N2O Accumulation Research Articles

Long-term effects of acetylene on denitrifying N2O production: Biomass performance and microbial community

Maximizing production and use of N2O produced by the process of denitrification has emerged as a promising concept for energy recovery from nitrogen. Acetylene is well known as an inhibitor of N2O reductase activity, however, to date, few studies have utilized long-term cultivation with acetylene to achieve denitrifying N2O recovery. This work assessed the performance of N2O production and shifts in the denitrifying community in response to long-term exposure of acetylene during nitrite denitrification. Batch tests showed that long-term exposure to acetylene at high concentrations (volume fraction >40 %) resulted in high N2O accumulation, up to 98.6 %. High-throughput 16S rRNA gene analysis suggested that Saprospiraceae dominated the microbial community at the end of the cultivation period with its abundance increasing from the initial 3.2% to 42.2%. Metagenomic analysis revealed that after the long-term cultivation under high level acetylene, the relative abundance of nosZ in the community did not change significantly. Instead, the nosZⅡ-harboring bacteria became dominant. Batch tests in series demonstrated that N2O reduction could be restored rapidly upon removal of acetylene. This study describes an effective method for high yield production of N2O during nitrite denitrification that can be used to enhance energy recovery and improves our understanding of the long-term effect of acetylene on the denitrifying community.

Characteristics and mechanism of simultaneous nitrate and phenol removal by a newly isolated Cupriavidus oxalaticus T2

Phenol and nitrate in wastewater are harmful to organisms and the environment due to their ecotoxicity. Biological treatment is considered as an environmentally friendly approach for their removal. At present, limited studies explored the capability of single bacterium with simultaneous phenol degradation and aerobic denitrification. In this work, a Cupriavidus oxalaticus T2 with the capability of simultaneous phenol degradation and denitrification under aerobic conditions was isolated from activated sludge. Single factor tests demonstrated that the denitrification optimal conditions of T2 were the carbon source of sodium succinate, C/N ratio of 10, 30 °C, shaking speed of 150 rpm, and pH of 6–7. Strain T2 could remove 100 mg/L NO3−-N within 12 h under optimal conditions without obvious accumulation of NO2−-N and NH4+-N. Moreover, it could simultaneously remove 69 mg/L of NO3−-N and 1000 mg/L of phenol. This study revealed that the strain degraded phenol via ortho-pathway and removed nitrate involved in two pathways: denitrification and dissimilatory nitrate reduction to ammonium. A new strain with the capability of simultaneous nitrate removal and phenol degradation was characterized, suggesting its potential application in further eco-friendly treatment of wastewater.

Isolated heterotrophic nitrifying and aerobic denitrifying bacterium for treating actual refinery wastewater with low C/N ratio

Heterotrophic nitrifying and aerobic denitrifying bacteria that have been widely isolated from complicated activated sludge microflorae demonstrate dominant advantages in simultaneous removal of ammonium and nitrogen oxides under aerobic conditions. However, owing to the need of organic carbon to support bacterial growth, nitrogen removal of actual industrial wastewater with low carbon-to-nitrogen (C/N) ratio remains a challenge. Here, Pseudomonas mendocina Y7 was identified and presented to effectively remove nitrogen of actual refinery wastewater with low C/N ratio. The isolated bacterium showed high removal efficiency of NH4+-N, NO2--N, and NO3--N up to about 90% in single (100mg/L) or mixed (200mg/L) nitrogen source media at low C/N ratio of 6 when it was cultivated for 12 or 21h. According to PCR amplification, the heterotrophic nitrification and aerobic denitrification capability of strain Y7 was attributed to the functional genes of amoA, hao, napA, and nirS. In activated sludge process for treating actual refinery wastewater with low C/N ratio, compared to abundant accumulation of NO2--N and NO3--N only using the activated sludge, strain Y7 significantly improved the removal efficiency of NH4+‒N and total nitrogen (with influent concentrations of about 40 and 55mg/L) from about 47% and 22% to about 85% and 73%, respectively, without the accumulation of nitrogen oxides. Microbial community structure analysis revealed that strain Y7 could coexist well with other microorganisms in the activated sludge and maintain highly efficient and steady nitrogen removal in continuous treatment system. This discovery provides a promising treatment approach toward actual nitrogen-rich industrial wastewater.

New insight on the regulation of N2O production in aerobic condition: An N2O metabolic perspective based on enzymatic analysis of nitrous oxide reductase

Nitrous oxide reductase (N2OR) (EC 1.7.2.4) is the only enzyme that can catalyze nitrous oxide (N2O) decomposition to nitrogen gas, which connects tightly with N2O accumulation. But it is known little about N2O metabolism based on the enzyme in biological nitrogen removal of wastewater, especially in aerobic conditions. This paper studies the biosynthesis and metabolism of N2O through the analysis of the activity of the enzyme in aerobic conditions. It is revealed that the main metabolic mechanism of N2O was heterotrophic denitrification, which was mainly affected by electron supply (the amount of organic matter) and the external environment (pH value), while the escaped N2O mainly came from nitrifiers. This indicates that the primary reason for the emission of N2O in the aerobic phase was that the N2O consumption rate of heterotrophic denitrifiers was lower than the N2O production rate of nitrifiers. Considering the removal efficiency of nitrogen, the enzyme activity and the reduction of N2O emissions, it is feasible to keep the C/N ratio of inflowing around 6.5 in A/O SBR.

Nitric oxide and nitrite removal by partial denitrifying hollow-fiber membrane biofilm reactor coupled with nitrous oxide generation as energy recovery

ABSTRACT Nitrogen oxide (NOx) emissions cause significant impacts on the environment and must therefore be controlled even more stringently. This requires the development of cost-effective removal strategies which simultaneously create value-added by-products or energy from the waste. This study aims to treat gaseous nitric oxide (NO) by hollow-fibre membrane biofilm reactor (HFMBfR) in the presence of nitrite () and evaluate nitrous oxide (N2O) emissions formed as an intermediate product during the denitrification process. Accumulated N2O can be utilised in methane oxidation as an oxidant to produce energy. In the first stage of the study, the HFMBfR was operated by feeding only gaseous NO as the nitrogen source. During this period, the best performance was achieved with 92% NO removal efficiency (RE). In the second stage, both NO gas and were supplied to the system, and 91% NO and 99% reduction were achieved simultaneously with the maximum N2O generation of 386 ± 31 ppm. Lower influent carbon to nitrogen (C/N) ratios, such as 4.5 and 2.0, and higher −N loading rate of 158 mg N day−1 favoured N2O generation. An improved NO removal rate and N2O accumulation were seen with the increasing amount of in the medium. The 16S rDNA sequencing analysis revealed that Alicycliphilus denitrificans and Pseudomonas putida were the dominant species. The study shows that an HFMBfR can be successfully used to eliminate both and gaseous NO and simultaneously generate N2O by adjusting the system parameters such as C/N ratio, and loading.

Nitrification performance evaluation of activated sludge under high potassium ion stress during high-ammonia nitrogen organic wastewater treatment

The recycling reverse osmosis (RO) membrane concentrate of some high-ammonia nitrogen (NH4+-N) organic wastewater to the biological unit could cause potassium ion (K+) accumulation, thereby affecting the removal of NH4+-N by activated sludge. Thus, the effects of high K+ stress on activated sludge nitrification performance was studied. The results showed that the high K+ stress promoted the floc sludge to produce more extracellular polymers (EPS), which accelerated the sludge sedimentation and enriched the biomass in sequential batch reactors (SBRs). The ammonia oxidation process and nitrite (NO2--N) oxidation process were further analyzed in the nitrification process. High K+ stress enriched ammonia oxidizing bacteria (AOB), which ensured the efficient ammonia oxidation process in SBRs, and ensured the removal rate of NH4+-N was maintained above 93%. However, high K+ stress (15g/L KCl) inhibited the activity of NO2−-N oxidizing bacteria (NOB) and reduced the abundance of NOB, thus leading to the accumulation of NO2−-N, and finally worsened the nitrification performance of activated sludge. In short, the performance of activated sludge will not be inhibited when the K+ in the wastewater does not exceed 5.23 g/L. The results could provide a reference for the optimization of the biological performance in treating high-NH4+-N organic wastewater with activated sludge coupled RO membrane treatment process.

Paddy-upland rotation with Chinese milk vetch incorporation reduced the global warming potential and greenhouse gas emissions intensity of double rice cropping system.

It is a common practice to maintain soil fertility based on the paddy-upland rotation with green manure in the subtropical region of China. However, rare studies are known about greenhouse gas (GHG) emissions from the paddy-upland rotation with green manure incorporation. Therefore, we conducted a field experiment of two years to compared with the effect of two kinds of green manure (CV: Chinese milk vetch and OR: Oilseed rape), and two kinds of cropping system (DR: double rice system and PR: paddy-upland rotation) on greenhouse gases emissions. We have found that the annual accumulation of CH4 of Chinese milk vetch-rice-sweet potato || soybean was significantly reduced by 32.95%∼63.22% compared with other treatments, mainly because Chinese milk vetch reduced the abundance of methanogens by reducing soil C/N ratio. Meanwhile increasing soil permeability resulting from paddy-upland rotation also reduced soil CH4 emission. However, The annual accumulation of N2O of Chinese milk vetch-rice-sweet potato || soybean was increased by 17.39%∼870.11% compared with other treatments, mainly attributed to paddy-upland rotation decreased soil pH and nosZ abundance and increased nirK and nirS, thus enhancing N2O emission, meanwhile the Chinese milk vetch incorporation and its interaction with the paddy-upland rotation has greatly enhanced the contents of NO3--N and abundance of ammonia-oxidizing archaea (AOA). The area-scaled global warming potential (GWP) and the biomass-scaled greenhouse gas emissions intensity (GHGI) of Chinese milk vetch-rice-sweet potato || soybean was reduced by 19.01%∼50.69% and 5.38%∼35.77% respectively. Thereby, the Chinese milk vetch-rice-sweet potato || soybean cropping system was suitable for agricultural sustainable development.

Effects of Straw Mulching and Nitrogen Reduction on the Distribution of Soil Nitrogen and Groundwater Nitrogen Pollution

To explore the effects of straw mulching and reduced nitrogen fertilization on the temporal and spatial patterns of soil nitrogen, groundwater nitrogen pollution, and summer maize yield, field experiments were carried out in the Hetao irrigation district in 2017 and 2018. The experiment involved the following seven treatments:a control (CK) treatment involving conventional fertilization and traditional tillage, and conventional nitrogen applications reduced by 30% (N1), 20% (N2), and 10% (N3) coupled with either straw surface covering (B) or deep straw burial (S). The results showed that the distribution of soil nitrogen in the CK treatment varied depending on soil depth, with an overall decreasing trend. In the 0-20 cm soil layer under straw surface covering (B) treatments, soil nitrogen was superficially accumulated. NO3--N and NH4+-N content increased by an average of 22.2% and 42.7% compared to the CK treatment, respectively, which decreased significantly at first and then increased slightly with depth. In the 20-40 cm deep soil layer under the deep straw burial (S) treatments, soil nitrogen accumulated and the content of NO3--N and NH4+-N increased by an average of 29.8% and 48.1%, respectively, compared to the CK treatment. Nitrogen accumulation first and then decreased significantly with depth. Nitrogen accumulation under the different straw mulching regimes increased with an increase in the application of reduced nitrogen. After the harvest of summer maize, the accumulation of NO3--N and NH4+-N in the >80 cm soil layer under the B treatments was 19.9%-58.2% and 31.1%-61.7% lower than that of the CK treatment, respectively. This compared to reductions of 36.7%-70.9% and 82.6%-89.2% for the S treatments, respectively. Only the BN3 treatment increased accumulation compared with CK by 0.4% on average, while the SN2 treatment resulted in a 9.3% increase. Summer maize yield and relative indexes were also improved relative to the other treatments. Nonlinear fitting of yield and application reduction showed that deep straw burial was better than surface covering at increasing summer maize production. The effect of deep straw burial and 14%-20% application reduction was better. Straw mulching with reduced nitrogen fertilization can limit nitrogen leaching and thereby reduce the risk of groundwater pollution. After the harvest, groundwater quality was classified in the Ⅱ class, with the risk of nitrogen contamination being lowest under deep straw burial with>20% reduced nitrogen fertilization. These observations show that deep straw deep alongside 14%-20% application reduction could effectively alleviate nitrogen leaching and reduce the risk of nitrogen pollution in groundwater. This approach can help improve the ecological environment and summer maize yields in the Hetao irrigation district.

Improvement of Alcaligenes sp.TB performance by Fe-Pd/multi-walled carbon nanotubes: Enriched denitrification pathways and accelerated electron transport

Aiming at the accumulation of NO2−-N and N2O during denitrification process, this work focused to improve the denitrification performance by Pd-Fe embedded multi-walled carbon nanotubes (MWCNTs). The main conclusions were as follows: 30 mg/L Pd-Fe/MWCNTs have shown an excellent promotion on denitrification (completely TN removal at 36 h). Meanwhile, enzyme activity results indicated that the generation of NO2−-N, NH4+-N by Pd-Fe/MWCNTs led the occur of short-cut denitrification by increasing 203.9% the nitrite reductase activity. Furthermore, electrochemical results and index correlation analysis confirmed that the electron exchange capacity (1.401 mmol eg−1) of Pd-Fe/MWCNTs was positively related to nitrite reductase activity, indicating its crucial role in electron transport activity (0.46 μg O2/(protein·min) at 24 h) during denitrification process by Pd-Fe/MWCNTs played a role of chemical reductant and redox mediator.

Nitrite removal by Acinetobacter sp.TX: a candidate of curbing N2O emission

ABSTRACT The nitrite removal pathway in Acinetobacter sp. TX5 was explored through the key gene identification and the corresponding enzyme purification, after which the capability to reduce nitrite by immobilized beads was investigated in a fixed-bed reactor. Results revealed that a nosZ gene encoding nitrous oxide reductase (N2OR) exists in TX5 cells, and a N2OR responsible for the reduction of N2O to N2 was purified successfully with a molecular weight of 70.05 kDa, a purification fold of 16.30 and a recovery rate of 5.17%. For TX5 immobilization, the optimal values of polyvinyl alcohol (PVA), spent mushroom substrate (SMS) and Aci (TX5) obtained by response surface methodology (RSM) were 6.32%, 2.92% and 4.57%, respectively. In a fixed-bed reactor packed with immobilized TX5, the removal efficiency (RE) achieved 90% (at 50 h) for -N and 85% (at 96 h) for total nitrogen (TN). On the basis of these results, a nitrite removal pathway in TX5 was proposed. Overall, Acinetobacter sp. TX5 might be a promising candidate for nitrite removal with an ability to suppress N2O accumulation.

Preparation of S-doped g-C3N4 with C vacancies using the desulfurized waste liquid extracting salt and its application for NOx removal

In this study, graphitized carbon nitride photocatalysts were successfully prepared by using the desulfurized waste liquid extracting salt from coking plants. The NOx removal efficiency of g-C3N4 synthesized using ammonium thiocyanate (NH4SCN), ammonium thiosulfate ((NH4)2S2O3) and ammonium sulfate ((NH4)2SO4) with different mass ratios were evaluated. The results showed that due to the addition of (NH4)2S2O3 and (NH4)2SO4 a large amount of sulfur-containing gas was released during pyrolysis, which significantly increased the specific surface area and pore volume of the photocatalysts. The release of sulfur-containing gas also resulted in the incorporation of sulfur, and due to this doping process the band gap of the photocatalyst was widened to 2.94 eV, thus enhancing its visible light response. C vacancies were formed in the hepazine ring, which increased the electron density around the N–(C)3 bridge bond and formed an electron trapping center, thus inhibiting the recombination of charge carriers. The photocatalysts had increased O2 adsorption capacity and increased number of amino groups on their surface. This facilitated the rapid generation and consumption of reactive oxygen species, inhibited the accumulation of NO2 on the surface, reduced the production of NO2, thus improving the NOx removal efficiency of the photocatalysts. The principle of using “waste for waste control” was achieved, and a new method for nitrogen oxide removal was provided.

Biological Reduction of Nitric Oxide for Efficient Recovery of Nitrous Oxide as an Energy Source.

Chemical absorption-biological reduction based on Fe(II)EDTA is a promising technology to remove nitric oxide (NO) from flue gases. However, limited effort has been made to enable direct energy recovery from NO through production of nitrous oxide (N2O) as a potential renewable energy rather than greenhouse gas. In this work, the enhanced energy recovery in the form of N2O via biological NO reduction was investigated by conducting short-term and long-term experiments at different Fe(II)EDTA-NO and organic carbon levels. The results showed both NO reductase and N2O reductase were inhibited at Fe(II)EDTA-NO concentration up to 20 mM, with the latter being inhibited more significantly, thus facilitating N2O accumulation. Furthermore, N2O accumulation was enhanced under carbon-limiting conditions because of electron competition during short-term experiments. Up to 47.5% of NO-N could be converted to gaseous N2O-N, representing efficient N2O recovery. Fe(II)EDTA-NO reduced microbial diversity and altered the community structure toward Fe(II)EDTA-NO-reducing bacteria-dominated culture during long-term experiments. The most abundant bacterial genus Pseudomonas, which was able to resist the toxicity of Fe(II)EDTA-NO, was significantly enriched, with its relative abundance increased from 1.0 to 70.3%, suggesting Pseudomonas could be the typical microbe for the energy recovery technology in NO-based denitrification.

Influence of land use and change in the proportion of electron donors required for denitrification on N2O in groundwater.

Nitrate (NO3-) and nitrous oxide (N2O) accumulate in groundwater in relation to human activities and pose multiple threats to the global environment (harming human health and atmospheric damage). This study focused on the evaluation of groundwater NO3-, N2O, and its indirect emission factor under different land use types (agricultural land, urban land, and forest) and response mechanism of major anions to dissolved N2O within groundwater in Dexing which has the largest copper mine in Asia. Specifically, this work used self-organizing maps (SOMs) to identify which anion conditions (NO3-, SO42-, F-, Cl-) and water quality parameters were suitable for the accumulation of groundwater N2O. Finally, we found that the shallow groundwater of agricultural land has a high concentration of NO3- and N2O and the agricultural activity has a significant effect on the temporal and spatial variation of N2O in groundwater. The result of SOMs combined with the positive correlation between N2O and NO3-/SO42- suggested that the electron donor required for denitrification has a significant effect on N2O accumulation. In this respect, when an increased proportion of reduced sulfur is available as an electron donor for autotrophic denitrification, this results in lower concentrations of N2O in groundwater. Through the comprehensive evaluation of the anion conditions and N2O in groundwater under different land use types, this study case can help to estimate the N2O indirect emission from groundwater, so as to constrain the global nitrogen budget.

Performance and microbial community structure of a full-scale ANITATMMox bioreactor for treating reject water located in Finland

The aim of this work was to study the operational performance and the microbial community dynamics during the start-up of ANITATMMox technology implemented at full-scale wastewater treatment plant in Finland to treat reject water from anaerobic digesters. The average ammonium removal in the studied setup reached around 90%, withstanding ammonium loads up to 0.13 g N m−2h−1. The nitrite concentration in the effluent did not exceed 10 mg L−1, and there was a slight accumulation of NO3−-N during the operation which was controlled. Thus, the result showed a robust success to high ammonium loading in presence of organic matter. The sequencing showed a heterogeneous microbial population where Methanosaeta, WCHA1-57 genus, Sphingobacteriia, Chlorobia and diverse unknown fungi were found as dominant phylotypes. Moreover, members of the Brocadiaceae family were dominant in the adhered biomass, mostly represented by Candidatus Scalindua, rarely reported in WWTPs. Overall, the results demonstrated a drastic effect of region-specific operational conditions on carrier biofilm microbial communities as it was demonstrated by the microbial studies.

Winter N2O accumulation and emission in sub-boreal grassland soil depend on clover proportion and soil pH

Inclusion of legume species into grass leys reduces nitrogen (N) fertilizer need but increases the risk of freeze-thaw induced N2O emissions. We investigated how liming and presence of clover affect N2O accumulation under snowpack and its emission during freeze-thaw cycles in autumn and spring under sub-boreal conditions. A field experiment was performed in southern Norway in limed and control plots containing grasses only (fertilized with 270 kg N ha−1 yr−1), a grass-red clover mixture (fertilized with 140 kg N ha−1 yr−1) and unfertilized pure red clover. Soil air samples were collected at 8, 24, and 40 cm depths and analyzed for gas concentrations including N2O, and N2O fluxes measured by a fast-chamber robot. Red clover produced more N2O than the grass-only plots during freeze-thaw cycles in autumn and spring and accumulated more N2O under snow cover (emissions were not measured during this period). Contrary to expectations, limed red clover plots emitted more N2O than control plots during freeze-thaw cycles. Liming reduced subnivean N2O accumulation in grass-only but not in grass-clover or pure clover plots. After spring fertilization, grass-only plots had larger N2O emissions than red clover plots. Our data suggest that winter-sensitive, N-rich clover biomass fuels decomposition and nitrification, thereby increasing NO3 - and depleting O2, resulting in increased N2O emissions from denitrification. Although liming of pure clover leys exacerbated the risk of high N2O emissions during freeze-thaw, this effect was not observed in grass-clover mixtures. Interestingly, grass-clover mixtures also emitted less N2O than expected from their proportions and the emissions recorded in pure grass and clover stands. This warrants further studies into off-season functional diversity effects on N cycling and N2O loss in temperate and boreal forage production.

Microbial controls on net production of nitrous oxide in a denitrifying woodchip bioreactor.

Denitrifying woodchip bioreactors are potential low-cost technologies for the removal of nitrate (NO3 - ) in water through denitrification. However, if environmental conditions do not support microbial communities performing complete denitrification, other N transformation processes will occur, resulting in the export of nitrite (NO2 - ), nitrous oxide (N2 O), or ammonium (NH4 + ). To identify the factors controlling the relative accumulation of NO2 - , N2 O, and/or NH4 + in denitrifying woodchip bioreactors, porewater samples were collected over two operational years from a denitrifying woodchip bioreactor designed for removing NO3 - from mine water. Woodchip samples were collected at the end of the operational period. Changes in the abundances of functional genes involved in denitrification, N2 O reduction, and dissimilatory NO3 - reduction to NH4 + were correlated with porewater chemistry and temperature. Temporal changes in the abundance of the denitrification gene nirS were significantly correlated with increases in porewater N2 O concentrations and indicated the preferential selection of incomplete denitrifying pathways ending with N2 O. Temperature and the total organic carbon/NO3 - ratio were strongly correlated with NH4 + concentrations and inversely correlated with the ratio between denitrification genes and the genes indicative of ammonification (Σnir/nrfA), suggesting an environmental control on NO3 - transformations. Overall, our results for a denitrifying woodchip bioreactor operated at hydraulic residence times of 1.0-2.6d demonstrate the temporal development in the microbial community and indicate an increased potential for N2 O emissions with time from the denitrifying woodchip bioreactor.

Greenhouse gases and biogeochemical diel fluctuations in a high-altitude wetland

The landscapes of high-altitude wetland ecosystems are characterized by different kinds of aquatic sites, including ponds holding conspicuous microbial life. Here, we examined a representative pond of the wetland landscape for dynamics of greenhouse gases, and their association with other relevant biogeochemical conditions including diel shifts of microbial communities' structure and activity over two consecutive days. Satellite image analysis indicates that the area of ponds cover 238 of 381.3 Ha (i.e., 62.4%), representing a significant landscape in this wetland. Solar radiation, wind velocity and temperature varied daily and between the days sampled, influencing the biogeochemical dynamics in the pond, shifting the pond reservoir of inorganic versus dissolved organic nitrogen/phosphorus bioavailability, between day 1 and day 2. Day 2 was characterized by high dissolved organic nitrogen/phosphorus and N2O accumulation. CH4 presented a positive excess showing maxima at hours of high radiation during both days. The microbial community in the sediment was diverse and enriched in keystone active groups potentially related with GHG recycling including bacteria and archaea, such as Cyanobacteria, Verrucomicrobia, Rhodobacterales and Nanoarchaeaota (Woesearchaeia). Archaea account for the microbial community composition changes between both days and for the secondary productivity in the water measured during day 2. The results indicate that an intense recycling of organic matter occurs in the pond systems and that the activity of the microbial community is correlated with the availability of nutrients. Together, the above results indicate a net sink of CO2 and N2O, which has also been reported for other natural and artificial ponds. Overall, our two-day fluctuation study in a representative pond of a high-altitude wetland aquatic landscape indicates the need to explore in more detail the short-term besides the long-term biogeochemical variability in arid ecosystems of the Andes plateau, where wetlands are hotspots of life currently under high anthropogenic pressure.

Organic nitrogen addition causes decoupling of microbial nitrogen cycles by stimulating gross nitrogen transformation in a temperate forest soil

External inorganic and organic nitrogen (N) inputs can contrastingly affect the transformation and availability of N in forest soils. Studies have mainly focused on the effects of inorganic N enrichment, whereas little is known about the effects of organic N input on soil gross N transformation and the underlying microbial mechanisms. Here we conducted a laboratory 15N tracing study in a temperate needle-broadleaved mixed forest with a fertilization rate of 0, 20, 60, and 120 kg urea-N ha−1 yr−1 over three years. We investigated the key drivers of soil N transformation processes using a 15N tracing model in the context of selected soil chemical properties and microbial characteristics. Urea addition did not change soil gross N mineralization rates, while stimulating mineralization of labile organic N (MNlab). Urea addition at a rate of 120 kg N ha−1 yr−1 significantly increased autotrophic nitrification and gross nitrification rates by 88% and 96%, respectively. In contrast, all the three levels of urea addition significantly reduced gross microbial N immobilization by 28% to 52%, leading to an increase in the accumulation of soil NO3−-N in the top 10 cm soil layer by 38% to 88%. The changes in autotrophic nitrification were primarily driven by acid-tolerant ammonia-oxidizing archaea (AOA). Fungi were responsible for the change in heterotrophic nitrification under organic N enrichment. Gross N transformation rates were predominately regulated by AOA and fungal abundances as well as soil NO3−-N content under high level of organic N addition. The response of soil N transformation to exogenous organic N input depended on N addition level with the threshold rate being estimated to be 60–120 kg N ha−1 yr−1. All lines of evidence showed that the temperate needle-broadleaved mixed forest is moving towards an opener microbial N cycle under elevated organic N deposition. Our finding suggests that the effect of organic N input on soil gross N transformation is different from that of inorganic N input, which should be considered in ecosystem process models.

Impact of Feedlot Manure and Nitrogen Additions on Forage Yields, Nutrient Balance and Soil Nitrate, Phosphate and Salinity

ABSTRACT Safe disposal of feedlot manure is an increasing problem throughout the world. A field experiment was established on a duplex sandy loam soil (Alfisol) in Northern NSW, Australia to determine the dry matter (DM) crop response and the fate of nutrients derived from cattle feedlot manure and inorganic fertilizer applications. The five treatments imposed over a three year period were no manure or inorganic fertilizer (NM), a moderate annual manure application (20–25 t DM ha−1) -/+ N (MA, MAN) a high initial application in year 1 (60 t DM ha−1 + N, HIN) and an inorganic fertilizer treatment (single superphosphate, KCl, urea, I) applied in each year. Vegetative yields of forage sorghum (Sorghum bicolor cv. Super-Dan) and triticale (Triticosecale spp. cv. Madonna) were recorded in crops grown in sequence over a three-year period. A mass balance approach was used to quantify nutrient gains to, and losses from the system. Total crop yield was 14.22 t dry matter (DM) ha−1 in the NM treatment with an approximate threefold increase with HIN and I. Removal of nutrients (kg ha−1) in harvested forage was highest for N, K and S in the I treatment (767 N, 1447 K, 76 S) and highest for P in the HIN treatment (189 P). There was no accumulation of NO3 in the soil layers to the B horizon in the manure treatments which contrasts with I. In the manure treatments there was a 6 to 8 fold increase over the initial value in Colwell bicarbonate extractable P in the 0–10 cm soil horizon in the MAN and HIN treatments, a decline in exchangeable K, an increase in EC and a loss of P sorption capacity. Supplemental inorganic nitrogen has been found to increase the nutrient benefits of feedlot manure. Accumulation of soil phosphate and salts, and a decline in soil K was found and needs to be closely monitored to maintain productivity and the environment.

Effects of Temperature and Substrate Concentration on N2O Release of ANAMMOX Process

The greenhouse gas N2O is released during the biological nitrogen removal process. ANAMMOX (anaerobic ammonium oxidation) is regarded as a promising nitrogen removal process for treating municipal wastewater, and the N2O emission patterns and mechanisms need further investigation. In this study, batch tests were performed to study the release of N2O at different temperatures and substrate concentrations, and the microbial mechanisms of N2O emission were discussed. The results showed that the increase of the influent substrate concentration of the ANAMMOX process promoted the release of N2O. At 35℃, when the influent nitrite increased from 40 mg·L-1 to 60 mg·L-1, 120 mg·L-1, the maximum accumulated concentration of N2O increased from 0.5 mg·L-1 to 1.5 mg·L-1 and 2.4 mg·L-1, accounting for 0.85%, 1.43%, and 1.11% of the total nitrogen removal, respectively. Lowering temperature had an obvious inhibitory effect on ANAMMOX activity. The specific ANAMMOX activity at 15℃ was only 6% of that at 30℃. Furthermore, the intracellular ATP concentration was reduced. At 15℃, the intracellular ATP concentration was 4% of that at 30℃. The decrease in temperature led to a decrease in the release of N2O in the ANAMMOX process. When the temperature decreased, the denitrification rate would decrease, leading to a lower N2O production rate and lower N2O accumulation. 16S rRNA amplicon sequencing showed that ANAMMOX bacteria Candidatus Brocadia and Ca. Jettenia were enriched, accounting for 6.9%-13.8% and 1.4%-2.6% of microbial community, respectively. Abundant heterotrophic bacteria were also found in the microbial community. The accumulation of N2O in the ANAMMOX process was mainly attributed to denitrifying bacteria producing and consuming N2O. This study provides support for controlling N2O emission during the ANAMMOX process for treating municipal wastewater.