Evaluation of Jordanian zeolite tuff as a controlled slow-release fertilizer for NH4 +

 The exchange and release properties of the natural phillipsite tuff from the Aritain area in Jordan were evaluated by studying the exchange properties of this natural zeolite in the NH₄ ⁺–Na⁺ system. Exchange isotherms at 18, 35, and 50  °C showed that phillipsite exchanged NH₄ ⁺ preferably over Na⁺ at all temperatures. However, the selectivity coefficient for NH₄ ⁺ decreased with decreasing temperature. The release of NH₄ ⁺ from phillipsite saturated with ammonium sulfate took place in two stages characterized by different SO₄ ^2– : NH₄ ⁺ ratios. Aritain phillipsite from NE Jordan could be processed and used as NH₄ ⁺ slow-release fertilizers. The use of NH₄ ⁺-phillipsite tuff offers an option to the widely used soluble NH₄-fertilizers in agciculture to avoid environmental problems associated with nitrogen contamination of surface water and groundwater. Received: 19 December 1996 · Accepted: 13 May 1997     

Isotopic Approach to Determining the Fate of Ammonium Regenerated from Sediments in a Eutrophic Sub-estuary of Waquoit Bay,MA

We measured fluxes of NH₄⁺ and NO₃⁻ and δ¹⁵N of NH₄⁺, sediment, and porewater NH₄⁺ from incubated sediment cores along a nitrate gradient and in different seasons from Childs River, MA. NH₄⁺ flux was low at the downstream site with the lowest concentration of organic matter (high salinity) but otherwise did not differ along the estuary. The δ¹⁵N of regenerated NH₄⁺ ranged from +6.1‰ to +15.3‰ but did not vary significantly with season or salinity; the mean for the entire estuary was +10.4 ± 0.5‰. Based on differences between the δ¹⁵N of regenerated NH₄⁺ and sediment, and expected isotopic fractionation due to remineralization, we concluded that nitrification occurred after remineralization of NH₄⁺. Differences between the δ¹⁵N of regenerated NH₄⁺ and the δ¹⁵N of porewater NH₄⁺ provided further evidence of nitrification. We estimated that 11% to 48% of remineralized NH₄⁺ underwent coupled nitrification–denitrification before release into the water column. In spite of losses to denitrification, NH₄⁺ flux released 1.4 mol N m⁻² year⁻¹ to the water column and could provide 42% of phytoplankton nitrogen requirements.     

The Effects of Varying Salinity on Ammonium Exchange in Estuarine Sediments of the Parker River,Massachusetts

We examined the effects of seasonal salinity changes on sediment ammonium (NH₄ ⁺) adsorption and exchange across the sediment–water interface in the Parker River Estuary, by means of seasonal field sampling, laboratory adsorption experiments, and modeling. The fraction of dissolved NH₄ ⁺ relative to adsorbed NH₄ ⁺ in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH₄ ⁺ adsorption by reducing the exchangeable pool from 69% to 14% of the total NH₄ ⁺ in the upper estuary sediments that experience large (0–20) seasonal salinity shifts. NH₄ ⁺ dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under 10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic numerical model for pore water chloride and NH₄ ⁺ diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH₄ ⁺ profiles that fit measured profiles very well and described a seasonal pattern of NH₄ ⁺ flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH₄ ⁺ release from the sediments. Salinity-mediated desorption and fluxes of NH₄ ⁺ from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore be important in supporting estuarine productivity when watershed inputs of N are low.     

Investigation of ammonia removal from polluted waters by Clinoptilolite zeolite

Ammonia nitrogen compounds in the wastewaters and effluents have harmful effects on water resources. Ion exchange with zeolites is a separation process for ammonia removal from effluents. The objective of this research was to study the efficiency of an ammonia removal and the factor affecting to this process. The Clinoptilolite was obtained from Semnan mines at the north part of Iran. The samples were grounded and sieved based on the U.S. standard mesh number 20, 30, 40 and conditioned by ammonia sulfate and sodium chloride solutions. The characteristics of samples for ammonia removal and the selectivity sequence for adsorbing interfering cations were then determined. Results shown that the average ion exchange capacity of zeolite in batch and continuous systems were 6.65–16 and 16.31–19.5 mg NH₄ ⁺ /g zeolite weight, respectively. In study on the zeolite for selective cations showed the ranking of K⁺, NH₄ ⁺, Na⁺, Ca²⁺ and Mg²⁺ respectively. Results indicated that high level of regeneration (95–98%) might be achieved with NaCl solution. Based on the results, Clinoptilolite may be effective applied in wastewater treatment, both from technical and economical aspects.     

Denitrification and the stoichiometry of nutrient regeneration in Waquoit Bay, Massachusetts

To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N₂ fluxes to the stoichiometry of nutrient and O₂ fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO₄ ³⁻ (0.2 mol P m⁻² yr⁻¹) and NH₄ ⁺ (1.6 mol N m⁻² yr⁻¹) flux and the Redfield N∶P ratio of 16 suggested an annual deficit of 1.5 mol N m⁻² yr⁻¹. Denitrification predicted from O₂∶NH₄ ⁺ flux ratios and measured as N₂ flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m⁻² yr⁻¹). Denitrification accounted for low N∶P ratios of benthic flux and removed 32–37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N₂ flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N₂ flux and did not alter denitrification efficiency. Increased N₂ flux co-varied with O₂ and NH₄ ⁺ fluxes. N₂ flux (261±60 μmol m⁻² h⁻¹) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458±70 μmol m⁻² h⁻¹), and O₂ uptake rate was higher in chambers deployed on macroalgal beds (14.6±2.2 mmol m⁻² h⁻¹) than on bare sediments (9.6±1.5 mmol m⁻² h⁻¹). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.     

Inorganic nitrogen dynamics in intertidal rocky biofilms and sediments of the Douro River estuary (Portugal)

In this study rates of oxygen, ammonium (NH₄ ⁺), nitrate (NO₃ ⁻), nitrite (NO₂ ⁻), and nitrous oxide (N₂O) fluxes, nitrogen (N) fixation, nitrification, and denitrification were compared between two intertidal sites for which there is an abundant global literature, muddy and sandy sediments, and two sites representing the rocky intertidal zone where biogeochemical processes have scarcely been investigated. In almost all sites oxygen production rates greatly exceeded oxygen consumption rates. During daylight, NH₄ ⁺ and NO₃ ⁻ uptake rates together with ammonification could supply the different N requirements of the primary producer communities at all four sites; N assimilation by benthic or epilithic primary producers was the major process of dissolved inorganic nitrogen (DIN) removal; N fixation, nitrification, and denitrification were minor processes in the overall light DIN cycle. At night, distinct DIN cycling processes took place in the four environments, denitrification rates ranged from 9 ± 2 to 360 ± 30 μmol N₂ m⁻² h⁻¹, accounting for 10–48% of the water column NO₃ ⁻ uptake; nitrification rates varied from 0 to 1712 ± 666 μmol NH₄ ⁺ m⁻² h⁻¹. A conceptual model of N cycle dynamics showed major differences between intertidal sediment and rocky sites in terms of the mean rates of DIN net fluxes and the processes involved, with rocky biofilm showing generally higher fluxes. Of particular significance, the intertidal rocky biofilms released 10 times the amount of N₂O produced in intertidal sediments (up to 17 ± 6 μmol N₂O m⁻² h⁻¹), representing the highest N₂O release rates ever recorded for marine systems. The biogeochemical contributions of intertidal rocky substrata to estuarine and coastal processes warrant future detailed investigation.     

Mechanism of efficient nutrient removal and microbial analysis of a combined anaerobic baffled reactor–membrane bioreactor process

A combined ABR–MBR process consisting of an anaerobic baffled reactor (ABR) combined with an aerobic membrane bioreactor (MBR) treating municipal wastewater was investigated at controlled pH range 6.5–8.5 and at constant temperature 25 ± 1 °C. Total nitrogen (TN), ammonia (NH₄ ⁺–N), total phosphorus (TP), and chemical oxygen demand (COD) removal performances were evaluated by analyzing the mechanism for efficient nutrient removal. The results showed that the average removal rates of COD, NH₄ ⁺–N, TN, and TP reached 93, 99, 79, and 92 %, respectively, corresponding with the COD, NH₄ ⁺–N, TN, and TP effluent of 24 (18–31), 0.4 (0–0.8), 10.6 (8.8–12.9), and 0.31 (0.1–0.5) mg/L under the operational condition of hydraulic retention time (HRT) 7.5 h, recycle ratio 200 %, and dissolved oxygen 3 mg/L. The MBR enhanced NH₄ ⁺–N, TN, and TP removal rates of 13, 10, and 18 %, respectively, and the membrane retention reduced TP 0.17 mg/L. The process was able to maintain a stable performance with high-quality effluent. Analysis of the results by fluorescence in situ hybridization showed that the abundance of ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, and phosphorus accumulating organisms as percentages of all bacteria in each compartment was stable. The enriched microorganisms in the system appear to be the main drivers of the process efficient for nutrient removal.     

Atmospheric deposition of nitrogen: Implications for nutrient over-enrichment of coastal waters

Atmospheric deposition of nitrogen (AD-N) is a significant source of nitrogen enrichment to nitrogen (N)-limited estuarine and coastal waters downwind of anthropogenic emissions. Along the eastern U.S. coast and eastern Gulf of Mexico, AD-N currently accounts for 10% to over 40% of new N loading to estuaries. Extension of the regional acid deposition model (RADM) to coastal shelf waters indicates that 11, 5.6, and 5.6 kg N ha⁻¹ may be deposited on the continental shelf areas of the northeastern U.S. coast, southeast U.S. coast, and eastern Gulf of Mexico, respectively. AD-N approximates or exceeds riverine N inputs in many coastal regions. From a spatial perspective, AD-N is a unique source of N enrichment to estuarine and coastal waters because, for a receiving water body, the airshed may exceed the watershed by 10–20 fold. AD-N may originate far outside of the currently managed watersheds. AD-N may increase in importance as a new N source by affecting waters downstream of the oligohaline and mesohaline estuarine nutrient filters where large amounts of terrestrially-supplied N are assimilated and denitrified. Regionally and globally, N deposition associated with urbanization (NO_x, peroxyacetyl nitrate, or PAN) and agricultural expansion (NH₄ ⁺ and possibly organic N) has increased in coastal airsheds. Recent growth and intensification of animal (poultry, swine, cattle) operations in the midwest and mid-Atlantic regions have led to increasing amounts of NH₄ ⁺ emission and deposition, according to a three decadal analysis of the National Acid Deposition Program network. In western Europe, where livestock operations have dominated agricultural production for the better part of this century, NH₄ ⁺ is the most abundant form of AD-N. AD-N deposition in the U.S. is still dominated by oxides of N (NO_x) emitted from fossil fuel combustion; annual NH₄ ⁺ deposition is increasing, and in some regions is approaching total NO₃ ⁻ deposition. In receiving estuarine and coastal waters, phytoplankton community structural and functional changes, associated water quality, and trophic and biogeochemical alterations (i.e, algal blooms, hypoxia, food web, and fisheries habitat disruption) are frequent consequences of N-driven eutrophication. Increases in and changing proportions of various new N sources regulate phytoplankton competitive interactions, dominance, and successional patterns. These quantitative and qualitative aspects of AD-N and other atmospheric nutrient sources (e.g., iron) may promote biotic changes now apparent in estuarine and coastal waters, including the proliferation of harmful algal blooms, with cascading impacts on water quality and fisheries.     

Study on Nitrogen Dynamics at the Sediment–Water Interface of Dongting Lake,China

Eutrophication of lakes and reservoirs has become a worldwide environmental problem, and nitrogen (N) has been recognized as one of the key factors responsible for eutrophication. Nitrogen adsorbed on sediments may be released via chemical and biological processes under changing environmental conditions. Spatial distributions of concentrations of ammonia nitrogen (NH₄ ⁺–N), nitrate nitrogen (NO₃ ^?–N) and total nitrogen (TN) were investigated in sediments and overlying water of Dongting Lake, the second largest freshwater lake in China. The concentration of TN in the sediments exhibited strong spatial variation with relatively high values in the eastern part and relatively low values in the southern part of the lake. The TN concentration in the water of different regions of Dongting Lake was affected by the internal load of sediment N. The vertical distribution of TN in sediment cores showed a decreasing trend with an increase in depth. Concentrations of NH₄ ⁺–N in the sediment cores decreased with the depth increase until 6–8 cm and then increased slowly. However, concentrations of NO₃ ^?–N in the sediment cores showed an opposite trend from those of NH₄ ⁺–N. A kinetic release experiment of NH₄ ⁺–N showed that the maximum release rate occurred in the first 5 min and the amount of NH₄ ⁺–N release reached 77.93–86.34 % of the total amount in 0–10 min. The release of NH₄ ⁺–N in the surface sediments of Dongting Lake fits a first-order kinetics function.     

Nitrogen Sources and Rates of Phytoplankton Uptake in Different Regions of Hong Kong Waters in Summer

Phytoplankton uptake rates of ammonium (NH₄ ⁺), nitrate (NO₃ ⁻), and urea were measured at various depths (light levels) in Hong Kong waters during the summer of 2008 using ¹⁵N tracer techniques in order to determine which form of nitrogen (N) supported algal growth. Four regions were sampled, two differentially impacted by Pearl River discharge, one impacted by Hong Kong sewage discharge, and a site beyond these influences. Spatial differences in nutrient concentrations, ratios, and phytoplankton biomass were large. Dissolved nutrient ratios suggested phosphorus (P) limitation throughout the region, largely driven by high N loading from the Pearl River in summer. NH₄ ⁺ and urea made up generally ≥50% of the total N taken up and the f ratio averaged 0.26. Even at the river-impacted site where concentrations of NO₃ ⁻ were >20 μM N, NH₄ ⁺ comprised >60% of the total N uptake. Inhibition experiments demonstrated that NO₃ ⁻ uptake rates were reduced by 40% when NH₄ ⁺ was >5 μM N. The relationship between the total specific uptake rates of N (sum of all measured substrates, V, per hour) and the chlorophyll a-specific rates (micromolars of N per microgram of Chl a per hour) varied spatially with phytoplankton biomass. Highest uptake rates and biomass were observed in southern waters, suggesting that P limitation and other factors (i.e., flushing rate) controlled production inshore and that the unincorporated N (mainly NO₃ ⁻) was transported offshore. These results suggest that, at the beginning of summer, inshore algal blooms are fueled primarily by NH₄ ⁺ and urea, rather than NO₃ ⁻, from the Pearl River discharge. When NH₄ ⁺ and urea are depleted, then NO₃ ⁻ is taken up and can increase the magnitude of the bloom.     

Inorganic and Organic Nitrogen Uptake by Phytoplankton and Bacteria in Hong Kong Waters

Measurements of uptake rates of inorganic (NO₃⁻ and NH₄⁺) and organic (urea, glycine, and glutamic acid) N, and indirect estimates of total N uptake by bacteria, were made in four contrasting environments in sub-tropical Hong Kong waters in summer of 2008. In addition, the effects of several days of rain on N uptake rates were studied in eastern waters. Although ambient NO₃⁻ was the dominant form of N in Hong Kong waters, the dominant N form taken up by phytoplankton was usually NH₄⁺ and organic N, including urea and amino acids, rather than NO₃⁻. Hence, because of the low NO₃⁻ uptake, there was a long turnover time for NO₃⁻ (100 days), and most of the NO₃⁻ was apparently transported offshore into deeper shelf waters. In eastern waters where NH₄⁺ was undetectable, NO₃⁻ uptake rates were positively correlated with phytoplankton cell size. In contrast, potential rates of glutamic acid uptake were negatively correlated with phytoplankton size. N uptake rates in the smaller size fraction (0.7–2.8 μm) were less affected by the rain event, and smaller phytoplankton appeared to outcompete larger cells after several days of rain. The surface (PN)-specific N uptake rates in the >8-μm fraction decreased from 0.02 to 0.0001 h⁻¹, while the smaller fraction only exhibited a one- to threefold decrease after the rainfall. In contrast, bacterial production and N uptake were not affected by the rain event, and bacteria N uptake accounted for 10–60% of the total N uptake by phytoplankton.     

C-N elemental and isotopic investigation in agricultural soils: Insights on the effects of zeolitite amendments

In this paper we present an elemental and isotopic investigation of carbon and nitrogen in the soil-plant system. Plants grown in an unamended soil were compared to plants grown in a soil amended with natural and NH₄⁺-enriched zeolitites. The aim was to verify that zeolitites at natural state increase the chemical fertilization efficiency and the nitrogen transfer from NH₄⁺-enriched zeolitites to plants. Results showed that plants grown on plots amended with zeolitites have generally a δ¹⁵N approaching that of chemical fertilizers, suggesting an enhanced nitrogen uptake from this specific N source with respect to the unamended plot. The δ¹⁵N of plants grown on NH₄⁺-enriched zeolitites was strongly influenced by pig-slurry δ¹⁵N (employed for the enrichment process), confirming the nitrogen transfer from zeolitites to plants. The different agricultural practices are also reflected in the plant physiology as recorded by the carbon discrimination factor, which generally increases in plots amended with natural zeolitites, indicating better water/nutrient conditions.     

IR spectroscopic characterization of NH4-analcime

 The IR spectrum of ammonium-exchanged natural analcime (basalt, Nidym River, Siberian platform) exhibits several features that suggest a lowered symmetry for the NH₄ ⁺ ion and that this is influenced by hydrogen bonding within the framework. These features are: the pronounced splitting into three components and high-frequency shift of the ν₄-bending mode; appearance of the ν₁-stretching mode which is predicted to become IR-active when the ideal T _d symmetry of NH₄ ⁺ ion is violated, and the low-frequency shift of the ν₁- and ν₃-stretching modes. The absence of absorption lines in the 1800–2400-cm⁻¹ region indicates that hydrogen bonding between the framework and the NH⁺ ₄ ion is very weak. The three-component splitting of the ν₄-bending mode indicates that the symmetry of NH⁺ ₄ ion is lower than C_3v. This implies that at least two N–H bonds of the NH⁺ ₄ ion are disturbed by hydrogen bonding. Computer analysis of the normal vibrations of the NH⁺ ₄ molecule for different symmetry types (using harmonic approximation) indicates that the best fit to the observed ν₄ triplet frequencies for C₁ symmetry implies a deviation of the valent angle ∠H–N–H from ideal T _d symmetry of around ±2.5°. The factors governing the behaviour of the NH⁺ ₄ ion in the analcime structure are discussed. The geometry of the nearest environment of the NH⁺ ₄ ion in the analcime structure is analyzed with respect to the present IR data. Received: 2 January 2002 / Accepted: 26 June 2002 Acknowledgements We thank Dr. D. Harlov and an anonymous reviewer for their helpful comments, as well as Dr. I.A. Belitsky and Dr. S.V. Goryainov for discussion of the material. This work is supported by RFBR grants 01-05-65414, 00-05-65305 and 02-05-65313.     

Characterization and environmental application of a Chilean natural zeolite

The use of natural zeolites for environmental applications is gaining new research interests mainly due to their properties and significant worldwide occurrence. The present work describes the characterization of a natural Chilean zeolite and the results as adsorbent for ammonia from aqueous solutions. The zeolitic-rich tuff sample, mainly composed of clinoptilolite and mordenite, consisted of 13 μm mean volumetric particle diameter, 55 m² g⁻¹ (methylene blue adsorption) and 177 m² g⁻¹ (nitrogen adsorption) of specific surface area. Particles were negatively charged over a broad pH range (with or without ammonia) and 1.02 meq NH₄⁺ g⁻¹ cation-exchange capacity. The ammonia removal appears to proceed through ion-exchange and rapid kinetics (rate constant of 0.3 min⁻¹) at neutral pH value, with removal capacities up to 0.68 meq NH₄⁺ g⁻¹. The Langmuir isotherm model provided excellent equilibrium data fitting (R²=0.97). Results indicate a significant potential for the Chilean natural zeolite as an adsorbent/ion-exchange material for wastewater treatment and water reuse applications.     

Effects of salinity on NH4 + adsorption capacity, nitrification, and denitrification in Danish estuarine sediments

The regulatory effect of salinity on nitrogen dynamics in estuarine sediments was investigated in the Randers Fjord estuary, Denmark, using sediment slurries and intact sediment cores and applying ¹⁵N-isotope techniques. Sediment was sampled at three representative stations varying in salinity, and all experiments were run at 0‰, 10‰, 20‰, and 30‰. The sediment NH₄ ⁺ adsorption capacity decreased markedly at all stations when salinity was increased from 0‰ to 10‰; further increase showed little effect. In situ nitrification and denitrification also decreased with increasing salinities, with the most pronounced reduction of approximately 50% being observed when the salinity was raised from 0‰ to 10‰. The salinity-induced reduction in NH₄ ⁺ adsorption capacity and stimulation of NH₄ ⁺ efflux has previously been argued to cause a reduction in nitrification activity since the nitrifying bacteria become limited by NH₄ ⁺ availability at higher salinities. However, using a potential nitrification assay where NH₄ ⁺ was added in excess, it was demonstrated that potential nitrification activity also decreased with increasing salinity, indicating that the inhibitory salinity effect may also be a physiological effect on the microorganisms. This hypothesis was supported by the finding that denitrification based on NO₃ ⁻ from the overlying water (D_w), which is independent of the nitrification process, and hence NH₄ ⁺ availability, also decreased with increasing salinity. We conclude that changes in salinity have a significant effect on nitrogen dynamics in estuarine sediments, which must be considered when nitrogen transformations are measured and evaluated.     

Interstitial water and hydrochemistry of a mangrove forest and adjoining water system, south west coast of India

 Eh, pH, salinity, total alkalinity, dissolved O₂, NO₂ ^–, PO₄ ^–3, SiO₂ and NH₄ ⁺ of waters from a mangrove forest, an estuary and a creek connecting the mangrove forest and the estuary have been measured. Further, the chemistry of interstitial waters of surficial and core sediments from the mangrove forest have been analyzed for the above parameters, except dissolved oxygen. To understand the flux of nutrients from the mangrove forest to the adjoining estuary, creek waters were monitored during tidal phases. PO₄ ^–3, SiO₂ and NH₄ ⁺ were found to be at elevated levels in mangrove waters whereas NO₂ ^– shows no variation compared to the estuary. Dissolved O₂ is low in mangrove waters. PO₄ ^–3, NH₄ ⁺ and SiO₂ are several times higher in interstitial waters than in overlying waters. Several fold enrichment of PO₄ ^–3, NH₄ ⁺ and, to some extent, SiO₂ were measured in creek waters during ebbing relative to flooding, indicating that mangroves act as a perennial source for the above nutrients. Received: 26 May 1998 · Accepted: 21 July 1998     

Distribution and isotope composition of nitrogen in Bay of Quinte (Lake Ontario) sediments

The concentrations and isotopic compositions of the various forms of nitrogen in silty clay sediments from the Bay of Quinte (Lake Ontario) have been determined. The total organic-N content is high throughout the sediment profiles and generally decreases with depth. On the contrary, exchangeable NH⁺₄-N concentration is quite low and tends to increase with depth in two out of three sediment cores examined. The concentration of non-exchangeable NH⁺₄-N and the 6 N HCl hydrolyzable NH⁺₄-N are relatively constant with depth. Among the N fractions analyzed, the exchangeable NH⁺₄-is most enriched in ¹⁵N. In most cases, the δ ¹⁵ N values of the N fractions remain relatively constant with sediment depth. There is no apparent correlation of δ ¹⁵ N values with the N concentration for any of the individual N fractions. The observed ranges in the δ ¹⁵ N values are: exchangeable NH⁺₄, + 5–+10‰; 6 N HCl hydrolyzable total N and 6 N HCl hydrolyzable NH⁺₄-N, + 3.5–+5.5‰.     

Transformation of ammonium nitrogen and response characteristics of nitrifying functional genes in tannery sludge contaminated soil

High concentrations of ammonium nitrogen released from tannery sludge during storage in open air may cause nitrogen pollution to soil and groundwater. To study the transformation mechanism of NH₄⁺-N by nitrifying functional bacteria in tannery sludge contaminated soils, a series of contaminated soil culture experiments were conducted in this study. The contents of ammonium nitrogen (as NH₄⁺-N), nitrite nitrogen (as NO₂^?-N) and nitrate nitrogen (as NO₃^?-N) were analyzed during the culture period under different conditions of pollution load, soil particle and redox environment. Sigmodial equation was used to interpret the change of NO₃^?-N with time in contaminated soils. The abundance variations of nitrifying functional genes (amoA and nxrA) were also detected using the real-time quantitative fluorescence PCR method. The results show that the nitrification of NH₄⁺-N was aggravated in the contaminated silt soil and fine sand under the condition of lower pollution load, finer particle size and more oxidizing environment. The sigmodial equation well fitted the dynamic accumulation curve of the NO₃^?-N content in the tannery sludge contaminated soils. The Cr(III) content increased with increasing pollution load, which inhibited the reproduction and activity of nitrifying bacteria in the soils, especially in coarse-grained soil. The accumulation of NO₂^?-N contents became more obvious with the increase of pollution load in the fine sand, and only 41.5% of the NH₄⁺-N was transformed to NO₃^?-N. The redox environment was the main factor affecting nitrification process in the soil. Compared to the aerobic soil environment, the transformation of NH₄⁺-N was significantly inhibited under anaerobic incubation condition, and the NO₃^?-N contents decreased by 37.2%, 61.9% and 91.9% under low, medium and high pollution loads, respectively. Nitrification was stronger in the silt soil since its copy number of amoA and nxrA genes was two times larger than that of fine sand. Moreover, the copy numbers of amoA and nxrA genes in the silt soil under the aerobic environment were 2.7 times and 2.2 times larger than those in the anaerobic environment. The abundance changes of the amoA and nxrA functional genes have a positive correlation with the nitrification intensity in the tannery sludge-contaminated soil.     

Ammonium excretion by benthic invertebrates and sediment-water nitrogen flux in the Gulf of Mexico near the Mississippi River outflow

Benthic macroinvertebrate biomass and ammonium excretion rates were measured at four stations in the Gulf of Mexico near the Mississippi River mouth. Calculated areal excretion rates were then compared to sediment-water nitrogen fluxes measured in benthic bottom lander chambers at similar stations to estimate the potential importance of macroinvertebrate excretion to sediment nitrogen mineralization. Excretion rates for individual crustaceans (amphipods and decapods) was 2–21 nmoles NH₄ ⁺ (mg dry weight)^?1 h^?1. The mean excretion rates for the polychaetes, Paraprionaspio pinnata [6–12 nmoles NH₄ ⁺ (mg dry weight)^?1h^?1] and Magelona sp. [27–53 nmoles NH₄ ⁺ (mg dry weight)^?1h^?1], were comparable or higher than previous measurements for similar size benthic or pelagic invertebrates incubated at the same temperature (22±1°C). Although the relatively high rates of excretion by these selective feeders may have been partially caused by experimental handling effects (e.g., removal from sediment substrates), they probably reflected the availability of nitrogen-rich food supplies in the Mississippi River plume. When the measured weight-specific rates were extrapolated to total areal biomass, areal macroinvertebrate excretion estimates ranged from 7 μmole NH₄ ⁺ m^?2h^?1 at a 40-m deep station near the river mouth to 18 μmole NH₄ ⁺ m^?2h^?1 at a shallower (28-m deep) station further from the river mouth. The net flux of ammonium and nitrate from the sediments to the water measured in bottom lander chambers in the same region were 15–53 μmole NH₄ ⁺ m^?2h^?1 and ?25–21 μmole NO₃ ^? m^?2h^?1. These results suggest that excretion of NH₄ ⁺ by macroinvertebrates could be a potentially important component of benthic nitrogen regeneration in the Mississippi River plume-Gulf shelf region.     

Tracing the sources of nitrate in karstic groundwater in Zunyi,Southwest China: a combined nitrogen isotope and water chemistry approach

Nitrate (NO₃ ⁻) is major pollutant in groundwater worldwide. Karst aquifers are particularly vulnerable to nitrate contamination from anthropogenic sources due to the rapid movement of water in their conduit networks. In this study, the isotopic compositions (δ¹⁵N–NO₃ ⁻, δ¹⁵N–NH₄ ⁺) and chemical compositions(e.g., NO₃ ⁻, NH₄ ⁺, NO₂ ⁻, K⁺) were measured in groundwater in the Zunyi area of Southwest China during summer and winter to identify the primary sources of contamination and characterize the processes affecting nitrate in the groundwater. It was found that nitrate was the dominant species of nitrogen in most of the water samples. In addition, the δ¹⁵N–NO₃ ⁻ values of water samples collected in summer were lower than those collected in winter, suggesting that the groundwater received a significant contribution of NO₃ ⁻ from agricultural fertilizer during the summer. Furthermore, the spatial variation in the concentration of nitrate and the δ¹⁵N–NO₃ ⁻ value indicated that some of the urban groundwater was contaminated with pollution from point sources. In addition, the distribution of δ¹⁵N–NO₃ ⁻ values and the relationship between ions in the groundwater indicated that synthetic and organic fertilizers (cattle manure) were the two primary sources of nitrate in the study area, except in a few cases where the water had been contaminated by urban anthropogenic inputs. Finally, the temporal and spatial variation of the water chemistry and isotopic data indicated that denitrification has no significant effect on the nitrogen isotopic values in Zunyi groundwater.