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Acidification of coastal waters, synergistically driven by increasing atmospheric carbon dioxide (CO 2 ) and intensive land-derived nutrient inputs, exerts significant stresses on the biogeochemical cycles of coastal ecosystem. However, the combined effects of anthropogenic nitrogen (N) inputs and aquatic acidification on nitrification, a critical process of N cycling, remains unclear in estuarine and coastal ecosystems. Here, we showed that increased loading of ammonium (NH 4 ) in estuarine and coastal waters alleviated the inhibitory effect of acidification on nitrification rates but intensified the production of the potent greenhouse gas nitrous oxide (N + ) in estuarine and coastal waters alleviated the inhibitory effect of acidification on nitrification rates but intensified the production of the potent greenhouse gas nitrous oxide (N 2 O), thus accelerating global climate change. Metatranscriptomes and natural N 2 O isotopic signatures further suggested that the enhanced emission of N 2 O may mainly source from hydroxylamine (NH 2 ) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N 2 - ) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N 2 O emissions, thereby enhancing our ability to predict the feedbacks of estuarine and coastal ecosystems to climate change and human perturbations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Nitrification inhibitors (NIs) and drip irrigation are recommended to mitigate trace gas emissions from agricultural soils. However, studies comparing the effect of different NIs on the release of trace gases from soils with contrasting textures under subsurface (SBD) and surface (SD) drip irrigation are lacking. Therefore, this study aimed to assess the effectiveness of three NIs in mitigating nitrous oxide (N 2 O), carbon dioxide (CO 2 ), and methane (CH 4 ) emissions from two soils with different textures under SBD, with pipe buried in 10 cm depth, and SD. Two greenhouse experiments were carried out with silt loam and loamy sand soil textures cultivated with wheat under SBD and SD to assess the effectiveness of the NIs Dicyandiamide (DCD), 3,4-Dimethylpyrazole phosphate (DMPP), and 3-Methylpyrazol combined with Triazol (MP + TZ). Ammonium sulfate was applied at a rate of 0.18 g N kg soil -1 . The measured variables were daily and cumulative N 2 O-N, CO 2 -C, and CH 4 -C emissions, as well as soil NH 4 + -N and NO 3 - -N concentrations. The NIs and SBD had additive effects on reducing N 2 O-N emissions in the silt loam, but not in the loamy sand soil texture. Under SBD, total N 2 O-N emissions were 44% and 52% lower than under SD in the silt loam and loamy sand soil textures, respectively. Moreover, DMPP kept the highest NH 4 + -N concentrations and promoted the lowest N 2 O-N emissions relative to SD. Among the investigated NIs, DMPP has the highest effectiveness in retarding nitrification and mitigating N 2 -C and CH 4 -C total emissions were not affected by the treatments. Our findings supported the hypothesis that SBD decreases N 2 O-N emissions relative to SD. Among the investigated NIs, DMPP has the highest effectiveness in retarding nitrification and mitigating N 2 O-N release under the studied treatments. Finally, in coarse-textured soils, the use of NIs could be sufficient to significantly abate N 2 O-N emissions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Comammox Nitrospira and canonical ammonia-oxidizing bacteria (cAOB) generally coexist in activated sludge. In present study, the effect of comammox Nitrospira on N 2 O production during nitrification of activated sludge was investigated. Comammox Nitrospira and cAOB were separately enriched in two nitrifying reactors, with respective relative abundance of approximately 98% in ammonia oxidizer community. The N 2 O emission factor (EF) of nitrification in comammox Nitrospira dominated reactor was 0.35%, consistently lower than that (2.2%) in cAOB dominated reactor. When increasing the relative abundance of comammox Nitrospira in ammonia oxidizer community, the N 2 O EF of nitrification decreased exponentially, which suggested that comammox Nitrospira not only decreased N 2 O production directly but also might have reduced N 2 O yield by cAOB. When cAOB dominated the ammonia oxidizer community of sludge, decreasing pH to 6.3, lowering DO to less than 0.5 mg/L, and increasing nitrite concentration enhanced N 2 O EF dramatically. When comammox Nitrospira became the dominant ammonia oxidizer, however, the N 2 O EF correlated to nitrite insignificantly and a low DO of 0.2 mg/L and weakly acidic pH (6.3) decreased N 2 O EF by approximately 70% and 60%, respectively. These results imply that enhancing the relative abundance of comammox Nitrospira in sludge is an effective way to reducing N 2 O emissions and can also offset the promoting effects of acidic pH, low DO, and high nitrite concentration on N 2 O production during nitrification., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Low-carbon approaches to agriculture constitute a pivotal measure to address the challenge of global climate change. In agroecosystems, rhizosphere exudates are significantly involved in regulating the nitrogen (N) cycle and facilitating belowground chemical communication between plants and soil microbes to reduce direct and indirect emissions of greenhouse gases (GHGs) and control N runoff from cultivated sites into natural water bodies. Here, we discuss specific rhizosphere exudates from plants and microorganisms and the mechanisms by which they reduce N loss and subsequent N pollution in terrestrial and aquatic environments, including biological nitrification inhibitors (BNIs), biological denitrification inhibitors (BDIs), and biological denitrification promoters (BDPs). We also highlight promising application scenarios and challenges in relation to rhizosphere exudates in terrestrial and aquatic environments., Competing Interests: Declaration of interests None declared by authors., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

In their review: An examination of the ability of plantain (Plantago lanceolata L.) to mitigate nitrogen leaching from pasture systems, Eady et al. (2024) dispute both the historic estimates of typical urine patch nitrogen (N) load and leaching and the evidence for the N leaching reduction mechanisms of plantain; and question the recommended levels of plantain required to achieve N leaching reduction. We reject the suggestion that the urine patch has little influence on N leaching, and that average annual N leaching from dairy farms is 6 kg N/ha. We agree that the low dry matter content of plantain is the dominant and best documented effect of plantain on urine N dilution. We reject that there is no evidence for the effect of plantain on nitrogen partitioning to urine, and on potential nitrification rate in the urine patch. We point to empirical evidence of statistically significant reductions in nitrate leaching from plantain at paddock scale, at levels as low as 21% plantain of dry matter eaten. Current research will improve understanding of the mechanisms and magnitude of the effect of plantain on N loss, paving the way for recognition of other forage-based N loss reduction options, and ongoing development of mechanistic models that are adaptable to other forages. [ABSTRACT FROM AUTHOR]

The aim of this study was to explore the effect of biochar on N2O emissions in soils with different pH levels. Soils with five pH levels (4.0, 5.1, 5.8, 6.6, and 7.2) were incubated in two conditions, with 0% biochar (CK) and 1% biochar (BC), for 23 days. N2O emissions were measured at nine time points, and soil chemical properties, AOA-amoA, AOB-amoA, nirK, nirS, and nosZ, were analyzed. Partial least squares path modelling (PLS-PM) was used to assess the effect of nitrification and denitrification pathways on potential N2O emissions. The results showed that biochar reduced N2O emissions in highly acidic soil (pH 4.0) but increased emissions in soils with pH values ranging from 5.1 to 7.2. In highly acidic soils, decreased N2O emission was associated with increased soil pH (p < 0.05) and decreased dissolved organic carbon content (p < 0.05), leading to higher nosZ gene abundance (p < 0.05). Meanwhile, in acidic to neutral soils, biochar application increased soil pH (6.6–11.7%), dissolved organic nitrogen (5.9–29.5%), dissolved organic carbon (8.6–41.0%), stimulated AOB-amoA, nirK, nirS gene abundance (p < 0.05), and thus increased N2O emissions. The results verified the influence of nitrification and denitrification genes on N2O production in soils with different pH values. In conclusion, biochar had different effects on N2O emissions based on soil pH, highlighting the need to consider pH when using biochar to mitigate N2O emissions in subtropical citrus orchards. [ABSTRACT FROM AUTHOR]

Coastal nutrient loads from point sources such as rivers are mostly well-monitored. This is not the case for diffuse nutrient inputs from coastal catchments unconnected to rivers, despite the potential for high inputs due to intensive land use. The German Baltic Sea coastline consists of numerous peatlands that have been diked and drained. However, some of the dikes have been removed in order to re-establish the hydrological connection to the Baltic Sea, restore local biodiversity, and promote natural CO2 uptake. Since these peatlands were used for agriculture, their rewetting may release accumulated nutrients, leading to nutrient export into the Baltic Sea and intensified coastal eutrophication. Data on these potential nutrient exports are mostly lacking. Therefore, this study investigated nutrient exports from two former agricultural, coastal peatlands: Drammendorfer Wiesen, rewetted in 2019, and Karrendorfer Wiesen, rewetted in 1993. Nutrients (NO3–, NO2–, NH4+, PO43–), nitrous oxide (N2O), particulate organic matter (POM, comprising POC and PON; δ13C-POC), chlorophyll-a, and nitrification rates were analyzed in surface water and porewater sampled weekly to monthly in 2019 and 2020 to compare the effects of different time scales after rewetting on nutrient cycling and potential exports. NH4+, NO2−, and PO43− concentrations were higher in the porewater than in the overlying water at both sites, while nutrient concentrations were generally higher at the recently rewetted Drammendorfer Wiesen than at the Karrendorfer Wiesen. NO3− concentrations in porewater, however, were lower than in the overlying water, indicating NO3− retention within the peat, likely due to denitrification. Nitrification rates and N2O concentrations were generally low, except for a high N2O peak immediately after rewetting. These results suggest that denitrification was the dominant process of N2O production at the study sites. Both peatlands exported nutrients to their adjacent bays of the Baltic Sea; however, N exports were 75% lower in the longer-rewetted peatland. Compared to major Baltic Sea rivers, both sites exported larger area-normalized nutrient loads. Our study highlights the need to monitor the impact of rewetting measures over time to obtain accurate estimates of nutrient exports, better assess negative effects on coastal waters, and to improve peatland management. [ABSTRACT FROM AUTHOR]

Lime (CaCO3) application has led to an inconsistent mitigation of greenhouse gas emissions (GHGs) from agricultural soils. In addition, only a few studies have investigated the impact of gypsum (CaSO4) on the release of GHGs. Therefore, here we investigated the impact of initial soil pH and nitrogen sources on CaSO4 and CaCO3 effectiveness in reducing the production of GHGs. We hypothesized that (i) liming presents the highest effectiveness in reducing GHGs when performed in acidic soil and under NO3−‐N fertilizer application, and (ii) application of CaSO4 is not effective in reducing GHG irrespective of the initial soil pH or nitrogen (N) source. Firstly, an incubation experiment was carried out to evaluate the effect of initial soil pH using acidic soil and neutral soil amended with CaSO4 and CaCO3. Subsequently, a second incubation experiment was conducted to investigate the effect of N source by applying calcium nitrate and urea in an acidic soil amended with CaSO4, CaCO3 and CaSO4 + CaCO3. The measured variables were soil pHH2O$$ {\mathrm{pH}}_{{\mathrm{H}}_2\mathrm{O}} $$, daily and cumulative emissions of N2O‐N, CO2‐C and CH4‐C, as well as NH4+‐N and NO3−‐N concentrations. CaCO3 did not affect N2O‐N emission under neutral soil pH, and NO3−‐N fertilizer application. However, CaCO3 increased N2O‐N release under acidic soil (+96%), and urea fertilizer application (+97%). Furthermore, CaCO3 and CaSO4 reduced CO2‐C emission from the neutral pH soil by 31%, and CaSO4 decreased the mean cumulative N2O‐N release from the soil fertilized with urea‐N by 43%. In conclusion, this study did not confirm liming as an alternative to reduce N2O‐N release under different initial soil pH and N sources. Furthermore, our results indicated that CaSO4 may decrease GHG emissions. Finally, these results suggest caution for considering liming as a strategy to mitigate N2O‐N emission from agriculture. [ABSTRACT FROM AUTHOR]

The excessive application of nitrogen in tea plantations leads to severe soil acidification and N2O emission boosting. To promote sustainable agriculture, nanoparticles (NPs) have emerged as alternative fertilizers, but their effects on soil nitrification and greenhouse gas emissions in tea plantations remain unclear. In this study, the effects of NP type (ZnO-NPs and Fe2O3-NPs) and dose (0, 1, 10, and 100 mg·kg−1) on soil N2O emissions were investigated via a lab incubation trial. Soil pH, ammonium, and nitrate changes were also monitored during the incubation period. The abundance of functional genes related to nitrification and denitrification processes was analyzed as well. The results showed that ZnO-NPs led to a decrease in N2O emissions. The reduction effect was stronger with increasing dose and resulted in a 33% reduction at an addition rate of 100 mg·kg−1. The cumulative N2O emissions had significantly positive correlations with NH4+-N and NO3−-N. ZnO-NP addition showed a significantly negative effect on Ammonia-Oxidizing Archaea (AOA) but a positive effect on Ammonia-Oxidizing Bacteria (AOB) gene abundance. In contrast, Fe2O3-NPs showed an insignificant impact on N2O emissions and soil N content, as well as nitrification–denitrification gene abundance, regardless of different doses. These results imply that the application of ZnO-NPs may inhibit nitrification through the retarding of AOA activity. This study provided us with a potential practice to reduce N2O emissions in tea plantations by applying ZnO-NPs, but the efficiency of this reduction needs further examination under ambient conditions before field application. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a potent greenhouse gas and a strong ozone‐depleting substance. Ocean is an important natural source of atmospheric N2O, which has attracted increasing attention recently as to understand the biogeochemical cycles of nitrogen in marine systems. Nitrification is one of the major N2O production pathways during which N2O is produced as a side product and hydroxylamine (NH2OH) is produced as an obligate intermediate. However, few studies have reported the NH2OH variability in natural seawater and its relationship between N2O remains ambiguous. In summer 2022, spatial distribution and influencing factors of N2O and NH2OH in the northern South China Sea (SCS) were investigated. Dissolved N2O varied from 5.79 to 34.80 nmol L−1 in water column and generally increased with water depth above 800 m. N2O was correlated with dissolved oxygen and nitrate, inferring that N2O was mainly produced via nitrification. NH2OH varied from below detection limit to 3.91 nmol L−1. The adoption of multiple standard additions can largely reduce the bias in NH2OH measurement. Structural equation modeling showed that NH2OH has a potential effect favoring excessive N2O. Surface N2O saturation ranged between 102% and 278%, indicating that the northern SCS was a net source of atmospheric N2O in summer. Plain Language Summary: Hydroxylamine (NH2OH) is an important intermediate product and a precursor of N2O in nitrification, but debates persist regarding NH2OH variability and N2O production. We investigated the distribution, variation and regulatory mechanism of dissolved N2O and NH2OH in the northern SCS, analyzed their potential relationship during nitrification, and provided a new perspective for the greenhouse gas research in the future. We have also proved that a shift in recovery factor can cause large error and thus necessitate the introduction of multiple calibration curves in NH2OH measurement. We have found a weak but positive correlation between NH2OH and ∆N2O, indicating the potential contribution of NH2OH to N2O production during ammonia oxidation. Key Points: A shift in recovery factor from different depths necessitates the introduction of multiple calibration curves in NH2OH measurementA weak correlation between NH2OH and excessive N2O indicated the potential NH2OH oxidation to N2O production via nitrificationThe northern South China Sea (SCS) is a net source of atmospheric N2O [ABSTRACT FROM AUTHOR]

Knowledge of the ecological mechanisms governing N2O cycling in marine sediments lags that of water columns and terrestrial soils, leaving much to be learned about how microbial community dynamics relate to variability in sediment N2O fluxes. The present study assesses these relationships across two distinct environments by focusing on the community structure and activity of N2O reducing microorganisms. The N2O sink capacity of minimally impacted Bermudian mangrove sediments was first estimated using trace‐level microsensors and profile interpretation modeling. Molecular data obtained from these sediments were then compared with those from the Northeast Subarctic Pacific (NESAP) outer continental margin, where previous measurements suggest considerable N2O effluxes. Net N2O uptake was observed for mangrove sediments under ambient and elevated dissolved inorganic nitrogen concentrations (−0.22 ± 0.15 to −0.30 ± 0.26 μmol N2O m−2 d−1), suggesting the microbial potential for N2O consumption exceeded the potential for production via combined nitrification and denitrification. Targeting of bacterial nosZI and nosII gene clusters for quantification using qPCR indicated higher abundance and expression of non‐denitrifier nosZII genes in mangrove sediments demonstrating net N2O uptake. Net N2O production in NESAP sediments was associated with higher abundance and expression of nosZI genes associated with canonical denitrifiers. These results suggest that organisms possessing atypical nosZII genes may act as important N2O scavengers in low‐nitrogen coastal sediments. Plain Language Summary: Nitrous oxide (N2O) is potent greenhouse gas that contributes to ozone depletion in the stratosphere, and atmospheric concentrations a rising as a result of human activities. The production and consumption of N2O in the environment is mediated by microorganisms, and the balance between the two processes is governed by environmental factors such as concentrations of dissolved oxygen and fixed nitrogen. However, little is known about the relationships between microbial community dynamics and rates of N2O production and consumption, especially in sediment environments. We found that minimally impacted mangrove sediments of Bermuda are net consumers of N2O, and continue to consume N2O following short term nitrogen enrichments. This N2O sink capacity was linked to higher abundances and activity levels of atypical N2O reductase genes when compared to sediments that produce N2O. These genes facilitate the consumption of N2O, and frequently occur in microbes that do not have the capacity to produce N2O via denitrification. This study provides further insight into the role of microbial community structure in regulating sediment N2O cycling, and argues for targeted conservation and restoration of coastal systems where sediment environments are in close proximity to the atmosphere. Key Points: The N2O sink capacity of minimally impacted Bermudian mangrove sediments is resilient against modest, short‐term nutrient enrichmentHigh prevalence of non‐denitrifier nosZII genes in mangrove sediments implies a crucial role as N2O scavengers in low‐nitrogen systemsNet N2O production in NESAP sediments is associated with higher expression of nosZI genes associated with canonical denitrifiers [ABSTRACT FROM AUTHOR]

Aim: Incorporating non-bloat legumes into grass pastures can reduce enteric methane and alter cattle urinary urea N output by increasing protein intake. Deposition of high urea N urine influences soil N-cycling microbes and potentially N2O production. We studied how urine urea N concentration affects soil nitrifier and denitrifier abundances, activities, and N2O production. Methods: 15N13C-labelled urea dissolved in cattle urine was added at 3.5 and 7.0 g L−1 to soils from a grazed, non-bloat legume pasture and incubated under controlled conditions. CO2, N2O, 13C-CO2, and 15N-N2O production were quantified over 240 h, along with nitrifier and denitrifier N-cycling genes and mRNA transcripts. Results: High urea urine increased total N2O relative to the control; low urea was not significantly different from the control or the high urea treatment. As a result, N2O-N emission factors were not significantly different between the low urea treatment (1.17%) and high urea treatment (0.94%). Doubling urea concentration doubled CO2-Curea and N2O-Nurea but not total N2O-N. Urine addition initially inhibited then increased AOB transcripts and gene abundances. nirK and nirS transcript abundances indicated that denitrification by ammonia oxidizers and/or heterotrophic denitrifiers dominated N2O production. Urine addition increased nosZ-II vs. nosZ-I transcripts, improving soil N2O reduction potential. Conclusion: Characterizing this interplay between nitrifiers and denitrifiers improves the understanding of urine patch N2O sinks and source dynamics. This mechanistic information helps to explain the constrained short-term N2O emissions observed in response to excess urine N excretion from cattle consuming high protein diets, e.g. non-bloat legumes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Acidification, coverage, microbe N conversion, adsorption combined for NH 3 reduction. • Rich H 2 SO 4 and sulfate remained on the vermiculite (VM) after 5 M H+ modification. • Giant CO 2 bubbles due to initial slurry acidification promoted the cover floating. • Vermiculite after 5 M H+ modification performed highest NH 3 reduction (68 %). • NH 4 +/ NH 3 adsorption by VM contributed little (4.7%) to the overall NH 3 mitigation. Animal slurry storage is an important ammonia (NH 3) emission source. Sulfuric acid (H 2 SO 4)-modified vermiculite coverage is a new promising technology for controlling NH 3 emission from slurry storage. However, the underlying mechanisms in controlling the mitigation effect remain unclear. Here, a series of experiments to determine the effect of H 2 SO 4 on the modified vermiculite properties, floating persistence, and NH 3 mitigation effect was conducted. Results showed that abundant H 2 SO 4 and sulfate remained on the outer surface and in the extended inner pores of the vermiculite with acidifying H+ concentrations higher than 5 M. An initial strong instantaneous acidification of surface slurry released rich carbon dioxide bubbles, strengthening cover floating performance. An acidification in the vermiculite cover layer and a good coverage inhibition interacted, being the two leading mechanisms for mitigating NH 3 during initial 40–50 days of storage. The bacterial-amoA gene dominated the conversion of NH 3 to nitrous oxide after 50 days of storage. Vermiculite with 5 M H+ modification reduced the NH 3 emissions by 90 % within the first month of slurry storage and achieved a 64 % mitigation efficiency throughout the 84 days period. With the development of the aerial spraying equipment such as agricultural drones, acidifying vermiculite coverage hold promise as an effective method for reducing NH 3 emission while absorbing nutrients from liquid slurry storage tank or lagoon. This design should now be tested under field conditions. [ABSTRACT FROM AUTHOR]

This study characterized nitrogen removal and nitrous oxide (N2O) emissions from lab-scale anoxic/aerobic sequencing batch reactor (AOSBR) and intermittent aeration sequencing batch reactor (IASBR), respectively, for treating synthetic municipal wastewater. The N2O emission was evaluated in a simulated cycle, and batch conditions of aerobic nitrification, simultaneous nitrification and denitrification (SND), and aerobic denitrification. The results show that nitrogen removal was enhanced in IASBR compared to AOSBR, with 94.2% and 67.9% of total inorganic nitrogen removal efficiency in IASBR and AOSBR, respectively. In the simulated cycle, the emission factors (of oxidized ammonium) were 4.9% and 0.6% in AOSBR and IASBR, respectively. Under batch conditions, the N2O emission factors during SND were obviously higher than that during aerobic nitrification and denitrification. The N2O emission factors during SND ranging 0.68–11.68% in AOSBR and 1.25–5.13% in IASBR. Furthermore, N2O emission under batch conditions was affected by the aeration ratios. Moderate and high aeration ratios used in this study stimulated the N2O emission from SND. The N2O emission was enhanced with the nitrite accumulation during aerobic nitrification when the nitrite-oxidizing bacteria was inhibited by a chemical inhibitor. Aerobic denitrification via nitrite could be the main pathway of N2O generation from SND processes. The findings from our study can help further understand N2O emission mechanisms and guide the optimization of the current wastewater treatment process for minimizing N2O emission. [ABSTRACT FROM AUTHOR]

The aim of this study was to explore the effect of biochar on N2O emissions in soils with different pH levels. Soils with five pH levels (4.0, 5.1, 5.8, 6.6, and 7.2) were incubated in two conditions, with 0% biochar (CK) and 1% biochar (BC), for 23 days. N2O emissions were measured at nine time points, and soil chemical properties, AOA-amoA, AOB-amoA, nirK, nirS, and nosZ, were analyzed. Partial least squares path modelling (PLS-PM) was used to assess the effect of nitrification and denitrification pathways on potential N2O emissions. The results showed that biochar reduced N2O emissions in highly acidic soil (pH 4.0) but increased emissions in soils with pH values ranging from 5.1 to 7.2. In highly acidic soils, decreased N2O emission was associated with increased soil pH (p < 0.05) and decreased dissolved organic carbon content (p < 0.05), leading to higher nosZ gene abundance (p < 0.05). Meanwhile, in acidic to neutral soils, biochar application increased soil pH (6.6–11.7%), dissolved organic nitrogen (5.9–29.5%), dissolved organic carbon (8.6–41.0%), stimulated AOB-amoA, nirK, nirS gene abundance (p < 0.05), and thus increased N2O emissions. The results verified the influence of nitrification and denitrification genes on N2O production in soils with different pH values. In conclusion, biochar had different effects on N2O emissions based on soil pH, highlighting the need to consider pH when using biochar to mitigate N2O emissions in subtropical citrus orchards.

Nitrous oxide (N2O) is a potent greenhouse gas, and its mitigation is a pressing task in the coming decade. However, it remains unclear which specific process between concurrent nitrification and denitrification dominates worldwide N2O emission. We snagged an opportunity to ascertain whence the N2O came and which were the controlling factors on the basis of 1315 soil N2O observations from 74 peer‐reviewed articles. The average N2O emission derived from nitrification (N2On) was higher than that from denitrification (N2Od) worldwide. The ratios of nitrification‐derived N2O to denitrification‐derived N2O, hereof N2On:N2Od, exhibited large variations across terrestrial ecosystems. Although soil carbon and nitrogen content, pH, moisture, and clay content accounted for a part of the geographical variations in the N2On:N2Od ratio, ammonia‐oxidizing microorganisms (AOM):denitrifier ratio was the pivotal driver for the N2On:N2Od ratios, since the AOM:denitrfier ratio accounted for 53.7% of geographical variations in N2On:N2Od ratios. Compared with natural ecosystems, soil pH exerted a more remarkable role to dictate the N2On:N2Od ratio in croplands. This study emphasizes the vital role of functional soil microorganisms in geographical variations of N2On:N2Od ratio and lays the foundation for the incorporation of soil AOM:denitrfier ratio into models to better predict N2On:N2Od ratio. Identifying soil N2O derivation will provide a global potential benchmark for N2O mitigation by manipulating the nitrification or denitrification. [ABSTRACT FROM AUTHOR]

The excessive application of nitrogen in tea plantations leads to severe soil acidification and N2O emission boosting. To promote sustainable agriculture, nanoparticles (NPs) have emerged as alternative fertilizers, but their effects on soil nitrification and greenhouse gas emissions in tea plantations remain unclear. In this study, the effects of NP type (ZnO-NPs and Fe2O3-NPs) and dose (0, 1, 10, and 100 mg·kg−1) on soil N2O emissions were investigated via a lab incubation trial. Soil pH, ammonium, and nitrate changes were also monitored during the incubation period. The abundance of functional genes related to nitrification and denitrification processes was analyzed as well. The results showed that ZnO-NPs led to a decrease in N2O emissions. The reduction effect was stronger with increasing dose and resulted in a 33% reduction at an addition rate of 100 mg·kg−1. The cumulative N2O emissions had significantly positive correlations with NH4+-N and NO3−-N. ZnO-NP addition showed a significantly negative effect on Ammonia-Oxidizing Archaea (AOA) but a positive effect on Ammonia-Oxidizing Bacteria (AOB) gene abundance. In contrast, Fe2O3-NPs showed an insignificant impact on N2O emissions and soil N content, as well as nitrification–denitrification gene abundance, regardless of different doses. These results imply that the application of ZnO-NPs may inhibit nitrification through the retarding of AOA activity. This study provided us with a potential practice to reduce N2O emissions in tea plantations by applying ZnO-NPs, but the efficiency of this reduction needs further examination under ambient conditions before field application.

A large amount of greenhouse gas nitrous oxide (N 2 O) will be produced during the biological nitrogen removal process for organic wastewater of low C/N ratio. One of the effective methods to solve this problem is to incorporate inexpensive carbon source. In this study, raw wastewater (RW) from pig farm, that was not anaerobically digested, was utilized as exogenous carbon in both A/O and SBR aerobic reactor to treat liquid digestate with high ammonia nitrogen and low C/N ratio. The results showed that N 2 O emission in SBR was higher than that of A/O process under the same nitrogen load. The N 2 O conversion in the biological nitrogen removal process was investigated by the strategy of integrating stable isotope method and metagenomics. The δO18-N 2 O, δN15-N 2 O, and SP values of the SBR were closer to the denitrification values of Ammonia-Oxidizing Bacteria (AOB) than those of A/O. The abundance of AOB in the SBR reactor was higher than that in the A/O reactor, while the abundance of denitrifying bacteria was lower. The amoA/B/C gene abundance in the SBR was greater than that in the A/O, and the NOS gene abundance was the opposite. The results indicated that both AOB denitrification and bacterial denitrification led to the increase of N 2 O emissions of the SBR. [Display omitted] • RW as source of exogenous carbon source improved nitrogen removal while reducing N 2 O emissions. • Insufficient carbon source would result in more N 2 O emission. • A/O had more N 2 O emission reduction potential than SBR. • The denitrification of ammonia oxidizing bacteria (AOB) resulted in more N 2 O emission from the SBR. [ABSTRACT FROM AUTHOR]

Marine systems are active regions for producing and emitting nitrous oxide (N 2 O), a potent greenhouse gas. In October 2018, samples were collected in the western tropical North Pacific (WTNP) to study the distributions, emissions, and influencing factors of N 2 O. The N 2 O concentrations in surface seawater showed little variation, ranging from 6.2 to 7.9 nmol L−1 (corresponding to N 2 O saturation range of 104–125%), with an average of 6.7 ± 0.6 nmol L−1. The vertical N 2 O distribution is a mirror image of dissolved oxygen (DO), increasing with depth from the surface to a maximum in the vicinity of the DO minimum. The sea-to-air fluxes of N 2 O ranged from 0.1 to 7.5 μmol m−2 d−1, with an average of 1.7 ± 2.2 μmol m−2 d−1, indicating that the WTNP was a net source of N 2 O to the atmosphere. Nitrification is the main process for N 2 O production. The presence of the Mindanao Eddy noticeably changes the vertical profiles of N 2 O in the water column, allowing the N 2 O-rich deep water to reach the subsurface layers, but has little effect on the surface N 2 O concentration. After Typhoon "Yutu" passed through, N 2 O concentrations in surface and subsurface water increased dramatically. The surface concentration increased by about 20%, and the sea-to-air flux of N 2 O increased by about 56%. Despite the short duration of the typhoon, its effect on N 2 O distribution and sea-to-air flux was more pronounced than that of eddies. • Mindanao Eddy transport deep N 2 O to the subsurface layer but not the surface. • The passage of Typhoon "Yutu" enhanced surface N 2 O concentrations by about 20% and sea-to-air N 2 O fluxes by about 56%. • Western Tropical North Pacific acts as a net source of N 2 O to the atmosphere. [ABSTRACT FROM AUTHOR]

[Display omitted] • The N 2 O comes from denitrification of nitrite produced by nitrification in composting. • Nitrosomonas carrying amoA and nirK participate in nitrification and denitrification. • Dicyandiamide delayed N 2 O emissions for through delayed nitrification in mature phase. • Dicyandiamide increases the N 2 O produced by amoA dominated incomplete nitrification. • 10% dicyandiamide reduces N 2 O production of denitrification by inhibiting hao and nitrite. Nitrous oxide (N 2 O) emissions contribute to nitrogen loss and exacerbate the greenhouse effect during composting. Investigating the efficacy and underlying mechanisms of dicyandiamide (DCD) in reducing N 2 O emissions can inform composting strategies aimed at mitigating global climate change. In this study, 5 % or 10 % DCD (based on TN content in the initial mixture) was added during the maturation phase of composting, resulting in delayed N 2 O production. In the control (CK) treatment without DCD, N 2 O production primarily stemmed from denitrification, facilitated by ammonia-oxidizing bacteria (AOB) providing nitrite substrate. The DCD-treated groups exhibited prolonged NH 4 + retention times, and the significant N 2 O emissions were postponed by 4–9 days instead of being consistently inhibited. Notably, the Nitrosomonas genus emerged as the primary AOB contributing to N 2 O emissions, engaging in both nitrification and nitrifier denitrification processes, particularly through Nitrosomonas europaea carrying the nirK gene. DCD delayed nitrification via the amoA gene and persistently hindered the enrichment of hao genes. With a 10 % DCD addition, the predominant pathway for N 2 O production shifted from denitrification, dominated by nirK and norB genes, to incomplete nitrification, predominantly influenced by Nitrosomonas carrying the amoA gene. This study offers a novel perspective on elucidating the mechanism by which DCD influences N 2 O production in composting. [ABSTRACT FROM AUTHOR]

We investigated the potential of earthworm casts to emit N2O, hypothesizing that emission levels are influenced by the species of earthworm and their ecological category. This study examined casts a broad taxonomic and ecological coverage of tropical earthworms, i.e., 16 different species across four ecological categories. We quantified the potential nitrification, N2O production and consumption as well as the abundance of N-related microbial functional groups, including ammonia-oxidizers, nitrite-reducers, and distinct clades of N2O-reducers, along with casts chemical properties to determine cast organic matter quality and substrate availability. Earthworm casts exhibited significantly higher concentrations of carbon, nitrogen, and nitrate compared to control soil, while humification index were lower. A negative correlation between humification index and potential N₂O production suggests that more labile substrates in the casts promote higher N₂O flux. Net potential N₂O emissions were higher in the casts of 7 out of 16 species compared to control soil, and all species’ casts showed higher gross potential N₂O production, with substantial interspecific variability. The abundance of nitrite and N₂O reducers was significantly higher in the casts and positively correlated with potential N₂O emissions. Casts from epigeic and mixed categories displayed higher carbon and nitrogen content, abundance of nitrite and N₂O reducers, ammonia-oxidizing bacteria, and potential N₂O production compared to anecic and endogeic categories, which exhibited higher values of humification index. Structural equation modeling indicated that gross potential N₂O production was primarily explained by the abundance of nitrite reducers and substrate availability indicators such as humification index and nitrate concentration. Our study demonstrates significant interspecific variability in N₂O potential emissions from a broad range of tropical earthworm casts, influenced by species feeding behavior, microbial communities, and substrate availability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two validated models to reveal role of comammox bacteria in granular PN/A system. • AOB-based PN/A achieved > 80 % TN removal at higher DO with 1.7 %∼2.8 % N 2 O production. • Comammox bacteria-based PN/A removed > 80 % TN in a wider range of DO and NH 4 +. • Comammox bacteria-based PN/A produced negligible N 2 O but was more sensitive to DO. In this study, two bioprocess models were first constructed with the newly-discovered comammox process described as one-step and two-step nitrification and evaluated against relevant experimental data. The validated models were then applied to reveal the potential effect of comammox bacteria on the granular bioreactor particularly suitable for undertaking partial nitritation/anammox (PN/A) under different operating conditions of bulk dissolved oxygen (DO) and influent NH 4 +. The results showed although comammox bacteria-based PN/A could achieve > 80.0 % total nitrogen (TN) removal over a relatively wider range of bulk DO and influent NH 4 + (i.e., 0.25–0.40 g‐O 2 /m3 and 470–870 g‐N/m3, respectively) without significant nitrous oxide (N 2 O) production (< 0.1 %), the bulk DO should be finely controlled based on the influent NH 4 + to avoid the undesired full nitrification by comammox bacteria. Comparatively, conventional ammonium‐oxidizing bacteria (AOB)-based PN/A not only required higher bulk DO to achieve > 80.0 % TN removal but also suffered from 1.7 %∼2.8 % N 2 O production. [ABSTRACT FROM AUTHOR]

The newly discovered complete ammonia oxidizer (comammox Nitrospira) is able to single-step nitrification capability, and increased our understanding of soil nitrogen cycling. However, the response of comammox and ammonia-oxidizing bacteria (AOB) and archaea (AOA) to long-term fertilization and rhizosphere effects in paddy soils and their relative contribution to the nitrification-derived N 2 O emissions is still unclear. Here, we collected rhizosphere and bulk soils with thirty years of different fertilization strategies, i.e., non-fertilization (CK), chemical N, P, and K application (NPK), and NPK plus pig manure application (NPKM), respectively, to test changes in nitrification potential rate (PNR), N 2 O emission fluxes, abundance of ammonia oxidizers and their significant drivers. The result showed that NPKM significantly increased soil C and N levels, the proportion of soil middle-size particles (40.35–148.00 μm class), and soil PNR, but decreased soil N 2 O emissions, especially in the drying time of paddy (P < 0.05), compared to NPK fertilization. NPKM had the highest values of AOA, AOB, and comammox clade A amoA gene copy numbers (P < 0.05), but clade B was increased by the NPK in the rhizosphere soil. Furthermore, fertilization showed greater effects on ammonia oxidizers (except for clade B) than the rhizosphere effect. Mantel test showed that SOM, TP, pH, NH 4 +, and NO 3 − were main abiotic factors causing the niche separation among ammonia oxidizers. Linear regression analysis and structural equation model (SEM) showed that both PNR and N 2 O emission fluxes were significantly associated with the abundance of AOB and comammox clade A (P < 0.05). Therefore, our results underline the importance of AOB together with comammox clade A in nitrification and N 2 O production in long-term organic fertilized paddy fields, which could provide new ideas for the mitigation of N 2 O emission by adopting organic fertilization scenarios in Chinese paddy fields. [Display omitted] • Organic fertilization significantly increased AOA, AOB, and comammox abundances. • The rhizosphere showed negligible impacts on AOA, AOB and comammox clade A. • SOM, NH 4 +-N, and pH were the main drivers of abundance of AOA, AOB and comammox. • AOB and comammox clade A contributed most to N 2 O fluxes in long-term fertilized paddy fields. [ABSTRACT FROM AUTHOR]

The seasonal variability, pathways, and sea-to-air fluxes of nitrous oxide (N 2 O) and methane (CH 4) in the coastal environment, where coastal upwelling and mudbanks co-exist are presented based on the monthly time-series measurements from November 2021 to December 2022. Upwelling-driven hypoxic water's shoreward propagation and persistence were the major factors controlling the N 2 O concentrations, while the freshwater influx and sedimentary fluxes modulate CH 4 concentrations. The N 2 O concentrations were high during the southwest monsoon (up to 35 nM; 19 ± 8 nM)), followed by spring inter-monsoon (up to 19 nM; 10 ± 5 nM), and lowest during the northeast monsoon (up to 13 nM; 8 ± 2 nM), whereas the CH 4 levels were high during the spring inter-monsoon (8.4 to 65 nM), followed by southwest monsoon (6.8 to 53.1 nM) and relatively lower concentrations during the northeast monsoon (3.3 to 32.6 nM). The positive correlations of excess N 2 O with Apparent Oxygen Utilisation (AOU) and the sum of nitrate and nitrite (NO x) indicate that nitrification is the primary source of N 2 O in the mudbank regime. The negative correlation of CH 4 concentrations with salinity indicates considerable input of CH 4 through freshwater influx. CH 4 exhibited a highly significant positive correlation with Chlorophyll- a throughout the study period. Furthermore, it displayed a statistically significant positive correlation with phosphate (PO 4 3−) during the northeast monsoon while a strong negative correlation with PO 4 3− during the spring inter-monsoon, pointing towards the role of aerobic CH 4 production pathways in the mudbank regime. N 2 O and CH 4 exhibited a contrasting seasonal pattern of sea-to-air fluxes, characterised by the highest N 2 O fluxes during the southwest monsoon (hypoxia) (13 ± 10 μM m−2 d−1), followed by spring inter-monsoon (12 ± 16 μM m−2 d−1), and the lowest during the northeast monsoon (0.6 ± 3 μM m−2 d−1). Conversely, the highest sea-to-air fluxes of CH 4 were noticed during the spring inter-monsoon (74 ± 56 μM m−2 d−1), followed by the southwest monsoon (45 ± 35 μM m−2 d−1), and the lowest values during the northeast monsoon (19 ± 16 μM m−2d−1). Long-term time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in dynamic nearshore systems. • This study investigated the seasonal variability and sea-to-air fluxes of N 2 O and CH 4 in mudbanks. • Upwelling-induced nearshore hypoxia was the major factor controlling the N 2 O concentrations. • The inter-relationship of N 2 O with NOx and AOU indicated nitrification as the pathway of N 2 O production. • N 2 O showed the highest N 2 O sea-to-air fluxes during the SWM, followed by NEM and SIM. • The highest sea-to-air fluxes of CH 4 were noticed during the SWM and the lowest NEM. [ABSTRACT FROM AUTHOR]

• Exploring nitrogen's dual role: life-giver in agriculture, yet a threat in warfare and biodiversity. • From farm experiences to global impact, exploring nitrogen's historical, environmental, and industrial journey. • Navigating nitrogen complexities: fertilizers, environmental impact, and innovative technological solutions. • A retrospective on the evolving human-nitrogen relationship. • Rethinking nitrogen's role for sustainability, climate, and global food security. This retrospective article reflects on the complex and evolving relationship between humans and nitrogen over several decades. Raised on a Flemish farm, the author's early experiences with nitrogen in agriculture – both its benefits and dangers – laid the foundation for a lifelong interest in this element. The article traverses a broad range of topics related to nitrogen, highlighting its critical role in various historical, agricultural, environmental, and industrial contexts. The narrative begins with a historical overview of nitrogen's role in agriculture and warfare. The development of industrial processes like the Haber and Ostwald methods transformed nitrogen into a key ingredient for both fertilizers and explosives. The dual nature of nitrogen – as a life-giver in agriculture and a destructive component in warfare and also in biodiversity – is an important theme. The article delves into the environmental impacts of nitrogen, particularly in the context of modern agriculture and industrialization. Issues like fertilization, water contamination, and the challenges of managing nitrogenous waste highlight the complex interplay between human activities and environmental health. Technological advancements are explored, including the development of bioaugmentation methods and the potential of genetic engineering in optimizing nitrogen fixation. Throughout the narrative, personal anecdotes are weaved with scientific information, offering a unique perspective on the historical and contemporary challenges of managing nitrogen. The discussion extends to the broader implications of nitrogen management in the context of sustainability, climate change, and global food security and its overall regulatory space. All these considerations call for a re-evaluation of our relationship with nitrogen, advocating for innovative solutions and systemic thinking to address the multifaceted challenges posed by this essential, yet often problematic element. [ABSTRACT FROM AUTHOR]

Anthropogenic input of excess nutrients stimulates massive nitrous oxide (N 2 O) production in estuaries with distinct seasonal variations. Here, nitrogen isotopic and isotopomeric signatures were utilized to investigate the seasonal dynamics of N 2 O production and nitrification at the middle reach of the eutrophic Pearl River Estuary in the south of China. Elevated N 2 O production primarily via ammonia oxidation (> 1 nM-N d−1) occurred from April to November, along with increased temperature and decreased dissolved oxygen concentration. This consistently oxygenated water column showed active denitrification, contributing 20–40 % to N 2 O production. The water column microbial N 2 O production generally constituted a minor fraction (10–15 %) of the estuarine water-air interface efflux, suggesting that upstream transport and tidal dilution regulated the dissolved N 2 O inventory in the middle reach of the estuary. Nitrification (up to 3000 nM-N d−1) played a critical role in bioavailable nitrogen conversion and N 2 O production, albeit with N 2 O yields below 0.05 %. • Seasonal N 2 O production peaked from April to November, primarily due to ammonia oxidation, with a significant contribution from denitrification. • Seasonal variations in temperature and dissolved oxygen were key regulators of N 2 O production. • In-situ microbial production accounted for a fraction (10-15%) of N 2 O emission at the water-air interface. [ABSTRACT FROM AUTHOR]

Rivers play a significant role in the global nitrous oxide (N 2 O) budget. However, the microbial sources and sinks of N 2 O in river systems are not well understood or quantified, resulting in the prolonged neglect of nitrification. This study investigated the isotopic signatures of N 2 O, thereby quantifying the microbial source of N 2 O production and the degree of N 2 O reduction in the Yellow River. Although denitrification has long been considered to be the dominant pathway of N 2 O production in rivers, our findings indicated that denitrification only accounted for 18.3% (8.2%–43.0%) of the total contribution to N 2 O production in the Yellow River, with 50.2%–80.2% being concurrently reduced. The denitrification contribution to N 2 O production (R2 = 0.44, p < 0.01) and N 2 O reduction degree (R2 = 0.70, p < 0.01) were positively related to the dissolved organic carbon (DOC) content. Similar to urban rivers and eutrophic lakes, denitrification was the primary process responsible for N 2 O production (43.0%) in certain reaches with high organic content (DOC = 5.29 mg/L). Nevertheless, the denitrification activity was generally constrained by the availability of electron donors (average DOC = 2.51 mg/L) throughout the Yellow River basin. Consequently, nitrification emerged as the primary contributor in the well-oxygenated Yellow River. Additionally, our findings further distinguished the respective contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N 2 O emissions. Although AOB dominated the N 2 O production in the Yellow River, the AOA specie abundance (AOA/(AOA + AOB)) contributed up to 32.6%, which resulted in 25.6% of the total nitrifier-produced N 2 O, suggesting a significant occurrence of AOA in the oligotrophic Yellow River. Overall, this study provided a non-invasive approach for quantifying the microbial sources and sinks to N 2 O emissions, and demonstrated the substantial role of nitrification in the large oligotrophic rivers. [Display omitted] • Nitrification was the primary contributor to N 2 O in the well-oxygenated Yellow River. • DOC content determined the denitrification activity and N 2 O reduction degree. • Denitrification was generally constrained by electron donors in the Yellow River. • 50.2%–80.2% N 2 O was reduced to N 2 in the Yellow River. The quantification of the microbial sources to N 2 O emissions highlights the important role of nitrification to N 2 O emission in large rivers. [ABSTRACT FROM AUTHOR]

[Display omitted] • First monthly nitrous oxide (N 2 O) measurements conducted in the NW Iberian Upwelling in 2017 and 2018. • Marked seasonal cycle of N 2 O concentrations and air-sea fluxes is reported. • Ventilation driven by the upwelling and remineralization modulated the spatial and seasonal pattern. • Higher N 2 O concentration were found in Ria of Vigo compared to Ria of A Coruña. • The NW Iberian Upwelling system acts a moderate source of N 2 O to the atmosphere. Despite their small spatial extent, coastal upwelling systems are an important source of oceanic nitrous oxide (N 2 O) to the atmosphere. To date, hot-spot N 2 O emissions have been reported for low oxygen waters of the eastern boundary upwelling systems at their tropical latitudes, but there is a limited number of studies in their "oxygenated" temperate latitudes. This is the first study of the N 2 O cycle in the NW Iberian Upwelling system, where we investigated the seasonality of the N 2 O concentrations and their emissions to the atmosphere, along with the spatial differences in this coastal region in response to the upwelling. Monthly observations were collected from February 2017 to July 2018, in two hydrographic sections within the Ría of Vigo and Ría of A Coruña, two coastal embayments with contrasting response to the upwelling of the Eastern North Atlantic Central Water (ENACW) in the region. N 2 O concentrations ranged between 8.56 to 12.53 nmol kg−1 (94–121 % of saturation) in the shelf, and from 8.62 to 17.60 nmol kg−1 (94–203 % of saturation) inside the rías, with the highest N 2 O concentration at the bottom, which increase as the upwelling progress from April to October. The air-sea fluxes of N 2 O varied between −1.6 to 3.26 µmol m−2 d−1 in the shelf and −1.53 to 7.00 µmol m−2 d−1 inside the rías. Local differences on the ventilation and remineralization pattern drives the seasonality of N 2 O and differences between Ria of Vigo and Ria of A Coruña, being the higher values of N 2 O concentrations and air-sea fluxes registered in the inner Ria of Vigo. Our study reports the N 2 O emissions of an upwelling system in a temperate latitude, where the upwelling waters are central waters relatively well ventilated in terms of oxygen content, behaving as a moderate low net source of N 2 O to the atmosphere compared to tropical upwelling latitudes, characterised by a lower oxygen content. [ABSTRACT FROM AUTHOR]

The discovery of comammox Nitrospira has altered our perception of the nitrogen biogeochemical cycle. However, their functional importance compared to canonical ammonia oxidisers (i.e., ammonia-oxidising bacteria (AOB) and archaea (AOA)) in agricultural soils remains elusive, especially in acidic soils. Here, we assessed the functional importance of these functional guilds in nitrification and nitrous oxide (N 2 O) emissions in three acidic agricultural soils by using a range of nitrification inhibitors (acetylene, 3,4-dimethylpyrazole phosphate (DMPP) and different concentrations of 1-octyne) and monitored their community assemblage and population dynamics. The sensitivity of comammox Nitrospira clade A to 1-octyne varied across soils, highlighting that the inappropriate use of 1-octyne can lead to misestimation of comammox activity. AOA were key NH 3 oxidisers in the three soils, while AOB also contributed significantly to nitrification in one soil. In contrast, comammox Nitrospira always played a minor role in ammonia oxidation and N 2 O emissions, likely due to their low abundances, restricted cellular kinetic properties and N 2 O production mechanisms. Together, this study demonstrates that comammox Nitrospira play a less important role in ammonia oxidation and N 2 O production in acidic agricultural soils than AOA and AOB, thereby providing important novel insights into the mitigation of nitrogen fertiliser loss and N 2 O emissions. • Comammox play a minor role in NH 3 oxidation and N 2 O emissions in acidic arable soils. • AOB are functionally active in acidic agricultural soils. • The sensitivity of comammox Nitrospira clade A to 1-octyne varies across soils. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe-modified biochar addition enhanced the total gross NH 4 + immobilization while decreasing the autotrophic nitrification. • AOB dominated N 2 O emissions in paddy soils with or without biochar or Fe-modified biochar amendment under aerobic conditions. • Fe-modified biochar addition decreased total N 2 O emissions mainly by inhibiting AOB activity. We investigated the potential of ferric iron-modified biochar to lessen autotrophic nitrification and lower nitrous oxide (N 2 O) emissions in paddy soils. A 15N tracing incubation was conducted to investigate the changes in soil gross nitrogen (N) transformations under various biochar amendments (control, unmodified biochar, and Fe-modified biochar). Acetylene and 1-octyne were used to assess the relative contributions of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to N 2 O emission from paddy soil. The Fe-modified biochar increased the rate of NH 4 + immobilization by 26 % and 383 % compared to the control and unmodified biochar treatments, respectively. The gross rate of autotrophic nitrification was reduced to 5.43 μg N g−1 d−1 in the Fe-modified biochar treatment, compared to 6.74 μg N g−1 d−1 in the control treatment and 9.38 μg N g−1 d−1 in the unmodified biochar treatment. Soil pH had varying effects on N 2 O emissions involving AOB and AOA. The N 2 O yields of AOA were more sensitive to Fe-modified biochar applications. AOB, specifically the Nitrosopira -AOB genus, dominated N 2 O production in all treatments. Overall, this study suggests that Fe-modified biochar holds greater potential than unmodified biochar in reducing N 2 O emissions from paddy soils by stimulating NH 4 + adsorption, restraining autotrophic nitrification rates, and AOB-dominant N 2 O production pathways. [ABSTRACT FROM AUTHOR]

• MABRs were exposed to different aeration patterns and scouring frequencies. • Oxygen dynamics were analyzed at different depths during intermittent aeration. • Aeration and scouring had a major impact on the oxygen profiles within the biofilms. • N 2 O emissions differed between the different MABR operation strategies. This study investigated the effects of aeration and scouring strategies on the performance of Membrane Aerated Biofilm Reactors (MABRs) and the distribution of oxygen and nitrous oxide in the biofilm. Four flat sheet MABRs were operated with synthetic feed under different conditions: two with intermittent aeration (iMABR) and two with continuous aeration (cMABR). Scouring was induced by bubbling dinitrogen gas through the reactor bulk at low and high frequencies (LF and HF). In the iMABRs, a partial nitritation biofilm initially developed, but the biofilm adapted to the aeration strategy over time and became nitrifying. The cMABRs directly developed a nitrifying biofilm without a significant phase of partial nitritation. Limiting oxygen availability improved the overall performance with regards to total nitrogen (TN) removal by providing a better environment for anaerobic ammonium oxidation (Anammox) while limiting complete nitrification. Oxygen profiles were measured in the iMABR over time at different biofilms depths, showing that intermittent aeration led to various oxygen concentrations and temporal variations in the oxygen availabilities at different depths of the biofilm. Also, N 2 O emissions from the MABRs differed greatly between the different systems, but still remained lower compared to other reactor configurations for nitrogen removal, making the MABR technology a worthy alternative. The results showed large differences between the operating strategies of the MABRs and can help to gain more insight into the specific properties of MABRs for nitrogen removal. [Display omitted] [ABSTRACT FROM AUTHOR]

Nitrous oxide (N 2 O) emissions from soils are a significant environmental concern due to their contribution to greenhouse gas emissions. Biochar has been considered as a promising soil amendment for its potential to influence soil processes. Iron modification of biochar has been extensively discussed for its ability to enhance adsorption of pollutants, yet its impact on mitigating soil N 2 O emissions remains poorly understood. In the present study, corn straw (CB) and wood (WB) biochars were treated with FeSO 4 /FeCl 3 (SCB and SWB) and Fe(NO 3) 3 (NCB and NWB). The effects of these biochars on soil N 2 O emissions were investigated using soils with varying fertility levels over a 35-day incubation period at 20 °C. Results revealed significant variations in biochar surface chemistry depending on biochar feedstock and iron salts. Compared to pristine biochars, NWB and NCB exhibited higher pH, total N content, and dissolved NO 3 –N concentrations (246 ± 17 and 298 ± 35 mg kg−1, respectively), but lower bulk and surface C content. In contrast, SWB and SCB demonstrated acidic pH and elevated dissolved NH 4 –N concentrations (5.38 ± 0.43 and 4.19 ± 0.22 mg kg−1, respectively). In forest soils, NWB and NCB increased cumulative N 2 O emission by 28.5% and 67.0%, respectively, likely due to the introduction of mineral nitrogen evidenced by significant positive correlation with NO 3 –N or NH 4 –N. Conversely, SWB and SCB reduced emissions in the same soil by 28.5% and 6.9%, respectively. In agricultural soil, most biochars, except SWB, enhanced N 2 O emissions, possibly through the release of labile organic carbon facilitating denitrification. These findings underscore the significance of changes in biochar surface chemistry and the associated potential risk in triggering soil N 2 O emissions. This study highlights the need for a balanced design of biochar that considers both engineering benefits and climate change mitigation. • Corn straw and wood biochars were modified by FeSO 4 /FeCl 3 and Fe(NO 3) 3. • The changes in biochar surface chemistry depend on feedstock and iron salts. • Fe(NO 3) 3 -treated biochars significantly enhanced soil N 2 O emissions by 12% and 67%. • FeSO 4 /FeCl 3 -treated wood biochar suppressed soil N 2 O emissions by 29% and 17%. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electric field improved the growth and survival rate of BSFL. • Electric field mitigated the emissions of NH 3 by 24.55% and N 2 O by 66.71%. • Electric field suppressed the microbial nitrification and denitrification. As a potential strategy for the disposal of various organic wastes, bioconversion with black soldier fly larvae (BSFL) emits large amounts of ammonia (NH 3) and nitrous oxide (N 2 O), especially at the late stage. Recently, application of electric field has been proposed as an effective strategy to mitigate NH 3 and N 2 O emissions in the composting progress, but its effect remains to be explored in BSFL-based bioconversion. Here, this study firstly evaluated the effect of 1, 3, and 5 V direct-current voltage on the bioconversion of dairy manure by BSFL. The results showed that 3 V treatment significantly enhanced the growth and survival rate of BSFL and significantly reduced the emissions of NH 3 and N 2 O by 24.55 % and 66.71 %, respectively. Consistently, 3 V treatment significantly reduced the contents of NH 4 +-N, NO 3 –-N, and NO 2 –-N, as well as the relative abundance of key genes (amoA , narG , nirS , and norB) for microbial nitrification and denitrification in the substrate relative to CK at day 6. Hence, electric field can stimulate BSFL growth and mitigate NH 3 and N 2 O emissions, which might be ascribed to the effect of current on the activity of BSFL, and the suppression of microbial nitrogen transformation while promoting the degradation of organic matter. This study novelly demonstrates that electric field is a novel and environment-friendly strategy to facilitate bioconversion of organic wastes by BSFL with higher biomass production and lower nitrogen loss. [ABSTRACT FROM AUTHOR]

Ammonia-oxidation process directly contribute to soil nitrous oxide (N 2 O) emissions in agricultural soils. However, taxonomy of the key nitrifiers (within ammonia oxidising bacteria (AOB), archaea (AOA) and complete ammonia oxidisers (comammox Nitrospira)) responsible for substantial N 2 O emissions in agricultural soils is unknown, as is their regulation by soil biotic and abiotic factors. In this study, cumulative N 2 O emissions, nitrification rates, abundance and community structure of nitrifiers were investigated in 16 agricultural soils from major crop production regions of China using microcosm experiments with amended nitrogen (N) supplemented or not with a nitrification inhibitor (nitrapyrin). Key nitrifier groups involved in N 2 O emissions were identified by comparative analyses of the different treatments, combining sequencing and random forest analyses. Soil cumulative N 2 O emissions significantly increased with soil pH in all agricultural soils. However, they decreased with soil organic carbon (SOC) in alkaline soils. Nitrapyrin significantly inhibited soil cumulative N 2 O emissions and AOB growth, with a significant inhibition of the AOB Nitrosospira cluster 3a.2 (D11) abundance. One Nitrosospira multiformis -like OTU phylotype (OTU34), which was classified within the AOB Nitrosospira cluster 3a.2 (D11), had the greatest importance on cumulative N 2 O emissions and its growth significantly depended on soil pH and SOC contents, with higher growth at high pH and low SOC conditions. Collectively, our results demonstrate that alkaline soils with low SOC contents have high N 2 O emissions, which were mainly driven by AOB Nitrosospira cluster 3a.2 (D11). Nitrapyrin can efficiently reduce nitrification-related N 2 O emissions by inhibiting the activity of AOB Nitrosospira cluster 3a.2 (D11). This study advances our understanding of key nitrifiers responsible for high N 2 O emissions in agricultural soils and their controlling factors, and provides vital knowledge for N 2 O emission mitigation in agricultural ecosystems. • Soil N 2 O emissions increase with soil pH increase in fertilised cropland soils. • High N 2 O emissions in alkaline soils are derived from AOB-dominated nitrification. • Nitrification-related N 2 O emissions are negatively correlated with SOC content. • Nitrosospira cluster 3a.2 is the main AOB taxa responsible for high N 2 O emissions. • OTU34 could be a candidate biomarker for predicting nitrification-related N 2 O emissions. [ABSTRACT FROM AUTHOR]

Microplastics (MPs) pollution are found to be increasing in vegetable soils and potentially affecting N 2 O production and their associated pathways; however, its specific effects remain unclear. Here, we selected two common MPs, PE and PP at four different concentration levels of 0, 0.5, 1.5 and 3%, and conducted several incubation experiments aiming to explore soil bacterial and fungal N 2 O production. Results showed that the bacteria were the main contributors for the production of N 2 O, regardless of the absence or presence of MPs; and its contribution was decreased with increasing concentrations of PE and PP. The nosZ clade I and II genes were positively correlated with N 2 O reduction rates, indicating a combined regulation on soil N 2 O reduction. PE significantly inhibited the bacterial nitrification and denitrification, but did not affect the total N 2 O production rates; while PP significantly reduced both the bacterial and fungal N 2 O production rates. The resistance of fungal N 2 O production to MPs pollution was stronger than that of the bacterial N 2 O production. It highlights that the MPs pollution could reduce the potential of N 2 O production and reduction, and thus disturb soil nitrogen cycling system; while the inhibition on N 2 O production via bacteria and fungi varies with different types of MPs. This study is conducive to an improved and more comprehensive understanding of the ecological impacts of MPs within the agroecosystem. [Display omitted] • PE microplastic inhibited bacterial but did not affect the total N 2 O production. • PP microplastic decreased both the bacterial and fungal N 2 O productions. • N 2 O reduction potential was suppressed by both PP and PE microplastics. • Both the nosZ clade I and II genes dominated the N 2 O reduction in vegetable soils. [ABSTRACT FROM AUTHOR]

The marked salinity and alkaline pH of coastal saline soil profoundly impact the nitrogen conversion process, leading to a significantly reduced nitrogen utilization efficiency and substantial gaseous nitrogen loss. The application of soil amendments (e.g. biochar, manure, and gypsum) was proved to be effective for the remediation of saline soils. However, the effects of the three amendments on soil nitrogen transformation in soils with various salinity levels, especially on NH 3 volatilization and N 2 O emission, remain elusive. Here, we reported the effects of biochar, manure, and gypsum on NH 3 volatilization and N 2 O emission under four natural salinity gradients in the Yellow River Delta. Also, high-throughput sequencing and qPCR analysis were performed to characterize the response of nitrification (amoA) and denitrification (nirS , nirK , and nosZ) functional genes to the three amendments. The results showed that the three amendments had little effect on NH 3 volatilization in low- and moderate-salinity soils, while biochar stimulated NH 3 volatilization in high-salinity soils and reduced NH 3 volatilization in severe-salinity soils. Spearman correlation analysis demonstrated that AOA was significantly and positively correlated with the NO 3 −-N content (r = 0.137, P < 0.05) and N 2 O emissions (r = 0.174, P < 0.01), which indicated that AOA dominated N 2 O emissions from nitrification in saline soils. Structural equation modeling indicated that biochar, manure, and gypsum affected N 2 O emission by influencing soil pH, conductivity, mineral nitrogen content, and functional genes (AOA- amoA and nosZ). Two-way ANOVA further showed that salinity and amendments (biochar, manure, and gypsum) had significant effects on N 2 O emissions. In summary, this study provides valuable insights to better understand the effects of gaseous N changes in saline soils, thereby improving the accuracy and validity of future GHG emission predictions and modeling. • The amendments indirectly affected N 2 O emissions through influencing soil properties and functional genes. • Nitrososphaera AOA dominated N 2 O emissions from nitrification in saline soils. • Biochar and manure could reduce N 2 O emissions, while gypsum has the risk of increasing N 2 O emission in saline soils. • The amendments had little effect on NH 3 volatilization in saline soils. [ABSTRACT FROM AUTHOR]

[Display omitted] Copper (Cu) is a key cofactor in ammonia monooxygenase functioning responsible for the first step of nitrification, but its excess availability impairs soil microbial functions and plant growth. Yet, the impact of Cu on nitrogen (N) cycling and process-related variables in cropland soils remains unexplored globally. Through a meta -analysis of 1209-paired and 319-single observations from 94 publications, we found that Cu (Cu addition or Cu-polluted soil) reduced soil potential nitrification by 33.8% and nitrite content by 73.5% due to reduced soil enzyme activities of nitrification and urease, microbial biomass content, and ammonia oxidizing archaea abundance. The response ratio of potential nitrification decreased with increasing Cu concentration, soil total N, and clay content. We further noted that soil potential nitrification inhibited by 46.5% only when Cu concentration was higher than 150 mg kg−1, while low Cu concentration (less than 150 mg kg−1) stimulated soil nitrate by 99.0%. Increasing initial soil Cu content stimulated gross N mineralization rate due to increased soil organic carbon and total N, but inhibited gross nitrification rate, which ultimately stimulated gross N immobilization rate as a result of increased the residence time of ammonium. This resulted in a lower ratio of gross nitrification rate to gross N immobilization rate, implying a lower potential risk of N loss as evidenced by decreased nitrous oxide emissions with increasing initial soil Cu content. Our analysis offers initial global evidence that Cu has an important role in controlling soil N availability and loss through its effect on N production and consumption. [ABSTRACT FROM AUTHOR]

Elevated atmospheric carbon dioxide (CO 2) concentration generally stimulates nitrous oxide (N 2 O) emissions from upland soils, leading to a promoted feedback on climate change. However, the underlying mechanisms on how elevated CO 2 (eCO 2) stimulates N 2 O emissions from nitrogen (N)-fertilized upland soils remain poorly known. Here, we investigated the effects of eCO 2 on soil N 2 O production pathways and associated gross N transformation rates, as well as on nitrifying and denitrifying microbes. Based on a 13 years of free-air CO 2 enrichment (FACE) platform, we found that N 2 O emissions were significantly increased by 46.7% under eCO 2 in a typical summer-maize field. The 15N tracing experiment showed that autotrophic nitrification rate was significantly increased by 11.6% under eCO 2 , coincided with the significantly higher abundance and shift composition of ammonia-oxidizing bacteria (AOB) under eCO 2. Also, eCO 2 enhanced gross ammonium (NH 4 +) immobilization rate but reduced gross nitrate (NO 3 −) immobilization rate. Autotrophic nitrification accounted for approximately 78% of N 2 O emissions, followed by heterotrophic nitrification (about 19%) and denitrification (about 3%). It was found that eCO 2 increased carbon (C) deposition in soil, which could affect the availability of soil labile organic C and N and alter the composition and activity of nitrifiers. These changes promoted the N 2 O production derived from autotrophic nitrification, resulting in higher N 2 O emissions under eCO 2. We could deduce that nitrifier denitrification contributed to the increased N 2 O emissions greatly under eCO 2 , derived by strongly stimulated ammonia oxidation and microbial respiration (i.e. higher soil CO 2 emissions) caused suboxic conditions (i.e. lower soil oxygen (O 2) concentration), along with higher abundance ratios of (nirK + nirS)/ nosZ. Our results suggest that controlling autotrophic nitrification appropriately is crucial for mitigating N 2 O emissions from upland soils under rising atmospheric CO 2 concentration. [Display omitted] • Elevated CO 2 significantly increased soil N 2 O emissions from the maize field. • Autotrophic nitrification was the main N 2 O production pathway. • Elevated CO 2 significantly enhanced autotrophic nitrification and N 2 O emission product ratio. • Elevated CO 2 significantly increased soil nitrifiers and denitrifiers abundances. [ABSTRACT FROM AUTHOR]

This study examined the factors controlling the intra- and inter-seasonal variations of dissolved methane (CH 4) and nitrous oxide (N 2 O) in the southeastern Arabian Sea (SEAS). Time-series measurements of CH 4 , N 2 O and allied biogeochemical parameters were carried out during the monthly campaigns in the coastal waters and a seasonal campaign in the shelf waters of the SEAS. The southwest monsoon period (SWM) brought drastic changes in the regional hydrography through the incursion of hypoxic waters due to coastal upwelling, which increased N 2 O concentrations substantially but reduced CH 4 levels. The ranges of N 2 O and CH 4 during the upwelling period were 8–89 nM and 9–165 nM, respectively, and the non-upwelling period was 2–27 nM and 5–271 nM, respectively. The significant positive correlations of N 2 O with apparent oxygen utilisation (AOU), the sum of dissolved nitrate and nitrite (NO 2 − + NO 3 −) and excess N 2 O (ΔN 2 O), as well as a negative correlation with dissolved oxygen indicates that nitrification is the major process in this region during the non-upwelling period. In contrast, during SWM, N 2 O did not correlate with NO 2 − + NO 3 −; however, it exhibited a significant negative correlation with dissolved nitrite (under hypoxia), suggesting the possibility of nitrifier-denitrification as an active process during hypoxia. The high (low) levels of CH 4 recorded during the oxic spring inter-monsoon (hypoxic during the SWM) period showed a direct dependency on the changes in the benthic community. The high abundance of the adult macrofauna and active bioturbation resulted in high sedimentary CH 4 release, which led to enhanced water column CH 4 concentrations (17–271 nM) during the spring inter-monsoon period. In addition, the breakdown of methylated organic compounds under nutrient-limited conditions may also support the elevated CH 4 levels in surface waters. A low macrofaunal abundance and reduced bioturbation led to a considerable reduction of subsurface CH 4 concentrations during hypoxia. Overall, the SEAS is found to be a net source of CH 4 and N 2 O to the atmosphere, with its sea-to-air fluxes ranging from 1.7 to 85.8 μM m−2 d−1 (19.88 ± 22.20 μM m−2 d−1) for N 2 O and 4–756 μM m−2 d−1(133 ± 158 μM m−2 d−1) for CH 4. • Seasonal variability and sea-to-air fluxes of CH 4 and N 2 O in the SEAS were linked to upwelling and freshwater influx. • Under hypoxia, N 2 O concentrations were substantially high, and CH 4 levels were low. • Nitrification was the major process regulating the N 2 O during the non-upwelling season. • Nitrifier-denitrification might act as a N 2 O source during hypoxia. • Low macrofaunal abundance and bioturbation led to a decrease in CH 4 concentrations during hypoxia. [ABSTRACT FROM AUTHOR]

Water management strategies are critical in regulating nitrous oxide (N 2 O) emissions from paddy soils under rice cultivation. Biochar is widely used as an amendment to decrease soil N 2 O emissions. However, the impacts of biochar amendment on N 2 O emissions under different water management strategies in paddy soils have not been investigated thoroughly, and the underlying mechanisms remain poorly understood. In particular, the effects of mid-season water drainage, a strategy used for water conservation, need to be better understood. In this study, a pot experiment was conducted including six treatments: alternating dry-wet conditions during the mid-season period without and with 2% (w/w) biochar (AWD1 and AWD2, respectively), continuous flooding during the mid-season period without (CF1) and with 2% (CF2) biochar, and regular mid-season drainage during the mid-season period without (CON1) and with 2% (CON2) biochar. All treatments received inorganic nitrogen, phosphorus and potassium fertilizers (NPK) administered in a split application. We found that fluxes of N 2 O emission peaked after the two fertilizers' additions. No significant difference in the first N 2 O flux peak was found among different treatments. There also was no difference in the second N 2 O flux peak in both CF treatments, whereas biochar addition significantly decreased the second N 2 O flux peak in AWD2 and CON2 treatments. This was mainly due to increased nosZ gene copies and decreased ratios of denitrification and amoA genes to nosZ gene copies. Biochar amendments resulted in significant stimulation of nifH gene copies in the AWD2 treatment. Lastly, biochar increased rice yields in all treatments. Our study suggested that AWD2 was the optimal management strategy for mitigating N 2 O emissions and improving rice production in this tropical paddy soil. [Display omitted] • Biochar addition decreased cumulative N 2 O emissions in all treatments. • Biochar addition only promoted cumulative CH 4 emissions in the AWD treatments. • Biochar addition decreased the second N 2 O flux peak mainly by increasing nosZ gene. • Gene copy ratios of (nirS + nirK)/ nosZ and of amoA/nosZ drive N 2 O emissions. • The AWD2 treatment had the lowest greenhouse gas intensity and the highest crop yield. [ABSTRACT FROM AUTHOR]