Contrasting response of comammox Nitrospira, ammonia oxidising bacteria, and archaea to soil pH and nitrogen inputs

This study aimed to investigate the effect of soil pH change, and nitrogen amendment on ammonia oxidiser abundance and comammox Nitrospira community composition. The experimental design used soil mesocosms placed in a temperature-controlled incubator for 90 days. A Templeton silt loam was used as its physiochemical properties are typical of the region's dairy farms. The results showed that comammox Nitrospira clade B preferred the natural (pH 6.1-6.2) soil pH with no applied nitrogen. Furthermore, synthetic urine (N700) decreased the abundance of comammox Nitrospira clade B. This may have been because the large amounts of available ammonia in the N700 treatments inhibited the growth of comammox Nitrospira. These results suggest that while comammox Nitrospira clade B are present in New Zealand dairy farm soils, but their role in nitrification in the very high nitrogen environment under a urine patch in grazed pastures may be limited. Further research is needed to confirm this. In contrast to comammox, the AOB community (dominated by Nitrosospira) responded positively to the application of synthetic urine. The response was greatest in the high pH soil (7.1), followed by the natural and then the low pH (4.9) soils. This may be due to the difference in ammonia availability. At high pH, the ammonia/ammonium equilibrium favours ammonia production. Calculated ammonia availability in the N700 treatments accurately predicted the AOB amoA gene abundance. Interestingly, the AOA community abundance (which was predominantly made up of Thaumarchaeota group I.1b clade E) seemed to prefer the natural and high pH soils over the low pH. This may be due to the specific lineage of AOA present. AOA did not respond to the application of nitrogen.

The significant contribution of comammox bacteria to nitrification in a constructed wetland revealed by DNA-based stable isotope probing

The discovery of Comammox bacteria (CMX) has changed our traditional concept towards nitrification, yet its role in constructed wetlands (CWs) remains unclear. This study investigated the contributions of CMX and two canonical ammonia-oxidizing microorganisms, ammonia-oxidizing bacteria (AOB) and archaea to nitrification in four regions (sediment, shoreside, adjacent soil, and water) of a typical CW using DNA-based stable isotope probing. The results revealed that CMX not only widely occurred in sediment and shoreside zones with high abundance (5.08 × 104 and 6.57 × 104 copies g-1 soil, respectively), but also actively participated in ammonia oxidation, achieving ammonia oxidation rates of 1.43 and 2.00 times that of AOB in sediment and shoreside, respectively. Phylogenetic analysis indicated that N. nitrosa was the dominant and active CMX species. These findings uncovered the crucial role of CMX in nitrification of sediment and shoreside, providing a new insight into nitrogen cycle of constructed wetlands.

Biofilm viability and microbial community of non-inoculated moving bed biofilm reactor in Nile tilapia Oreochromis niloticus cultivation

The aim of this study was to evaluate two moving bed biofilm reactors (MBBR) without nitrifying bacteria inoculation. Biofilms and viable bacterial colonies were evaluated after 124 days. MBBR bioreactors received water from Oreochromis niloticus fish farming and water quality parameters were monitored daily. Four distinct phases with different fish stocking density were established.: phase 1 (2.40 kg m-3), phase 2 (4.95 kg m-3), phase 3 (8.71 kg m-3) and phase 4 (12.23 kg m-3). The successful maturation of the bioreactors occurred around on the 100th experimental day when the nitration rate increased to 57 % in MBBR1 and 38 % in MBBR2. 105 species were identified in the biofilms, which were grouped into 65 genera, three of which were essential: Pseudomonas (21.7 %), Nitrospira (15.1 %) and Gemmobacter (11.2 %). MBBR start-up without bacterial inoculation is time-consuming, however, strengthened by important nitrifying groups.

Activity and community structure of nitrifiers and denitrifiers in nitrogen-polluted rivers along a latitudinal gradient

Nitrogen (N) cycling in rivers is particularly active and dynamic due to excess nutrient inputs worldwide. However, the multidimensional spatial patterns of the activity and community structure of N-cycling microorganisms in rivers remain unclear, limiting our understanding of river ecological functions, especially N removal capacity. Here, we measured the nitrification and denitrification rates and identified nitrifying and denitrifying microorganisms using high-throughput sequencing of archaeal amoA, bacterial amoA, nirK, and nirS genes in channel sediments, riparian rhizosphere soils, and riparian bulk soils of 30 N-polluted rivers across China. Results showed that in the lateral dimension, nitrification rates in sediments did not differ significantly from those in rhizosphere and bulk soils, but denitrification rates were higher in sediments than in bulk soils. However, the archaeal amoA gene abundance in sediments was considerably lower than that in rhizosphere and bulk soils, and bacterial amoA gene abundance in sediments was greater than that in rhizosphere soils. In the vertical dimension, both nitrification and denitrification rates in riparian bulk soils decreased with soil depth, and topsoils harbored more nitrifying and denitrifying microbes than subsoils. Denitrification but not nitrification rates increased with latitude and altitude but decreased with increasing mean annual temperature and precipitation. Overall, these results provide new insights into the multidimensional spatial patterns of river N cycling at a large scale, which is crucial to evaluating the N removal function of global rivers.

Comparison of nitrification performance in SBR and SBBR with response to NaCl salinity shock: microbial structure and functional genes

Ammonia removal by nitrifiers at the extremely high salinity poses a great challenge for saline wastewater treatment. Sequencing batch reactor (SBR) was conducted with a stepwise increase of salinity from 10 to 40 g-NaCl·L-1, while sequencing batch biofilm reactor (SBBR) with one-step salinity enhancement, their nitrification performance, microbial structure and interaction were evaluated. Both SBR and SBBR can achieve high-efficiency nitrification (98% ammonia removal) at 40 g-NaCl·L-1. However, SBBR showed more stable nitrification performance than SBR at 40 g-NaCl·L-1 after a shorter adaptation period of 4-15 d compared to previous studies. High-throughput sequencing and metagenomic analysis demonstrated that the abundance and capability of conventional ammonia-oxidizing bacteria (Nitrosomonas) were suppressed in SBBR relative to SBR. Gelidibacter, Anaerolineales were the predominant genus in SBBR, which were not found in SBR. NorB and nosZ responsible for reducing NO to N2O and reducing N2O to N2 respectively had s strong synergistic effect in SBBR. This study will provide a valuable reference for the startup of nitrification process within a short period of time under the extremely high NaCl salinity.

The influence of using different types of modified vermiculite cover on ammonia mitigation from animal slurry storage: The role of sulfuric acid

Animal slurry storage is an important ammonia (NH3) emission source. Sulfuric acid (H2SO4)-modified vermiculite coverage is a new promising technology for controlling NH3 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 H2SO4 on the modified vermiculite properties, floating persistence, and NH3 mitigation effect was conducted. Results showed that abundant H2SO4 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 NH3 during initial 40-50 days of storage. The bacterial-amoA gene dominated the conversion of NH3 to nitrous oxide after 50 days of storage. Vermiculite with 5 M H+ modification reduced the NH3 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 NH3 emission while absorbing nutrients from liquid slurry storage tank or lagoon. This design should now be tested under field conditions.

Higher stochasticity in comammox Nitrospira community assembly in upland soils than the adjacent paddy soils at a regional scale

Understanding the assembly mechanisms of microbial communities, particularly comammox Nitrospira, in agroecosystems is crucial for sustainable agriculture. However, the large-scale distribution and assembly processes of comammox Nitrospira in agricultural soils remain largely elusive. We investigated comammox Nitrospira abundance, community structure, and assembly processes in 16 paired upland peanuts and water-logged paddy soils in south China. Higher abundance, richness, and network complexity of comammox Nitrospira were observed in upland soils than in paddy soils, indicating a preference for upland soils over paddy soils among comammox Nitrospira taxa in agricultural environments. Clade A.2.1 and clade A.1 were the predominant comammox Nitrospira taxa in upland and paddy soils, respectively. Soil pH was the most crucial factor shaping comammox Nitrospira community structure. Stochastic processes were found to predominantly drive comammox Nitrospira community assembly in both upland and paddy soils, with deterministic processes playing a more important role in paddy soils than in upland soils. Overall, our findings demonstrate the higher stochasticity of comammox Nitrospira in upland soils than in the adjacent paddy soils, which may have implications for autotrophic nitrification in acidic agricultural soils.

Climate controls on nitrate dynamics and gross nitrogen cycling response to nitrogen deposition in global forest soils

Understanding the patterns and controls regulating nitrogen (N) transformation and its response to N enrichment is critical to re-evaluating soil N limitation or availability and its environmental consequences. Nevertheless, how climatic conditions affect nitrate dynamics and the response of gross N cycling rates to N enrichment in forest soils is still only rudimentarily known. Through collecting and analyzing 4426-single and 769-paired observations from 231 15N labeling studies, we found that nitrification capacity [the ratio of gross autotrophic nitrification (GAN) to gross N mineralization (GNM)] was significantly lower in tropical/subtropical (19%) than in temperate (68%) forest soils, mainly due to the higher GNM and lower GAN in tropical/subtropical regions resulting from low C/N ratio and high precipitation, respectively. However, nitrate retention capacity [the ratio of dissimilatory nitrate reduction to ammonium (DNRA) plus gross nitrate immobilization (INO3) to gross nitrification] was significantly higher in tropical/subtropical (86%) than in temperate (54%) forest soils, mainly due to the higher precipitation and GNM of tropical/subtropical regions, which stimulated DNRA and INO3. As a result, the ratio of GAN to ammonium immobilization (INH4) was significantly higher in temperate than in tropical/subtropical soils. Climatic rather than edaphic factors control heterotrophic nitrification rate (GHN) in forest soils. GHN significantly increased with increasing temperature in temperate regions and with decreasing precipitation in tropical/subtropical regions. In temperate forest soils, gross N transformation rates were insensitive to N enrichment. In tropical/subtropical forests, however, N enrichment significantly stimulated GNM, GAN and GAN to INH4 ratio, but inhibited INH4 and INO3 due to reduced microbial biomass and pH. We propose that temperate forest soils have higher nitrification capacity and lower nitrate retention capacity, implying a higher potential risk of N losses. However, tropical/subtropical forest systems shift from a conservative to a leaky N-cycling system in response to N enrichment.

Enhancing the relative abundance of comammox nitrospira in ammonia oxidizer community decreases N2O emission in nitrification exponentially

Comammox Nitrospira and canonical ammonia-oxidizing bacteria (cAOB) generally coexist in activated sludge. In present study, the effect of comammox Nitrospira on N2O 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 N2O 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 N2O EF of nitrification decreased exponentially, which suggested that comammox Nitrospira not only decreased N2O production directly but also might have reduced N2O 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 N2O EF dramatically. When comammox Nitrospira became the dominant ammonia oxidizer, however, the N2O EF correlated to nitrite insignificantly and a low DO of 0.2 mg/L and weakly acidic pH (6.3) decreased N2O 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 N2O emissions and can also offset the promoting effects of acidic pH, low DO, and high nitrite concentration on N2O production during nitrification.

Mineral and organic fertilisation influence ammonia oxidisers and denitrifiers and nitrous oxide emissions in a long-term tillage experiment

Nitrous oxide (N2O) emissions from different agricultural systems have been studied extensively to understand the mechanisms underlying their formation. While a number of long-term field experiments have focused on individual agricultural practices in relation to N2O emissions, studies on the combined effects of multiple practices are lacking. This study evaluated the effect of different tillage [no-till (NT) vs. conventional plough tillage (CT)] in combination with fertilisation [mineral (MIN), compost (ORG), and unfertilised control (CON)] on seasonal N2O emissions and the underlying N-cycling microbial community in one maize growing season. Rainfall events after fertilisation, which resulted in increased soil water content, were the main triggers of the observed N2O emission peaks. The highest cumulative emissions were measured in MIN fertilisation, followed by ORG and CON fertilisation. In the period after the first fertilisation CT resulted in higher cumulative emissions than NT, while no significant effect of tillage was observed cumulatively across the entire season. A higher genetic potential for N2O emissions was observed under NT than CT, as indicated by an increased (nirK + nirS)/(nosZI + nosZII) ratio. The mentioned ratio under NT decreased in the order CON > MIN > ORG, indicating a higher N2O consumption potential in the NT-ORG treatment, which was confirmed in terms of cumulative emissions. The AOB/16S ratio was strongly affected by fertilisation and was higher in the MIN than in the ORG and CON treatments, regardless of the tillage system. Multiple regression has revealed that this ratio is one of the most important variables explaining cumulative N2O emissions, possibly reflecting the role of bacterial ammonia oxidisers in minerally fertilised soil. Although the AOB/16S ratio aligned well with the measured N2O emissions in our experimental field, the higher genetic potential for denitrification expressed by the (nirK + nirS)/(nosZI + nosZII) ratio in NT than CT was not realized in the form of increased emissions. Our results suggest that organic fertilisation in combination with NT shows a promising combination for mitigating N2O emissions; however, addressing the yield gap is necessary before incorporating it in recommendations for farmers.

Impact of salinity and organic matter on the ammonia-oxidizing archaea and bacteria in treating hypersaline industrial wastewater: amoA gene abundance and ammonia removal contributions

Studies published recently proposed that ammonia-oxidizing archaea (AOA) may be beneficial for hypersaline (salinity > 50 g NaCl L-1) industrial wastewater treatment. However, knowledge of AOA activity in hypersaline bioreactors is limited. This study investigated the effects of salinity, organic matter, and practical pickled mustard tuber wastewater (PMTW) on AOA and ammonia-oxidizing bacteria (AOB) in two sequencing batch biofilm reactors (SBBRs). Results showed that despite observed salinity inhibition (p < 0.05), both AOA and AOB contributed to high ammonia removal efficiency at a salinity of 70 g NaCl L-1 in the two SBBRs. The ammonia removal efficiency of SBBR2 did not significantly differ from that of SBBR1 in the absence of organic matter (p > 0.05). Batch tests and quantitative real-time PCR (qPCR) reveal that salinity and organic matter inhibition resulted in a sharp decline in specific ammonia oxidation rates and amoA gene copy numbers of AOA and AOB (p < 0.05). AOA demonstrated higher abundance and more active ammonia oxidation activity in hypersaline and high organic matter environments. Salinity was positively correlated with the potential ammonia oxidation contribution of AOA (p < 0.05), resulting in a potential transition from AOB dominance to AOA dominance in SBBR1 as salinity levels rose. Moreover, autochthonous AOA in PMTW promoted the abundance and ammonia oxidation activities of AOA in SBBR2, further elevating the nitrification removal efficiency after feeding the practical PMTW. AOA demonstrates greater tolerance to the challenging hypersaline environment, making it a valuable candidate for the treatment of practical industrial wastewater with high salinity and organic content.

Effective chloramine management without "burn" in biofilm affected nitrifying tanks using a low dose of copper

This paper highlights the potential to effectively inhibit nitrification and restore chloramine levels using a low copper concentration in a biofilm-affected (surface-to-volume ratio 16 m-1) continuous-flow laboratory-scale chloraminated system. High nitrite and low chloramine containing tanks are always recovered with chlorine "burn" by water utilities. The "burn" is not only costly and operationally complex, but also compromises the water quality, public health, and customer relations. A laboratory system comprising five reactors connected in series was operated. Each reactor simulated conditions typically encountered in full-scale systems. Low amount of copper (0.1-0.2 mg-Cu L-1) was dosed once per day into nitrified reactors. At any given time, only one reactor was dosed with copper. Not only inhibition of nitrification, chloramine decay associated with bulk water, biofilm and sediments also improved. However, the improvement was quicker and more significant when the influent to the reactor contained a high chloramine and a low nitrite concentration. Ammonia oxidising microbes exhibited resilience when exposed to low copper and chloramine concentrations for an extended period. Chloramine decay due to planktonic microbes and chemical reactions in bulk water decreased more rapidly than decay attributed to biofilm and sediments. The concept "biostable residual chlorine" explained how copper and chloramine can inhibit nitrification. Once nitrification was inhibited, the chloramine supplied from upstream effectively continued to suppress downstream nitrification, and this effect lasted more than 50 days even at 22 °C. The findings could be used to develop short-term copper dosing strategies and prevent negative impacts of nitrification and breakpoint chlorination.

Impact of iron-modified biochars on soil nitrous oxide emissions: Variations with iron salts and soil fertility

Nitrous oxide (N2O) 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 N2O emissions remains poorly understood. In the present study, corn straw (CB) and wood (WB) biochars were treated with FeSO4/FeCl3 (SCB and SWB) and Fe(NO3)3 (NCB and NWB). The effects of these biochars on soil N2O 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 NO3-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 NH4-N concentrations (5.38 ± 0.43 and 4.19 ± 0.22 mg kg-1, respectively). In forest soils, NWB and NCB increased cumulative N2O emission by 28.5% and 67.0%, respectively, likely due to the introduction of mineral nitrogen evidenced by significant positive correlation with NO3-N or NH4-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 N2O 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 N2O emissions. This study highlights the need for a balanced design of biochar that considers both engineering benefits and climate change mitigation.

A preliminary study on benthic nutrient exchange across sediment-water interfaces in a shallow marine protected area of the Northwestern Arabian Gulf

Sedimentary processes are expected to play a crucial role in macronutrient cycling of the shallow Arabian Gulf. To investigate this aspect, sediment cores were collected from the shallow intertidal and subtidal expanses of the first Marine Protected Area (MPA) of Kuwait in the Northwestern Arabian Gulf (NAG). Porewater nutrient profiling and whole core incubation experiments were conducted to measure the nutrient fluxes, both with and without the addition of the nitrification inhibitor allylthiourea (ATU). The porewater data confirmed the potential of sediments to host multiple aerobic and anaerobic pathways of nutrient regeneration. The average (±SD) of net nutrient fluxes from several incubation experiments indicated that ammonium (NH4+) predominantly fluxed out of the sediment (3.81 ± 2.53 mmol m-2 d-1), followed by SiO44- (3.07 ± 1.21 mmol m-2 d-1). In contrast, the average PO43- flux was minimal, at only 0.06 ± 0.05 mmol m-2 d-1. Fluxes of NO3- (ranged from 0.07 ± 0.005 to 1.16 ± 0.35 mmol m-2 d-1) and NO2- (0.03 ± 0.003 to 0.71 ± 0.21 mmol m-2 d-1) were moderate, which either reduced or reversed in the presence of ATU (-0.001 ± 0.0001 to 0.01 ± 0.0001 mmol m-2 d-1 and -0.001 ± 0.0003 to 0.006 ± 0.001 mmol m-2 d-1 for NO3- and NO2- respectively). Thus, this study provides preliminary experimental evidence that nitrification can act as a source of NO3- and NO2- as well as contribute towards the relatively high concentrations of NO2- (>1 in the gulf waters.

Effects of dredging wastewater input history and aquaculture type on greenhouse gas fluxes from mangrove sediments along the shorelines of the Jiulong River Estuary, China

Dredging wastewater (DW) from aquaculture ponds is a major disturbance factor in mangrove management, and its effects on the greenhouse gas (GHG) fluxes from mangrove sediment remain controversial. In this study, we investigated GHG (N2O, CH4, and CO2) fluxes from mangrove sediment at typical aquaculture pond-mangrove sites that were stimulated by DW discharged for different input histories and from different farm types. The GHG fluxes exhibited differing cumulative effects with increasing periods of DW input. The N2O and CH4 fluxes from mangrove sediment that received DW inputs for 17 y increased by ∼10 and ∼1.5 times, respectively, whereas the CO2 flux from mangrove sediment that received DW inputs for 11 y increased by ∼1 time. The effect of DW from shrimp ponds on the N2O flux was significantly larger than those of DW from fish/crab ponds and razor clam ponds. Moreover, the total global warming potentials (GWPs) at the field sites with DW inputs increased by 29-129% of which the CO2 flux was the main contributor to the GWP (85-96%). N2O as a proportion of CO2-equivalent flux increased from 2% to 12%, indicating that N2O was an important contributor to the increase in GWP. Overall, DW increased the GHG fluxes from mangrove sediments, indicating that the contribution of mangroves to climate warming was enhanced under DW input. It also implies that the carbon sequestration potential of mangrove sediments may be threatened to some extent. Therefore, future assessments of the carbon sequestration capacity of mangroves at regional or global scales should consider this phenomenon.

Pilot-scale investigation of performance and microbial community in a novel system combining fixed and suspended activated sludge

The conventional activated sludge (CAS) process is a widely used method for wastewater treatment due to its effectiveness and affordability. However, it can be prone to sludge abnormalities such as sludge bulking/foaming and sludge loss, which can lead to a decrease in treatment efficiency. To address these issues, a novel bag-based fixed activated sludge (BBFAS) system utilizing mesh bags to contain the sludge was developed for low carbon/nitrogen ratio wastewater treatment. Pilot-scale experiments demonstrated that the BBFAS system could successfully avoid the sludge abnormalities. Moreover, it was not affected by mass transfer resistance and exhibited significantly higher nitrogen removal efficiency, surpassing that of the CAS system by up to 78%. Additionally, the BBFAS system demonstrated comparable organic matter removal efficiency to CAS system. 16S rRNA gene high-throughput sequencing revealed that the bacterial community structure within the BBFAS system was significantly different from that of the CAS system. The bacteria associated with ammonium removal were more abundant in the BBFAS system than in the CAS system. The abundance of Nitrospira in the BBFAS could reach up to 6% and significantly higher than that in the CAS system, and they were likely responsible for both ammonia-oxidizing and nitrite-oxidizing functions. Clear stratification of microbial communities was observed from the outer to inner layers of the bag components due to the gradients of dissolved oxygen and other substrates. Overall, this study presents a promising approach for avoiding activated sludge abnormalities while maintaining high pollutant removal performance.

Effects of temperature on nitrifying membrane-aerated biofilms: An experimental and modeling study

Temperature is known to have an important effect on the morphology and removal fluxes of conventional, co-diffusional biofilms. However, much less is known about the effects of temperature on membrane-aerated biofilm reactors (MABRs). Experiments and modeling were used to determine the effects of temperature on the removal fluxes, biofilm thickness and morphology, and biofilm microbial community structure of nitrifying MABRs. Steady state tests were carried out at 10 °C and 30 °C. MABRs grown at 30 °C had higher ammonium removal fluxes (5.5 ± 0.9 g-N/m2/day at 20 mgN/L) than those grown at 10 °C (3.4 ± 0.2 g-N/m2/day at 20 mgN/L). The 30 °C biofilms were thinner and rougher, with a lower protein to polysaccharides ratio (PN/PS) in their extracellular polymeric substance (EPS) matrix and greater amounts of biofilm detachment. Based on fluorescent in-situ hybridization (FISH), there was a higher relative abundance of nitrifying bacteria at 30 °C than at 10 °C, and the ratio of AOB to total nitrifiers (AOB + NOB) was higher at 30 °C (95.1 ± 2.3%) than at 10 °C (77.2 ± 8.6 %). Anammox bacteria were more abundant at 30 °C (16.6 ± 3.7 %) than at 10 °C (6.5 ± 2.4 %). Modeling suggested that higher temperatures increase ammonium oxidation fluxes when the biofilm is limited by ammonium. However, fluxes decrease when oxygen becomes limited, i.e., when the bulk ammonium concentrations are high, due to decreased oxygen solubility. Consistent with the experimental results, the model predicted that the percentage of AOB to total nitrifiers at 30 °C was higher than at 10 °C. To investigate the effects of temperature on biofilm diffusivity and O2 solubility, without longer-term changes in the microbial community, MABR biofilms were grown to steady state at 20 °C, then the temperature changed to 10 °C or 30 °C overnight. Higher ammonium oxidation fluxes were obtained at higher temperatures: 1.91 ± 0.24 g-N/m2/day at 10 °C and 3.19 ± 0.40 g-N/m2/day at 30 °C. Overall, this work provides detailed insights into the effect of temperature on nitrifying MABRs, which can be used to better understand MABR behavior and manage MABR reactors.

Multiple isotopes decipher the nitrogen cycle in the cascade reservoirs and downstream in the middle and lower Yellow River: Insight for reservoir drainage period

Intensive anthropogenic activities, such as excessive nitrogen input and dam construction, have altered the nitrogen cycle in the global river system. However, the focus on the source, transformation and fate of nitrogen in the Yellow River is still scarce. In this study, the multiple isotopes (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+ and δ15N-PN) were deciphered to explore the nitrogen cycling processes and the driving factors in the thermally stratified cascade reservoirs (Sanmenxia Reservoir: SMXR and Xiaolangdi Reservoir: XLDR) and Lower Yellow River (LYR) during the drainage period of the XLDR. In the SMXR, algal bloom triggered the assimilation process in the upper layer before the SMX Dam, followed by remineralization and subsequent nitrification processes in the lower water layers. The nitrification reaction in the XLDR progressively increased along both longitudinal and vertical directions to the lower layer of the XLD Dam, which was linked to the variation in the water residence time of riverine, transition and lentic zones. The robust nitrification rates in the lower layer of the lentic zone coincided with the substantial depletion of nitrate isotopic composition and enrichment of both δ15N-PN and δ15N-NH4+, indicating the longer water residence time not only promoted the growth of the nitrifying population but also facilitated the remineralization to enhance NH4+ availability. In the LYR, the slight nitrate assimilation, as indicated by nitrate isotopic composition and fractionation models, was the predominant nitrogen transformation process. The Bayesian isotope mixing model results showed that manure and sewage was the dominant nitrate source (50 %) in the middle and lower Yellow River. Notably, the in-reservoir nitrification was a significant nitrate source (27 %) in the XLDR and LYR. Our study deepens the understanding of anthropogenic activities impacting the nitrogen cycle in the river-reservoir system, providing valuable insight into water quality management and nitrogen cycle mechanisms in the Yellow River.

Effects of intensive chlorine disinfection on nitrogen and phosphorus removal in WWTPs

The increased use of disinfection since the pandemic has led to increased effective chlorine concentration in municipal wastewater. Whereas, the specific impacts of active chlorine on nitrogen and phosphorus removal, the mediating communities, and the related metabolic activities in wastewater treatment plants (WWTPs) lack systematic investigation. We systematically analyzed the influences of chlorine disinfection on nitrogen and phosphorus removal activities using activated sludge from five full-scale WWTPs. Results showed that at an active chlorine concentration of 1.0 mg/g-SS, the nitrogen and phosphorus removal systems were not significantly affected. Major effects were observed at 5.0 mg/g-SS, where the nitrogen and phosphorus removal efficiency decreased by 38.9 % and 44.1 %, respectively. At an active chlorine concentration of 10.0 mg/g-SS, the nitrification, denitrification, phosphorus release and uptake activities decreased by 15.1 %, 69.5-95.9 %, 49.6 % and 100 %, respectively. The proportion of dead cells increased by 6.1 folds. Reverse transcriptional quantitative polymerase chain reaction (RT-qPCR) analysis showed remarkable inhibitions on transcriptions of the nitrite oxidoreductase gene (nxrB), the nitrite reductase genes (nirS and nirK), and the nitrite reductase genes (narG). The nitrogen and phosphorus removal activities completely disappeared with an active chlorine concentration of 25.0 mg/g-SS. Results also showed distinct sensitivities of different functional bacteria in the activated sludge. Even different species within the same functional group differ in their susceptibility. This study provides a reference for the understanding of the threshold active chlorine concentration values which may potentially affect biological nitrogen and phosphorus removal in full-scale WWTPs, which are expected to be beneficial for decision-making in WWTPs to counteract the potential impacts of increased active chlorine concentrations in the influent wastewater.

Evolution of microbiome composition, antibiotic resistance gene loads, and nitrification during the on-farm composting of the solid fraction of pig slurry using two bulking agents

Composting is a nature-based method used to stabilize organic matter and to transform nitrogen from animal farm manure or solid fraction of slurry (SFS). The use of composted material as source of nutrients for agriculture is limited by its potential to facilitate the propagation of biological hazards like pathogens and antibiotic-resistant bacteria and their associated antibiotic-resistance genes (ARG). We show here an experimental on-farm composting (one single batch) of pig SFS, performed under realistic conditions (under dry continental Mediterranean climate) for 280 days, and using two different bulking agents (maize straw and tree pruning residues) for the initial mixtures. The observed reduction in potentially pathogenic bacteria (80-90%) and of ARG loads (60-100%) appeared to be linked to variations in the microbiome composition occurring during the first 4 months of composting, and concurrent with the reduction of water-soluble ammonium and organic matter loads. Nitrification during the composting has also been observed for both composting piles. Similar patterns have been demonstrated at small scale and the present study stresses the fact that the removal can also occur at full scale. The results suggest that adequate composition of the starting material may accelerate the composting process and improve its global performance. While the results confirm the sanitization potential of composting, they also issue a warning to limit ARG loads in soils and in animal and human gut microbiomes, as the only way to limit their presence in foodstuffs and, therefore, to reduce consumers' exposure.

Side effects of the addition of an adsorbent for the nitrification performance of a microbiome in the treatment of an antibiotic mixture

This work determined the effect of biochar (BC) as an adsorbent on the nitrifying microbiome in regulating the removal, transformation, fate, toxicity, and potential environmental consequences of an antibiotic mixture containing oxytetracycline (OTC) and sulfamethoxazole (SMX). Despite the beneficial role of BC as reported in the literature, the present study revealed side effects for the nitrifying microbiome and its functioning arising from the presence of BC. Long-term monitoring revealed severe disruption to nitratation via the inhibition of both nitrite oxidizers (e.g., Nitrospira defluvii) and potential comammox species (e.g., Ca. Nitrospira nitrificans). Byproducts (BPs) more toxic than the parent compounds were found to persist at a high relative abundance, particularly in the presence of BC. Quantitative structure-activity relationship modeling determined that the physicochemical properties of the toxic BPs significantly differed from those of OTC and SMX. The results suggested that the BPs tended to mobilize and accumulate on the surface of the solids in the system (i.e., the BC and biofilm), disrupting the nitrifiers growing at the interface. Collectively, this study provides novel insights, demonstrating that the addition of adsorbents to biological systems may not necessarily be beneficial; rather, they may generate side effects for specific bacteria that have important ecosystem functions.

Effects of microplastics on N2O production and reduction potential in crop soils of northern China

Microplastics (MPs) pollution are found to be increasing in vegetable soils and potentially affecting N2O 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 N2O production. Results showed that the bacteria were the main contributors for the production of N2O, 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 N2O reduction rates, indicating a combined regulation on soil N2O reduction. PE significantly inhibited the bacterial nitrification and denitrification, but did not affect the total N2O production rates; while PP significantly reduced both the bacterial and fungal N2O production rates. The resistance of fungal N2O production to MPs pollution was stronger than that of the bacterial N2O production. It highlights that the MPs pollution could reduce the potential of N2O production and reduction, and thus disturb soil nitrogen cycling system; while the inhibition on N2O 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.

AOB Nitrosospira cluster 3a.2 (D11) dominates N2O emissions in fertilised agricultural soils

Ammonia-oxidation process directly contribute to soil nitrous oxide (N2O) 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 N2O emissions in agricultural soils is unknown, as is their regulation by soil biotic and abiotic factors. In this study, cumulative N2O 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 N2O emissions were identified by comparative analyses of the different treatments, combining sequencing and random forest analyses. Soil cumulative N2O 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 N2O 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 N2O 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 N2O emissions, which were mainly driven by AOB Nitrosospira cluster 3a.2 (D11). Nitrapyrin can efficiently reduce nitrification-related N2O emissions by inhibiting the activity of AOB Nitrosospira cluster 3a.2 (D11). This study advances our understanding of key nitrifiers responsible for high N2O emissions in agricultural soils and their controlling factors, and provides vital knowledge for N2O emission mitigation in agricultural ecosystems.

Aerobic nitrous oxide emission in anoxic/aerobic and intermittent aeration sequencing batch reactors

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.

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

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

Recurrent multi-stressor floc treatments with sulphide and free ammonia enabled mainstream partial nitritation/anammox

Selective suppression of nitrite-oxidising bacteria (NOB) over aerobic and anoxic ammonium-oxidising bacteria (AerAOB and AnAOB) remains a major challenge for mainstream partial nitritation/anammox implementation, a resource-efficient nitrogen removal pathway. A unique multi-stressor floc treatment was therefore designed and validated for the first time under lab-scale conditions while staying true to full-scale design principles. Two hybrid (suspended + biofilm growth) reactors were operated continuously at 20.2 ± 0.6 °C. Recurrent multi-stressor floc treatments were applied, consisting of a sulphide-spiked deoxygenated starvation followed by a free ammonia shock. A good microbial activity balance with high AnAOB (71 ± 21 mg N L-1 d-1) and low NOB (4 ± 17 % of AerAOB) activity was achieved by combining multiple operational strategies: recurrent multi-stressor floc treatments, hybrid sludge (flocs & biofilm), short floc age control, intermittent aeration, and residual ammonium control. The multi-stressor treatment was shown to be the most important control tool and should be continuously applied to maintain this balance. Excessive NOB growth on the biofilm was avoided despite only treating the flocs to safeguard the AnAOB activity on the biofilm. Additionally, no signs of NOB adaptation were observed over 142 days. Elevated effluent ammonium concentrations (25 ± 6 mg N L-1) limited the TN removal efficiency to 39 ± 9 %, complicating a future full-scale implementation. Operating at higher sludge concentrations or reducing the volumetric loading rate could overcome this issue. The obtained results ease the implementation of mainstream PN/A by providing and additional control tool to steer the microbial activity with the multi-stressor treatment, thus advancing the concept of energy neutrality in sewage treatment plants.

N-methyl pyrrolidone manufacturing wastewater as the electron donor for denitrification: From bench to pilot scale

Actual wastewater generated from N-methylpyrrolidone (NMP) manufacture was used as electron donor for tertiary denitrification. The organic components of NMP wastewater were mainly NMP and monomethylamine (CH3NH2), and their biodegradation released ammonium that was nitrified to nitrate that also had to be denitrified. Bench-scale experiments documented that alternating denitrification and nitrification realized effective total‑nitrogen removal. Ammonium released from NMP was nitrified in the aerobic reactor and then denitrified when actual NMP wastewater was used as the electron donor for endogenous and exogenous nitrate. Whereas TN and NMP removals occurred in the denitrification step, dissolved organic carbon (DOC) and CH3NH2 removals occurred in the denitrification and nitrification stages. The genera Thauera and Paracoccus were important for NMP biodegradation and denitrification in the denitrification reactor; in the nitrification stage, Amaricoccus and Sphingobium played key roles for biodegrading intermediates of NMP, while Nitrospira was responsible for NH4+ oxidation to NO3-. Pilot-scale demonstration was achieved in a two-stage vertical baffled bioreactor (VBBR) in which total‑nitrogen removal was realized sequential anoxic-oxic treatment without biomass recycle. Although the bench-scale reactors and the VBBR had different configurations, both effectively removed total nitrogen through the same mechanisms. Thus, an N-containing organic compound in an industrial wastewater could be used to drive total-N removal in a tertiary-treatment scenario.

Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3-) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3- content. Multiple isotopes (δD, δ18O-H2O, δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3- concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3- being the predominant form of nitrogen pollution. Moreover, the average NO3- levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3- accumulation in rivers and subsidence areas, which was further supported by the linear relationship between δ15N-NO3- and δ15N-NH4+. The redox conditions and the relationship between δ15N-NO3- and δ18O-NO3-, and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3- assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.

Coexistence of microplastics and Cd alters soil N transformation by affecting enzyme activity and ammonia oxidizer abundance

Interactions between heavy metal and microplastics represent a serious threat to ecosystems and human health, but the effect of their coexistence on the soil N transformation processes is unclear. The mechanism in which metal-polluted soil reacts to additional microplastics stress and their toxicology interactions on soil N transformation were determined by investigating the dynamics of soil microbial N transformation in response to Cd stress and different doses of polythene (PE) microplastics by conducting a 14 days aerobic 15N microcosmic incubation experiment. The gross nitrification rates (n_gross) were decreased by 7.47% and 12.5% in the 1% and 2% (w/w) PE groups, respectively, through the direct effect on enzyme activity (β-glucosidase, N-acetylglucosaminidase, and leucine-aminopeptidase) and the abundance and community composition of ammonia oxidizer. It also exerted indirect effect by reducing nitrification substrate concentrations. PE microplastics (>1% [w/w]) significantly increased the gross N immobilization rate, and this change could have been driven by C/N stoichiometry. Cd stress alone led to a rapid short-term mineralization-immobilization turnover (1.67 times of the control). However, such effect was offset when Cd coexisted with PE microplastics, possibly because Cd was directly adsorbed by PE microplastics, and/or microplastics satisfied the C demand by microorganisms under Cd stress. Our findings demonstrated that the coexistence of microplastics and Cd significantly altered soil N nitrification and immobilization, which would change the N bioavailability in soil and alter the effect N cycling on the ecological environment.

Removal of nitric oxide in bioreactors: a review on the pathways, governing factors and mathematical modelling

The constant surge in nitric oxide in the atmosphere results in severe environmental degradation, negatively impacting human health and ecosystems, and is presently a global concern. Widely used physicochemical technologies for nitric oxide (NO) removal comes with high installation and operational costs and the production of secondary pollutants. Thus, biological treatment has been emphasized over the last two decades, but the poor solubility of NO in water makes it a challenging issue. The present article reviews the various technical aspects of biological treatment of nitric oxide, including the removal pathways and reactor configurations involved in the process. The most widely used technologies in this regard are chemical adsorption processes followed by biological reactors like biofilters, biotrickling filters and membrane bioreactors that enhance NO solubility and offer the flexibility and scope of further improvement in process design. The effect of various experimental and operational parameters on NO removal, including pH, carbon source, gas flow rate, gas residence time and presence of inhibitory components in the flue gas, is also discussed along with the developed mathematical models for predicting NO removal in a biological treatment system. There is an extensive scope of investigation regarding the development of an economical system to remove NO, and an exhaustive model that would optimize the process considering maximum practical parameters encountered during such operation. A detailed discussion made in this article gives a proper insight into all these areas.

Effect of carbon and nitrogen mineralization of chitosan and its composites with hematite/gibbsite on soil acidification of an Ultisol induced by urea

Chitosan is a biodegradable polymer with a vast range of applications. Along with its metal composites, chitosan has been applied in the remediation of polluted soils as well as a biofertilizer. However, little attention has been given to the degradation of chitosan composites in soil and how they affect soil respiration rate and other physicochemical parameters. In this study, the degradation of chitosan and its composites with gibbsite and hematite in an acidic Ultisol and the effect on urea (200 mg N kg-1) transformation were investigated in a 70-d incubation experiment. The results showed that the change trends of soil pH, N forms, and CO2 emissions were similar for chitosan and its composites when applied at rates <5 g C kg-1. At a rate of 5 g C kg-1, the C and N mineralization trends suggested that the chitosan-gibbsite composite was more stable in soil and this stability was owed to the formation of a new chemical bond (CH-N-Al-Gibb) as observed in the Fourier-transform infrared spectrum at 1644 cm-1. The mineralization of the added materials significantly increased soil pH and decreased soil exchangeable acidity (P < 0.01). This played an important role in decreasing the amount of H+ produced during urea transformation in the soil. The soil's initial pH was an important factor influencing C and N mineralization trends. For instance, increasing the initial soil pH significantly increased the nitrification rate and chitosan decomposition trend (P < 0.01) and thus, the contribution of chitosan and its composites to increase soil pH and inhibit soil acidification during urea transformation was significantly decreased (P < 0.01). These findings suggest that to achieve long-term effects of chitosan in soils, applying it as a chitosan-gibbsite complex is a better option.

Initial evidence on the effect of copper on global cropland nitrogen cycling: A meta-analysis

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.

Short term grazing increased growing-season N2O production and decreased its reduction potential by reducing the abundance and expression of nosZ clade II gene in a semi-arid steppe

Understanding nitrous oxide (N2O) production as well as reduction in response to grazing and mowing is essential for designing better management strategies to improve sustainability of grassland ecosystems. We evaluated how four years of grazing or mowing altered N2O production and reduction potential, gene abundance, and expression of microbial functional groups pertinent to N2O production in situ on a typical grassland in Inner Mongolia. In our study, we found that grazing dramatically raised soil ammonium (NH4+-N) and nitrate (NO3--N) concentrations, AOB gene abundance and potential of N2O production through nitrification (NN2O) and denitrification (DN2O) in summer, but lessened the expression of nosZ clade II gene in all seasons. Mowing had minor effect on soil inorganic nitrogen (N) concentrations. Mowing diminished the quantity of denitrification genes (narG and nosZ), expression of nosZ and nosZ clade II genes, and DN2O concentration. The expression and abundance of nosZ clade II gene were related to DN2. These results suggested that short-term grazing could enhance N2O production potential in peak growing season, while the reduction in abundance and expression of nosZ calde II gene might be an important contributor to the enhanced N2O production of semi-arid typical steppe grasslands.

A review on the fate of micro and nano plastics (MNPs) and their implication in regulating nutrient cycling in constructed wetland systems

This review discusses the micro-nano plastics (MNPs) and their interaction with physical, chemical and biological processes in a constructed wetland (CW) system that is typically used as a nature-based tertiary wastewater treatment for municipal as well as industrial applications. Individual components of the CW system such as substrate, microorganisms and plants were considered to assess how MNPs influence the CW processes. One of the main functions of a CW system is removal of nutrients like nitrogen (N) and phosphorus (P) and here we highlight the pathways through which the MNPs influence CW's efficacy of nutrient removal. The presence of morphologically (size and shape) and chemically different MNPs influence the growth rate of microorganisms important in N and P cycling, invertebrates, decomposers, and the plants which affect the overall efficiency of a CW treatment system. Certain plant species take up the MNPs, and some toxicity has been observed. This review focuses on two significant aspects: (1) the presence of MNPs in a significant concentration affects the efficiency of N and P removal, and (2) the removal of MNPs. Because MNPs reduce the enzyme activities in abundance and overproduction of ROS oxidizes the enzyme active sites, resulting in the depletion of proteins, ultimately inhibiting nitrogen and phosphorus removal within the substrate layer. The review found that the majority of the studies used sand-activated carbon (SAC), granular-activated carbon (GAC), rice straw, granular limestone, and calcium carbonate, as a substrate for CW treatment systems. Common plant species used in the CW include Phragmites, Arabidopsis thaliana, Lepidium sativum, Thalia dealbata, and Canna indica, which were also found to be dominant in the uptake of the MNPs in the CWs. The MNPs were found to affect earthworms such as Eisenia fetida, Caenorhabditis elegans, and, Enchytraeus crypticus, whereas Metaphire vulgaris were found unaffected. Though various mechanisms take place during the removal process, adsorption and uptake mechanism effectively emphasize the removal of MNPs and nitrogen and phosphorus in CW. The MNPs characteristics (type, size, and concentration) play a crucial role in the removal efficiency of nano-plastics (NPs) and micro-plastics (MPs). The enhanced removal efficiency of NPs compared to MPs can be attributed to their smaller size, resulting in a faster reaction rate. However, NPs dose variation showed fluctuating removal efficiency, whereas MPs dose increment reduces removal efficiency. MP and NPs dose variation also affected toxicity to plants and earthworms as observed from data. Understanding the fate and removal of microplastics in wetland systems will help determine the reuse potential of wastewater and restrict the release of microplastics. This study provides information on various aspects and highlights future gaps and needs for MNP fate study in CW systems.

How does biochar influence soil nitrification and nitrification-induced N2O emissions?

Nitrification is a major pathway of N2O production, especially in aerobic soils. The amendment of soils with biochar has been suggested as a promising solution to regulate soil N cycle and reduce N2O emissions. However, there is a lack of comprehensive and quantitative understanding of biochar impacts on soil nitrification and nitrification-induced N2O emissions. In this study, a meta-analysis was conducted using data compiled across 95 peer-reviewed studies. Results showed that biochar in general significantly increased soil nitrification rate by 56 %, with overall no significant effect on nitrification-induced N2O emissions, suggesting that biochar likely restricted the fraction of nitrified N emitted as N2O emissions. The abundance of ammonia-oxidizing bacteria (AOB) was significantly increased by 37 % following biochar addition, but that of ammonia-oxidizing archaea (AOA) did not change significantly, indicating that the impact of biochar on AOB rather than AOA may play an important role in soil nitrification. The impacts of biochar on soil nitrification processes were heterogeneous depending on soil properties. Biochar increased soil nitrification rate and AOB abundance to a larger extent in poorly pH-buffered soils such as those with acidic pH (<5), low organic carbon (<10 g kg-1), or poor texture (rich in either sand or clay), which may be attributed to the liming and structural effects of biochar that regulate soil pH and water-air status. The overall no significant effect of biochar on nitrification-induced N2O emissions was due to a positive effect in acidic soils, a negative effect in alkaline soils, and little effect in neutral soils. This study provides a comprehensive insight into how different factors mediate the response of soil nitrification processes to biochar amendment, which contributes to a new understanding of biochar function in regulating soil N2O emissions, and can assist in designing biochar projects that would benefit soil N cycle while minimizing undesirable side effects.

Exploring nutrient removal mechanisms in column-type SBR with simultaneous nitrification and denitrification

A comprehensive investigation utilized a column-type sequencing batch reactor (SBR) to efficiently remove nutrients throughout various phases of its operational cycle by forming granules. This study assessed the influence and mechanisms of a simultaneous nitrification and denitrification (SND) system employing a column-type sequential batch reactor (SBR). The primary focus was on elucidating the functional groups involved in nitrogen transformation and removal within the extracellular polymeric substances (EPS). The research findings demonstrate the superior performance of the SBR process compared to the control group. It achieved an impressive SND efficiency of 69%, resulting in a remarkable 66% total nitrogen removal. Furthermore, a detailed analysis unveiled that the SBR process had a beneficial impact on the composition and properties of EPS. This impact was observed through increased EPS content and enhanced capacity to transport, convert, and retain nitrogen effectively. Additionally, after initial acclimatization, the SBR process showed its effectiveness in removing nutrients (88-98%) and COD (93%) from the generated wastewater within a hydraulic retention time (HRT) of 6 h. A statistically significant difference between the treatments for the investigated mixing ratios was found by univariate analysis of variance (ANOVA). Machine learning (CatBoost model) was employed to understand each parameter's relationship and predict the outcomes in measurable quantity. The findings of the SBR trials showed that the concentration of generated wastewater and the HRT impacted the treatment efficiency. However, the effluent may still need other physicochemical processes, such as membrane filtering, coagulation, electrocoagulation, etc., as post-treatment options, even though COD, nutrients, and turbidity have been entirely or significantly effectively removed. Overall, this work offers insightful information on the critical function of the SBR bacterial community in promoting SND.

Combined application of Pseudomonas fluorescens and urea can mitigate rapid acidification of cropland Ultisol

Rhizobacteria maintain a healthy soil required for crop growth. This has led to increased interest in the use of bacteria-based biofertilizers in agriculture as they improve soil nutrient content and protect plants against pathogens. However, the effect of bacteria inoculum on N transformation and soil physicochemical properties during urea fertilization remains unexploited. Thus, this study investigated the effect of Pseudomonas fluorescens on urea N transformation in an acidic Ultisol within a 70-d incubation period. The results revealed that (1) soil pH peaked on d 5 (pH 5.58) and 20 (pH 6.23) and rapidly decreased till d 62 (pH 4.10) and 50 (pH 4.93) for urea and urea + bacteria treatments, respectively, and remained constant thereafter. After 70 d, the pH of the bacteria-treated Ultisol remained higher (0.78 pH units) than that of urea-treated Ultisol; (2) the change in soil pH was in agreement with the mineralization trend of N, as the concentration of NH4+-N peaked on d 5 (134.2 mg N kg-1) and 20 (423 mg N kg-1) before decreasing to 62.1 and 276.1 mg N kg-1 on d 70 in urea-treated and bacteria-treated Ultisol, respectively; and (3) P. fluorescens consumed protons produced during nitrification to retard rapid decrease in soil pH, decreased soil exchangeable acidity (33.3 %), increased soil effective cation exchange capacity (32.8 %), and increased the solubility of soil exchangeable base cations (68.4 %, Ca2+ + Mg2+ + K+ + Na+). Thus, bacterial inoculum could promote N mineralization, enhance nutrient solubility, and retard soil acidification during N transformation in soils.

Reducing nitrogen leaching using wood vinegar treated in urea-fertilized soil

Wood vinegar (WV) is known to retard the release of ammonium (NH4+) from urea by inhibiting urea hydrolysis. However, the effect of WV on nitrogen leaching in soil is not known, and there are few studies on the effect of WV on microbial activity although WV exhibits antibacterial properties against pathogens in agriculture. Therefore, the purpose of this study was to investigate the effect of WV on controlling nitrogen leaching and soil microbial activity. Soils were treated with urea and WV, and the available inorganic nitrogen concentrations in the soil were compared with those from soils treated with N-(n-butyl)thiophosphoric triamide (NBPT), a commonly used urease inhibitor. The nitrate concentration in the soil was significantly decreased in the WV treatment, although the ammonium concentration was not affected by the WV treatment. Basal soil respiration was significantly increased in the WV and NBPT treatments although the microbial biomass was increased in the urea only treatment. The ammonium nitrogen concentration in the leachate was not significantly different in the WV and urea-treated soil compared to the urea-only treatment. However, the nitrate leaching increased in the soil treated only with urea at 16 days after the treatment although there was no statistically significant difference in the total leached nitrate. Therefore, WV can be used to reduce nitrogen leaching and enhance soil microbial activity.

Runoff of foliar-applied natural fungicides in aquaponics: Implications for fish and nitrification

Aquaponics is a method of producing food in a sustainable manner through the integration of aquaculture and hydroponics, which allows simultaneous cultivation of fish and economic crops. The use of natural fungicides are crucial to the sustainable control of diseases in aquaponics. We assessed the potential impacts of natural fungicides, such as clove oil and lecithin, as well as a synthetic fungicide, tebuconazole, following foliar application in aquaponics. This study examined the runoff rates of the fungicides in decoupled aquaponics, and the subsequent effects of the runoffs on nitrification processes and Nile tilapia (Oreochromis niloticus). The runoffs of the foliar-applied fungicides, clove oil, lecithin, and tebuconazole, were detected in aquaponics water at a percentage runoff rate of 0.3 %, 2.3 %, and 0.3-0.8 % respectively. In the biofilter, lecithin altered the ammonium levels by increasing ammonium-nitrogen levels by 7 mg L-1, 6 h post application. Clove oil, on the other hand, showed no significant effect on ammonium, nitrite, and nitrate-nitrogen. Similarly, the toxicity test showed that eugenol had no significant effects on the hematological, biochemical and antioxidative activities of O. niloticus. Conversely, tebuconazole exhibited significant and persistent effects on various biochemical parameters, including lactate, albumin, and total protein, as well as hematological parameters like hemoglobin and MCH. The use of lecithin and tebuconazole should only be limited to decoupled aquaponics.

Influence of ammonia nitrogen management strategies on microbial communities in biofloc-based aquaculture systems

Controlling ammonia nitrogen is very important in intensive aquaculture. This study evaluated how different management strategies, i.e., chemoautotrophic (control), heterotrophic bacterial enhancement using carbon in glucose or polyhydroxy butyrate-hydroxy valerate (PHBV), and mature biofloc application, affect water quality and microbial community structure and composition. The management strategies were examined during the domestication and fish culture stages. In the domestication stage, the average NO2--N concentration, pH, and DO in the glucose-added groups were significantly lower than those in the control and PHBV groups. All water quality parameters differed significantly among treatment groups in the culture stage. Carbon additions decreased both bacterial richness and diversity in the fish culture stage. Both principal coordinate analysis and hierarchical cluster analysis grouped the 33 bacteria community samples from the two stages into four clusters, which were closely related to management strategy. The dominant taxa of the clusters were identified using linear discriminant analysis effect size (LEfSe). The biomarkers of Cluster I included Marinomonas, Photobacterium, and Vibrio. Porticoccus and Clade-1a were identified as the biomarkers of Cluster II. Marivia, Leucothrix, and Phaeodactylibacter were identified as the biomarkers of Cluster IV. The Cluster I biomarkers were positively correlated with NO2--N, while those of Cluster IV were positively correlated with NO3--N. The redundancy analysis showed that the bacterial communities and biomarkers were influenced by water quality parameters. Quantitative real-time PCR analysis revealed significant differences in the abundances of the amoA and nxrB genes among treatments and between the two stages. The abundance of the amoA gene was higher in the control group than in the carton-added treatments at the ends of both stages. This study provides an important theoretical basis for the selection of efficient ammonia nitrogen control strategies in aquaculture systems.

Constructing a model including the cryptic sulfur cycle in Chesapeake Bay requires judicious choices for key processes and parameters

A new biogeochemical model for Chesapeake Bay has been developed by merging two published models - the ECB model of Da et al. (2018) that has been calibrated for the Bay but only simulates nitrogen, carbon and oxygen and the BioRedoxCNPS model of al Azhar et al. (2014) and Hantsoo et al. (2018) that includes cryptic sulfur cycling. Comparison between these models shows that judicious choices are required for key processes and parameters. This manuscript documents the sources of differences between the two published models in order to select the most realistic configuration for our new model.•This study focuses on three sets of differences-processes only included in ECB (burial and dissolved organic matter), processes only included in BioRedoxCNPS (explicit dynamics for hydrogen sulfide, sulfate and nitrite, light attenuation that does not include CDOM or sediments), and differences in parameters common to the two codes.•Sensitivity studies that highlight particular choices (absorption by dissolved organic matter, nitrification rates, stoichiometric ratios) are also shown.•The new model includes sulfur cycling and has comparable skill in predicting oxygen as ECB, but also has improved simulation of nitrogen species compared with both original codes.

Effect of co-application of straw and various nitrogen fertilizers on N2O emission in acid soil

In order to explore the alteration of N transformation and N2O emissions in acid soil with the co-application of straw and different types of nitrogen (N) fertilizers, an incubation experiment was carried out for 40 days. There are totally five treatments in the study: (a) without straw and N fertilizer (N0), (b) straw alone application (SN0), (c) straw with NH4Cl (SN1), (d) straw with NaNO3 (SN2), and (e) straw with NH4NO3 (SN3). N2O emissions, soil physicochemical properties, and abundance/activity of ammonia-oxidizing archaea (AOA) were measured. The results showed that the combined application of straw and N enhanced N2O emissions, particularly, SN2 and SN3 treatments. Moreover, the soil pH was lower in co-application treatments and the average decreasing rate was 9.69%. Specially, the pH was lowest in the SN1 treatment. The results of correlation analysis indicated a markedly negative relationship between pH and N2O, as well as a negative relationship between pH and net mineralization rate. These findings suggest that pH alteration can affect the N transformation process in soil and thus influence N2O emissions. In addition, the dominant AOA at the genus level in the SN2 treatment was Nitrosopumilus, and Candidatus nitrosocosmicus in the SN3 treatment. The reshaped AOA structure can serve as additional evidence of the changes in the N transformation process. In conclusion, as the return of straw, the cumulation of N2O from arable acid soil depends on the form of N fertilizer. It is also important to consider how N fertilizer is applied to reduce the possibility of N being lost in the soil as gas.

Depth profiles of biological aerated contactors: Characterizing microbial activity treating reduced contaminants

The biological treatment process consisting of an aerated contactor and filter is effective for groundwaters containing elevated ammonia and other reduced contaminants, including iron, manganese, arsenic, and methane. Depth profiles characterizing microbial activity across aerated contactors are lacking. A 1-year pilot study comparing gravel- and ceramic-packed contactors was conducted, and media depth profile samples were collected at the conclusion of the study. Media and water samples also were collected from pilot-scale aerated contactors at 4 other water systems. Water quality, media surface metals concentrations, and a suite of biofilm parameters were analyzed. Media surface metals concentrations were greatest at the influent end. ATP concentrations, extracellular polymeric substances, and extracellular enzyme activities tended to be similar across depth. Bacteria and functional genes involved in contaminant oxidation co-occurred and tended to decrease across depth, but were not correlated to the media metals concentration. Microbial community composition changed with depth, and the diversity either decreased or remained similar. The microbial activity profiles through aerated contactors differed from what is typically reported for groundwater biofilters, suggesting that the different reactor flow and dissolved oxygen profiles impacted the microbial community.

Physiological adaptation and population dynamics of a nitrifying sludge exposed to ampicillin

Antibiotics in wastewater treatment plants can alter the physiological activity and the structure of microbial communities through toxic and inhibitory effects. Physiological adaptation, kinetic, and population dynamics behavior of a nitrifying sludge was evaluated in a sequential batch reactor (SBR) fed with 14.4 mg/L of ampicillin (AMP). The addition of AMP did not affect ammonium consumption (100 mg NH4+-N/L) but provoked nitrite accumulation (0.90 mg NO2--N formed/mg NH4+-N consumed) and an inhibition of up to 67% on the nitrite oxidizing process. After 30 cycles under AMP feeding, the sludge recovered its nitrite oxidizing activity with a high nitrate yield (YNO3-) of 0.87 ± 0.10 mg NO3--N formed/mg NH4+-N consumed, carrying out again a stable and complete nitrifying process. Increases in specific rate of nitrate production (qNO3-) showed the physiological adaptation of the nitrite oxidizing bacteria to AMP inhibition. Ampicillin was totally removed since the first cycle of addition. Exposure to AMP had effects on the abundance of bacterial populations, promoting adaptation of the nitrifying sludge to the presence of the antibiotic and its consumption. Nitrosomonas and Nitrosospira always remained within the dominant genera, keeping the ammonium oxidizing process stable while an increase in Nitrospira abundance was observed, recovering the stability of the nitrite oxidizing process. Burkholderia, Pseudomonas, and Thauera might be some of the heterotrophic bacteria involved in AMP consumption.

Seasonal linkages between soil nitrogen mineralization and the microbial community in broadleaf forests with Moso bamboo (Phyllostachys edulis) invasion

Plant invasions significantly alter the microbiome of the soil in terms of fungal and bacterial communities, which in turn regulates ecosystem processes and nutrient dynamics. However, it is unclear how soil microbial communities, nitrogen (N) mineralization, and their linkages respond to plant invasions over the growing season in forest ecosystems. The present study investigated the seasonal associations between the microbial composition/function and net N mineralization in evergreen broadleaf, mixed bamboo-broadleaf, and Moso bamboo (Phyllostachys edulis) forests, depicting uninvaded, moderately invaded, and heavily invaded forests, respectively. The ammonification and nitrification rates in the bamboo forest were significantly higher than those in the broadleaf and mixed bamboo-broadleaf forests during the spring season only. The forest type and seasonal variation significantly influenced the net rates of ammonification and nitrification and the abundances of bacterial apr and AOB amoA, fungal cbhI and lcc genes, as well as the microbial composition. Moreover, the partial least squares path model revealed that bamboo invasion enhanced net ammonification through increasing total N and fungal-to-bacterial ratio, and enhanced net nitrification through modifying the bacterial composition and increasing the fungal-to-bacterial ratio during spring. However, microbial parameters had no significant effect on net ammonification and nitrification during autumn. We conclude that shifts in the microbial abundance and composition following bamboo invasion facilitated soil N mineralization during spring, contributing to the rapid growth of Moso bamboo at the beginning of the growth season and its invasion into adjacent subtropical forests.

The driving effects of nitrogen deposition on nitrous oxide and associated gene abundances at two water table levels in an alpine peatland

Alpine peatlands are recognized as a weak or negligible source of nitrous oxide (N2O). Anthropogenic activities and climate change resulted in the altered water table (WT) levels and increased nitrogen (N) deposition, which could potentially transition this habitat into a N2O emission hotspot. However, the underlying mechanism related with the effects is still uncertain. Hence, we conducted a mesocosm experiment to address the response of growing-season N2O emissions and the gene abundances of nitrification (bacterial amoA) and denitrification (narG, nirS, norB and nosZ) to the increased N deposition (20 kg N ha-1 yr-1) at two WT levels (WT-30, 30 cm below soil surface; WT10, 10 cm above soil surface) in the Zoige alpine peatland, Qinghai-Tibetan Plateau. The results showed that the WT did not affect N2O emissions, and this was attributed with the limitation of soil NO3-. The higher WT level increased denitrification (narG and nirS gene abundance) resulting in the depletion of soil NO3-, but the consequent NO3- deficiency further limited denitrification, while the WT did not affect nitrification (bacterial amoA gene abundance). Meanwhile, the N deposition increased N2O emissions, regardless of WT levels. This was associated with the N-deposition induced increase in denitrification-related gene abundances of narG, nirS, norB and nosZ at WT-30 and narG at WT10. Additionally, the N2O emission factor assigned to N deposition was 1.3 % at WT-30 and 0.9 % at WT10, respectively. Our study provided comprehensive understanding of the mechanisms referring N2O emissions in response to the interactions between climate change and human disturbance from this high-altitude peatland.

DN3

Recently, the functional microorganisms capable of eliminating nitrogenous waste have been applied in mariculture systems. As a potential candidate for treating mariculture wastewater, strain DN3 eliminated 100% of ammonia and nitrate and 96.61%-100% of nitrite within 72 h, when single nitrogen sources at concentrations of 0-50 mg/L. Strain DN3 also exhibited the efficient removal performance of mixed-form nitrogen (ammonia, nitrate, and nitrite) at salinity 30 ‰, C/N ratio 20, and 180 rpm. The nitrogen assimilation pathway dominated inorganic nitrogen metabolism, with less nitrogen (14.23%-25.02% of TN) lost into the air via nitrification and denitrification, based on nitrogen balance analysis. Moreover, the bacterial nitrification pathway was explored by enzymatic assays and inhibition assays. These complex nitrogen assimilation and dissimilation processes were further revealed by bacterial genome analysis. These results provide important insight into nitrogen metabolism of Halomonas sp. and theoretical support for treating mariculture wastewater with strain DN3.

Nitrate sources and transformation processes in groundwater of a coastal area experiencing various environmental stressors

In coastal salinized groundwater systems, contamination from various nitrate (NO3) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO3 sources and identify potential NO3 transformtation processes. Effective field-based NO3 studies in coastal areas are needed to improve the understanding of NO3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ18OH2O, δ2HH2O, δ15NNO3, δ18ONO3, and δ11B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO3 sources and their contributions, and (ii) to describe the potential NO3 transformation processes. The measured NO3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO3 indicates that manure, soil organic matter, and sewage are the potential sources of NO3, while δ11B values constrain the NO3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO3 dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both 15N and 18O in the remaining NO3, although isotopic mass balances between the measured and the theoretical δ18ONO3 values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management.

Comparison of typical nitrite oxidizing bacteria suppression strategies and the effect on nitrous oxide emissions in a biofilm reactor

In mainstream partial nitritation/anammox (PN/A), suppression of nitrite oxidizing bacteria (NOB) and mitigation of N2O emissions are two essential operational goals. The N2O emissions linked to three typical NOB suppression strategies were tested in a covered rotating biological contactor (RBC) biofilm system at 21 °C: (i) low dissolved oxygen (DO) concentrations, and treatments with (ii) free ammonia (FA), and (iii) free nitrous acids (FNA). Low emerged DO levels effectively minimized NOB activity and decreased N2O emissions, but NOB adaptation appeared after 200 days of operation. Further NOB suppression was successfully achieved by periodic (3 h per week) treatments with FA (29.3 ± 2.6 mg NH3-N L-1) or FNA (3.1 ± 0.3 mg HNO2-N L-1). FA treatment, however, promoted N2O emissions, while FNA did not affect these. Hence, biofilm PN/A should be operated at relatively low DO levels with periodic FNA treatment to maximize nitrogen removal efficiency while avoiding high greenhouse gas emissions.

Douglas-fir plantations impact stream and groundwater chemistry in western Europe: Insights from three case studies in France and Luxembourg

In rural areas, nitrate concentrations in surface waters most often originate from the leaching of excess N fertilizer in agricultural lands, whereas forested catchments often have good water quality. However, Douglas-fir plantations may induce nitrogen cycle unbalances which may lead to an excess of nitrate production in the soil. We hypothesize that the excess of production of nitrate in the soil and nitrate leaching to streamwater is greater in catchments planted with Douglas fir. We used paired catchments in both France and Luxembourg with different land covers (Douglas-fir, Spruce, Deciduous, Grassland and clearcut) which were monitored over a 3-5 year period in order to assess the effect of Douglas-fir plantations on the chemical composition of surface water. Nitrate concentration in the soil and groundwater were also monitored. The results show that nitrate concentrations in streams draining Douglas-fir catchments were two to ten times higher than in streams draining other land covers, but were similar to the clearcut catchment. Nitrate concentrations under Douglas-fir in groundwater (up to 50 mg L-1) and in the soil were also higher than under all other land covers. Soil nitrate concentration was related to stream nitrate concentration. This suggests that soil processes, through excessive nitrate production under Douglas-fir, are driving the nitrate concentration in the stream water and our hypothesis of a transfer of a fairly large proportion of this excessive production from the soil to the stream is supported. This study also shows that nitrate concentrations in surface and ground waters in rural areas could also originate from Douglas fir forested catchments. The impact of Douglas-fir is nevertheless reduced downstream through a dilution effect: mixing tree species at the catchment scale could thus be a solution to mitigate the effect of Douglas-fir on nitrate concentration in surface waters.