AOB Activity Research Articles

Environmental factors militating against the activities of ammonia oxidizing archaea and ammonia oxidizing bacteria in soil

This review aims to scrutinize the effect of ammonia concentration, temperature, pH, drought and inhibitors on activity of AOA and AOB in soil. Ammonia concentration in soil increases during drought due to the reduced soil water content and, with desiccation stress or a combination of both factors result in greater inhibition of AOA than AOB during drought. AOA were more susceptible to increased desiccation stress than AOB, irrespective of initial soil ammonium concentration and AOA cultures were more sensitive than AOB to osmotic stress which represent an additional niche differentiating factor between AOA and AOB in soil. Activity and growth of AOA and AOB observed in soil amended with high ammonium concentration at different temperature, suggesting that AOA can contribute to nitrification in highly fertilized soil. The selective inhibition of AOA by simvastatin in culture and in soil provides evidence for oxidation of ammonia by AOB at low ammonium concentration. The findings advance our understanding of the influence of ammonium supply, temperature and osmotic stress on soil nitrification and its role in controlling the availability of ammonium-based fertilizers for plant uptake.

Conservation strip-till modifies rhizosphere ammonia-oxidizing archaea and bacteria, increases nitrate accumulation and promotes maize growth at grain filling stage

Strip-till (ST) is a conservation tillage wherein the “inter-row” is not tilled and remains covered with protective residues, while the “row zone” is strip tilled. In comparison to conventional ridge tillage (CT), maize yield under ST is maintained due to its growth recovery at grain filling stage (GFS), though its growth lag at early growth stage in cold regions. To explore microbial mechanisms underlying the growth recovery, compared with CT, we investigated relationships among rhizosphere ammonia-oxidizing archaea and bacteria (AOA and AOB), rhizosphere soil characteristics, and maize growth under ST through a two-year field experiment in a Mollisol of Northeast China. Compared with CT, ST led to rhizosphere NO3--N accumulation at 0–5 cm location, which positively correlated with maize N accumulation and growth at GFS. ST significantly affected rhizosphere AOA and AOB communities at 0–5 and 20–30 cm locations, and decreased AOA abundance, norank_p_Thaumarchaeota relative abundance and bio-markers, but increased AOB abundance, richness, Nitrosospira relative abundance and bio-markers at 0–5 cm location. ST obviously increased the number of OTU hubs belonging to AOB Nitrosospira in networks and their complexity at 0–5 cm location. AOB community play a more important function role than AOA at 0–5 cm location. Rhizosphere NO3--N accumulation, maize N accumulation and growth recovery were closely related with AOB (especially Nitrosospira), but not AOA. It is concluded that, in contrast to CT, ST mainly stimulate rhizosphere AOB activity at 0–5 cm location (especially Nitrosospira) during grain filling stage, which contributes to more rhizosphere NO3--N accumulation and maize N absorption and faster growth. Our findings provide a theoretical basis for efficient crop-soil management to increase maize productivity under conservation tillage system.

Activity of Ammonium-Oxidizing Bacteria—An Essential Parameter for Model-Based N2O Mitigation Control Strategies for Biofilms

The reduction in N2O emissions is an important task in the control of wastewater treatment plants. Since local operating conditions, especially inside biofilms, are usually not known, models are an important tool in the development and implementation of control strategies. For a pilot-scale nitrifying biofilm reactor and an SBR, different operational strategies to reduce autotrophic nitrous oxide (N2O) formation were developed and tested by applying a combination of modeling and measurement. Both approaches highlighted the relevance of addressing the actual AOB activity as a sensitive control variable. The investigated strategies, therefore, focused on decreasing the AOB-related NH4 conversion rate, as autotrophic N2O formation is directly linked to AOB activity. The results showed that the biofilm system was more advantageous compared with suspended sludge systems. A higher AOB content resulted in a decrease in AOB activity, leading to fewer N2O emissions at the same reactor performance. The highest reduction in autotrophic N2O formation (SBR: 25%; Biofilm: 27%) was obtained by maximizing the aerated time per day and minimizing the number of aeration cycles (the suppression of nitrite-oxidizing bacteria still needed to be ensured). A higher biofilm thickness or a higher sludge mass in the SBR, however, did not have a noteworthy positive effect since no additional biomass could be kept in the system in the long term due to limited substrate availability. Besides nitritation, denitrification was also identified as a relevant source of N2O in both systems (biofilm: main source) due to the inhibition of N2O reduction by nitrous acid (elevated nitrite concentrations in combination with pH values < 7).

Partial Nitrification Characteristics of an Immobilized Carrier in Municipal Wastewater under Low-Temperature Shock: The Role of the Nitrifying Bacterial Community Structure

To analyze the effects of the nitrifying bacterial community structure on the partial nitrification (PN) characteristics in a PN-immobilized carrier in municipal wastewater under low-temperature shock, two PN-immobilized carriers with different nitrifying bacterial communities were investigated. The E1-immobilized carrier contained a high abundance of ammonia-oxidizing bacteria (AOB; 38.59%), and the E2-immobilized carrier had a low AOB abundance of 4.78%. The results of experiments with different dissolved oxygen (DO) concentrations showed that the oxygen-limited environment inside the immobilized carrier, generated by the high AOB abundance, was critical for achieving PN. The nitrite accumulation rate (NAR) decreased from 90.0–93.9% to 84.2–88.3% for the E1-immobilized carrier and from 86.0–90.4% to 81.7–85.8% for the E2-immobilized carrier under low-temperature shock (the temperature suddenly decreased from 25 ± 1 °C to 15 ± 1 °C). The decrease in the ammonia oxidation rate due to the decreased AOB activity led to a decrease in NAR. Moreover, NOB abundance in the E2-immobilized carrier increased because of the destruction of the oxygen-limiting region in the immobilized carrier due to the low AOB abundance. Increasing the abundance of AOB in the PN-immobilized carrier could reduce the adverse effects from the low-temperature shock. The results of this study can be used to further develop immobilization technology for efficient PN in mainstream wastewater treatment.

Anthropogenic land-use activities within watersheds reduce comammox activity and diversity in rivers

Nitrogen cycling plays a key role in maintaining river ecological functions which are threatened by anthropogenic activities. The newly discovered complete ammonia oxidation, comammox, provides novel insights into the ecological effects of nitrogen on that it oxidizes ammonia directly to nitrate without releasing nitrite as canonical ammonia oxidization conducted by AOA or AOB which is believed to play an important role in greenhouse gas generation. Theoretically, contribution of commamox, AOA and AOB to ammonia oxidization in rivers might be impacted by anthropogenic land-use activities through alterations in flow regime and nutrient input. While how land use pattern affects comammox and other canonical ammonia oxidizers remains elusive. In this study, we examined the ecological effects of land use practices on the activity and contribution of three distinctive groups of ammonia oxidizers (AOA, AOB, comammox) as well as the composition of comammox bacterial communities from 15 subbasins covering an area of 6166 km2 in North China. The results showed that comammox dominated nitrification (55.71%–81.21%) in less disturbed basins characterized by extensive forests and grassland, while AOB became the major player (53.83%–76.43%) in highly developed basins with drastic urban and agricultural development. In addition, increasing anthropogenic land use activities within the watershed lowered the alpha diversity of comammox communities and simplified the comammox network. Additionally, the alterations of NH4+-N, pH and C/N induced by land use change were found to be crucial drivers in determining the distribution and activity of AOB and comammox. Together, our findings cast a new light on aquatic-terrestrial linkages from the view of microorganism-mediated nitrogen cycling and can further be applied to target watershed land use management.

Advances in analysis, quantification and modelling of N2O emission in SBRs under various DO set points

Nitrous oxide (N2O), considered a major greenhouse gas (GHG) in wastewater treatment plants (WWTPs), is produced during both nitrification and denitrification processes; hence, it needs to be controlled by internal and external strategies. Various factors, such as DO, temperature, and pH, could be incorporated into the mitigation of emissions in WWTPs. In this research, potential operational strategies were investigated in order to find the optimal range for DO and temperature for controlling the N2O production during the nitrification process. In parallel, the activity of nitrite oxidizing bacteria (NOB) could also be limited under optimal conditions to make the process more cost-effective and energy-saving. In this regard, under a lab-scale environment, DO = 0.7 mg/l was detected as the optimal range for inhibiting NOB activity and maintaining AOB activity. Moreover, the importance of developing mathematical modelling methods has gained significant attention in order to better understand the possibility of minimizing GHG in WWTPs. In this study, advanced mathematical modelling methods were used for simulating the kinetics of the nitrification process to determine the interaction among different operating factors compared to nitrification rates.

A novel process for anammox pretreatment of municipal wastewater: semi-partial nitrification, biological phosphorus removal and recovery

Achieving simultaneous semi-partial nitrification and deep phosphorus removal is a preferred process technology for Anammox pretreatment. In this study, semi-partial nitrification combined with in-situ phosphorus recovery (PNPR) was used to treat municipal wastewater. The SRT conflict between the nitrification and phosphorus removal was resolved by in-situ phosphorus recovery every 20 cycles of Anaerobic/Oxid, and a supernatant with more than 10 times the influent phosphorus concentration was obtained, thus achieving bio-enhanced phosphorus removal and recovery with satisfactory semi-partial-nitrification effluent. Interestingly, the results showed that phosphorus removal and recovery process could improve the activity of AOB. The PNPR system’s nitrite accumulation rate (NAR) and phosphorus removal rate (PRR) were more than 90% each, whereas the relative abundance of AOB and PAOs increased from 0.04% to 0.74% and from 0.25% to 0.70%, respectively (P < 0.01). Furthermore, on average, the NO2–-Neff/NH4+-Neff value was 1.96, which laid the foundation for the subsequent anammox treatment.

Aeration strategy based on numerical modelling and the response mechanism of microbial communities under various operating conditions

Aeration system is the main energy consumer in a wastewater treatment process. In this paper, the Naive Bayes classification (NBC) algorithm and response surface method (RSM) were firstly used to establish a methodology to improve the aeration efficiency and estimate effluent quality. Lab-scale experiments were conducted to verify the model. The errors between experimental values and predicted values were 3.36, −0.67 and −3.78% at operating temperatures of 20, 30 and 35 °C, indicating the applicability. To further elucidate the biological mechanisms of the experimental results, the microbial community composition was investigated under various operating conditions, the results shows that aerobic heterotrophic bacteria (HET) activity and COD removal efficiency were promoted at 30 °C. AOB and NOB activity and NH4+-N removal efficiency were promoted at 30–35 °C. These findings together suggest that operating temperature is crucial for activated sludge treatment, which should be considered when regulating DO content or aeration rate in practical application.

Nutrients removal from low C/N actual municipal wastewater by partial nitritation/anammox (PN/A) coupling with a step-feed anaerobic-anoxic-oxic (A/A/O) system

In this study, a novel combined strategy was successfully established by partial nitritation/anammox (PN/A) within a step-feed A/A/O process integrated with fixed-biocarriers to treat municipal sewage for 200 days. The excellent nutrients removal performance of this system compared with national level of discharging standard were achieved: low total inorganic nitrogen (7.1 mg/L) and phosphorus (0.3 mg/L) in the effluent with the influent (51.1 and 4.2 mg/L) at C/N ratios of 3.4 ± 0.5, mainly attributed to the stable PN (oxic zone) and subsequently anammox effect (anoxic zone). Nitrogen mass balance indicated that anammox contribution in anoxic zones to nitrogen loss could be up to 42% at stable phase. Therefore, aeration and carbon cost could be greatly reduced under low DO, low C/N and aerobic hydraulic retention time (HRT) of 7.4 h condition. The low DO and anammox bacteria retention in anoxic chambers promoted the washout of NOB and combination of anammox and partial nitritation process. During long-term operation, the activity of AOB effectively maintained while that of NOB drastically reduced to 0.1 mg N / g MLSS / h resulting in high and stable nitrite accumulation ratios (about 90%). The achievement of partial nitritation was mainly due to low DO (0.4–0.5 mg/L) and effective retention of anammox bacteria even with a low temperature (14.5 °C). Notedly, anammox activity gradually increased both on the biocarriers and in the flocs while a higher anammox abundance was observed on the biocarriers (2.48%) than that in suspend flocs (0.03%). As above, this study indicated that the novel combined strategies could be applicable to mainstream anammox, and a pilot-scale reactor will be established to verify and promote the industrial application of mainstream anammox.

Impact of starvation conditions on the nitrifying performance and sludge properties in SBR system with a limited filamentous bulking state

Limited filamentous bulking (LFB) induced by low dissolved oxygen in activated sludge system is an effective energy saving process. However, starvation environment is liable to result in the unbalance between filaments and flocs, affecting the LFB system performance. The variations in nitrifying performance and properties of LFB sludge during 14 days of four starvation conditions (aerobic, alternating anaerobic/aerobic, anaerobic and anoxic) and their subsequent recovery were investigated in sequencing batch reactor (SBR) system. The results showed that the highest activity decay rates of ammonia- and nitrite-oxidizing bacteria (AOB and NOB) were observed under aerobic starvation condition, followed by anoxic, anaerobic, and alternating anaerobic/aerobic starvation conditions. In the reactivation period, the faster recovery of AOB activity and cell number, relative to NOB, particularly in aerobic case, led to temporary nitrite accumulation. Besides, the sludge settleability rapidly improved (SVI of ~30 mL/g) due to filamentous bacteria suppression under aerobic starvation, while the filaments (e.g. Type 0092) overgrew (SVI of ~250 mL/g) under anoxic starvation, triggering unexpected biomass loss and going against the nitrifying performance recovery of the system. In contrast, alternating anaerobic/aerobic and anaerobic starvations avoid pure aerobic or anoxic starvation condition, effectively maintaining the nitrifying performance and LFB state, and therefore are the best storage strategies for LFB sludge.

Treatment of Medium Ammonium Wastewater by Single-stage Partial Nitritation-ANAMMOX SMBBR

A SMBBR was established to treat medium ammonium under room temperature. Results showed that TN load can reach 0.16 kg·(m3·d)-1, and the average TN removal efficiency was (51.58±6.80)% in the SMBBR with an influent ammonia concentration of 100 mg·L-1 and DO of 0.4-0.7 mg·L-1. AOB, ANAMMOX, and NOB activity reached (2253.21±502.10) mg·(m2·d)-1, (4847.46±332.89) mg·(m2·d)-1, and (1455.17±473.83) mg·(m2·d)-1, and ANAMMOX and AOB bacteria were found to develop a good collaborative relationship. Quantitative PCR results showed that the relative abundance of ANAMMOX, AOB and NOB were 11.57%, 1.01% and 0.94%, respectively. The stable operation of single stage partial nitritation-ANAMMOX process provide an alternative technology for medium ammonia wastewater.

3, 4-Dimethylpyrazole phosphate is an effective and specific inhibitor of soil ammonia-oxidizing bacteria

A microcosm study was established using three arable soils differing in both physicochemical properties and AOB or AOA dominance of ammonia oxidization. Nitrification and N2O emissions were quantified under 10 Pa C2H2, 1-octyne, or 3,4-dimethylpyrazole phosphate (DMPP), and the community composition and abundance of AOA and AOB were assessed. Amendment of soil with DMPP had a comparable effect to 1-octyne on AOB activities, composition, and N2O emissions across soils. By blocking AOB activity, both DMPP and 1-octyne supported higher rates of AOA growth in both the acid and alkaline soils. Approximately, 50% of ammonia-oxidizing activity in the acid soil was attributed to AOB activity by phylotypes affiliated with Nitrosomonas communis. When AOB were blocked by DMPP or 1-octyne across all soils, recovery of AOA communities was associated with the increased abundance of Nitrososphaera viennesis.

Modelling AOB-NOB competition in shortcut nitrification compared with conventional nitrification-denitrification process

In particular, mainstream deammonification and/or shortened nitrification-denitrification via nitrite (so-called “nitrite shunt”) is a promising new treatment concept that has the potential to revolutionise how nitrogen removal is achieved at WWTPs. Understanding the role of the AOB/NOB competition in the nitrogen cycle in wastewater treatment systems will change operational strategies of the novel nitrogen removal processes. The key role in this process is inhibition of NOB activity undesirably affects AOB activity and leads to inefficient partial nitrification process and when used as pre-treatment for Anammox it can limit nitrite supply to Anammox bacteria. Successful NOB repression requires a combination of such factors as a low DO concentration, a rapid transition from aerobic to anoxic conditions, and tight control of Temperature and/or pH. The major driving force behind the successful NOB washout is the inhibition of those bacteria based on the difference in the growth rate between AOB and NOB. The obtained results from this study show the mechanisms and operating conditions (e.g. DO concentration, Temp.) leading to complete domination of AOB over NOB under aerobic conditions. This paper presents the perspectives on modelling AOB-NOB competition in shortcut nitrification. The combined deammonification, shortened nitrification-denitrification and/or nitritation-anammox process was compared with conventional nitrification-denitrification based on own experiments and literature data. Its successful application as shortcut nitrification technology and new control system will represent a paradigm shift for the wastewater industry, offering the opportunity for sustainable wastewater treatment, energy-neutral or even energy-positive facilities, and substantial reductions in treatment costs.

Short-cut Nitrification Start-up and Optimization of Operating Conditions Under Different Control Strategies

Low C/N domestic sewage was treated in a SBR. With an operating temperature of (25±0.5)℃, NO2--N accumulation rates reached 96.79%, 98.80%, and 98.78% after 69, 63, and 58 cycles, respectively. In each case, alternating modes of hypoxia/aerobic (four times), aerobic/anoxic (five times), and aerobic/anoxic (four times) were applied, respectively, with an alternating time of 30 min. At the same temperature, when the aerobic/anoxic time ratios were 30 min:30 min, 40 min:20 min, and 30 min:60 min, and the single-cycle alternating times were 5, 3, and 5, respectively, stable NO2--N accumulation rates reached 98.81%, 97.71%, and 94.64% after 63, 73, and 78 cycles, respectively. The activity of AOB was 96.30, 99.27, and 102.26, respectively. Simultaneous nitrification and denitrification occurred under the three aerobic/anoxic time ratios. The total amounts of nitrogen removed by synchronous nitrification and denitrification were 29.89, 28.77, and 29.78 mg·L-1. When temperatures were adjusted to 18, 25, and 30℃, and when the aerobic/anoxic time ratio was 30 min:30 min, the NO2--N accumulation rates were 99.58%, 99.21%, and 95.93% after 90, 64, and 61 cycles, respectively. The activity of the sludge (f) peaked when after 64, 40, and 48 cycles, while the sludge-settling performance was good.

Effect of ambient temperature variations on an indigenous microalgae-nitrifying bacteria culture dominated by Chlorella

Two outdoor photobioreactors were operated to evaluate the effect of variable ambient temperature on an indigenous microalgae-nitrifying bacteria culture dominated by Chlorella. Four experiments were carried out in different seasons, maintaining the temperature-controlled PBR at around 25 °C (by either heating or cooling), while the temperature in the non-temperature-controlled PBR was allowed to vary with the ambient conditions. Temperatures in the range of 15–30 °C had no significant effect on the microalgae cultivation performance. However, when the temperature rose to 30–35 °C microalgae viability was significantly reduced. Sudden temperature rises triggered AOB growth in the indigenous microalgae culture, which worsened microalgae performance, especially when AOB activity made the system ammonium-limited. Microalgae activity could be recovered after a short temperature peak over 30 °C once the temperature dropped, but stopped when the temperature was maintained around 28–30 °C for several days.

Effects of rhizosphere and long-term fertilisation practices on the activity and community structure of ammonia oxidisers under double-cropping rice field

ABSTRACTThe potential nitrification activity (PNA), population size and composition of AOB and AOA communities in both rhizosphere and bulk soil from a long-term (32 year) fertiliser field experiment conducted during early rice and late rice seasons were investigated by using the shaken slurry method and quantitative real-time polymerase chain reaction. The experiment begins in 1986, and including five treatments: without fertiliser input, chemical fertiliser alone, rice straw residue and chemical fertiliser, low organic manure rate and chemical fertiliser and high organic manure rate and chemical fertiliser. The results showed that the application of chemical fertiliser greatly enhanced PNA and AOB abundance, while application of rice straw residue and organic manure increased AOA abundance. Moreover, the results showed that the PNA and population sizes of AOB and AOA were higher in the rhizosphere than in the bulk soil. Cluster and redundancy analyses further indicated that the rhizosphere effect play a more important role in shaping AOA community structure than long-term fertilization. In summary, the results indicated that AOB rather than AOA functionally dominate ammonia oxidation in the double-cropping rice paddy soils, and that rhizosphere effect and fertiliser regime play different roles in the activity and community structures of AOB and AOA.

A strategy of high-efficient nitrogen removal by an ammonia-oxidizing bacterium consortium

An ammonia-oxidizing bacterium consortium showed approximately 100% removal of NH4+-N with an initial concentration of 262.28 ± 8.21 mg·L−1 within 10 days, and only 16.54 ± 0.52% of NH4+-N was converted to NO2−-N in this study. The consortium removed ammonium by heterotrophic nitrification and aerobic denitrification (HNAD) without N2O emission. The activity of AOB was not affected by low concentrations of FA or FNA, but completely inhibited by 0.04 mg HNO2·L−1. In a bioaugmentation treatment of eutrophic wastewater using the consortium, the removal efficiency of NH4+-N reached 90.85 ± 0.8% and 77.88 ± 1.86% at initial concentrations of 1.80 ± 0.04 mg·L−1 and 40.31 ± 0.57 mg·L−1, respectively, and the dissolved oxygen level had a significant impact on the consortium activity. No significant changes in the bacterial community structure were observed after the consortium addition, and local functional bacteria were enriched by aeration and contributed to ammonium nitrogen removal with AOB.

High-rate partial nitrification of semiconductor wastewater: Implications of online monitoring and microbial community structure

The aims of this study were to investigate the feasibility of partial nitrification (PN) in a sequencing batch reactor (SBR) fed with real wastewater discharged from the washing process at a semiconductor factory, to evaluate the efficacy of online control for nitrite buildup, and to examine the microbial community composition. To this end, online monitoring of NH4+ and NO3− concentrations in an SBR was implemented to track the extant activities of ammonia-oxidizing (AOB) and nitrite-oxidizing bacteria (NOB). The results showed that the rate of NH4+ oxidation by AOB increased during the first 20 days of the operation, and were maintained as high as a maximum volumetric rate of NH4+ removal of 2.17 kg-N/m3/day with an NH4+ removal efficiency of 94.1–99.8%. Successful nitrite buildup (> 99.9%) has been achieved after day 20 when AOB activity was 8.11-times higher than NOB activity. High-throughput sequencing of 16S rRNA genes manifested that the relative abundance of cluster 7 Nitrosomonas species in the family Nitrosomonadaceae increased from 0.2% to 50% over total bacteria. On the other hand, the relative abundance of Nitrospira spp. as the predominant NOB decreased from 1.0% to 0.05%, indicating NOB out-selection during SBR operations. A positive correlation was noted between the abundance of the dominant AOB and NH4+ oxidation rate, suggesting that a simple nitrifier community regulated high-rate PN.

System performance and microbial community succession in a partial nitrification biofilm reactor in response to salinity stress

The system performance and microbial community succession in a partial nitrification biofilm reactor in response to salinity stress was conducted. It was found that the NH4+-N removal efficiency decreased from 98.4% to 42.0% after salinity stress increased to 20 g/L. Specific oxygen uptake rates suggested that AOB activity was more sensitive to the stress of salinity than that of NOB. Protein and polysaccharides contents showed an increasing tendency in both LB-EPS and TB-EPS after the salinity exposure. Moreover, EEM results indicated that protein-like substances were the main component in LB-EPS and TB-EPS as self-protection in response to salinity stress. Additionally, humic acid-like substances were identified as the main component in the effluent organic matter (EfOM) of partial nitrification biofilm, whereas fulvic acid-like substances were detected at 20 g/L salinity stress. Microbial community analysis found that Nitrosomonas as representative species of AOB were significantly inhibited under high salinity condition.

Responses of nitrification and ammonia oxidizers to a range of background and adjusted pH in purple soils

Soil pH is often changed by anthropogenic activities such as agronomic management, land use or acidifying pollution, and is widely considered to be a dominant factor affecting soil nitrogen cycling. In this study, three purple soils originating from similar parent materials but varying in pH (5.7, 7.3, and 8.0), representing acidic, neutral and alkaline soils, were selected to determine the effect of background pH on net nitrification rate (NNR) and ammonia oxidizers (AOA and AOB). The background pH of each soil was modified to the pH of the other soils to investigate the effect of adjusted pH on NNR and ammonia oxidizers. Net nitrification rates varied significantly with adjusted pH in neutral soils but did not change in acidic and alkaline soils, suggesting that soil at neutral pH is more sensitive to changes in nitrification. The AOB abundance increased in neutral soils adjusted to high pH, whereas AOA decreased with increased pH in acidic and neutral soils, which indicated that the activity and abundance of AOA and AOB is the more important factor affecting nitrification in neutral soils. The ratios of AOA to AOB in the unmodified acidic, neutral and alkaline soils were 120, 1.55 and 0.07, respectively. The highest AOA and AOB abundances occurred in unmodified pH neutral soil. However, the highest NNR was found in alkaline soils (7.04 mg N kg−1 dry soil d−1), which was significantly higher than that in neutral and acidic soils (2.31 and − 0.23 mg N kg−1 dry soil d−1, respectively). These results indicate that substrate competition between AOA and AOB exists in neutral soils, which can provide insight and improve our understanding of microbial regulation of N cycling in terrestrial ecosystems.