NH3 Flux Research Articles

Ammonia volatilization losses were small after mowing field peas in dry conditions

Engel, R., Jones, C. and Wallander, R. 2013. Ammonia volatilization losses were small after mowing field peas in dry conditions. Can. J. Soil Sci. 93: 239–242. Ammonia losses following termination of peas (Pisum sativum L.) by mowing were measured using a micrometeorological mass-balance approach. Field trials were conducted during two seasons in a semiarid climate. Plant N in the above ground biomass was 105 and 79 kg N ha−1 in 2011 and 2012, respectively. Vertical NH3 flux estimates were nominal (0.3 to 1.7 g N ha−1 h−1) in the 2 wk following mowing. Cumulative NH3 loss represented 0.3 to 0.5% of the N in plant biomass, indicating that N fertility was not diminished by NH3 volatilization in this dry climate.

Ammonia volatilization losses were small after mowing field peas in dry conditions

Engel, R., Jones, C. and Wallander, R. 2013. Ammonia volatilization losses were small after mowing field peas in dry conditions. Can. J. Soil Sci. 93: 239-242. Ammonia losses following termination of peas (Pisum sativum L.) by mowing were measured using a micrometeorological mass-balance approach. Field trials were conducted during two seasons in a semiarid climate. Plant N in the above ground biomass was 105 and 79 kg N ha-1 in 2011 and 2012, respectively. Vertical NH3 flux estimates were nominal (0.3 to 1.7 g N ha-1 h-1) in the 2 wk following mowing. Cumulative NH3 loss represented 0.3 to 0.5% of the N in plant biomass, indicating that N fertility was not diminished by NH3 volatilization in this dry climate.

Emissions of NO and NH3 from a Typical Vegetable-Land Soil after the Application of Chemical N Fertilizers in the Pearl River Delta

Cropland soil is an important source of atmospheric nitric oxide (NO) and ammonia (NH3). Chinese croplands are characterized by intensive management, but limited information is available with regard to NO emissions from croplands in China and NH3 emissions in south China. In this study, a mesocosm experiment was conducted to measure NO and NH3 emissions from a typical vegetable-land soil in the Pearl River Delta following the applications of 150 kg N ha−1 as urea, ammonium nitrate (AN) and ammonium bicarbonate (ABC), respectively. Over the sampling period after fertilization (72 days for NO and 39 days for NH3), mean NO fluxes (± standard error of three replicates) in the control and urea, AN and ABC fertilized mesocosms were 10.9±0.9, 73.1±2.9, 63.9±1.8 and 66.0±4.0 ng N m−2 s−1, respectively; mean NH3 fluxes were 8.9±0.2, 493.6±4.4, 144.8±0.1 and 684.7±8.4 ng N m−2 s−1, respectively. The fertilizer-induced NO emission factors for urea, AN and ABC were 2.6±0.1%, 2.2±0.1% and 2.3±0.2%, respectively. The fertilizer-induced NH3 emission factors for the three fertilizers were 10.9±0.2%, 3.1±0.1% and 15.2±0.4%, respectively. From the perspective of air quality protection, it would be better to increase the proportion of AN application due to its lower emission factors for both NO and NH3.

The role of highly sratified nutrient-rich small estuaries as a source of dissolved inorganic nitrogen to coastal seawater, the Qishon (SE Mediterranean) case

We studied the role of small, highly stratified, sulfate and nutrient enriched estuaries, as a source or sink of inorganic nitrogen species, using the Qishon estuary at the Mediterranean coast of Israel, as a case study. Measurements of nutrient concentrations, δ15N and δ18O of nitrate+nitrite, δ13CDIC and δ18OH2O were performed during 2008–2009 along the upper-fresh and lower-saline water masses, as well as sediment porewater depth-profiles. Such estuaries are characterized by relatively low removal flux of NO3- (via sedimentary denitrification) and enhanced (×3) upward flux of NH4+ (via sulfate reduction), attributed to the penetration of seawater of low NO3- and high dissolved oxygen and sulfate concentrations. The role of such small estuaries in releasing dissolved inorganic nitrogen, especially in sensitive oligotrophic areas as the Levantine basin and in the long-term, as a result of enhanced seawater penetration due to the expected sea level rise, has important environmental policy implications.

Processes of ammonia air–surface exchange in a fertilized <i>Zea mays</i> canopy

Abstract. Recent incorporation of coupled soil biogeochemical and bi-directional NH3 air–surface exchange algorithms into regional air quality models holds promise for further reducing uncertainty in estimates of NH3 emissions from fertilized soils. While this represents a significant advancement over previous approaches, the evaluation and improvement of such modeling systems for fertilized crops requires process-level field measurements over extended periods of time that capture the range of soil, vegetation, and atmospheric conditions that drive short-term (i.e., post-fertilization) and total growing season NH3 fluxes. This study examines the processes of NH3 air–surface exchange in a fertilized corn (Zea mays) canopy over the majority of a growing season to characterize soil emissions after fertilization and investigate soil–canopy interactions. Micrometeorological flux measurements above the canopy, measurements of soil, leaf apoplast and dew/guttation chemistry, and a combination of in-canopy measurements, inverse source/sink, and resistance modeling were employed. Over a period of approximately 10 weeks following fertilization, daily mean and median net canopy-scale fluxes yielded cumulative total N losses of 8.4% and 6.1%, respectively, of the 134 kg N ha−1 surface applied to the soil as urea ammonium nitrate (UAN). During the first month after fertilization, daily mean emission fluxes were positively correlated with soil temperature and soil volumetric water. Diurnally, maximum hourly average fluxes of ≈ 700 ng N m−2 s−1 occurred near mid-day, coincident with the daily maximum in friction velocity. Net emission was still observed 5 to 10 weeks after fertilization, although mid-day peak fluxes had declined to ≈ 125 ng N m−2 s−1. A key finding of the surface chemistry measurements was the observation of high pH (7.0–8.5) in leaf dew/guttation, which reduced the ability of the canopy to recapture soil emissions during wet periods. In-canopy measurements near peak leaf area index (LAI) indicated that the concentration of NH3 just above the soil surface was highly positively correlated with soil volumetric water, which likely reflects the influence of soil moisture on resistance to gaseous diffusion through the soil profile and hydrolysis of remaining urea. Inverse source/sink and resistance modeling indicated that the canopy recaptured ≈ 76% of soil emissions near peak LAI. Stomatal uptake may account for 12–34% of total uptake by foliage during the day compared to 66–88% deposited to the cuticle. Future process-level NH3 studies in fertilized cropping systems should focus on the temporal dynamics of net emission to the atmosphere from fertilization to peak LAI and improvement of soil and cuticular resistance parameterizations.

Fine Scale Modeling of Agricultural Air Quality over the Southeastern United States Using Two Air Quality Models. Part I. Application and Evaluation

Two air quality models, the U.S. EPA Community Multiscale Air Quality (CMAQ) model and ENVIRON’s Comprehensive Air Quality Model with extensions (CAMx), are evaluated for their applications in simulating ambient air quality, in particular, the fate and transport of agriculturally-emitted NH3 over an area in the southeastern U.S. in January and July 2002 using a fine-scale horizontal grid resolution of 4-km. Both models moderately overpredict maximum 1-hr and 8-hr ozone (O3) and fine particulate matter (PM2.5) in January, due likely to a weaker vertical mixing and insufficient dry and wet removal of PM2.5 species simulated by the models. They either slightly underpredict or overpredict O3 but significantly underpredict PM2.5 in July. The large underprediction in PM2.5 is due to an excess wet deposition removal of sulfate, an excess dry deposition removal of precursors, and an underestimation of emissions of primary PM and precursors of secondary PM and secondary organic aerosol concentrations. Both models show large biases in the simulated concentrations of several gases (e.g., CO in CAMx, NO in CMAQ, NO2 in both models in both months and NH3 by both models in July) and PM species (in particular, nitrate in both months and carbonaceous PM in July), visibility indices, and dry and wet deposition fluxes. They also show some inaccuracies in reproducing temporal variations of NH3, PM2.5, dry and wet deposition fluxes. Differences in model performance between the two models are attributed to different model treatments such as vertical mixing, wet and dry deposition, SOA formation, and PM size representations. These results indicate a need to improve accuracies of the emissions and measurements of NH3, the emissions of primary PM and precursors of secondary PM, as well as model treatments of vertical mixing and dry and wet removal processes.

Four-year monitoring of atmospheric ammonia using passive samplers at a single-crop rice paddy field in central Japan

The aim of this study was to clarify the atmosphere-paddy exchange of ammonia (NH3) at a single-crop rice paddy field in central Japan. Measurements of NH3 air concentrations were carried out using passive samplers at two heights above the ground surface (4.0 and 1.5 m) on a weekly basis over a four-year period. The mean NH3 air concentrations ± standard deviations at 4.0 m were 2.5 ± 0.8 and 3.0 ± 1.0 (20°C, 1013 hPa) in the cropping and fallow seasons, respectively. Sharp increases in the NH3 concentrations coincided with the application of poultry manure and post-harvest fieldburning activities; those values exceeded the 95% range of the weekly mean concentrations. The NH3 exchange fluxes were calculated using a gradient method in which the flux was expressed as the product of the difference in air concentrations multiplied by the diffusion velocity between the two heights. The mean NH3 exchange fluxes ± standard deviations were 0.016 ± 0.022 and 0.019 ± 0.115 μg N msof deposition in the cropping and fallow seasons, respectively; where the weekly mean flux was assumed to be zero if the difference in the air concentrations between the two heights was not significant (p < 0.05). In general, the paddy field acted as a net sink of atmospheric NH3, with an annual net deposition of 7–9 kg N hayr-. However, the application of poultry manure in February 2008 induced a strong NH3 emission of approximately 10 kg N haduring three weeks, which nearly counterbalanced the annual deposition for an entire year. Passive samplers are a convenient option for long-term monitoring, although the weekly mean exchange fluxes of NH3 had a systematic error; a case study showed a 66% overestimation for the weekly mean exchange flux.

Estimate of dry deposition fluxes of nutrients over the East China Sea: The implication of aerosol ammonium to non-sea-salt sulfate ratio to nutrient deposition of coastal oceans

Atmospheric deposition is one of important sources for nutrients to the surface ocean. Previous estimates for dry deposition fluxes of nutrients have mainly employed a single-mode particle model, and here we attempt to use size-segregated samples collected at Huaniao Island of the East China Sea (ECS) and dry deposition velocities derived from particle size and meteorological conditions of each sampling day. The dry deposition fluxes of NO3−, NH4+, and SP are estimated to be 6080, 10,006, and 26 μmol m−2 yr−1 respectively over the ECS using size-segregated samples. It is found that assuming a constant deposition velocity could overestimate the dry flux of NO3− by a factor of 6 while underestimate the flux of NH4+, which would alter the dry flux ratio of NH4+/NO3− from 1.6 to 0.1 with potential effects on the primary production and phytoplanktonic structure in the ECS. For coastal oceans influenced significantly by NH3 sources, aerosol NH4+ to non-sea-salt (nss-) SO42− ratio could be high and excess NH4+ may drag 34–54% of NO3− to fine mode aerosols, which may cause a large overestimation of dry flux of NO3− over the ocean by assuming its deposition velocity similar to that of coarse particle.

Estimation of Nitrogen and Phosphorus Release Rates at Sediment-Water Interface of Nansi Lake, China

At present, Nansi Lake restoration is maily focused on reducing extraneous pollution, however, it is unclear about the endogenous pollution. In this study, twelve intact sediment cores were collected from four sub-lakes (Nanyang Lake, Dushan Lake, Zhaoyang Lake and Weishan Lake) in Nansi Lake, and the fluxes of NH4+-N, PO43--P, NO3--N, TN and TP at sediment-water interface were calculated based on static incubation of sediment cores with a laboratory-scale benthic chamber. The incubation results showed fluxes of PO43--P, NO3--N, TN and TP in Nanyang Lake were the highest and as follows: 2.73, 7.55, 44.43 and 3.06 mg/m2.d, respectivly, and the flux of NH4+-N in Nanyang Lake, Dushan Lake and Zhaoyang Lake had little difference ranged from 8.99 to 10.19 mg/m2.d. This study indicated that during the sampling period sediment acted as a source of nitrogen as well as phosphorus to the overlying water body in Nansi Lake.

Characterization of aerosol emissions from wastewater aeration basins

The emission of particulate matter (PM10 and PM2.5) and ammonia (NH3) by aeration processes at wastewater treatment plants (WWTPs) with and without odor control units was examined. Local concentrations of PM2.5, PM10, and NH3 at the aeration basins were within urban ranges. Emission fluxes of NH3 and PM2.5 for a medium-sized WWTP were determined to be 136 g day−1 and 43 g day−1, respectively, which are not substantial emission fluxes for urban environments. Odor control treatment using a granulated activated carbon bed reduced aerosol and NH3 emissions substantially. Detection of sterols, in particular the fecal sterol campesterol, in the PM clearly demonstrates aerosolization of wastewater components in the aeration process. The presence of campesterol in PM2.5 at a remote fenceline location in a WWTP facility illustrates that wastewater components are aerosolized in the fine PM fraction and transported beyond the facilities. Implications: Wastewater treatment plants are potential emission sources of particulate matter and gases. This study characterized particulate matter emissions from aeration basins and quantified emissions fluxes of particulate matter and NH3. While fine and coarse particles as well as NH3 are being emitted, the overall emissions are small compared to other urban sources. However, fecal steroid presence in particles at the fence of a treatment plant demonstrates that wastewater material is getting aerosolized and transported beyond the facilities.

Impact of declining atmospheric deposition on forest soil solution chemistry in Flanders, Belgium

Throughout Europe and the USA, forest ecosystem functioning has been impacted by long-term excessive deposition of acidifying compounds. In this study, we report on trends in stand deposition and soil solution fluxes of inorganic nitrogen (N) and sulphur (S) compounds over a 17-year period (1994–2010) in five ICP Forests monitoring plots in Flanders, northern Belgium. Deposition was dominated by N, and primarily NH4+. Deposition of SO42− and NH4+ declined by 56–68% and 40–59% respectively. Deposition of NO−3 decreased by 17–30% in deciduous forest plots, but remained stable in coniferous forest plots. The decrease of N and S deposition was parallelled by a simultaneous decline in base cation (BC = Ca2+ + K+ + Mg2+) deposition, resulting in a 45–74% decrease of potentially acidifying deposition. Trends in soil solution fluxes of NH4+, NO3−, SO42− and BC mirrored declining depositions. Nitrate losses below the rooting zone were eminent in both coniferous forest plots and in one deciduous forest plot, while net SO42− release was observed in two deciduous forest plots. Critical limits for BC/Al ratio were exceeded at the three plots on sandy soils with lower cation exchange capacity and base saturation. Soil solution acid neutralizing capacity increased but remained negative, indicating that soil acidification continued, as the start of recovery was delayed by a simultaneous decrease of BC depositions and short-term soil buffering processes. Despite substantial reductions, current N deposition levels still exceed 4–8 times the critical load for safeguarding sensitive lichen species, and are still 22–69% above the critical load for maintaining ground vegetation diversity.

Effectiveness of urease inhibition on the abatement of ammonia, nitrous oxide and nitric oxide emissions in a non-irrigated Mediterranean barley field

Urea is considered the cheapest and most commonly used form of inorganic N fertilizer worldwide. However, its use is associated with emissions of ammonia (NH3), nitrous oxide (N2O) and nitric oxide (NO), which have both economic and environmental impact. Urease activity inhibitors have been proposed as a means to reduce NH3 emissions, although limited information exists about their effect on N2O and NO emissions. In this context, a field experiment was carried out with a barley crop (Hordeum vulgare L.) under Mediterranean conditions to test the effectiveness of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) on reducing these gaseous N losses from surface applied urea. Crop yield, soil mineral N concentrations, dissolved organic carbon (DOC), denitrification potential, NH3, N2O and NO fluxes were measured during the growing season. The inclusion of the inhibitor reduced NH3 emissions in the 30d following urea application by 58% and net N2O and NO emissions in the 95d following urea application by 86% and 88%, respectively. NBPT addition also increased grain yield by 5% and N uptake by 6%, although neither increase was statistically significant. Under the experimental conditions presented here, these results demonstrate the potential of the urease inhibitor NBPT in abating NH3, N2O and NO emissions from arable soils fertilized with urea, slowing urea hydrolysis and releasing lower concentrations of NH4+ to the upper soil layer.

Investigating the stomatal, cuticular and soil ammonia fluxes over a growing tritical crop under high acidic loads

Abstract. Ammonia concentration and fluxes were measured above a growing triticale field for two months during May and June 2010 at the NitroEurope crop site in Grignon (Fr-Gri) near Paris, France. The measurement campaign started 15 days following a 40 kg N ha−1 application of an ammonium nitrate solution. A new mini-wedd (Wet Effluent Denuder) flow injection analyser with three channels (ROSAA, RObust and Sensitive Ammonia Analyser) was used to measure NH3 fluxes using the aerodynamic gradient method. The measured ammonia concentrations varied from 0.01 to 39 μg NH3 m−3 and were largely influenced by advection from the nearby farm. The ammonia fluxes ranged from –560 to 220 ng NH3 m−2 s−1 and averaged –29 ng NH3 m−2 s−1. During some periods the large deposition fluxes could only be explained by a very small surface resistance, which may be partly due to the high concentrations of certain acid gases (HNO3 and SO2) observed in this suburban area. Ammonia emissions were also observed. The canopy compensation point Cc was around 1.5 μg NH3 m−3 on average. The canopy emission potential Γc (Cc normalised for the temperature response of the Henry equilibrium) decreased over the course of the measurement campaign from Γc = 2200 to Γc = 450, the latter value being close to the median stomatal emission potential (Γs) and lower than the median ground emission potential (Γg) for managed ecosystems reported in the literature. The temporal dynamics of the measured NH3 flux compared well with the Surfatm-NH3 model using fitted parameters. The subjectivity of the model fitting is discussed based on a sensitivity analysis.

Eddy covariance measurement of ammonia fluxes: Comparison of high frequency correction methodologies

Several methods are available for evaluating and correcting the underestimation of trace gas fluxes measured by eddy covariance (EC) method and due to the design and the setup of the employed equipment. Different frequency correction factors (CF) to apply to the measured EC ammonia (NH3) fluxes have been applied using the following approaches: (i) the spectral theoretical transfer function (CFTheor) with and without phase shift; (ii) the in situ ogive method (CFO); and (iii) the inductance correction method (CFL). The NH3 fluxes were measured in an experimental field located in south Italy, above a sorghum crop submitted to Mediterranean semi-arid climate and fertilized with urea. A fast analyser based on Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) coupled with a quantum cascade (QC) laser was used to measure NH3 concentration at high frequency. The results showed that correction of the values of NH3 EC fluxes over natural surfaces are necessary for taking into account the dumping of high frequency contribution due to the coupling of ammonia fast analyser by QC-TILDAS and sonic anemometer. In particular, the calculated flux losses ranged between −23% (inductance method, with the CF threshold selected to 2.5) and −43% (experimental-ogive method); while the two theoretical transfer function approaches gave comparable loss estimates, i.e. −30 and −31% for the methods with time lag and phase shift, respectively.

Ammonia Volatilization in a Semi—Arid Rangeland in Inner Mongolia

Abstract: Gaseous N losses via ammonia (NH3) volatilization were measured during the 2005 and 2006 growing seasons in a long—term (17 years) experiment with five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha -1). We aim to understand the seasonal variations in ammonia production, the potential carry—over effects of previous grazing and the underlying regulating processes involved. It was found that rates of NH3 volatilization varied seasonally, ranging from 0.88 to 3.52 g N ha-1d-1 during the measurement period, with higher values in spring and early summer and lower values in late summer and autumn. Soil pH value, NH-N concentration, moisture and bulk density exerted controls on NH3 volatilization over the two growing seasons. However, the constraining effect of bulk density on NH3 flux, driven by grazing, muted the supporting effect of above other three environmental factors. Even though no statistically significant effects of different grazing intensities on rates of NH3 volatilization ...

Measuring and modelling ammonia emissions from a regular pattern of cattle urine patches

Ammonia (NH3) emissions were measured during summer from a circular area of short pasture on a slightly acid stony sandy loam soil, treated with 156 cattle urine patches of realistic size and a nitrogen (N) content of 15gN each. Horizontal fluxes of NH3 were sampled at five heights in the centre of the treated circle. Three micrometeorological methods were used to derive the NH3 emission rate from these horizontal fluxes: the mass-budget (MB) method, the backward-Lagrangian stochastic (BLS) method, and the ZINST (height, z, independent of stability) method. Soil temperature was measured and soil samples were taken from within selected urine patches to provide pH, ammoniacal-N (NHx-N) and moisture contents as input parameters for a volatilisation model. The model describes a chain of three processes: the phase equilibrium between aqueous NHx in the soil solution and gaseous NH3 at the liquid-air interface (within the soil pores), the diffusion of gaseous NH3 in the soil layer, and the diffusion of gaseous NH3 in the atmospheric surface layer between ground and sampling height. The two diffusion processes are parameterised by resistances as functions of soil and wind flow parameters, respectively.With the MB method, 25.7 (±0.5) % of the applied urine-N was found to have volatilised as NH3 over the first 6 d. After that, the pH had dropped below 7, implying that NH3 emissions were probably insignificant. The ZINST method provided emission rate estimates that did not differ systematically from the MB method, with 50% larger random error. The BLS method underestimated the emission rate by 10 to 24% (dependent on measurement height), because of the requirement to specify mean wind speed and mean concentration as input parameters, instead of the measured mean horizontal flux.The volatilisation model was not tested in predictive mode because urine penetration depth could not be determined accurately enough. Instead, from the measured NH3 emissions and the model input parameters, the temporal evolution of the soil resistance was computed as well as an “effective source depth” associated with that. The median value of this depth was 2mm, with a typical error of a factor 2. It is concluded that the soil resistance model is physically realistic, but for accurate prediction of NH3 emission rates, the vertical distribution of NHx-N within the urine patches must be well-characterised. If this is provided, then soil sampling together with wind speed measurements will suffice to estimate NH3 emissions.

Quantitative low-energy electron diffraction analysis of the GaN[formula omitted] (1 × 1) reconstruction

The atomic structure of the GaN(0001¯) (1×1) reconstruction was investigated by quantitative low-energy electron diffraction (LEED). The GaN(0001¯) surface was annealed in an NH3 atmosphere and cooled down without any NH3 flux. LEED patterns with 6-fold symmetry were measured after annealing. The diffraction symmetry was explained by the presence of two domains on the GaN surface. LEED intensity–voltage (I–V) curves were acquired up to 500eV and calculated in the framework of the dynamical theory of electron scattering. Good agreement between the experimental and theoretical curves was achieved for the bare GaN(0001¯) (1×1) surface. Relaxation of the surface atomic layers decreased Pendry's R-factor to 0.21±0.03. The plane relaxations derived from the LEED analysis were found to be in qualitative agreement with ab initio calculations. By using previously suggested models with adlayers on the bare GaN(0001¯) (1×1) surface, much worse R-factors were obtained. It was concluded that the measured LEED I–V curves could be used to determine the bare GaN polarity.

Atmospheric Wet and Dry Depositions of Ions over an Urban Location in South-West India

Wet deposition (WD) and Dry deposition (DD) samples were collected during a period of 4 year (2006 to 2009), at four different sites representing different surroundings around Pune city in southwest India. The samples were collected on a daily basis for WD and weekly basis for DD. These samples were analyzed for major ionic components e.g. Cl−, NO3−, SO42−, Na+, K+, NH4+, Ca2+ and Mg2+. Both the WD and DD were alkaline (pH > 5.6) at all the four sites. The WD fluxes of all the ionic components were higher than the DD fluxes, except at the traffic junction Swargate, where majority of the species appeared with much higher DD fluxes than WD fluxes (68% for NO3−, 63% for Ca2+, 60% for Mg2+, 57% for K+). WD flux of NH4+ is higher (64–80%) than the DD flux at three locations and slightly lower (48%) at a high altitude location. In case of sea salt (Na and Cl), WD fluxes were higher (63–90%) than the DD fluxes at all the four locations. The dominant ion in DD was NO3− at Pashan (semi-urban) and Sinhagad (high altitude), Ca2+ at Swargate (traffic junction) and SO42− at Bhosari (industrial). The difference in deposition fluxes between the four sites was attributed to the effect of the local sources. Deposition velocities of SO42− and NH4+ were < 1 cm/s while Ca2+, Mg2+, Na+, K+, NO3−, and Cl− exhibited deposition velocities ≥ 1 cm/s. At one of the sites Pashan, where the earlier data is available; DD rates showed increase in all the chemical components, except for NH4+ after a period of about 2 decades.

The direct role of enzyme hydrolysis on ammonium regeneration rates in estuarine sediments

Benthic ammonium (NH4) regeneration in coastal marine sediments has a fundamental role in nitrogen (N) cycling and N supply to primary producers. Nitrogen regeneration involves benthic microbial mineralization of organic-N, which, in turn, depends on protein hydrolysis. These processes were examined in Aransas Bay (Texas, USA) sediments by monitoring NH4 evolution as a function of enzyme activity in controlled sediment slurries. Casein and tannic acid were added to evaluate the direct role of aminopeptidase on NH4 production and the effects of a polyphenolic enzyme inhibitor, respectively. Casein additions increased the NH4 concentration from 19 ± 0.3 to 737 ± 150 μM in 120 h, a final concentration 4.3-fold higher than that of control samples and 2.9-fold higher than that of samples with casein and tannic added together. Lower NH4 concentration in samples with tannic acid indicated that inhibiting aminopeptidase activity reduced NH4 production rates. The concentration of the regenerated NH4 related directly to aminopeptidase activity in controls (r = 0.86, p < 0.01), casein-enriched (r = 0.89, p < 0.01), and casein plus polyphenol treatments (r = 0.71, p < 0.01) over the first 72 h. The results demonstrate the importance of aminopeptidase in regenerating NH4 in sediments and provide insights about mechanisms of enzyme hydrolysis and NH4 fluxes in estuarine sediments.

Ammonia Volatilization from Urea and Mitigation by NBPT following Surface Application to Cold Soils

Urea is frequently surface applied to winter wheat (Triticum aestivum L.) during cold weather months (October–April) in the semiarid northern Great Plains. This study was conducted to quantify NH3 volatilization loss from surface-applied urea (100 kg N ha−1) and urea amended with N-(n-butyl) thiophosphoric triamide (NBPT) during this period. Ammonia emissions were quantified by a micrometeorological integrated horizontal flux method with samplers placed on a mast in the center of circular plots (20-m radius). Cumulative NH3 losses from urea varied but averaged 20.5% of applied N across 12 trials. The largest losses (30–44% of applied N) occurred after urea was applied to high-water-content soil surfaces, followed by a period of slow drying with little or no precipitation. Emissions occurred for a prolonged period often lasting >42 d. Periods (1–2 wk) of high NH3 flux (>30 g N ha−1 h−1) frequently occurred when mean daily soil temperatures (1-cm depth) were –2 to 5°C. In one trial, 24.3% of applied N was lost after urea was applied to a 140-mm snowpack. Ammonia losses were moderated by applying urea to dry soil surfaces. If precipitation events that followed were light (<8 mm), losses were reduced to 10 to 20% of applied N. If the events were heavy (>18 mm), then losses were <10%. Coating urea with NBPT (1 g kg−1) reduced cumulative NH3 losses by 66%. Volatilization protection lasted 2 to 3 wk on acidic soils, and >7 wk on an alkaline soil. This study demonstrated that significant NH3 losses from surface-applied urea could occur during cold weather months.