NH3 Flux Research Articles

Crop residue interactions with fertilizer rate enhances volatilization loss and reduces nitrogen use efficiency in irrigated maize and potato

ABSTRACT Ammonia (NH3) fluxes and their soil-related factors were quantified in irrigated maize and potato fields by two split-plot experiments with and without crop residue (CR) together with four fertilizer N rates: N zero (control), farmers’ N (FP), recommended N (RD) and 25% above RD (1.25 RD). CR application together with RD increased NH3 fluxes in maize by 8% in comparison to RD without CR. The NH3 emission factor in maize ranged from 29% in the farmers’ practice (FP-without CR) to 34% in 1.25 RD coupled with CR. Yield-scaled NH3 fluxes increased with N fertilizer application rate. Nitrogen use efficiency (NUE) in maize was 53% higher in FP without CR than in other treatment combinations. In potato, CR coupled with FP reduced NH3 fluxes by 23%. Crop residue and N fertilization rate interaction were significant for NUE, where 1.25 RD without CR increased NUE by 19% when compared with FP-without CR. Stepwise multiple linear regressions showed that mineral N and pH explained 95% of the variances of NH3 fluxes. NH3 fluxes in maize are a significant N loss pathway which is driven mainly by fertilizer N rate, soil mineral N and pH.

Nitrogen Recovery from Landfill Leachate Using Lab- and Pilot-Scale Membrane Contactors: Research into Fouling Development and Membrane Characterization Effects.

Membrane contactor technology affords great opportunities for nitrogen recovery from waste streams. This study presents a performance comparison between lab- and pilot-scale membrane contactors using landfill leachate samples. Polypropylene (PP) and polytetrafluoroethylene (PTFE) fibers in different dimensions were compared in terms of ammonia (NH3) recovery on a lab scale using a synthetic ammonium solution. The effect of pre-treating the leachate with tannin coagulation on nitrogen recovery was also evaluated. An ammonia transfer on the lab and pilot scale was scrutinized using landfill leachate as a feed solution. It was found that PTFE fibers performed better than PP fibers. Among PTFE fibers, the most porous one (denoted as M1) had the highest NH3 flux of 19.2 g/m2.h. Tannin pre-treatment reduced fouling and increased NH3, which in turn improved nitrogen recovery. The mass transfer coefficient of the lab-scale reactor was more than double that of the pilot reactor (1.80 × 10−7 m/s vs. 4.45 × 10−7 m/s). This was likely attributed to the difference in reactor design. An analysis of the membrane surface showed that the landfill leachate caused a combination of inorganic and organic fouling. Cleaning with UV and 0.01 M H2O2 was capable of removing the fouling completely and restoring the membrane characteristics.

Optimized fertigation mitigates N2O and NO emissions and enhances NH3 volatilizations in an intensified greenhouse vegetable system

Gaseous reactive nitrogen (GNr) losses (N2O, NO emissions and NH3 volatilization, etc.) in greenhouse vegetable fields (GVF) have received considerable critical attention. Much uncertainty still exists about GNr losses reduction under increasing vegetables demand and environmental policies in China. An experiment spanning four greenhouse vegetable growing seasons (cucumber and tomato) was conducted in North China Plain (NCP). It included four treatments, i.e., flood irrigation with no urea-N application (FU0), flood irrigation with conventional urea-N application (FUN), drip fertigation with conventional urea-N application (DUN), and drip fertigation with 50% reduction of urea-N application (DRUN). Obvious emission peaks of N2O, NO and NH3 flux were captured in the GVF after each fertilization and irrigation event. For FUN treatment, the mean annual area-scaled emissions of N2O, NO, and NH3 were 11.48, 1.66, and 44.90 kg N ha−1 yr−1, respectively; and the mean annual yield-scaled emissions of N2O, NO, and NH3 were 181.86, 23.04, and 800.07 g N t−1 yr−1, respectively. On the mean annual area-scaled and yield-scaled, the N2O emissions were significantly (P < 0.05) lower by 40.1 % and 32.3 %, and the NO emissions were lower by 23.5 % and 28.5 %, but the NH3 volatilization were significantly (P < 0.05) higher by 31.7 % and 49.0 %, respectively, for the DUN treatment than FUN. Also compared with FUN, in DRUN treatment, the mean annual area-scaled N2O emissions, NO emissions and NH3 volatilization were significantly reduced by 46.7 %, 52.4 % and 22.0 %, respectively; and the mean annual yield-scaled N2O emissions, NO emissions and NH3 volatilization were significantly reduced by 40.7 %, 57.1 % and 24.5 %, respectively. Moreover, compared with FUN treatment, DUN and DRUN treatment both alleviated the global temperature potential (GTP) at 20- and 100-year timescales. Overall, compared with conventional water and fertilizer regimes, only changing irrigation mode reduced N2O and NO emissions but increased NH3 volatilization losses in greenhouse cucumber-tomato cultivation system; reducing N application while changing irrigation methods had significant emission reduction effects on N2O, NO and NH3 at both the area-scaled and yield-scaled, and which is an effective water and fertilizer managements for greenhouse cucumber-tomato cultivation system.

NITRIFICATION INHIBITORS IMPACT ON NITROUS OXIDE EMISSION AND AMMONIA VOLATILIZATION: A SUSTAINABLE MEASURE TOWARD A HYGIENIC ENVIRONMENT

Nitrogen (N) application to agricultural fields warrants curtailing nitrous oxide (N2O) emission and ammonia (NH3) volatilization for improved use efficiency with a less environmental footprint of N. As a means of mitigating N2O emissions, the efficacy of nitrification inhibitors (NIs) is well established but the efficacy of NIs in reducing NH3 volatilization is not well understood. The study investigated the efficacy of neem oil, pomegranate leaf extract, and dicyandiamide (DCD) coating on prilled urea for reducing N2O emissions and the trend of NH3 release using static air closed chamber technique in an incubation room. The results showed that all NIs reduced N2O flux in the order of 37%–42% by DCD urea, 19%–34% by neem oil coated urea (NOCUs), and 11%–16% by pomegranate leaf extract coated urea (PLECU). However, over uncoated urea, 43%–54% NH3 flux was increased by DCD, 10%–32% by NOCUs, and significantly the least increase (5%–14%) in NH3 cumulative flux was shown by PLECU. Dicyandiamide significantly reduced N2O flux more than all other treatments, and PLECU showed the least increase in NH3 emission when compared with other coated treatments. Hence, it is suggested that neem oil and pomegranate leaf extract could be used successfully not only for mitigating N2O emission, but also lessen environmental damages in association with managed N intense agriculture. Moreover, research focus on the increase in NH3 volatilization using DCD needs serious attention, especially in alkaline calcareous soils.

Assessing the representativity of NH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; measurements influenced by boundary-layer dynamics and the turbulent dispersion of a nearby emission source

Abstract. This study presents a fine-scale simulation approach to assess the representativity of ammonia (NH3) measurements in the proximity of an emission source. Close proximity to emission sources (&lt; 5 km) can introduce a bias in regionally representative measurements of the NH3 molar fraction and flux. Measurement sites should, therefore, be located a significant distance away from emission sources, but these requirements are poorly defined and can be difficult to meet in densely agricultural regions. This study presents a consistent criterion to assess the regional representativity of NH3 measurements in proximity to an emission source, calculating variables that quantify the NH3 plume dispersion using a series of numerical experiments at a fine resolution (20 m). Our fine-scale simulation framework with explicitly resolved turbulence enables us to distinguish between the background NH3 and the emission plume, including realistic representations of NH3 deposition and chemical gas–aerosol transformations. We introduce the concept of blending distance based on the calculation of turbulent fluctuations to systematically analyze the impact of the emission plume on simulated measurements, relative to this background NH3. We perform a suite of systematic numerical experiments for flat homogeneous grasslands, centered around the CESAR Observatory at Cabauw, to analyze the sensitivity of the blending distance, varying meteorological factors, emission/deposition and NH3 dependences. Considering these sensitivities, we find that NH3 measurements at this measurement site should be located at a minimum distance of 0.5–3.0 and 0.75–4.5 km from an emission source for NH3 molar fraction and flux measurements, respectively. The simulation framework presented here can easily be adapted to local conditions, and paves the way for future ammonia research to integrate simulations at high spatio-temporal resolutions with observations of NH3 concentrations and fluxes.

Hydrochar and microplastics disturb soil dissolved organic matter and prominently mitigate ammonia volatilization from wheat growing soil

The impacts of microplastics (MPs) pollution on the nitrogen (N) cycle in soil ecosystems have attracted worldwide attention. Meanwhile, dissolved organic matter (DOM)-rich hydrochar (HBC) is being tried as an emerging soil conditioner for gaseous N reduction load to the atmosphere. However, the influence of MPs and HBC on soil DOM and ammonia (NH3) volatilization in wheat soils remains poorly understood. In this study, a soil column experiment was established in wheat growing soils to determine the impacts of HBC, MPs (i.e., polyethylene and polyacrylonitrile), and the co-occurrence of HBC and MPs (HBC + MPs) on soil NH3 volatilization, physicochemical properties and wheat growth. In comparison with the control (CK, without MPs or HBC), HBC, MPs, and HBC + MPs reduced cumulative NH3 volatilization by 45.4%, 66.7–67.4% and 29.8–58.7%, respectively. Soil DOM fulvic acid-like and humic aid substance abundances of the other treatments were increased by 18.5–56.2% and 12.7–39.3%, respectively, compared to CK. Besides, soil NH3 fluxes were positively correlated to soil NH4+-N concentration at basal fertilization and soil NO3−-N concentration at first supplementary fertilization. HBC, MPs and HBC + MPs treatments promoted soil urease activity and plant N uptake by 6.5–24.2% and 31.9–74.3%, respectively, compared to CK. This study provides an insight into the variation of NH3 volatilization caused by anthropogenic carbon in wheat soil ecosystems.

Methane, Nitrous Oxide, and Ammonia Emissions on Dairy Farms in Spain with or without Bio-Activator Treatment

Intensive livestock farming substantially impacts the environment, especially farm and slurry management. Slurries are significant sources of greenhouse gases and ammonia. The present study was conducted in an intensive livestock production system in Galicia, Spain. The measurements were taken at six different farms in that region along with one control using common management practices in Galicia without the addition of a bio-activator. This study aimed to quantify GHGs and NH3 fluxes and their reductions during slurry treatment using a dynamic chamber through FTIR analysis and to examine the potential of usage of bio-activators for slurry management. In addition, gas concentrations were measured at the barns and compared with their slurry management and architectural volume to obtain influences on their management and the architectural volume of the barns. Additionally, the effects of using a bio-activator in the barns inside the facility areas were addressed. Moreover, qPCR analysis was conducted to understand the correlations between syncoms and methanogen populations when a bio-activator is added to the slurry with at least a 30% reduction in methanogenic populations. The outcomes suggest encouraging results for GHG reductions in the livestock sector, giving farmers future options for climate change mitigation among their standard practices.

Ammonia fluxes over an agricultural field in growing and fallow periods using relaxed eddy accumulation

Asia has a considerable risk of nitrogen deposition effects on ecosystems due to the high deposition amounts. Ammonia (NH3) emissions, primarily from agricultural production, have increased in East Asia, and the increasing contribution of NH3 deposition to the total nitrogen deposition is concerning. However, NH3 deposition in this region is uncertain due to lacking flux measurements and the complex bidirectional nature of NH3 atmosphere−surface exchange. To better understand the NH3 deposition in this region, we investigated NH3 fluxes over an agricultural field in Tokyo, Japan. Fluxes were determined using the relaxed eddy accumulation method incorporating the denuder/filter-pack sampling technique and continuously conducting daytime and nighttime measurements in soybean-growing (July 28–August 5, 2020) and fallow (March 4–March 10, 2021) periods. NH3 fluxes were mostly downward in both periods, indicating that the agricultural field was a sink for NH3 during the periods (mean flux = −0.091 μg m−2 s−1). The deposition velocities in the growing period were lower than in the fallow period possibly due to the smaller turbulence. The deposition velocities correlated positively with friction velocity, indicating that the turbulence is the dominant factor driving NH3 deposition. We compared inferred fluxes using a compensation point model with the measured fluxes. The model did not reproduce the observation results when using default inputs, but showed a mean NH3 emission of 0.054 μg m−2 s−1. Revised inputs considering on-site soil and foliage information improved the model's results; however, the inferred fluxes were still smaller than the measured fluxes (mean flux = −0.017 μg m−2 s−1). These results indicate that there were possibly other processes enhancing NH3 deposition in the agricultural field.

4D-Var Inversion of European NH3 Emissions Using CrIS NH3 Measurements and GEOS-Chem Adjoint With Bi-Directional and Uni-Directional Flux Schemes.

We conduct the first 4D‐Var inversion of NH3 accounting for NH3 bi‐directional flux, using CrIS satellite NH3 observations over Europe in 2016. We find posterior NH3 emissions peak more in springtime than prior emissions at continental to national scales, and annually they are generally smaller than the prior emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH3 emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close (<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the Netherlands, and Switzerland are generally 10%–20% smaller than when treating NH3 fluxes as uni‐directional emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly mean in‐situ observations, our posterior NH3 emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH3 and bulk NH x wet deposition throughout most of Europe, whereas evaluation against hourly measurements at a background site shows the bi‐directional scheme better captures observed diurnal variability of surface NH3. This contrast highlights the need for accurately simulating diurnal variability of NH3 in assimilation of sun‐synchronous observations and also the potential value of future geostationary satellite observations. Overall, our top‐down ammonia emissions can help to examine the effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping us understand the uncertainty in top‐down NH3 emissions estimates associated with treatment of NH3 surface exchange.

A significant diurnal pattern of ammonia dry deposition to a cropland is detected by an open-path quantum cascade laser-based eddy covariance instrument

Large uncertainties and challenges remain in the quantification of land-atmosphere exchanges of ammonia (NH3), mainly due to limitations in measurement techniques. In this study, the eddy covariance (EC) technique based on an open-path quantum cascade laser (QCL) spectrometer was used to measure the NH3 fluxes at a typical cropland in the North China Plain, one of the global NH3 hotspots. It is the first attempt of measuring both NH3 volatilization and deposition fluxes at an agricultural ecosystem using the open-path instrument-based EC technique. During the one-month experimental period, the NH3 fluxes were characterized by enhanced emissions after fertilizer application and persistent dry depositions after ploughing practice, with the flux magnitude varying from −0.293 to 7.29 (median: −0.042) mg N m−2 h−1. Nearly 93% of the half-hourly fluxes (absolute values) exceeded the mean instrumental flux detection limit of 9.6 μg N m−2 h−1. Nitrogen loss rate from the wheat field through NH3 volatilization was 0.57–0.71% relative to the applied nitrogen fertilizers. Ammonia deposition dominated the period after the ploughing practice, showing an average deposition rate of 0.014 kg N ha−1 d−1 with a significant diurnal pattern. Both the local emission sources and meteorological conditions influenced the variation of NH3 dry deposition. The deposition fluxes measured by the EC technique were nearly one third the results modelled by the widely used inferential model method, but the difference needs to be validated by more observations that includes different surface and meteorological conditions. This study suggests that this open-path QCL-based EC instrument is an effective tool for long-term and high-frequency measurements of ecosystem NH3 exchanges. It also enables cross-validation between different methods to improve the understanding in the atmospheric reactive nitrogen cycling.

Clinoptilolite (E567), a natural zeolite, inclusion in heavy-pig diets: Effect on the productive performance and gaseous emissions during fattening and manure storage

Intensive pig rearing systems produce several air pollutant emissions, mainly associated with housing and slurry storage. Dietary strategies based on the use of feed additives can effectively mitigate such impacts. This work has been aimed at evaluating the effectiveness of dietary zeolites in mitigating ammonia (NH3), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions from piggery and slurry storage on finishing pig farms. An experimental trial, in which three groups of approximately 500 pigs each were reared, has been carried out on a commercial pig farm. The three groups were fed the same diet, with the addition of 0 g/kg (Z0, control), 10 g/kg (Z1), and 20 g/kg (Z2) of micronized clinoptilolite (E567), respectively. The emissions from housing facilities and the live and slaughtering animal performances, were assessed. In addition, manure samples were collected during the rearing period to evaluate, at a laboratory scale, the NH3, CO2, CH4, and N2O emission potential during the subsequent slurry storage phase prior to land application. The results have shown that the addition of dietary zeolite can be considered a valid strategy to reduce gaseous emissions from pig houses without affecting animal performances or the system’s overall productivity. Treatment Z2 gave the best results and resulted in a 25% and 36% reduction of NH3 and CO2 equivalent emission fluxes, respectively, compared to those recorded for the control. The laboratory- scale experiment revealed no significant effect of dietary clinoptilolite inclusion on NH3 or the greenhouse gas emission potential during slurry storage.

Characteristics of NH4+ and NO3- Fluxes in Taxodium Roots under Different Nitrogen Treatments.

To understand the characteristics of net NH4+ and NO3− fluxes and their relation with net H+ fluxes in Taxodium, net fluxes of NH4+, NO3− and H+ were detected by a scanning ion-selective electrode technique under different forms of fixed nitrogen (N) and experimental conditions. The results showed that higher net NH4+ and NO3− fluxes occurred at 2.1–3.0 mm from the root apex in T. ascendens and T. distichum. Compared to NH4+ or NO3− alone, more stable net NH4+ and NO3− fluxes were found under NH4NO3 supply conditions, of which net NH4+ flux was promoted at least 1.71 times by NO3−, whereas net NO3− flux was reduced more than 81.66% by NH4+ in all plants, which indicated that NH4+ is preferred by Taxodium plants. T. ascendens and T. mucronatum had the largest net NH4+ and total N influxes when NH4+:NO3− was 3:1. 15N Atom% and activities of N assimilation enzymes were improved by single N fertilization in the roots of T. distichum. In most cases, net H+ fluxes were tightly correlated with net NH4+ and NO3− fluxes. Thus, both N forms and proportions could affect N uptake of Taxodium. These findings could provide useful guidance for N management for better productivity of Taxodium plants.

Various quantification methods for estimating ammonia volatilization from wheat-maize cropping system

Ammonia volatilization (AV) dominates the pathway of nitrogen (N) fertilizer losses in crops throughout the world. However, different methods are highly responsible for the different measurements of AV. The existing techniques were separated into static chamber methods (SCM), dynamic chamber methods (DCM), calibrated Dräger-tube method (DTM) and micrometeorological methods (MMM), which were analyzed by a meta-study of 595 observations from 33 published studies. An exponential relationship (P < 0.01) was found between AV and the N fertilizer applied to wheat and maize using all the methods. The amount of AV using SCM was the lowest. The AV monitored by DCM was 24.5%–55.0% (wheat) and 46.9%–65.0% (maize) lower than that for the DTM. Additionally, the AV measured by DTM did not differ significantly in the wheat season but was 58.9% lower (P < 0.05) in the maize season than that in the MMM. To reveal the influencing factors responsible that were for DCM and DTM, a field experiment was conducted during the period of Oct. 2016 to Oct. 2017. The study indicated that the AV was 15.8%–28.3% (wheat, P < 0.05) and 36.7%–44.2% (maize, P < 0.05) lower when monitored by the DCM than when estimated by DTM. The concentration of soil NH4+−N, air temperature, and wind speed positively correlated with the NH3 fluxes. In addition, there was a significant linear correlation (P < 0.01) between the AV measured by DCM and DTM when the wind speed was <1.5 m s−1. This study highlighted the fact that wind speed was the main factor that caused the large difference between DCM and DTM. Herein, DTM or MMM was first recommended, and DCM was accepted when wind speed was <1.5 m s−1 for quantitative estimates of AV. However, only a straight comparison between DCM and DTM under the same field experiment was done, the other comparisons only being based on similar fertilization and environmental conditions. Consequently, the differences between methods have to be treated carefully.

Investigation on the Mocvd-Growth and Optical Properties of Ingan/Gan Quantum Wells by Modulating Nh3 Flux

Investigation on the Mocvd-Growth and Optical Properties of Ingan/Gan Quantum Wells by Modulating Nh3 Flux

Determination of Water Quality Characteristics and Nutrient Exchange Flux at the Sediment—Water Interface of the Yitong River in Changchun City, China

In this paper, the characteristics of water pollution in Yitong River were analyzed by the comprehensive pollution index method. Combined with the pore water concentration gradient method and Fick’s first law, the release characteristics of nutrients at the sediment–water interface of Yitong River (Jilin Province, China) were studied. The results showed that the distribution trend of nitrogen and phosphorus content in the overlying and interstitial water of the Yitong River was the same, and the highest values appeared at the S3 and S5 points in the urban section. The water quality was mainly affected by nitrogen pollutants in domestic sewage. The evaluation results of the water quality comprehensive pollution index showed that the pollution degree of interstitial water in urban areas was much higher than that of the overlying water, and the endogenous nitrogen and phosphorus pollutants had the risk of diffusion to the overlying water. The exchange flux analysis of ammonia nitrogen (NH4+-N), total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP) in water showed that the diffusion flux of NH4+-N ranged from 0.03 to 6.52 mg·(m2·d)−1, and the sediment was the “source” of ammonia nitrogen pollutants. The range of TDN diffusion flux was −1.57 to 11.6 mg·(m2·d) −1, and the difference between points was large. The sediment was both the “source” and “sink” of nitrogen pollutants. The range of TDP diffusion flux was −0.05 to 0.22 mg·(m2·d) −1. Except for point S8, the TDP diffused from sediment into the water body. Among all the sampling points, the diffusion fluxes of NH4+-N, TDN, and TDP at the S3 point were the largest, the release rate of endogenous pollutants was the most rapid, and the pollution to the water quality was the most serious. The results are of great significance to the exchange flux of nutrients at the sediment–water interface of rivers and the prevention and control of water eutrophication. It also provides a reference for the study of nutrient exchange flux at the sediment–water interface of rivers and other surface water bodies worldwide.

Nitrogen and Phosphorus Diffusion Fluxes: Insight from High-Resolution Technology and Hydrodynamic Modeling

Nitrogen and phosphorus are key elements in controlling eutrophication in the aquatic system. Water and sediment samples were collected from Hongfeng Lake, a seasonally stratified reservoir in southwest China, in winter and summer. Diffusion fluxes of NH4+, NO3−, and labile P in summer using diffusive gradients in thin films technology were 3.4, −37.2, and 0.9 mg m−2 day−1, respectively, based on Fick’s first law. The diffusion flux of labile P was 2.05 mg m−2 day−1 in winter. The contributions fraction of the labile P diffusion flux from sediment to the overlying water were higher in winter than those in summer, because of the relatively lower external input, concentrations and higher diffusion fluxes in winter. After the external input decreased, all of the three diffusion fluxes were lower than the previous record. To understand the influence effect of hydrodynamics, environmental fluid dynamics code modeling was used to simulate the flow and temperature field in winter and summer. Modeling results showed that velocity in summer was higher than that in winter due to concentrated rainfall within the catchment. Moreover, the velocity and temperature in the euphotic zone were higher than that of the hypolimnion in summer. Less variation of velocity and temperature in vertical profile in winter than that in summer was observed, which may be attributable to the high specific heat capacity and the low heat conductivity of water. There was no significant correlation among velocity, hydrochemistry, nitrogen, and phosphorus concentrations. Hydrodynamics, solar radiation, and water depth affect the position of the thermocline, which was consequently to water temperature, hydrochemistry, dissolved nitrogen, and phosphorus concentration. Correlation analysis suggested that the higher bottom velocity and total bed shear may accelerate labile P, NH4+, and NO3− diffusion fluxes. These results provide evidence and suggestions for preventing and controlling reservoir eutrophication and water safety management.

Effects of Seasonal Thermal Stratification on Nitrogen Transformation and Diffusion at the Sediment-Water Interface in a Deep Canyon Artificial Reservoir of Wujiang River Basin

Watershed-scale nitrogen pollution in aquatic systems has become a worldwide concern due to its continuous impact on water quality deterioration, while the knowledge of key influencing factors dominating nitrogen transportation and transformation at the sediment-water interface (SWI) remains limited, especially in impounded rivers with an artificial reservoir. Hence, for a better understanding of the effects of thermal stratification on nitrogen transformation, we investigated the nitrogen species and isotopes in the sediment of a deep reservoir in Southwest China. Our results confirmed a significant difference in nitrogen species and isotopic composition in sediment between those in the thermal stratification period and non-thermal stratification period and indicated that the sediment biogeochemical process and transportation were clearly linked to the variations in water temperature and dissolved oxygen dominated by the process of thermal stratification. Significant seasonal differences in NH4+-N and NO3−-N in pore water of the upper layer (0–19 cm) revealed that nitrification exhausted NH4+ in the non-stratified period (NSP), and a potential low mineralization rate appeared when compared with those in the stratified period (SP). Seasonal differences in nitrogen species and isotope fractionation of δ15N-PON (about 2.3‰ in SP) in the upper layer sediment indicated a higher anaerobic mineralization rate of organic matter in SP than that in NSP. The diffusion fluxes of NH4+-N at SWI were 9.48 and 15.66 mg·m−2·d−1 in NSP and SP, respectively, and annual NH4+-N diffusion accounted for 21.8% of total storage in the reservoir. This study demonstrated that the nitrogen cycling processes, especially nitrification, denitrification, and mineralization, have been largely altered along with the changes in dissolved oxygen and that the diffusion of nitrogen species varied with the presence of the oxygen. The results contribute to the future study of watershed nitrogen budget evaluation and suggest that the endogenous nitrogen released from the sediment-water interface should be emphasized when aiming to fulfil water management policies in deep reservoirs.

Winter mixing accelerates decomposition of sedimentary organic carbon in seasonally hypoxic coastal seas

Deltaic systems are characterized by the highest sedimentation rates in the globe. Meanwhile, sedimentary organic matter therein can be efficiently decomposed so that these depositional systems may deviate substantially from the oft-quoted correlation between net sediment accumulation and preservation of organic matter. The exact mechanisms that cause such a deviation in any given case, however, remain poorly understood. In this study, we utilize a novel 224Ra/228Th disequilibrium method to examine sediment oxygen consumption and the release of diagenetic products of organic matter along the major mud wedge system in the inner shelf of the East China Sea. Our sampling campaign was carried out in two contrasting seasons: the summer when seasonal hypoxia was at its peak and physical conditions were relatively quiescent, and the winter when the water column was well oxygenated by intense winter mixing and underlying deposits were subjected to reworking. Unexpectedly, during summer 2017 when the seafloor received the annual maximum supply of organic matter, sediment oxygen consumption rates and benthic fluxes of NH4+ were relatively low, ranging from 6 to 59 mmol O2 m−2 d−1 and from 1.6 to 13 mmol N m−2 d−1, respectively. In contrast, during winter 2018 sediment oxygen consumption rates and benthic fluxes of NH4+ surged to 44–690 mmol O2 m−2 d−1 and 22–58 mmol N m−2 d−1, respectively. We have also identified an exponential relationship between amplification factor of sediment surface area and oxygen concentration in the bottom water. This relationship suggests that kinetic energy dissipation in the water column not only controlled air-sea exchange and seawater mixing, but also intensified sediment–water interaction. Importantly, sediment oxygen consumption rates (FO2) in the mud wedge can be empirically described using a modified form of Michaelis-Menten kinetics, suggesting that FO2and the associated benthic consumption and production of chemicals are controlled by both the transport and reaction processes. We have further demonstrated that a large portion of the organic matter deposited over the seafloor in summer is likely decomposed in winter. Overall, this study highlights intense winter mixing as an important mechanism that causes the highly efficient decomposition of sedimentary organic matter in coastal seas.

Influences of gallium and nitrogen partial pressure on step-bunching and step-meandering morphology of InGaN quantum barrier layer

As the templates for the growth of the quantum well layer, the morphology of the quantum barrier layer is crucial. Through a cross-comparison among samples grown under different conditions, the effect of TMGa flux, NH3 flux, indium content, and temperature on morphology of InGaN quantum barrier grown by Metal-Organic Chemical Vapor Deposition (MOCVD) is studied. It is found that TMGa and NH3 flux increments are counterbalancing each other in terms of adatom diffusion. Without balanced V/III fluxes, the surface roughens either by spiral hillocks or by strong step meandering. The morphology can be quite different even under a similar V/III ratio. LED samples using the V/III unbalanced quantum barriers, compared to the V-III balanced ones, present inferior optical properties.

Physiological Characteristics and Transcriptomic Dissection in Two Root Segments with Contrasting Net Fluxes of Ammonium and Nitrate of Poplar Under Low Nitrogen Availability

To investigate physiological and transcriptomic regulation mechanisms underlying the distinct net fluxes of NH4+ and NO3- in different root segments of Populus species under low nitrogen (N) conditions, we used saplings of Populus × canescens supplied with either 500 (normal N) or 50 (low N) μM NH4NO3. The net fluxes of NH4+ and NO3-, the concentrations of NH4+, amino acids and organic acids and the enzymatic activities of nitrite reductase (NiR) and glutamine synthetase (GS) in root segment II (SII, 35-70 mm to the apex) were lower than those in root segment I (SI, 0-35 mm to the apex). The net NH4+ influxes and the concentrations of organic acids were elevated, whereas the concentrations of NH4+ and NO3- and the activities of NiR and GS were reduced in SI and SII in response to low N. A number of genes were significantly differentially expressed in SII vs SI and in both segments grown under low vs normal N conditions, and these genes were mainly involved in the transport of NH4+ and NO3-, N metabolism and adenosine triphosphate synthesis. Moreover, the hub gene coexpression networks were dissected and correlated with N physiological processes in SI and SII under normal and low N conditions. These results suggest that the hub gene coexpression networks play pivotal roles in regulating N uptake and assimilation, amino acid metabolism and the levels of organic acids from the tricarboxylic acid cycle in the two root segments of poplars in acclimation to low N availability.