Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.     

Nitrogen,phosphorus and sulfur mineralization as affected by soil depth in rangeland ecosystems

The potential mineralization and immobilization of soil nitrogen (N), phosphorus (P) and sulfur (S) are relatively high in natural ecosystems. This study was conducted to investigate the changes in essential plant macronutrients; N, P, and S status in response to different soil depth in rangeland ecosystems in vitro. The net nutrient mineralization was measured during 90 days at different depths (0–15, 15–30, 30–45 and 45–60 cm), using kinetic models to estimate the release rate. The net ammonification and mineralization of P and S were described using parabolic diffusion equation, while the power function equation was used to describe the net nitrification. The results indicated that the amount of released ammonium (NH₄ ⁺) decreased with time and depth and the rates of net ammonification were negative in all samples. Conversely, nitrification increased with time and depth and the rates were all positive. The net mineralization for both P and S reduced with time. The concentration of mineralized SO₄ ^2? increased with depth like nitrate (NO₃ ^?). Accumulation of SO₄ ^2? and NO₃ ^? in subsurface soils and NH₄ ⁺ and P at surface horizons can increase the potential of their loss by leaching or volatilization.     

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.     

Oyster (<Emphasis Type="Italic">Crassostrea virginica</Emphasis>) Aquaculture Shifts Sediment Nitrogen Processes toward Mineralization over Denitrification

Filter-feeding bivalves, like oysters, couple pelagic primary production with benthic microbial processes by consuming plankton from the water column and depositing unassimilated material on sediment. Conceptual models suggest that at low to moderate oyster densities, this deposition can stimulate benthic denitrification by providing denitrifying bacteria with organic carbon and nitrogen (N). While enhanced denitrification has been found at oyster reefs, data from oyster aquaculture are limited and equivocal. This study measured seasonal rates of denitrification, as well as dissimilatory nitrate reduction to ammonium (DNRA), and dissolved inorganic N fluxes at a rack and bag eastern oyster (Crassostrea virginica) aquaculture farm. Consistent with models, denitrification was enhanced within the farm, with an average annual increase of 350% compared to a reference site. However, absolute denitrification rates were low relative to other coastal systems, reaching a maximum of 19.2 μmol m^?2 h^?1. Denitrification appeared to be nitrate (NO₃ ^?) limited, likely due to inhibited nitrification caused by sediment anoxia. Denitrification may also have been limited by competition for NO₃ ^? with DNRA, which accounted for an average of 76% of NO₃ ^? reduction. Consequently, direct release of ammonium (NH₄ ⁺) from mineralization to the water column was the most significant benthic N pathway, with seasonal rates exceeding 900 μmol m^?2 h^?1 within the farm. The enhanced N processes were spatially limited however, with significantly higher rates directly under oysters, compared to in between oyster racks. For commercial aquaculture farms like this, with moderate oyster densities (100–200 oysters m^?2), denitrification may be enhanced, but nonetheless limited by biodeposition-induced sediment anoxia. The resulting shift in the sediment N balance toward processes that regenerate reactive N to the water column rather than remove N is an important consideration for water quality.     

The distribution of nitrogen speciation and sources of nitrate in the north of Taihu Lake

Meiliang Bay and Gonghu Bay, in the north of Taihu Lake, are important water sources for the city of Wuxi, and increased eutrophication now threatens the safety of drinking water. The distribution of nitrogen (N) speciation and source of N in the surface waters in the north of Taihu Lake is studied, which was an important first step in controlling N pollution. The result shows that the average concentration of ammonia (NH₄ ⁺) and nitrate (NO₃ ^?) of surface water in Meiliang Bay was 0.32 and 0.35 mg/L, while 0.21 and 0.74 mg/L of Gonghu Bay, in which both bays had serious nitrate pollution. The concentrations of NH₄ ⁺ and NO₃ ^? in the surface water of the two bays had a trend of gradual decrease from north to south. The maximum concentrations of NH₄ ⁺ and NO₃ ^? of two bays were observed near the inflowing rivers, and the maximum concentrations of NH₄ ⁺ in surface water of two bays were 0.49 and 0.61, and 0.77 and 1.38 mg/L of NO₃ ^?. The concentration of NH₄ ⁺ in the interstitial water of the two bays had a trend of gradual decrease from west to east, but NO₃ ^? had the opposite tendency. The maximum concentrations of NH₄ ⁺ in the interstitial water of the two bays were 5.88 and 4.64, and 3.58 and 7.18 mg/L of NO₃ ^?. The exchangeable NH₄ ⁺ content in the sediment of Meiliang Bay had a trend of gradual decrease from north to south, but Gonghu Bay showed the reverse. The exchangeable NO₃ ^? content in the sediment of Meiliang Bay had a trend of gradual decrease from east to west, but a decreasing trend from north to south was observed in Gonghu Bay. The maximum concentrations of exchangeable NH₄ ⁺ were determined, and the values were 96.25 and 74.90 mg/kg, as well as NO₃ ^? with the values of 12.06 and 7.08 mg/kg. Chemical fertilizer and domestic sewage were the major sources of nitrate in surface water of Gonghu Bay, contributing 39.16 and 47.79%, respectively. Domestic sewage was the major source of nitrate in Meiliang Bay, contributing 84.79%. The denitrification process in Gonghu Bay was more apparent than in Meiliang Bay. Mixing and dilution processes had important effects on changing the concentration of nitrate transportation in the two bays.     

Effect of soil-applied calcium carbide and plant derivatives on nitrification inhibition and plant growth promotion

The aim of this study was to evaluate the relative performance of three nitrification inhibitors (NIs) viz. calcium carbide (CaC₂), and plant derivatives of Pongamia glabra Vent. (karanj) and Melia azedarach (dharek) in regulating N transformations, inhibiting nitrification and improving N recovery in soil–plant systems. In the first experiment under laboratory incubation, soil was amended with N fertilizer diammonium phosphate [(NH₄)₂HPO₄] at a rate of 200 mg N kg^?1, N + CaC₂, N + karanjin, and N + M. azedarach and incubated at 22 °C for 56 days period. Changes in total mineral N (TMN), NH₄ ⁺–N and NO₃ ^?–N were examined during the study. A second experiment was conducted in a glasshouse using pots to evaluate the response of wheat to these amendments. Results indicated that more than 92 % of the NH₄ ⁺ initially present had disappeared from the mineral N pool by the end of incubation. Application of NIs i.e., CaC₂, karanjin, and M. azedarach resulted in a significant reduction in the extent of NH₄ ⁺ disappearance by 49, 32, and 13 %, respectively. Accumulation of NO₃ ^?–N was much higher in N amended soil 57 % compared to 11 % in N + CaC₂, 13 % in N + karanjin, and 18 % in N + M. azedarach. Application of NIs significantly increased growth, yield, and N uptake of wheat. The apparent N recovery in N-treated plants was 20 % that was significantly increased to 38, 34, and 37 % with N + CaC₂, N + karanjin, and N + M. azedarach, respectively. Among the three NIs tested, CaC₂ and karanjin proved highly effective in inhibiting nitrification and retaining NH₄ ⁺–N in the mineral pool for a longer period.     

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.     

Sediment Nitrogen Fixation: a Call for Re-evaluating Coastal N Budgets

Coastal ocean primary productivity is often limited by nitrogen (N) availability, which is determined by the balance between N sources (e.g., N-fixation, groundwater, river inputs, etc.) and sinks (e.g., denitrification, sediment burial, etc.). Historically, heterotrophic N-fixation in sediments was excluded as a significant source of N in estuarine budgets, based on low, indirectly measured rates (e.g., acetylene reduction assay) and because it was unnecessary to achieve mass balance. Many recent studies using net N₂ flux measurements have shown that sediment N-fixation can equal or exceed N₂ loss. In an effort to quantify N₂ production and consumption simultaneously, we measured N-fixation and denitrification directly in sediment cores from a temperate estuary (Waquoit Bay, MA). N-fixation, dissimilatory nitrate reduction to ammonium, and denitrification occurred simultaneously, and the net N₂ flux shifted from uptake (N-fixation) to efflux (denitrification) over the 120-h incubation. Evidence for N-fixation included net ²⁸N₂ and ³⁰N₂ uptake, ¹⁵NH₄ ⁺ production from ³⁰N₂ additions, ¹⁵N_{organic matter} production, and nifH expression. N-fixation from ³⁰N₂ was up to eight times higher than potential denitrification. However, N-fixation calculated from ¹⁵NO₃ ^? was one half of the measured fixation from ³⁰N₂, indicating that ¹⁵NO₃-isotope labeling calculations may underestimate N-fixation. These results highlight the dynamic nature of sediment N cycling and suggest that quantifying individual processes allows a greater understanding of what net N₂ fluxes signify and how that balance varies over time.     

Quantifying Sediment Nitrogen Releases Associated with Estuarine Dredging

Experimental studies of sediment pore water NH₄ ⁺ chemistry, adsorbed NH₄ ⁺ concentrations, sediment?Cwater NH₄ ⁺ exchange and N₂?CN flux were carried out to quantify the mass of labile N that can be released during large-scale dredging activities. Pore water NH₄ ⁺ concentrations below 0.5-m sediment depth averaged 5 ± 2 mmol L^?1 with average adsorbed NH₄ ⁺ concentrations of 11 ??mol g^?1. Elevated NH₄ ⁺ concentrations found in rapidly accreting dredge channels are partly a result of the rapid advective burial of both reactive organic matter and pore water. Elutriate tests, a dilution of sediment with site water, yielded adsorbed NH₄ ⁺ concentrations very similar to those using the more typical KCl extraction. Intact deep sediment sections exposed to overlying water, used to simulate postdredging conditions, showed high initial fluxes of ammonium and no development of coupled nitrification?Cdenitrification under the cold incubation conditions. Despite high concentrations and effluxes of NH₄ ⁺ during dredging, the amount of NH₄ ⁺ release during dredging was <0.5% of northern Chesapeake Bay sediment fluxes. The likelihood of large environmental effects of nitrogen release during the dredging of navigational channels in the Chesapeake Bay is low.     

Nutrient dynamics in pore water of tidal marshes near the Yangtze Estuary and Hangzhou Bay, China

Tidal marshes act as a buffer system for nutrients in the pore water and play important roles in controlling the budget of nutrients and pollutants that reach the sea. Spatial and seasonal dynamics of pore water nutrients were surveyed in three tidal marshes (Chongming Island, Hengsha Island, and Fengxian tidal flat) near the Yangtze Estuary and Hangzhou Bay from August 2007 to May 2008. Nutrient variations in pore water closely followed seawater quality in the estuaries, while the average concentration of NH₄ ⁺–N, the main form of inorganic nitrogen in pore water, was over two orders of magnitude higher than that in seawater which was dominated by nitrate. NH₄ ⁺–N export (13.81 μmol m^?2 h^?1) was lower than the import of (NO₃ ^?+NO₂ ^?)–N (?24.17 μmol m^?2 h^?1) into sediment over the 1-year period, hence reducing N-eutrophication in coastal waters. The export of SiO₃ ^2?–Si and PO₄ ^3?–P from tidal marshes regulated nutrient level and composition and lifted the ratio beyond potentidal element limitation in the coastal system. Moreover, macrophyte plants (Spartina alterniflora and Phragmites australis) played significant roles in controlling nutrient concentration in pore water and its exchange between marshes and estuaries. Fengxian marsh was characterized by higher nutrient concentrations and fluxes than other marshes in response to the more serious eutrophication in Hangzhou Bay than in the Yangtze Estuary.     

Reciprocal influence of Kürtün Dam and wastewaters from the settlements on water quality in the stream Harşit,NE Turkey

This paper aims to reveal the reciprocal influence of Kürtün Dam and wastewaters from the settlements on the water quality in the stream Har?it, NE Turkey. Several key water-quality indicators were measured: water temperature (T), pH, dissolved oxygen (DO), electrical conductivity, water hardness, chemical oxygen demand (COD), ammonium nitrogen (NH₄ ⁺–N), nitrite nitrogen (NO₂ ^?–N), nitrate nitrogen (NO₃ ^?–N), total Kjeldahl nitrogen (TKN), total nitrogen (TN), orthophosphate phosphorus (PO₄ ^3?–P), and methylene blue active substances (MBAS). The monitoring and sampling studies were conducted every 15 days from March 2009 to February 2010 at two stations selected in the upstream and downstream of the Kürtün Dam. It was concluded that the Kürtün Dam Lake had a high-quality water in terms of T, pH, DO, COD, NH₄ ⁺–N, NO₂ ^?–N and NO₃ ^?–N values, but slightly polluted water with respect to TKN, PO₄ ^3?–P, and MBAS according to the Turkish Water Pollution Control Regulation. The dam improved the stream water quality by increasing the DO concentration, and decreasing the NO₂ ^?–N and PO₄ ^3?–P concentrations thanks to its hydraulic residence time despite the wastewater discharge by the nearby settlements. However, the wastewater discharge deteriorated the stream water quality increasing the COD, NH₄ ⁺–N, NO₃ ^––N, and TN concentrations.     

The dominant acetate degradation pathway/methanogenic composition in full-scale anaerobic digesters operating under different ammonia levels

Ammonia is a major environmental factor influencing biomethanation in full-scale anaerobic digesters. In this study, the effect of different ammonia levels on methanogenic pathways and methanogenic community composition of full-scale biogas plants was investigated. Eight full-scale digesters operating under different ammonia levels were sampled, and the residual biogas production was followed in fed-batch reactors. Acetate, labelled in the methyl group, was used to determine the methanogenic pathway by following the ¹⁴CH₄ and ¹⁴CO₂ production. Fluorescence in situ hybridisation was used to determine the methanogenic communities’ composition. Results obtained clearly demonstrated that syntrophic acetate oxidation coupled with hydrogenotrophic methanogenesis was the dominant pathway in all digesters with high ammonia levels (2.8–4.57 g NH₄ ⁺-N L^?1), while acetoclastic methanogenic pathway dominated at low ammonia (<1.21 g NH₄ ⁺-N L^?1). Thermophilic Methanomicrobiales spp. and mesophilic Methanobacteriales spp. were the most abundant methanogens at free ammonia concentrations above 0.44 g NH₃-N L^?1 and total ammonia concentrations above 2.8 g NH₄ ⁺-N L^?1, respectively. Meanwhile, in anaerobic digesters with low ammonia (<1.21 g NH₄ ⁺-N L^?1) and free ammonia (<0.07 g NH₃-N L^?1) levels, mesophilic and thermophilic Methanosaetaceae spp. were the most abundant methanogens.     

Sandy seepage faces as bioactive nitrate reactors: Biogeochemistry,microbial ecology and metagenomics

Subterranean estuaries are highly dynamic in processing dissolved inorganic nitrogen (DIN). Here we investigate DIN turnover in surface sediments (0–20 cm depth) at the higher, medium and lower intertidal of a seepage face, i.e., the outer “mouth” of the subterranean estuary, during four consecutive seasons in Sanggou Bay, China. Throughout the studied period, ammonium (NH₄⁺) and nitrite (NO₂^?) concentrations in the sampled porewaters did not vary significantly with depth or season. In contrast, peaks in porewater nitrate (NO₃^?) concentration and decreases in δ¹⁵N-NO₃^? and δ¹⁸O-NO₃^? were observed in the 15–20 cm depth (bottom) sediment, particularly during summer and autumn. Coupled with NO₃^? production, the sediment total nitrogen was also markedly peaking in the bottom layer of the studied seepage face. Together with abundant heterotrophic microbes in the sediment, this NO₃^? accumulation was linked to a reaction chain including organic matter decomposition, ammonification and nitrification. During winter, porewater enrichment in total nitrogen occurred closer to the surface of the seepage face but triggered also active NO₃^? production. This pattern reinforced the importance of pelagic organic matter supply on NO₃^? production. In the shallower depths of the seepage face (<12 cm), active net NO₃^? removal occurred except in winter. The isotopic fractionation (δ¹⁵N-NO₃^? and δ¹⁸O-NO₃^?) and metagenomic results revealed denitrification as the main pathway for NO₃^? reduction. Biological assimilation from benthic primary producers may also consume a fraction of NO₃^? at the sediment water interface. Both NO₃^? production and removal significantly varied in magnitude with season (?13.6 to 6.2 nmol cm^?3 h^?1). Substrate supply was the key driver for nitrate cycling, as evidenced by the high NO₃^? production rate in spring by comparison to autumn. The highest NO₃^? turnover rates were found in summer, suggesting the combined influence of advection rates and sediment microbiota composition. In spite of active removal (peak NO₃^? removal capability: 61%), a significant amount of NO₃^? was still transported from the seepage face into the bay waters. The magnitude of NO₃^? fluxes ranged from 312 to 476 kg N d^?1, accounting for approximately 15% of the total exogenous NO₃^? loading into the bay. NO₃^? isotopic fingerprint revealed chemical fertilizer as the main source of terrestrial NO₃^? in SGD, highlighting the importance of land use to coastal system nitrogen budgets.     

Functional Performance of Three Invasive <Emphasis Type="Italic">Marenzelleria</Emphasis> Species Under Contrasting Ecological Conditions Within the Baltic Sea

A 4-week laboratory experiment investigated the behaviour (survival and bioirrigation) and impact of the invasive polychaetes Marenzelleria viridis, M. neglecta and M. arctia on sediment-water solutes exchange, porewater chemistry, and Fe and P interactions in high-salinity sandy sediment (HSS) and low-salinity muddy sediment (LSM) from the Baltic Sea. M. viridis showed deep burrowing with efficient bioirrigation (11 L m^?2 day^?1) and high survival (71%) in HSS, while M. arctia exhibited shallow burrowing with high bioirrigation (12 L m^?2 day^?1) and survival (88%) in LSM. M. neglecta behaved poorly in both ecological settings (bioirrigation, 5–6 L m^?2 day^?1; survival, 21–44%). The deep M. viridis bioirrigation enhanced total microbial CO₂ (TCO₂) production in HSS by 175% with a net efflux of NH₄⁺ and PO₄^3?, at rates 3- to 27-fold higher than for the other species. Although the shallow and intense bioirrigation of M. arctia in LSM stimulated microbial TCO₂ production to some extent (61% enhancement), the nutrient fluxes close to zero indicate that it effectively prevented the P release. Porewater Fe:PO₄^3? ratios revealed that the oxidizing effect of M. arctia bioirrigation increased the PO₄^3? adsorption capacity of LSM twofold relative to defaunated controls while no buffering of PO₄^3? was detected in M. viridis HSS treatment. Therefore, the different behaviour of the three species in various environments and the sharp contrast between M. viridis and M. arctia effects on C, N and P cycling must be considered carefully when the ecological role of Marenzelleria species in the Baltic Sea is evaluated.     

Dynamics of NH4 + and NO3 − uptake in the water column of the Neuse River Estuary,North Carolina

In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^? uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2–3-wk intervals from March 1985 to February 1989. Significant dark NH₄ ⁺ uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH₄ ⁺ uptake ranged from 0.001 μmol N 1^?1 h^?1 to 4.2 μmol N 1^?1 h^?1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH₄ ⁺ uptake was significantly related to NH₄ ⁺ concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH₄ ⁺ uptake ranged from 0.1 mmol N m^?2 d^?1 to 133 mmol N m^?2 d^?1 and followed the trend of apparent uptake. Annual NH₄ ⁺ uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO₃ ^? was only 14% of maximum light uptake. Apparent NO₃ ^? uptake rates ranged from 0.001 μmol N 1^?1 h^?1 to 1.84 μmol N 1^?1 h^?1 with highest rates occurring in the oligohaline estuary. Apparent NO₃ ^? uptake was significantly related to NO₃ ^? concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO₃ ^? uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO₃ ^? uptake ranged from 0.1 mmol N m^?2 d^?1 to 53 mmol N m^?2 d^?1 and followed similar patterns of apparent uptake. Annual NH₄ ⁺ uptake was 11.39 mol N m^?2 yr^?1, 10.28 mol N m^?2 Yr^?1, 10.93 mol N m^?2 yr^?1, and 7.38 mol N m^?2 yr^?1, and 1.84 mol N m^?2 yr^?1, with the 4-yr mean being 10.0. Annual NO₃ ^? uptake was 3.12 mol N m^?2 yr^?1, 3.40 mol N m^?2 yr^?1, 1.96 mol N m^?2 yr^?1, and 1.84 mol N m^?2 yr^?1, with the 4-yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.     

Sediment Denitrification in Two Contrasting Tropical Shallow Lagoons

Sediment denitrification was monthly evaluated in two tropical coastal lagoons with different trophic states using the ¹⁵N isotope pairing technique. Denitrification rates were very low in both environments, always <5.0 μmol N₂ m^?2 h^?1 and were not significantly different between them. Oxygen consumption varied from 426 to 4248 μmol O₂ m^?2 h^?1 and was generally three times higher in the meso-eutrophic than the oligotrophic lagoon. The low denitrification activity was ascribed to both low water NO₃ ^? concentrations (<2.0 μM) and little nitrate supply from nitrification. There was no correlation of denitrification with nitrate or ammonium fluxes. Sediments in temperate environments with similar oxygen consumption rates usually presented a higher proportion of nitrification–denitrification rates. Sediment oxygen consumption was a good predictor of sediment denitrification in both studied lagoons.     

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.     

Effects of sediment texture on in-stream nitrogen uptake

A comparative experiment was conducted in two cross sections with sandy and sandy loam sediment textures along an agricultural drainage stream in eastern China to address the effects of sediment texture on in-stream nitrogen uptake efficiency. Using dimerous chambers for in situ incubations, NO₃-N and NH₄-N uptake metrics (i.e., areal uptake rate and uptake velocity) and associated hydrochemical variables in the enclosed sediment–water column system were measured for 8 days and two nights across April–July in 2011 and March–June in 2012. For the investigated sites, in-stream uptake accounted for 2–45 and 9–36 % of the initial NH₄-N and NO₃-N within the enclosed water column, respectively. Although similar daytime, diel and day-to-day (daytime) variation patterns of NO₃-N or NH₄-N uptake metrics were observed for the two sites, the sandy loam sediments had average net NO₃-N and NH₄-N uptake efficiency ~50 % higher and ~40 % lower than for the sandy sediments, respectively. As NO₃-N was the dominant nitrogen form in the studied water columns (typical of agricultural drainage rivers), the sandy loam sediment site had an average of about 47 % higher net uptake efficiency for dissolved inorganic nitrogen (i.e., NO₃-N + NH₄-N). This study demonstrates that sediment texture has a considerable effect on spatial variation of nitrogen uptake along the river system. Changing sediment texture due to anthropogenic modifications on catchment land use and stream channels has the potential to change stream nitrogen cycling as well as altering nitrogen inputs and forms to downstream aquatic ecosystems.     

Detection of tyrosine,trace metals and nutrients in cow dung: the environmental significance in soil and water environments

This study examined the dissolved organic matter (DOM) components of cow dung using a combination of fluorescence (excitation–emission matrix, EEM) spectroscopy and parallel factor (PARAFAC) modelling along with eleven trace metals using ICP-MS and nutrients (NH₄⁺ and NO₃^?) using an AA3 auto analyser. EEM–PARAFAC analysis demonstrated that cow dung predominantly contained only one fluorescent DOM component with two fluorescence peaks (Ex/Em = 275/311 nm and Ex/Em = 220/311 nm), which could be denoted as tyrosine by comparison with its standard. Occurrence of tyrosine can be further confirmed by the FTIR spectra. Trace metals analysis revealed that Na, K and Mg were significantly higher than Ca, Fe, Mn, Zn Sr, Cu, Ni and Co. The NH₄⁺ concentrations were substantially higher than NO₃^?. These results thus indicate that the dissolved components of the cow dung could be useful for better understanding its future uses in various important purposes.