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Little is known about the effects of nitrogen (N) fertilization rates on ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) and their differential contribution to nitrous oxide (N 2 O) production, particularly in greenhouse based high N input vegetable soils. Six N treatments (N1, N2, N3, N4, N5 and N6 representing 0, 293, 587, 880, 1173 and 1760 kg N ha -1  yr -1 , respectively) were continuously managed for three years in a typically intensified vegetable field in China. The aerobic incubation experiment involving these field-treated soils was designed to evaluate the relative contributions of AOA and AOB to N 2 O production by using acetylene or 1-octyne as inhibitors. The results showed that the soil pH and net nitrification rate gradually declined with increasing the fertilizer N application rates. The AOA were responsible for 44-71% of the N 2 O production with negligible N 2 O from AOB in urea unamended control soils. With urea amendment, the AOA were responsible for 48-53% of the N 2 O production in the excessively fertilized soils, namely the N5-N6 soils, while the AOB were responsible for 42-55% in the conventionally fertilized soils, namely the N1-N4 soils. Results indicated that overdose fertilization induced higher AOA-dependent N 2 O production than AOB, whereas urea supply led to higher AOB-dependent N 2 O production than AOA in conventionally fertilized soils. Additionally, a positive relationship existed between N 2 O production and NO 2 - accumulation during the incubation. Further mechanisms for NO 2 - -dependent N 2 O production in intensive vegetable soils therefore deserve urgent attention., (Copyright © 2018 Elsevier B.V. All rights reserved.)

An ex-situ 15 N–18O tracing experiment with soils collected from the valley and slope, respectively, of a subtropical secondary karst forest with three N addition levels, i.e., 0, 50, and 100 kg N ha−1 year−1 for each topographic position to investigate N2O production pathways. Autotrophic nitrification pathways (ammonia oxidation, nitrifier denitrification, and nitrification-coupled denitrification) accounted for > 70% of total N2O production, but denitrification pathways (heterotrophic denitrification and co-denitrification) were the minor source of N2O at both topographic positions. In the valley, chronic N addition stimulated ammonia oxidation-derived N2O, which was paralleled by increased ammonia-oxidizing archaea (AOA) amoA gene transcript abundance, but inhibited nitrifier denitrification- and nitrification-coupled denitrification–derived N2O along with suppressed ammonia-oxidizing bacteria (AOB) amoA gene transcript abundance and stimulated nosZII gene transcript abundance, respectively. On the slope, chronic N addition stimulated ammonia oxidation-derived N2O along with increased AOB amoA gene transcript abundance, and enhanced nitrifier denitrification-derived N2O congruent with increased AOB amoA and decreased nirK gene transcript abundances. In addition, chronic N addition reduced the relative contribution of heterotrophic denitrification to N2O production but had no significant influence on heterotrophic denitrification-derived N2O on the slope. Overall, our results provide a comprehensive view in terms of how topography-driven soil properties regulate N2O production and its pathways in a subtropical forest. [ABSTRACT FROM AUTHOR]

Topography can influence nitrous oxide (N2O) emission via its influences on soil nutrient availability, moisture, and microbial communities. Nevertheless, it is still unclear whether topography modulates the responses of soil N2O emissions to elevated N deposition. Here the N addition experiment was conducted in the valley and on the slope of a subtropical karst forest in southwest China. Nitrogen was applied as NH4NO3 in two levels, that is, 50 (moderate N) and 100 (high N) kg N ha−1 yr−1 with no N addition plots as the control. Nitrogen addition consistently increased N2O emission in the valley, but only high N addition significantly increased N2O emission on the slope in 2017. The cumulative N2O fluxes across the 3 years were 1.16 ± 0.24 kg N ha−1 in the valley and 1.50 ± 0.06 kg N ha−1 on the slope under the control. Nitrogen addition stimulated N2O emission by 88.7%–113.3% in the valley due to increased ammonium, nitrate and dissolved organic carbon availabilities and ammonia‐oxidizing bacteria (AOB) amoA abundance. High N addition stimulated N2O emission by 84.3% on the slope owing to increased nitrate and carbon availabilities, AOB amoA, and nirK abundances. The stimulation of N2O emission by moderate N addition was more pronounced in the valley than on the slope largely owing to the lower N status in the valley. This work highlights the importance of N status in regulating the responses of soil N2O emissions to elevated N deposition. Plain Language Summary: Atmospheric N deposition is identified as one of the strongest driving factors responsible for the increase of forest soil N2O emission, and topography can influence N2O emission via its influences on soil nutrient availability, moisture, and microbial communities. However, there is uncertainty about whether topography modulates the responses of soil N2O emissions to elevated N deposition. We investigated soil N2O emission and related microbial functional gene abundances at two topographic positions under three N addition levels in a subtropical region of China. We find that the N addition consistently increased N2O emission in the valley, but only stimulated N2O emission at the early stage on the slope. Stimulation of soil N2O emission by moderate N addition was more pronounced in the valley than on the slope. Our findings highlight the importance of N status in regulating the response of soil N2O emissions to elevated N deposition. Key Points: N addition increased soil N2O emission in the valley across the 3 yearsHigh N addition stimulated N2O emission on the slope at the early stageModerate N stimulated greater N2O emission in the valley than on the slope [ABSTRACT FROM AUTHOR]

Purpose: The contribution of hydroxylamine (NH2OH) and nitrite (NO2−) to nitric oxide (NO) and nitrous oxide (N2O) production remains unclear in vegetable production soils. Materials and methods: Soils collected from six typical greenhouse vegetable fields were incubated for 48 h following amendment with 1 mM NaNO2, 10 μM NH2OH, or 1 mM NaNO2 + 10 μM NH2OH. The importance of abiotic processes on the NO and N2O formation from the NH2OH and NO2− were studied by irradiating the soil samples with γ-irradiation. Results and discussion: NO2− amendment significantly stimulated NO production, while the NH2OH-dependent NO production was minimal. NH2OH stimulated more abiotic N2O production in alkaline soils than in acidic soils (p < 0.05), while NO2− stimulated more biotic N2O production in acidic soils than in alkaline soils (p < 0.05). The NH2OH- and NO2−-dependent sources produced biotic or abiotic N2O with site preference (SP) values of 27.4–36.5‰, which is similar to those from ammonia-oxidizing archaea (AOA) or ammonia-oxidizing bacteria (AOB) sources (25.1–34.2‰), indicating that abiotic N2O production were closely linked with biotic NH3 oxidation. The variability of NO2−+NH2OH-induced N2O production can be explained by the soil organic carbon and iron concentrations, whereas NO2−-induced NO production can be explained by the soil pH. Conclusions: NO2− addition dominated NO production in all soils. Furthermore, NO2− addition increased biotic N2O production in acidic soils, while NH2OH addition increased abiotic N2O production in alkaline soils. The presence of NO2− could significantly stimulate the abiotic conversion of NH2OH to N2O in soils with low soil organic carbon and high iron concentrations. Thus, assessing the abundance of NH2OH and NO2− could provide crucial information for understanding NO and N2O production procedures in vegetable soils. Highlights: • The chemical decomposition of NO2− dominated NO production in all soils. • The NH2OH stimulated abiotic N2O production in alkaline soils. • The NO2− stimulated biotic N2O production in acidic soils. • The SP for abiotic NO2−-/NH2OH-related N2O were in the same range as AOB/AOA sources. [ABSTRACT FROM AUTHOR]

[Display omitted] • Composting with cellulose and hemicellulose-degrading microorganisms inoculated. • Microbial inoculation promoted AOB and denitrifying bacteria derived N 2 O production. • Microbial inoculation increased AOB amoA and denitrifying nirK gene abundances. • Microbial inoculation enhanced N 2 O reduction to N 2 by increasing nosZ Ⅰ and nosZ ⅠⅠ. This study aims to clarify the mechanisms underlying effects of inoculating cellulose and hemicellulose-degrading microorganisms on nitrous oxide (N 2 O) emissions during composting with silkworm excrement and mulberry branches. Inoculation with cellulose and hemicellulose–degrading microorganisms resulted in significant increases of total N 2 O emission by 10.4 ± 2.0 % (349.1 ± 6.2 mg N kg−1 dw) and 26.7 ± 2.1 % (400.6 ± 6.8 mg N kg−1 dw), respectively, compared to the control (316.3 ± 3.6 mg N kg−1 dw). The stimulation of N 2 O emission was attributed to the enhanced contribution of ammonia-oxidizing bacteria (AOB) and denitrifying bacteria to N 2 O production, as evidenced by the increased AOB amoA and denitrifying nirK gene abundances. Moreover, microbial inoculation stimulated N 2 O reduction to N 2 owing to increased abundances of nosZ Ⅰ and nosZ ⅠⅠ genes. These findings highlight the necessity to develop cost-effective and environmentally friendly strategies to reduce N 2 O emissions when cellulose and hemicellulose-degrading microorganisms are inoculated during composting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Isotope mapping, qPCR, and inhibition methods to explore N 2 O emission mechanisms. • Biochar and CaCO 3 reduced contribution of AOB and fungi to N 2 O production. • Biochar and CaCO 3 stimulated N 2 O reduction to N 2. • Biochar and CaCO 3 decreased AOB amoA , fungal nirK and P450 gene abundances. • Biochar and CaCO 3 increased nosZ Ⅰ and nosZ ⅠⅠ gene abundances. To investigate the mechanisms underlying effects of biochar and calcium carbonate (CaCO 3) addition on nitrous oxide (N 2 O) emissions during composting, this paper conducted a systematic study on mineral nitrogen (N), dissolved organic carbon (C) and N, sources of N 2 O, and functional genes. Biochar and CaCO 3 addition decreased N 2 O emissions by 26.5–47.8% (9.5–96.9 mg N kg−1 dw) and 13.9–37.4% (12.0–121.0 mg N kg−1 dw) compared to the control (14.3–179.7 mg N kg−1 dw), respectively. The mitigation of N 2 O emission was caused by decreased contribution of ammonia-oxidizing bacteria (AOB) and fungi to N 2 O production due to diminished AOB amoA , fungal nirK and P450 gene abundances, or by stimulated N 2 O reduction to N 2 owing to increased abundances of nosZ Ⅰ and nosZ ⅠⅠ genes under biochar and CaCO 3 addition. The findings suggest that the addition of biochar or CaCO 3 is effective in mitigating N 2 O emission during composting. [ABSTRACT FROM AUTHOR]

Greenhouse vegetable soils are characterized by high N 2 O emissions, but the functional importance of potential ammonia (NH 3 ) oxidation (PAO) and nitrite (NO 2 − ) consumption (PNC) by ammonia-oxidizing archaea (AOA) and bacteria (AOB) is poorly understood. Inhibitors of 1-octyne and 2-pheny l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) in combination with potassium chlorate were used to examine the importance of AOA and AOB in NO 2 − production and consumption, to evaluate the soil-specific effects on N 2 O production under six long-term fertilization greenhouse vegetable soils across mainland China, and to predict their thermodynamic responses in a temperature gradient of 5–45 °C using the square root growth (SQRT) and macromolecular rate theory (MMRT) models. The ammonia oxidizers-driven PAO and PNC exhibited a strong response to temperature with maximum rates attained at 35 °C or 40 °C by AOA compared to 30 °C or 35 °C by AOB. Concomitantly, compared with ammonium amendment, NO 2 − addition stimulated N 2 O production by approximately 2–4-fold for AOA and 2–9-fold for AOB at 20–40 °C in four of six soils. AOA exhibited a broader range of temperatures (−33.2–56.8 °C vs −16.1–52.3 °C) and lower relative temperature sensitivity (Q 10 ) than AOB ( p  < 0.05). The heat capacity (ΔC P ‡ ) of AOA and AOB in PAO and PNC increased in relation to the optimum temperature (T opt ) and exhibited a greater negative value by AOB than AOA, indicating that specialized ammonia-oxidizer populations of PAO and PNC adapted to in situ soil environmental changes. Structural equation modeling revealed that potential AOA-driven N 2 O production was influenced by ΔC P ‡ and Q 10 and C/N, while AOB-driven N 2 O production by ΔC P ‡ and Q 10 and soil nitrate (NO 3 − ) content, all of which is influenced directly and indirectly by in situ climate parameters. Findings suggest the important roles of specific in situ climate and soil properties and provide new insights into the NO 2 − consumption in soil N 2 O production by ammonia-oxidizers. [ABSTRACT FROM AUTHOR]

• AOB are primarily responsible for soil N 2 O production. • N addition stimulated AOA–derived N 2 O in the organic horizon of the valley. • N addition suppressed AOA and AOB–derived N 2 O in the organic horizon of the slope. • N addition increased AOB–derived N 2 O in the mineral horizon of the valley. • N addition enhanced AOA and AOB–derived N 2 O in the mineral horizon of the slope. Anthropogenic nitrogen (N) deposition may substantially affect the contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) to soil nitrous oxide (N 2 O) production. Nevertheless, it is still unclear whether topography modulates the responses of AOA– and AOB–derived N 2 O to elevated N deposition. We conducted an ex-situ experiment with organic and mineral soils collected from the valley and slope, respectively, of a subtropical karst forest with three N addition levels, i.e., 0 (control), 50 and 100 kg N ha−1 year−1 for each topographic position. Soil ecoenzymatic stoichiometry indexes were calculated as indicators of microbial resource limitation. AOA– and AOB–derived N 2 O were distinguished by inhibitors. AOB was primarily responsible for soil N 2 O production under the control regardless of topographic position. For the organic horizon, N addition stimulated AOA–derived N 2 O by 112.5–138.2% in the valley because of increased N mineralization due to alleviated microbial carbon limitation; but suppressed AOA and AOB–derived N 2 O by 40.2–53.5% and 35.6–46.8%, respectively, on the slope because of decreased N mineralization attributed to aggravated microbial phosphorus limitation. For the mineral horizon, N addition enhanced AOB–derived N 2 O by 104.5–143% in the valley because of increased ammonia availability, but stimulated AOA and AOB–derived N 2 O by 149.8–1162.5% and 26–64.5%, respectively, on the slope because of increased N mineralization and ammonia availability owing to aggravated microbial C limitation and alleviated phosphorus limitation. Our results indicate that the mechanisms underlying the impacts of N deposition on soil N 2 O production by ammonia oxidizers are topography–dependent, so that topography-specific niche specialization between AOA and AOB should be integrated into Earth system models in order to better predict soil N 2 O production under elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]

Biochar may variably impact nitrogen (N) transformation and N-cycle-related microbial activities. Yet the mechanism of biochar amendment on nitrous oxide (N 2 O) emissions from agricultural ecosystems remains unclear. Based on a 6-year long-term biochar amendment experiment, we applied a dual isotope (15N–18O) labeling technique with tracing transcriptional genes to differentiate the contribution of nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD) and heterotrophic denitrification (HD) pathway to N 2 O production. Then the field experiment provided quantitative data on dynamic N 2 O emissions, soil mineral N and key functional marker gene abundances during the wheat growing season. By using 15N–18O isotope, biochar decreased N 2 O emission derived from ND (by 45–94%), HD (by 35–46%) and NCD (by 30–64%) compared to the values under N application. Biochar increased the relative contribution of NN to total N 2 O production as evidenced by the increase in ammonia-oxidizing bacteria, but did not influence the cumulative NN-derived N 2 O. The field experiment found that the majority of the N 2 O emissions peaked following fertilization, in parallel with soil NH 4 + and nitrite dynamics. Soil N 2 O emissions during the wheat growing stage were effectively decreased (by 38–48%) by biochar amendment. Based on the correlation analyses and random forest analysis in both microcosm and field experiments, the decrease in nitrite concentration (by 62–65%) and increase in N 2 O consumption were mainly responsible for net N 2 O mitigation, as evidenced by the decrease in the ratios of nitrite reductase genes/transcripts (nirS , nirK and fungal nirK) and N 2 O reductase gene/transcripts (nosZI and nosZII). Based on the extrapolation from microcosm to field, biochar significantly mitigated N 2 O emissions by weakening the ND processes, since NCD and HD contributed little during the N 2 O emission "peaks" following urea fertilization. Therefore, emphasis should be put on the ND process and nitrite accumulation during N 2 O emission peaks and extrapolated to all agroecosystems. [Display omitted] • Biochar decreased total N 2 O emissions in paddy field under 6-yr application. • Biochar weakly increased N 2 O emissions derived from nitrifier nitrification. • Biochar decreased N 2 O emissions via nitrifier denitrification. • Biochar increased nosZ but decreased nir & Fungal nirK at gene&transcripts levels. • High N 2 O consumption and low nitrite explained the low N 2 O flux in biochar plot. Biochar persistently decreased N 2 O emissions "peaks" due to decreased nitrifier denitrification along with high N 2 O consumption and low nitrite in biochar plots. [ABSTRACT FROM AUTHOR]

The promising application modes of organic fertilizer (OF) and chemical nitrogen (N) fertilizer (CF) could be the homogeneous granulation (HG: OF and CF are distributed spatially evenly) and spatial heterogeneous granulation (SG: OF and CF are distributed separately in space), where the N transformation processes, such as the nitrous oxide (N 2 O) emissions, are greatly influenced by the spatial distribution of the OF and CF, particularly. Currently, there is a lack of in-depth understanding about the microbial mechanisms of the SG and HG application on N 2 O emissions, and the related functional guilds (ammonia oxidizers and heterotrophic denitrifiers) respond to the granular fertilizer is yet not known. In the present study, we made CF (15N-(NH 4) 2 SO 4), cow compost and maize straw (2% or 8% based on the N proportion) into granular of 1 cm in diameter, in HG and SG forms, respectively, and then applied these granules in soils for 80 days incubation. Results showed that, compared with HG treatments, the SG treatment promoted the ammonium (NH 4 +), nitrate (NO 3 −) and microbial biomass carbon (MBC) intensities, and increased the N 2 O emissions possibly through ammonia oxidize bacteria dependent nitrification and fungal denitrification. In addition, the high maize residues proportion in organic fertilizer significantly mitigated N 2 O emissions by the coupled impacts of suppressed nitrification and enhanced denitrification enzyme activity with high C input. Overall, our results suggest that spatial heterogeneous granulation of and CF may induce higher risk of N 2 O emissions and the higher proportion of maize residues could potentially mitigate such increased emissions. • Spatial heterogeneous granulation increased the N 2 O emissions. • High maize residues proportion in organic fertilizer decreased N 2 O emissions. • AOB act as the N 2 O source, whereas nosZ Ⅱ act as the N 2 O sink. [ABSTRACT FROM AUTHOR]

The influence of temperature on soil ammonia (NH 3) and nitrite (NO 2 −) oxidation and related NO 2 − accumulation in soils remain unclear. The soil potential NH 3 oxidation (PAO) and NO 2 − oxidation (PNO) rates were evaluated over a temperature gradient of 5–45 °C in six greenhouse vegetable soils using inhibitors. The values of temperature sensitivity traits such as temperature minimum (T min), temperature optimum (T opt), and maximum absolute temperature sensitivity (T m_sens) were also fitted to the square root growth (SQRT) and macromolecular rate theory (MMRT) models. The ammonia-oxidizing archaea (AOA) and bacteria (AOB) were determined by quantifying amoA , and nitrite-oxidizing bacteria (NOB) were determined by quantifying the nxrA and nxrB. Both models identified that T opt for PAO (34.0 °C) was significantly greater than that for PNO (26.0 °C). The T m_sens (23.4 ± 2.1 °C) and T min (1.0 ± 2.0 °C) for PAO were higher than those for PNO (16.8 ± 3.2 °C and − 11.7 ± 6.7 °C). PAO was positively correlated with AOB- amoA at 20–30 °C and with AOA- amoA at 30–35 °C, while PNO was positively correlated with nxrB at 5–30 °C. Additionally, NO 2 − and N 2 O were positively correlated with the (AOA + AOB amoA) to (nxrA + nxrB) ratio, and the concentration of N 2 O was positively correlated with NO 2 − accumulation. These results highlight that elevated temperatures resulted in the uncoupling of NH 3 oxidation and NO 2 − oxidation, leading to NO 2 − accumulation, which could stimulate N 2 O emissions. Unlabelled Image • Ammonia- and nitrite-oxidation become decoupled with increasing temperature. • Potential ammonia-oxidation had a >8 °C higher T opt than the nitrite-oxidation rate. • NO 2 − and N 2 O increased with gene ratio (AOA + AOB amoA):(nxrA + nxrB) in nitrification. [ABSTRACT FROM AUTHOR]

• Manure and biochar mitigated N 2 O emissions via dual effects on denitrification. • NIs mitigated N 2 O emissions via inhibiting both nitrification and denitrification. • BNI increased nirS , nosZⅠ and fungal nirK transcripts following fertilization. • The higher NO 3 − content led to a lower N 2 O reduction during denitrification. A clear understanding of the effects of manure, biochar and nitrification inhibitors (NI, nitrapyrin-SNI and Sorghum bicolor L.-BNI) amendment on nitrous oxide (N 2 O) production pathways and consumption remains elusive under field conditions. A field experiment using an isotopocule mapping approach (δ15NSP N2O and δ18O N2O/H2O map) in conjunction with molecular techniques were conducted to understand the mechanisms of these options for N 2 O mitigation in a Tung choy (Ipomoea aquatic Forssk.) vegetable soil. The manure substitution and biochar not only decreased nitrification/fungal denitrification and bacterial denitrification/nitrifier denitrification-derived N 2 O emission, but also stimulated N 2 O reduction to N 2 during the denitrification process as evidenced from the decrease of ammonia-oxidizing bacteria (AOB) amoA and the increase of nirS , nosZⅠ , nosZⅡ and fungal nirK transcripts. Biochar had a greater potential to enhance this reduction with decrease of ammonia-oxidizing archaea (AOA) amoA transcripts. NIs mitigated the N 2 O derived from nitrification and/or bacterial denitrification/nitrifier denitrification demonstrated from the simultaneous decrease of soil NO 2 − and NO 3 − and increase of soil NH 4 + intensity together with the lower AOB amoA transcripts in NIs than in the urea. These results helped explain the observed differences in N 2 O emissions among these mitigation options, and laid the foundation for a better understanding of N 2 O production and reduction under field conditions. [ABSTRACT FROM AUTHOR]

• Drainage enhanced N 2 O emission via increased abundances of genes related to N 2 O production. • Labile C addition mitigated the increased N 2 O emission by drainage via increased nosZ abundance. • Nitrogen addition strongly amplified the increased N 2 O emission due to drainage. • N 2 O emission increased with NH 4 + depletion and NO 3 − accumulation following paddy soil drainage. • N 2 O emission was positively correlated with AOA and AOB abundances, and (nirK + nirS)/ nosZ ratio. Conversion from paddy field to upland cultivation, driven by economic benefits, generally increases N 2 O emission. However, the underlying mechanisms regarding the changes in N 2 O emission during the transition period of paddy soil drainage and conversion into upland remain unclear. To address this knowledge gap, a microcosm experiment was conducted to mechanistically elucidate the N 2 O emissions in response to paddy soil drainage and its interaction with carbon (C, glucose) and nitrogen (N, NH 4 Cl) availability. Results showed that N 2 O emissions were significantly lower in the submerged paddy soil (0.51 ± 0.03 mg N kg−1) compared to drained paddy soil (3.63 ± 0.66 mg N kg−1). The increased N 2 O emission was positively associated with soil NH 4 + depletion and NO 3 − accumulation, indicating increased nitrification and NO 3 − supply for denitrification following paddy soil drainage. Substrate C and N availability had little effect on N 2 O emission from submerged paddy soil, but strongly mediated the response of N 2 O emission to paddy soil drainage. High C availability largely mitigated the increased N 2 O emission by drainage, mainly resulting from increased N 2 O-reductase gene (nosZ) abundance. The increased N availability interacting with drainage strongly increased N 2 O emissions by 1.99–16.34 folds, primarily due to increased NO 3 − content and gene abundances of ammonia-oxidizing bacteria (AOB) and nitrite reductase (nirK and nirS). The N 2 O emissions were positively correlated with the abundances of ammonia-oxidizing archaea (AOA), AOB, nirK and nirS genes, and (nirK + nirS)/ nosZ ratio across treatments. Collectively, our findings suggested that the increased N 2 O emission due to paddy soil drainage is regulated by C and N availability, attributed to changes in NO 3 − content and abundances of nitrifying and denitrifying genes. [ABSTRACT FROM AUTHOR]

• Biochar controlled potential ammonia oxidation (PAO), N 2 O & NO production via soil pH. • Both N 2 O and NO production depended mainly on soil PAO. • Biochar-enriched soil functioned similarly as fresh biochar, while aged biochar not. • Biochar-enriched soil increased ammonia-oxidizing bacteria in acidic soil. Biochar amendment has been recommended as a potential strategy to mitigate nitrous oxide (N 2 O) and nitric oxide (NO) emissions for wheat production, but its mechanism and effective duration are not well understood. The 1-octyne and 2-pheny l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) in combination with potassium chlorate were used to evaluate the relative contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential ammonia oxidation (PAO) and N 2 O and NO production as affected by biochar. Acidic and alkaline soils were collected during wheat-growing season, and four treatments were installed in each soil type: CK, urea alone; BE, biochar-enriched soil for 2–6 years; FB, fresh biochar added to CK; and AB, aged biochar added to CK. The results showed that octyne and PTIO efficiently assessed AOB and AOA activities in soil incubation. The AOB-driven PAO in acidic soil was largely enhanced by increased soil pH in BE and FB treatments, whereas AOA-driven PAO was not. And the contribution of AOB to PAO exceeded 80% in alkaline soil. The N 2 O and NO production were positively correlated with PAO in both soils. BE treatment decreased the direct N 2 O and NO production in alkaline soil, while both BE and FB treatments decreased the N 2 O and NO yields in acidic soil, indicating that biochar mitigated soil N 2 O and NO emissions for wheat production. The lack of differences between AB and CK treatments indicated that aged biochar lost its initial effects on PAO, while the biochar-enriched soil amended with biochar years earlier still functioned similarly as fresh biochar. [ABSTRACT FROM AUTHOR]