Nitrogen Transformation Research Articles

Isotope analysis of nitrogen removal pathways and N2O production potential in the SDAD-anammox system under different N/S ratios

This study explored the impact of varying nitrate to sulfide (N/S) ratios on nitrogen removal efficiency (NRE) in the sulfide-driven autotrophic denitrification and anammox (SDAD-anammox) system. Optimal nitrogen removal was observed at N/S ratios between 1.5 and 2.0. Isotope tracing results showed that the contribution of anammox to nitrogen removal was enhanced with increasing N/S ratios, reaching up to 37 % at the N/S ratio of 2.5. Additionally, complex nitrogen pathways were identified, including dissimilatory nitrate reduction to ammonium (DNRA). Furthermore, isotope tracing was innovatively applied to investigate N2O emissions, demonstrating that higher N/S ratios significantly reduced N2O emissions, with the lowest emissions at N/S ratio of 2.5. Gene expression analysis indicated that nitrogen and sulfide transformation genes decreased with increasing N/S ratios, while anammox-related genes first increased and then decreased, reflecting the system's microbial dynamics. These findings offer insights into nitrogen transformation pathways and N2O production mechanisms in the SDAD-anammox process.

Enhancing collaboration of anammox with heterotrophic microbes mediated selectively by iron of different valences: Activities balance, metabolic mechanism, and functional genes regulation

The partial denitrification/anammox (PD/A) process is receiving increasing attention due to its cost-effectiveness advantages. However, effective strategies to alleviate organic matter inhibition and promote anammox activity have been proven to be a big challenge. This study investigated the effects of three types of iron (nano zero-valent iron (nZVI), Fe(II), and Fe(III)) on the PD/A process. It is worth noting that nZVI of 5–50 mg/L and Fe(III) of 5–120 mg/L promoted both PD and anammox activity. Long-term intermittent addition of nZVI (50 mg/L) resulted in a nitrogen removal efficiency of 98.2% in the mixotrophic PD/A system driven by iron and organic matter. The contribution of anammox for nitrogen removal reached as high as 93.8%. The organic carbon demand decreased due to the external electron donor provided by nZVI for PD. Multiple Fe–N metabolic pathways, primarily involving ammonia oxidation by Fe(III) and nitrate reduction by nZVI, play a crucial role in facilitating nitrogen transformation. Conversely, the direct addition of 30–120 mg/L Fe (II) resulted in a significant decrease in pH to below 5.0 and severe inhibition of PD and anammox activity. Following prolonged operation in the presence of nZVI, it was demonstrated that there is an enhancing effect on robust nitrite production for anammox. This was accompanied by a remarkable up-regulation of genes encoding nitrate reductase and iron-transporting proteins dominated by Thauera. Overall, this study has provided an efficient approach for advanced nitrogen removal through organic- and iron-driven anammox processes.

Mastering granular activated carbon filtration to remove organic micropollutants, antibiotic resistance and metals for municipal wastewater reuse

GAC filtration of municipal wastewater was optimized and intensified, making its implementation and operation directly after secondary clarification possible and relevant. GAC was first selected based on laboratory tests. Performances on organic micropollutants were linked to the repartition of BET surface between micropores and meso/macropores. At pilot scale, in order to limit the impact of head loss, downflow declogging sequences (DCS) were implemented and upflow filtration tested. 6 to 12 DCS per day led to a 4.7–5.5-fold increase of particles retention capacity between backwashes (cycle duration of 20–120 h), and upflow operations improved head loss evolution profile with only a slight GAC (<15 %) expansion. DCS allows backwash frequency reduction, enabling significant water savings. Both adaptations maintained high organic micropollutants removals compared to a review of 16 GAC studies at pilot or full-scale, results being in the upper range. A specific dose of 2.0–2.5 g GAC/gC was necessary to obtain an average removal of pharmaceuticals and benzotriazole of 80 % at 20 min contact time, which is comparable to PAC and low granulometry GAC. Higher doses are needed for PFAS but >80 % removals are achievable. Particles, TKN, particulate phosphorus and organic matter are well removed by GAC filtration in both configurations. Biological activity is observed through nitrogen transformation in the GAC bed. Heavy metals are greatly removed in GAC filtration, in particular Cd, Cu, Ni and Pb, probably through biosorption onto the biofilm, developed within the GAC bed. For wastewater reuse applications, GAC filtration has an added value through physicochemical quality improvement and fecal contamination indicators removal of 1 log, facilitating the implementation and optimizing the design of a post-disinfection. Antibiotic resistant bacteria and antibiotic resistance genes are also partially retained in GAC filtration. Finally, biological wastewater treatments combined to GAC filtration is a good solution to effectively treat organic micropollutants together with heavy metals and preparing post-disinfection for reuse.

Microbial nitrogen transformations in tundra soil depend on interactive effects of seasonality and plant functional types

Microbial nitrogen transformations in tundra soil depend on interactive effects of seasonality and plant functional types

Tracing nitrogen sources and transformation characteristics in a large basin with spatially heterogeneous pollution distribution

This study used dual stable isotopes to examine nitrate sources and geographical distribution in the Liao River Basin (LRB), one of China's seven major river basins. During a normal hydrological season in April 2021, water samples were taken from the main streams of the Liao River (MLR), Shuangtaizi River (STR), Hun River (HR), Taizi River (TZR), and Daliao River (DLR). Monitoring results indicated that 93% of the water samples had a total nitrogen level exceeding the Class IV limit (1.5 mg/L) of the ‘Environmental Quality Standards (EQS) for surface water’, indicating a serious nitrogen pollution status. 71.3% of the total nitrogen on average was in the form of nitrate. The scatterplots of δD-H2O and δ18O-H2O showed that water in TZR and DLR were mainly affected by precipitation, while MLR, STR and HR were additionally impacted by evaporation and groundwater. The overall δ15N and δ18O of NO3− varied from 7.7‰ to 17.9‰ and 0.6‰–11.2‰, respectively. The correlations between δ15N-NO3- and δ18O-NO3-, along with attribution results from the Bayesian isotopic mixing model, indicated a predominant role of manure/sewage (MS) pollution in affecting river nitrate, accounting for 78% of total nitrate in MLR and 72% in DLR. A positive correlation between δ15N-NO3- and δ18O-NO3- in MLR indicated the occurrence of denitrification process. Overall, attribution results showed that the primary nitrate sources varied in different river systems within such a large basin, mainly due to spatially varied land use and human activities. Tailored nitrogen management strategies should be implemented to address the main anthropogenic pressures.

Revealing the distribution characteristics and key driving factors of dissolved organic matter in Baiyangdian Lake inflow rivers from different seasons and sources

The river course is a transitional area connecting the source and receiving water bodies. The dissolved organic matter (DOM) in the river course is an important factor affecting the aquatic environment and ecological health. However, there are shortcomings in studying the differences and quantitative contributions of river DOM in different seasons and sources. In this study, ultraviolet-visible (UV–vis) and three-dimensional fluorescence spectra were used to characterize the optical properties, analyze the spatiotemporal changes, and establish the quantitative relationship between environmental factors and DOM in the inflow rivers of Baiyangdian Lake. The results showed that the relative DOM concentrations in summer and autumn were significantly higher than those in the other seasons (P < 0.001) and that the DOM source (SR < 1) was mainly exogenous. The fluorescence abundance of protein-like substances (C1 + C2 + C3) was the highest in spring, whereas that of humus C4 was the highest in autumn. Moreover, the inflow rivers exhibited strong autogenetic characteristics (BIX > 1) throughout the year. Self-organizing maps (SOM) indicated that the main driving factors of water quality were NO3−-N in spring, autumn, and winter and DO, pH, and chemical oxygen demand (COD) in summer. Random forest analysis showed that the fluorescent components (C1–C4) were closely related to the migration and transformation of nitrogen, and pH and nitrogen were the main predictors of each component. The Mantel test and structural equation model (SEM) showed that temperature and NO3−-N significantly influenced the DOM concentration, components, and molecular properties in different seasons. Moreover, the river source also affected the distribution mechanism of DOM in the water body. Our study comprehensively analyzed the response of DOM in inflow rivers in different seasons and water sources, providing a basis for further understanding the driving mechanisms of water quality.

Enhancing soil gross nitrogen transformation through regulation of microbial nitrogen-cycling genes by biodegradable microplastics

Microplastics (MPs) in agricultural plastic film mulching system changes microbial functions and nutrient dynamics in soils. However, how biodegradable MPs impact the soil gross nitrogen (N) transformations and crop N uptake remain significantly unknown. In this study, we conducted a paired labeling 15N tracer experiment and microbial N-cycling gene analysis to investigate the dynamics and mechanisms of soil gross N transformation processes in soils amended with conventional (polyethylene, PE) and biodegradable (polybutylene adipate co-terephthalate, PBAT) MPs at concentrations of 0 %, 0.5 %, and 2 % (w/w). The biodegradable MPs-amended soils showed higher gross N mineralization rates (0.5–16 times) and plant N uptake rates (16–32 %) than soils without MPs (CK) and with conventional MPs. The MPs (both PE and PBAT) with high concentration (2 %) increased gross N mineralization rates compared to low concentration (0.5 %). Compare to CK, MPs decreased the soil gross nitrification rates, except for PBAT with 2 % concentration; while PE with 0.5 % concentration and PBAT with 2 % concentration increased but PBAT with 0.5 % concentration decreased the gross N immobilization rates significantly. The results indicated that there were both a concentration effect and a material effect of MPs on soil gross N transformations. Biodegradable MPs increased N-cycling gene abundance by 60–103 %; while there was no difference in the abundance of total N-cycling genes between soils without MPs and with conventional MPs. In summary, biodegradable MPs increased N cycling gene abundance by providing enriched nutrient substrates and enhancing microbial biomass, thereby promoting gross N transformation processes and maize N uptake in short-term. These findings provide insights into the potential consequences associated with the exposure of biodegradable MPs, particularly their impact on soil N cycling processes.

Co-pyrolysis characteristics and pyrolysis oil analysis of Chlorella vulgaris and marine waste plastics with multi-metal MOFs-derived additive

In this study, MOFs-derived additives with microwave absorption and catalytic characteristics were employed in the microwave co-pyrolysis of Chlorella vulgaris (CV) and marine waste plastics (MW) to produce high-quality pyrolysis oil. The effects of additives (Ni-Cu@C, Ni-Co@C, Cu-Co@C and Ni-Cu-Co@C) under different addition amounts (10%, 20%, 30%) on co-pyrolysis characteristics, product yield and pyrolysis oil composition were investigated. Furthermore, the pyrolysis mechanism was analyzed. The results showed that the MOFs-derived additives improved the co-pyrolysis characteristics, especially at 30% addition amount. The maximum average weight loss rate (0.0273 wt.%/s) and pyrolysis oil yield (14.33%) obtained at 30% Ni-Cu-Co@C group. Moreover, the aromatic content (30.07%), deoxygenation efficiency (27.70%) and denitrogenation efficiency (80.03%) in pyrolysis oil were maximum at adding 30% Ni-Cu-Co@C. Ni-Cu-Co@C promoted the nitrogen transformation into gaseous products, while reduced the nitrogen contents in char and pyrolysis oil. Meanwhile, the nitrogen in char was dominated by pyridine-N and quaternary-N. Importantly, the activity of the Ni-Cu-Co@C remained 0.66 times of the initial activity after 5 cycles, demonstrating the excellent regenerative usability.

Study on the effect of conditioner on NOx precursor control behavior from sewage sludge pyrolysis: Focusing on conditioner assessments and in-situ fixation mechanism

The nitrogen transformation during sludge pyrolysis is affected by the dewater conditioner. However, the comparative analysis of the conditioner under identical pyrolysis conditions has been previously absent. In this study, Ca-, Fe- and Al-based conditioners were selected as the representatives. A comprehensive evaluation considering the cost of the conditioners and the product characteristics was conducted. Additionally, the in-situ fixation mechanism of the conditioner on nitrogen-containing gas was concurrently revealed. Among the six conditioners, CaO and AlCl3 were identified as the top performers, ranking first and second, respectively. Furthermore, Fe/Ca-based conditioners reduced NH3 and HCN release by 1.5 ∼ 5.53 % and 0 ∼ 1.55 %, respectively, by facilitating the conversion of amine-N to a more stable form in condensable fraction. Fe promoted volatile amine-N cyclization, while Ca encouraged its dehydrogenation. Both Fe/Ca-based conditioners increased 7.5 ∼ 14.8 % nitrogen retention in char, by inhibiting the decomposition of protein-N. Al-based conditioners had little effect on NH3 and HCN, but contributed to 2.3 ∼ 2.8 % production of stabilized nitrogen in char. The introduction of Cl in Fe/Ca/Al chloride conditioners would promote the decomposition of inorganic ammonium salts to produce NH3 at 30 ∼ 185 °C. And Cl also reacted with volatiles through electrophilic substitution reaction, leading to the formation of halogenated hydrocarbons in condensable fraction and the release of more NH3, HCN, and HNCO at 30 ∼ 465 °C. The findings of this study provide a detailed comparative analysis of various conditioners under uniform conditions and reveal the in-situ fixation mechanism of nitrogen-containing gas. This will provide guidance for the sludge conditioning-dewatering-drying integrated treatment and disposal.

Nitrogen migration and transformation during the pyrolysis of corn straw with iron-based catalyst

The pyrolysis of renewable biomass into biofuels and high-value chemicals showcases remarkable application value and extensive market potential. However, the pyrolysis of biomass with high nitrogen content results in the production of NOX precursors that contribute to environmental pollution. Therefore, it is proposed to introduce iron-based catalysts to change the formation pathway of NOX precursors and control their production.The study elucidates the influence of iron-based catalysts on the conversion of nitrogen-containing products between gas, liquid and solid phases during corn straw pyrolysis. First, the effects of iron-based catalyst types on pyrolysis kinetics of corn straw were estimated using Thermogravimetry-mass spectrometry (TG-MS). Based on this, the effects of catalyst type and dispersion on the migration path of three-phase nitrogen-containing products were investigated, and the chemical morphology of the catalyst during pyrolysis was monitored by in-situ XRD to clarify the main catalytic active phase for N2 formation. Results showed that the addition of Fe, FeCl3 and Fe2O3 changed the distribution ratio of three-phase nitrogen-containing products, and the gas phase yield was significantly improved. Notably, the chemical form of Fe2O3 will change and transform into FeOOH, Fe3O4 and α-Fe during pyrolysis. It was confirmed that α-Fe was the main active phase for catalyzing the conversion of nitrogen-containing substances. Higher N2 conversion can be achieved by preparing high purity α-Fe. The mechanism is that α-Fe catalyzes the secondary decomposition of tar nitrogen into NH3 and HCN. Simultaneously, α-Fe reacts with char nitrogen, NH3, and HCN to form FeXN, which decomposes to produce environmentally friendly N2. The research findings offer theoretical guidance for the control of nitrogen pollutant emissions and the targeted acquisition of pyrolysis products (For example, NH3 can be used as a carbon-free hydrogen-rich fue).

Efficient nitrogen removal of decentralized rural sewage: Pilot-scale study of a novel push-flow coupled bio-ecological system

The performance and mechanisms for pollutant removal were investigated in an extended pilot-scale operation. Results demonstrated that the PF-EFB system maintained stable and efficient performance after over 450 days of continuous operation. Under various operating conditions, the total nitrogen (TN) concentration was reduced to <2 mg/L, and the removal rate exceeded 90 %. The effluent chemical oxygen demand (COD) concentration remained below 40 mg/L. Additionally, a combination of nitrogen transformation and molecular biology analyses was conducted to explore the mechanisms. The key bacterial groups driving system performance were aerobic and anoxic nitrifying bacteria, which dominated the microbial population. Microbial community analysis showed that the abundance of aerobic denitrifying bacteria in the middle of the system was 40.5 %, while the abundance of anoxic denitrifying bacteria at the end was 19.0 %. This spatial variation in the distribution of aerobic and anoxic denitrifying bacteria along the flow direction inside the reactor, driven by environmental factors, is key to the system's efficient denitrification. Overall, the PF-EFB system effectively addressed challenges posed by water flow fluctuations and enhanced nitrogen removal efficiency. This innovative system offers significant benefits and serves as a valuable reference for rural sewage treatment.

Sedimentary organic matter load influences the ecological effects of submerged macrophyte restoration through rhizosphere metabolites and microbial communities

Organic matter (OM) accumulation in lake sediments has doubled owing to human activities over the past 100 years, which has negatively affected the restoration of submerged vegetation and ecological security. Changes in the pollution structure of sediments caused by plant recovery and rhizosphere chemical processes under different sediment OM levels are the theoretical basis for the rational application of plant rehabilitation technology in lake management. This study explored how Vallisneria natans mediates changes in sediment N and P through rhizospheric metabolites and microbial community and function under low (4.94 %) and high (17.35 %) sediment OM levels. V. natans promoted the accumulation of NH4-N in the high-OM sediment and the transformation of Fe/Al-P to Ca-P in the low-OM sediment. By analyzing 63 rhizospheric metabolites and the sediment microbial metagenome, the metabolites lactic acid and 3-hydroxybutyric acid and the genus Anammoximicrobium were found to mediate NH4-N accumulation in the high-OM sediment. Additionally, 3-hydroxy-decanoic acid, adipic acid, and the genus Bdellovibrionaceae mediated the transformation of Fe/Al-P to Ca-P in the low-OM sediment. The growth of V. natans enriched the abundance of functional genes mediating each step from nitrate to ammonia and the genes encoding urease in the high-OM sediment, and it up-regulated three genes related to microbial phosphorus uptake in the low-OM sediment. This study revealed the necessity of controlling endogenous pollution by recovering submerged macrophytes under high- and low-OM conditions from the perspective of the transformation of inorganic nitrogen and phosphorus.

Impact of permanganate with polyaluminium chloride on algae-laden karst water: Behaviors and disinfection by-products control

The removal of algal organic matter (AOM) through water treatment processes is a major approach of reducing the formation of disinfection by-products (DBP). Here, the formation of DBP from AOM in karst water under different combination of potassium permanganate (KMnO4) and polyaluminium chloride (PACl) was investigated. The effect of divalent ions (Ca2+ and Mg2+) on DBP formation was traced by AOM chemistry variations. For DBP formation after KMnO4 preoxidation, total carbonaceous DBPs (C-DBPs) decreased by 12.9% but nitrogen-containing DBPs (N-DBPs) increased by 18.8%. Conversely, the C-DBPs further increased by 3.3% but N-DBPs reduced by 10.7% after the addition of PACl besides KMnO4 preoxidation. The variations of aromatic protein-like, soluble microbial products-like compounds and ultraviolet absorbance at 254 nm (UV254) were highly correlated with the formation of DBPs, which suggest aromatic substances strongly affect DBP behaviors at different treatment conditions. In the presence of divalent ions (Ca2+ = 135.86 mg/L, Mg2+ = 18.51 mg/L), the combination of KMnO4 and PACl was more effective in controlling DBP formation compared to the situation without Ca2+ and Mg2+. Specifically, trichloromethane formation was largely inhibited compared to the other tested DBPs, which may refer to complexation of electron-donating groups via divalent ions. While Ca2+ and Mg2+ may not affect the nature of α-carbon and amine groups, so the variation of haloacetonitriles (HANs) was not obvious. The study enhances the understanding of the DBP formation patterns, transformation of carbon and nitrogen by preoxidation-coagulation (KMnO4-PACl) treatment in algae-laden karst water.

Plant phenology modulates and undersown cover crops mitigate N2O emissions

Mitigation of N2O emissions, a potent greenhouse gas, remain challenging due to knowledge gaps in plant-mediated nitrogen (N) transformation pathways, which limits ability to identify optimal approaches for efficient N utilization. We set up mesocosms with barley, Italian ryegrass, and barley in combination with Italian ryegrass to assess role of cover crop in N2O emission mitigation. Soil emitted N2O was collected simultaneously from the pots with plants at three growth stages: namely, vegetative, canopy expansion, and grain filling. The gas sample N2O contents, N in microbial biomass (MBN), mineral N content, and phospholipid fatty acid (PLFA) analysis in soils were determined at the three growth stages. Cumulatively, highest N2O was emitted from soil under Italian ryegrass (0.056 mg N g−1 soil) followed by barley (0.0051 mg N g−1 soil) and the least under barley and Italian ryegrass combination (0.0014 mg N g−1 soil). The high emissions under Italian ryegrass occurred at vegetative stage due to high reactive N availability. Strong emissions were observed at canopy expansion stage under barley and were linked to access to the large mineral N proportion redistributed to the lower depth as depicted by highest MBN (0.025 mg N g−1 soil) and decreased extractable N (0.0068 mg N g−1 soil). The high emissions under barley correlated with high fungal/bacterial ratio, pointing towards a fungal role in the emissions. The least soil N2O emissions under barley and Italian ryegrass combination were accompanied by elimination of variations induced by the plant growth stages. Absence of 18:2ω6,9 fungal PLFA biomarker under barley and Italian ryegrass combination indicates a potential inhibition and corresponds with reduced N2O emissions. Together, these results broaden our understanding on how plant-soil interactions drives N2O emissions processes and improves our ability to identify optimal plant-based emission mitigation approaches.

Discovery of Candidatus Nitrosomaritimum as a New Genus of Ammonia-Oxidizing Archaea Widespread in Anoxic Saltmarsh Intertidal Aquifers.

Ammonia-oxidizing archaea (AOA) are widely distributed in marine and terrestrial habitats, contributing significantly to global nitrogen and carbon cycles. However, their genomic diversity, ecological niches, and metabolic potentials in the anoxic intertidal aquifers remain poorly understood. Here, we discovered and named a novel AOA genus, Candidatus Nitrosomaritimum, from the intertidal aquifers of Yancheng Wetland, showing close metagenomic abundance to the previously acknowledged dominant Nitrosopumilus AOA. Further construction of ammonia monooxygenase-based phylogeny demonstrated the widespread distribution of Nitrosomaritimum AOA in global estuarine-coastal niches and marine sediment. Niche differentiation among sublineages of this new genus in anoxic intertidal aquifers is driven by salinity and dissolved oxygen gradients. Comparative genomics revealed that Candidatus Nitrosomaritimum has the genetic capacity to utilize urea and possesses high-affinity phosphate transporter systems (phnCDE) for surviving phosphorus-limited conditions. Additionally, it contains putative nosZ genes encoding nitrous-oxide (N2O) reductase for reducing N2O to nitrogen gas. Furthermore, we gained first genomic insights into the archaeal phylum Hydrothermarchaeota populations residing in intertidal aquifers and revealed their potential hydroxylamine-detoxification mutualism with AOA through utilizing the AOA-released extracellular hydroxylamine using hydroxylamine oxidoreductase. Together, this study unravels the overlooked role of priorly unknown but abundant AOA lineages of the newly discovered genus Candidatus Nitrosomaritimum in biological nitrogen transformation and their potential for nitrogen pollution mitigation in coastal environments.

Advances in Research of the Effects and Mechanisms of Polyethylene Microplastics on Soil Nitrogen Transformation

In the agricultural lands of China, polyethylene is the main component of microplastics （MPs）, with characteristics such as small size, wide distribution, easy accumulation, and difficult degradation. Therefore, it may have an impact on the elemental cycling process of the soil. On the basis of reviewing the key literatures in the past few years, this study systematically analyzed and summarized the key factors and processes of the polyethylene microplastics （PE-MPs） affecting soil nitrogen transformation. On the one hand, PE-MPs directly affected the activities of microorganisms and key enzymes related to soil nitrogen transformation by enriching microorganisms, selecting colonized microbial populations, and releasing additives. On the other hand, PE-MPs had indirect impacts on the activities of microorganisms and key enzymes related to soil nitrogen transformation by affecting soil physicochemical properties of soil and changing the microenvironment for microbial growth. Moreover, phthalates, an important additive of the MPs, may be the key factor affecting soil nitrogen transformation in the short-term. Finally, we posed key scientific issues that should be further studied in order to provide scientific support for nitrogen nutrition regulation and ecological risk assessment of soils contaminated by PE-MPs.

Increased nitrogen accumulation in mulberry trees due to the secretion of glomalin-related soil protein induced by arbuscular mycorrhizal fungi

In the initial stages of restoring areas affected by rocky desertification, plant survival is strongly influenced by nitrogen nutrition. Mycorrhization is a unique type of inter-root engineering that improves nitrogen acquisition efficiency by plant roots. We selected potted mulberry trees inoculated, two dominant arbuscular mycorrhizal fungi (AMF) with Funneliformis mosseae (Fm) and Rhizophagus intraradices (Ri), to clarify the effects of AMF on the root nitrogen content of mulberry trees. Meanwhile, the key factors of soil nitrogen changes caused by AMF were analyzed, based on the primary role of soil nitrogen as the source of root nitrogen. Simultaneously, the potential of AMF to promote the acquisition of different forms of nitrogen by mulberry roots was investigated. Our findings indicate that the inoculation of mulberry plants with Fm and Ri, improved plant height and increased nitrogen accumulation in the roots and shoots. Additionally, AMF regulates nitrogen transformation, significantly increasing soil nitrate nitrogen (NO3−-N) and dissolved organic nitrogen (DON) levels. The results indicated that soil NH4+-N, NO3−-N, and DON contributed to the observed changes in root nitrogen accumulation. The largest contribution (22.0 %) to the overall effect size was made by NO3−-N. AMF stimulated soil microbial activity and significantly increased soil glomalin-related soil protein (GRSP), enzyme activity, and soil microbial biomass (SMB). Urease activity and microbial biomass carbon (MBC) both increased exponentially by 118.7 % and 115.2 %, respectively. Higher GRSP, enzyme activity, and SMB were positively correlated with changes in soil nitrogen patterns, and GRSP had the most significant effect on changes in the soil nitrogen dynamics. Our study confirmed that inoculation with AMF not only regulates soil nitrogen dynamics but also diversifies plant nitrogen sources. This is achieved by increasing plant growth and enhancing soil microbial activity. Ultimately, this enhances plant root nitrogen nutrition. Therefore, AMF promote root nitrogen accumulation and enhance root nitrogen uptake through GRSP-regulated soil nitrogen, providing a theoretical basis for the management of rocky desertification.

Nitrogen transformation as affected by decomposition of 15N‐labeled cover crop shoots and roots

AbstractBackgroundIncorporation of cover crop (cc) shoot and root biomass can have different effects on nitrogen (N) dynamics and the transformation of soil‐derived N and cc N.AimsThe objective was to determine the effects of different ccs, cc compartments (roots and shoots), and pretreatment of cc biomass (fresh vs. dried) on mineralization processes and on the transformation of soil and cc N following incorporation into a silty loam soil.MethodsSoil columns with incorporated 15N‐labeled root and shoot biomass of two cc species (winter rye and oil radish) and different pretreatments (dried and fresh) were incubated for 70 days at a constant temperature and soil moisture (8°C, 40% water‐filled pore space). Carbon and N transformation dynamics were determined repeatedly, distinguishing between N originating from cc biomass and from soil.ResultsNet CO2 emission was related to the amount of soluble cell components added with ccs. Net N mineralization was negatively related to the C:N ratio of cc biomass. The incorporation of dried cc biomass caused higher initial soil respiration and N immobilization than fresh biomass. All treatments with cc incorporation showed increased N2O emission. Emitted N2O‐N consisted mainly of cc N (55%–57%) in treatments with fresh shoot biomass, whereas soil N was the main source of N2O (75%) in the treatment with fresh oil radish roots. Recovery of cc 15N was affected by crop compartment and pretreatment. At the end of the incubation, it was 17.5%–42.3% in soil NO3−, 0.1%–8.1% in microbial biomass N, and less than 0.23% of cc N was found in cumulative N2O emission.ConclusionThe incorporation of cc roots and shoots had different effects on N mobilization and immobilization processes and on the partitioning of cc N. These processes can be influenced significantly by pretreatment of the added plant biomass (dried vs. fresh).

Enhancement of nitrogen removal in constructed wetlands through boron mud and elemental sulfur addition: Regulation of sulfur and oxygen cycling

Constructed wetlands (CWs) utilizing elemental sulfur (S0) as an electron donor are effective for nitrate (NO3–-N) removal from low C/N wastewater. However, the limited bioavailability of chemical S0 (chem-S0) due to low water solubility and inadequate mass transfer impedes complete denitrification. In this study, boron mud waste and S0 (S0&BM) were processed and combined to convert chem-S0 to biological S0 (bio-S0) in CWs. The employment of S0&BM improved NO3–-N and total nitrogen (TN) removal rates by 15.36 % and 10.55 % compared to S0 alone. The 15N isotope tracing experiment unveiled the pathways of N2O production and demonstrated that S0&BM facilitated full denitrification, resulting in a higher percentage of nitrogen being degraded in a non-polluting form (54.70 %). Releasing trace amounts of boron and magnesium in S0&BM mitigated the excessive acidification caused by the sulfur autotrophic process and promoted biofilm formation, thereby pronouncing vertical oxygen fluctuations. Meanwhile, oxygen vacancies generated in the S0&BM promoted the closed-loop sulfur cycle, starting from the oxidation of chem-S0 to sulfate (SO42-), the important process of SO42--reducing bacteria using partial organic compounds as electron donors to reduce SO42- to sulfide (S2-) was realized, and finally the S2- was oxidized to bio-S0 with better properties. S0&BM enriched functional microbial communities in the biofilm related to nitrogen and sulfur transformation. Functional gene analysis showed an increase in the narG, nirS, norB, and nosZ genes supporting denitrification. The dsrA and sqr genes responsible for the sulfur cycle exhibited the highest abundance. This realization allowed for the utilization of waste materials for treatment, enhancing the energy efficiency of carbon reduction and pollution reduction in CWs.

Inorganic carbon metabolism enhanced hydrogen-driven denitrification: Evaluation of carbon fixation pathways and microbial traits

H2-driven denitrification offers a promising alternative for treating low C/N wastewater amidst global nitrogen pollution, due to its advantages of high removal efficiency, low energy consumption, and independence from exogenous organic carbon source. While inorganic carbon assimilation is crucial for the growth and reproduction of autotrophic microorganisms, it remains uncertain whether the fixation of inorganic carbon could impact H2-driven denitrification. This work demonstrated that the supplement of inorganic carbon efficiently improved the efficiency of nitrate removal to 90.3%, compared to 78.0% in the control. Further analysis indicated that inorganic carbon supplementation shifted the microbial structure from autotrophic-dominated (e.g., Hydrogenophaga and Rhodococcus with abundance of 2.3% and 2.6%) to mixotrophic-dominated (e.g., Thauera and Microscillaceae sp. with abundance of 5.6% and 32.8%). Additionally, although H2 boosted the carbon fixation via the reductive TCA cycle and Wood-Ljungdahl pathway, the Calvin-Benson cycle emerged as the predominant pathway, significantly boosted under inorganic carbon supplementation, along with the upregulation of critical enzyme expression including RuBisCo (EC 4.1.1.39), PGK (EC 2.7.2.3) and GAPDH (EC 1.2.1.12). Moreover, the expressions and activities of functional enzymes involved in nitrogen transformation, including nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR), as well as microbial electron transfer ability, were remarkably enhanced under sufficient inorganic carbon conditions. This work contributed to understanding the carbon metabolism mechanism in H2-driven denitrification, thereby facilitating the promotion of the efficient and low-carbon approach for nitrogen removal from wastewater.