-weight organic N compounds (HMWONCs) such as proteins and nucleic acids.

Elevated N2O emission by the rice roots: based on the abundances of narG and bacterial amoA genes

Rice fields are an important source of nitrous oxide (N₂O), where rice plants could act as a key factor controlling N₂O fluxes during the flooding-drying process; however, the microbial driving mechanisms are unclear. In this study, specially designed equipment was used to grow rice plants and collect emitted N₂O from the root-growing zone (zone A), root-free zones (zones B, C, and D) independently, at tillering and booting stages under flooding and drying conditions. Soil samples from the four zones were also taken separately. Nitrifying and denitrifying community abundances were detected using quantitative polymerase chain reaction (qPCR). The N₂O emission increased significantly along with drying, but the N₂O emission capabilities varied among the four zones under drying, while zone B possessed the highest N₂O fluxes that were 2.7~4.5 times higher than those from zones C and D. However, zone A showed N₂O consumption potential. Notably, zone B also harbored the highest numbers of narG-containing denitrifiers and amoA-containing nitrifiers under drying at both tillering and booting stages. This study demonstrates that drying caused significant increase in N₂O emission from rhizosphere soil, in which the higher abundance of AOB would help to produce more nitrate and significantly higher narG-containing microbes would drive more N₂O production and emission.

Microbial N Transformations and N2O Emission after Simulated Grassland Cultivation: Effects of the Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate (DMPP)

Grassland cultivation can mobilize large pools of N in the soil, with the potential for N leaching and N₂O emissions. Spraying with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) before cultivation was simulated by use of soil columns in which the residue distribution corresponded to plowing or rotovation to study the effects of soil-residue contact on N transformations. DMPP was sprayed on aboveground parts of ryegrass and white clover plants before incorporation. During a 42-day incubation, soil mineral N dynamics, potential ammonia oxidation (PAO), denitrifying enzyme activity (DEA), nitrifier and denitrifier populations, and N₂O emissions were investigated. The soil NO₃^- pool was enriched with ¹⁵N to trace sources of N₂O. Ammonium was rapidly released from decomposing residues, and PAO was stimulated in soil near residues. DMPP effectively reduced NH₄⁺ transformation irrespective of residue distribution. Ammonia-oxidizing archaea (AOA) and bacteria (AOB) were both present, but only the AOB amoA transcript abundance correlated with PAO. DMPP inhibited the transcription of AOB amoA genes. Denitrifier genes and transcripts (nirK, nirS, and clades I and II of nosZ) were recovered, and a correlation was found between nirS mRNA and DEA. DMPP showed no adverse effects on the abundance or activity of denitrifiers. The ¹⁵N enrichment of N₂O showed that denitrification was responsible for 80 to 90% of emissions. With support from a control experiment without NO₃^- amendment, it was concluded that DMPP will generally reduce the potential for leaching of residue-derived N, whereas the effect of DMPP on N₂O emissions will be significant only when soil NO₃^- availability is limiting.

IMPORTANCE

Residue incorporation following grassland cultivation can lead to mobilization of large pools of N and potentially to significant N losses via leaching and N₂O emissions. This study proposed a mitigation strategy of applying 3,4-dimethylpyrazole phosphate (DMPP) prior to grassland cultivation and investigated its efficacy in a laboratory incubation study. DMPP inhibited the growth and activity of ammonia-oxidizing bacteria but had no adverse effects on ammonia-oxidizing archaea and denitrifiers. DMPP can effectively reduce the potential for leaching of NO₃^- derived from residue decomposition, while the effect on reducing N₂O emissions will be significant only when soil NO₃^- availability is limiting. Our findings provide insight into how DMPP affects soil nitrifier and denitrifier populations and have direct implications for improving N use efficiency and reducing environmental impacts during grassland cultivation.

Simultaneous ammonia and nitrate removal in an airlift reactor using poly(butylene succinate) as carbon source and biofilm carrier

In this study, an airlift inner-loop sequencing batch reactor using poly(butylene succinate) as the biofilm carrier and carbon source was operated under an alternant aerobic/anoxic strategy for nitrogen removal in recirculating aquaculture system. The average TAN and nitrate removal rates of 47.35±15.62gNH4-Nm(-3)d(-1) and 0.64±0.14kgNO3-Nm(-3)d(-1) were achieved with no obvious nitrite accumulation (0.70±0.76mg/L) and the dissolved organic carbon in effluents was maintained at 148.38±39.06mg/L. Besides, the activities of dissimilatory nitrate reduction to ammonium and sulfate reduction activities were successfully inhibited. The proteome KEGG analysis illustrated that ammonia might be removed through heterotrophic nitrification, while the activities of nitrate and nitrite reductases were enhanced through aeration treatment. The microbial community analysis revealed that denitrifiers of Azoarcus and Simplicispira occupied the dominate abundance which accounted for the high nitrate removal performance. Overall, this study broadened our understanding of simultaneous nitrification and denitrification using biodegradable material as biofilm carrier.

Effects of the Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate on Nitrification and Nitrifiers in Two Contrasting Agricultural Soils

The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is a powerful tool that can be used to promote nitrogen (N) use efficiency and reduce N losses from agricultural systems by slowing nitrification. Mounting evidence has confirmed the functional importance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in nitrification and N2O production; however, their responses to DMPP amendment and the microbial mechanisms underlying the variable efficiencies of DMPP across different soils remain largely unknown. Here we compared the impacts of DMPP on nitrification and the dynamics of ammonia oxidizers between an acidic pasture soil and an alkaline vegetable soil using a (15)N tracing and (13)CO2-DNA-stable-isotope probing (SIP) technique. The results showed that DMPP significantly inhibited nitrification and N2O production in the vegetable soil only, and the transient inhibition was coupled with a significant decrease in AOB abundance. No significant effects on the community structure of ammonia oxidizers or the abundances of total bacteria and denitrifiers were observed in either soil. The (15)N tracing experiment revealed that autotrophic nitrification was the predominant form of nitrification in both soils. The (13)CO2-DNA-SIP results indicated the involvement of AOB in active nitrification in both soils, but DMPP inhibited the assimilation of (13)CO2 into AOB only in the vegetable soil. Our findings provide evidence that DMPP could effectively inhibit nitrification through impeding the abundance and metabolic activity of AOB in the alkaline vegetable soil but not in the acidic pasture soil, possibly due to the low AOB abundance or the adsorption of DMPP by organic matter.

IMPORTANCE

The combination of the (15)N tracing model and (13)CO2-DNA-SIP technique provides important evidence that the nitrification inhibitor DMPP could effectively inhibit nitrification and nitrous oxide emission in an alkaline soil through influencing the abundance and metabolic activity of AOB. In contrast, DMPP amendment has no significant effect on nitrification or nitrifiers in an acidic soil, potentially owing to the low abundance of AOB and the possible adsorption of DMPP by organic matter. Our findings have direct implications for improved agricultural practices through utilizing the nitrification inhibitor DMPP in appropriate situations, and they emphasize the importance of microbial communities to the efficacy of DMPP.

Land Spreading of Wastewaters from the Fruit-Packaging Industry and Potential Effects on Soil Microbes: Effects of the Antioxidant Ethoxyquin and Its Metabolites on Ammonia Oxidizers

Thiabendazole (TBZ), imazalil (IMZ), ortho-phenylphenol (OPP), diphenylamine (DPA), and ethoxyquin (EQ) are used in fruit-packaging plants (FPP) with the stipulation that wastewaters produced by their application would be depurated on site. However, no such treatment systems are currently in place, leading FPP to dispose of their effluents in agricultural land. We investigated the dissipation of those pesticides and their impact on soil microbes known to have a key role on ecosystem functioning. OPP and DPA showed limited persistence (50% dissipation time [DT50], 0.6 and 1.3 days) compared to TBZ and IMZ (DT50, 47.0 and 150.8 days). EQ was rapidly transformed to the short-lived quinone imine (QI) (major metabolite) and the more persistent 2,4-dimethyl-6-ethoxyquinoline (EQNL) (minor metabolite). EQ and OPP exerted significant inhibition of potential nitrification, with the effect of the former being more persistent. This was not reflected in the abundance (determined by quantitative PCR [qPCR]) of the amoA gene of ammonia-oxidizing bacteria (AOB) and archaea (AOA). Considering the above discrepancy and the metabolic pattern of EQ, we further investigated the hypothesis that its metabolites and not only EQ were toxic to ammonia oxidizers. Potential nitrification, amoA gene abundance, and amoA gene transcripts of AOB and AOA showed that QI was probably responsible for the inhibition of nitrification. Our findings have serious ecological and practical implications for soil productivity and N conservation in agriculturally impacted ecosystems and stress the need to include metabolites and RNA-based methods when the soil microbial toxicity of pesticides is assessed.

Influence of natural zeolite and nitrification inhibitor on organics degradation and nitrogen transformation during sludge composting

Sludge composting is one of the most widely used treatments for sewage sludge resource utilization. Natural zeolite and nitrification inhibitor (NI) are widely used during composting and land application for nitrogen conservation, respectively. Three composting reactors (A--the control, B--natural zeolite addition, and C--3,4-dimethylpyrazole phosphate (DMPP) addition) were established to investigate the influence of NI and natural zeolite addition on organics degradation and nitrogen transformation during sludge composting conducted at the lab scale. The results showed that, in comparison with the control, natural zeolite addition accelerated organics degradation and the maturity of sludge compost was higher, while the DMPP addition slowed down the degradation of organic matters. Meanwhile, the nitrogen transformation functional genes including those responses for nitrification (amoA and nxrA) and denitrification (narG, nirS, nirK, and nosZ) were quantified through quantitative PCR (qPCR) to investigate the effects of natural zeolites and DMPP addition on nitrogen transformation. Although no significant difference in the abundance of nitrogen transformation functional genes was observed between treatments, addition of both natural zeolite and DMPP increases the final total nitrogen content by 48.6% and 23.1%, respectively. The ability of natural zeolite for nitrogen conservation was due to the absorption of NH3 by compost, and nitrogen conservation by DMPP was achieved by the source reduction of denitrification. Besides, it was assumed that the addition of natural zeolite and DMPP may affect the activity of these genes instead of the abundance.

Impact of quinoline on activity and microbial culture of partial nitrification process

In this study, the effect of quinoline on nitrification in an activated sludge system was evaluated in batch assays. Quinoline was evaluated to partial nitrification process or to partial nitrification-quinoline degrading process. In partial nitrification assays, 50-150mg/L quinoline presence promoted the ammonium oxidation efficiency (200-100%). Nonetheless, the consumption efficiencies for quinoline were less than 66.6%. On the other hand, in partial nitrification-quinoline degrading process, the promotion effect of quinoline (50-150mg/L) diminished significantly, ammonium oxidation were similarity to the control. However, the consumption efficiencies for quinoline were nearly 100%. DGGE results showed that the bacteria communities varied significantly. Acidovorax sp. JS42 and Acidovorax ebreus TPSY were responsible for ammonium oxidation in partial nitrification process, while Nitrosomonas in partial nitrification-quinoline degrading process. This information might be useful for treating wastewaters of ammonium/quinoline by partial nitrification technology.

The core root microbiome of sugarcanes cultivated under varying nitrogen fertilizer application

Diazotrophic bacteria potentially supply substantial amounts of biologically fixed nitrogen to crops, but their occurrence may be suppressed by high nitrogen fertilizer application. Here, we explored the impact of high nitrogen fertilizer rates on the presence of diazotrophs in field-grown sugarcane with industry-standard or reduced nitrogen fertilizer application. Despite large differences in soil microbial communities between test sites, a core sugarcane root microbiome was identified. The sugarcane root-enriched core taxa overlap with those of Arabidopsis thaliana raising the possibility that certain bacterial families have had long association with plants. Reduced nitrogen fertilizer application had remarkably little effect on the core root microbiome and did not increase the relative abundance of root-associated diazotrophs or nif gene counts. Correspondingly, low nitrogen fertilizer crops had lower biomass and nitrogen content, reflecting a lack of major input of biologically fixed nitrogen, indicating that manipulating nitrogen fertilizer rates does not improve sugarcane yields by enriching diazotrophic populations under the test conditions. Standard nitrogen fertilizer crops had improved biomass and nitrogen content, and corresponding soils had higher abundances of nitrification and denitrification genes. These findings highlight that achieving a balance in maximizing crop yields and minimizing nutrient pollution associated with nitrogen fertilizer application requires understanding of how microbial communities respond to fertilizer use.

Responses of soil ammonia-oxidizing microorganisms to repeated exposure of single-walled and multi-walled carbon nanotubes

The impacts of carbon nanotubes (CNTs) including single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) on soil microbial biomass and microbial community composition (especially on ammonium oxidizing microorganisms) have been evaluated. The first exposure of CNTs lowered the microbial biomass immediately, but the values recovered to the level of the control at the end of the experiment despite the repeated addition of CNTs. The abundance and diversity of ammonium-oxidizing archaea (AOA) were higher than that of ammonium-oxidizing bacteria (AOB) under the exposure of CNTs. The addition of CNTs decreased Shannon-Wiener diversity index of AOB and AOA. Two-way ANOVA analysis showed that CNTs had significant effects on the abundance and diversity of AOB and AOA. Dominant terminal restriction fragments (TRFs) of AOB exhibited a positive relationship with NH4(+), while AOA was on the contrary. It implied that AOB prefer for high-NH4(+) soils whereas AOA is favored in low NH4(+) soils in the CNT-contaminated soil.

The Importance of the Microbial N Cycle in Soil for Crop Plant Nutrition

Nitrogen is crucial for living cells, and prior to the introduction of mineral N fertilizer, fixation of atmospheric N2 by diverse prokaryotes was the primary source of N in all ecosystems. Microorganisms drive the N cycle starting with N2 fixation to ammonia, through nitrification in which ammonia is oxidized to nitrate and denitrification where nitrate is reduced to N2 to complete the cycle, or partially reduced to generate the greenhouse gas nitrous oxide. Traditionally, agriculture has relied on rotations that exploited N fixed by symbiotic rhizobia in leguminous plants, and recycled wastes and manures that microbial activity mineralized to release ammonia or nitrate. Mineral N fertilizer provided by the Haber-Bosch process has become essential for modern agriculture to increase crop yields and replace N removed from the system at harvest. However, with the increasing global population and problems caused by unintended N wastage and pollution, more sustainable ways of managing the N cycle in soil and utilizing biological N2 fixation have become imperative. This review describes the biological N cycle and details the steps and organisms involved. The effects of various agricultural practices that exploit fixation, retard nitrification, and reduce denitrification are presented, together with strategies that minimize inorganic fertilizer applications and curtail losses. The development and implementation of new technologies together with rediscovering traditional practices are discussed to speculate how the grand challenge of feeding the world sustainably can be met.

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.

Comparative effects of 3,4-dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD) on ammonia-oxidizing bacteria and archaea in a vegetable soil

Nitrification inhibitors (NIs) 3,4-dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD) have been used extensively to improve nitrogen fertilizer utilization in farmland. However, their comparative effects on ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in agricultural soils are still unclear. Here, we compared the impacts of these two inhibitors on soil nitrification, AOA and AOB abundance as well as their community structure in a vegetable soil by using real-time PCR and terminal restriction fragment length polymorphism (T-RFLP). Our results showed that urea application significantly increased the net nitrification rates, but were significantly inhibited by both NIs, and the inhibitory effect of DMPP was significantly greater than that of DCD. AOB growth was more greatly inhibited by DMPP than by DCD, and the net nitrification rate was significantly related to AOB abundance, but not to AOA abundance. Application of urea and NIs to soil did not change the diversity of the AOA community, with the T-RFs remaining in proportions that were similar to control soils, while the community structure of AOB exhibited obvious shifts within all different treatments compared to the control. Phylogenetic analysis showed that all AOA sequences fell within group 1.1a and group 1.1b, and the AOB community consisted of Nitrosospira cluster 3, cluster 0, and unidentified species. These results suggest that DMPP exhibited a stronger inhibitory effect on nitrification than DCD by inhibiting AOB rather than AOA.

Inhibition of bacterial ammonia oxidation by organohydrazines in soil microcosms

Hydroxylamine oxidation by hydroxylamine oxidoreductase (HAO) is a key step for energy-yielding in support of the growth of ammonia-oxidizing bacteria (AOB). Organohydrazines have been shown to inactivate HAO from Nitrosomonas europaea, and may serve as selective inhibitors to differentiate bacterial from archaeal ammonia oxidation due to the absence of bacterial HAO gene homolog in known ammonia-oxidizing archaea (AOA). In this study, the effects of three organohydrazines on activity, abundance, and composition of AOB and AOA were evaluated in soil microcosms. The results indicate that phenylhydrazine and methylhydrazine at the concentration of 100 μmol g⁻¹ dry weight soil completely suppressed the activity of soil nitrification. Denaturing gradient gel electrophoresis fingerprinting and sequencing analysis of bacterial ammonia monooxygenase subunit A gene (amoA) clearly demonstrated that nitrification activity change is well paralleled with the growth of Nitrosomonas europaea-like AOB in soil microcosms. No significant correlation between AOA community structure and nitrification activity was observed among all treatments during the incubation period, although incomplete inhibition of nitrification activity occurred in 2-hydroxyethylhydrazine-amended soil microcosms. These findings show that the HAO-targeted organohydrazines can effectively inhibit bacterial nitrification in soil, and the mechanism of organohydrazine affecting AOA remains unclear.