Denitrification in coastal ecosystems: methods, environmental controls, and ecosystem level controls, a review

In this review of sediment denitrification in estuaries and coastal ecosystems, we examine current denitrification measurement methodologies and the dominant biogeochemical controls on denitrification rates in coastal sediments. Integrated estimates of denitrification in coastal ecosystems are confounded by methodological difficulties, a lack of systematic understanding of the effects of changing environmental conditions, and inadequate attention to spatial and temporal variability to provide both seasonal and annual rates. Recent improvements in measurement techniques involving ¹⁵ N techniques and direct N₂ concentration changes appear to provide realistic rates of sediment denitrification. Controlling factors in coastal systems include concentrations of water column NO ₃ ^– , overall rates of sediment carbon metabolism, overlying water oxygen concentrations, the depth of oxygen penetration, and the presence/absence of aquatic vegetation and macrofauna. In systems experiencing environmental change, either degradation or improvement, the importance of denitrification can change. With the eutrophication of the Chesapeake Bay, the overall rates of denitrification relative to N loading terms have decreased, with factors such as loss of benthic habitat via anoxia and loss of submerged aquatic vegetation driving such effects.     

Denitrification and N2O effluxes in the Bothnian Bay (northern Baltic Sea) river sediments as affected by temperature under different oxygen concentrations

Denitrification rates and nitrous oxide (N₂O) effluxes were measured at different temperatures and for different oxygen concentrations in the sediments of a eutrophied river entering the Bothnian Bay. The experiments were made in a laboratory microcosm with intact sediment samples. ¹⁵N-labelling was used to measure denitrification rates (Dw). The rates were measured at four temperatures (5, 10, 15 and 20°C) and with three oxygen inputs (<0.2, 5, and 10 mg O₂ l⁻¹). The temperature response was highly affected by oxygen concentration. At higher O₂ concentrations (5 and 10 mg O₂ l⁻¹) a saturation over 10°C was observed, whereas the anoxic treatment (<0.2 mg O₂ l⁻¹) showed an exponential increase in the temperature interval with a Q ₁₀ value of 3.1. The result is described with a combined statistical model. In contrast with overall denitrification, the N₂O effluxes from sediments decreased with increasing temperature. The N₂O effluxes had a lower response to oxygen than denitrification rates. The N₂O/N₂ ratio was always below 0.02. Increased temperatures in the future could enhance denitrification rates in boreal river sediments but would not increase the amount of N₂O produced.     

Retention and leaching losses of atmospherically-derived nitrogen in the aggrading coastal watershed of Waquoit Bay,MA

Extensive areas of the eastern United States are being exposed to elevated levels of nitrogen in precipitation, with levels of inorganic N in wet deposition ranging from 5 to over 20 times preindustrial, background levels. This increase in N loading to the terrestrial system, coupled with changes in land use in coastal regions in particular, has dramatically increased the level of nutrient loading from watersheds to the point that coastal waters are today among the most intensely fertilized ecosystems on earth. Studies in upland, aggrading forests have generally found that precipitation N inputs are efficiently sequestered in forest biomass and soil organic matter. However, acidic soils, sandy, porous parent substrates, and chronic inputs of salt spray common to coastal watersheds may all reduce the potential for N sequestration by the terrestrial community.We assessed the role of coastal forests in the long-term storage and retention of atmospherically-derived N in the watersheds of Waquoit Bay, MA, an increasingly eutrophic estuary on Cape Cod, by measuring precipitation inputs, storage, and lysimeter outputs below the rooting zone in a chronosequence of sites released from agriculture at different times. Calculated annual retention efficiencies were relatively low for an N-limited, aggrading forest (40–62%), and leaching losses did not vary with site age from young pine stands to mature beech forests. Nearly all nitrogen input was retained during summer months except in months with very high rainfall events. Nitrogen was released during the dormant-season in proportion to water flux through the forest floor. The composition of lysimeter output was 76% DON, 11% NO ₃ ^– , and 13% NH ₄ ^– . Total water flux and infiltration appear to be more important determinants of N retention in this sandy, coastal site than in more upland forest ecosystems; sandy systems may inherently have a low N retention efficiency.     

Denitrification measurements in aquatic sediments: A comparison of three methods

Measurements of denitrification using the acetylene inhibition,¹⁵N isotope tracer, and N₂ flux methods were carried out concurrently using sediment cores from Vilhelmsborg sø, Denmark, in an attempt to clarify some of the limitations of each technique. Three experimental treatments of overlying water were used: control, nitrate enriched, and ammonia enriched water. The N₂ flux and¹⁵N tracer experiments showed high rates of coupled nitrification/denitrification in the sediments. The acetylene inhibition method did not capture any coupled nitrification/denitrification. This could be explained by acetylene inhibition of nitrification. A combined¹⁵N tracer/acetylene inhibition experiment demonstrated that acetylene inhibition of N₂O reduction was incomplete and the method, therefore, only measured approximately 50% of the denitrification due to nitrate from the overlying water. Similar rates of denitrification due to nitrate in the overlying water were measured by the N₂ flux method and the acetylene inhibition method, after correcting for the 50% efficiency of acetylene inhibition. Rates of denitrification due to nitrate from the overlying water measured by the¹⁵N tracer method, however, were only approximately 35% or less of those measured by the acetylene inhibition or N₂ flux methods.     

柘林湾表层沉积物中的生物可利用磷

对2002年7月至2003年7月间采集的柘林湾表层沉积物样品中的总磷(TP)、有机磷(OP)及各形态无机磷进行分析。结果表明,各形态磷的空间分布与调查海区沉积物粒径有关,并受沿岸污水排放及近岸养殖排污等人为活动的影响。表层沉积物各形态磷的平面分布基本呈现为湾内(除S1、S7站外)高于湾外的总体趋势。无机磷中以自生磷(Au-P)为主要存在形态,占总磷的47.3%,有机磷(OP)含量次之,占总磷的19.5%,铁结合磷(Fe-P)最低。其中,生物可利用磷(包括Ex-P、Fe-P、OP和部分Au-P)占TP的35.7%~83.0%,在国内外海湾中处于较高水平。     

Microbial metabolism in surface sediments and its role in the immobilization of phosphorus in oligotrophic lake sediments

Rapid microbial metabolism and a large phosphorus uptake potential were observed in surface sediments of Lake George, New York. This sediment (termed the flocculent layer) also exhibited a phosphorus limited condition and a large reservoir of inorganic phosphorus associated with humic substances. These observations suggest that the empirically observed phosphorus retention in oligotrophic lake sediments may be promoted by a rapid cycling of phosphorus between microflora and its associated organic matter.     

Photographic demonstration of the annual nature of a varve type common in N. Swedish lake sediments

Several lakes in northern Sweden have laminated sediments, of which many are interpreted as varved (annually laminated). In one of these lakes, a core of the recent sediment has been sampled annually since 1979 (except 1984). These cores verify that one varve, comprising of a thick summer layer (often colour-banded) and a thin winter layer, is formed each year. The cores also show, that, other than compaction, no change in visual appearance of the individual varves takes place after they have been overlain by new varves.     

Microbial characteristics of marine sediments in bathing area along Pesaro-Gabicce coast (Italy): a preliminary study

AIMS: This paper evaluates the presence of human pathogen micro-organisms in marine sediments in a coastal area suitable for bathing. In addition, the grain size analysis was performed in order to correlate the characteristics of the sediments and the microbial content. METHODS AND RESULTS: The samples were collected in two small bays along the central Adriatic coast, where breakwaters had been built for the purpose of halting marine erosion. Faecal contamination indicators, Salmonella and Vibrio species, enteric viruses were investigated using standard techniques for isolation and identification. The grain size was determined using calibrated sieves and 'Sedigraph' device. In some samples, the faecal contamination indices overstepped legislative limits. Salmonella was never found. Vibrio and enteric viruses were isolated: the micro-organisms were preferentially abundant in fine sediments. CONCLUSIONs: Marine sediments can represent an important reservoir of allochthonous and marine micro-organisms and the microbial charge correlates with the characteristics of the sediments. SIGNIFICANCE AND IMPACT OF THE STUDY: This study indicates that it is important to analyse marine sediments before defining the quality of coastal areas.     

Interannual patterns of water table height and groundwater derived nitrate in nearshore sediments

Nitrate concentration and microbial nitrogen transformations in ground-water-affected sediments of Great South Bay, NY were examined over several annual cycles. Nitrate concentrations are typically higher at 40 cm depth than at the surface, while salinity generally decreases with depth. Denitrification occurs through the sediment core and is organic substrate limited at depth while being nitrate limited near the sediment-water interface. Denitrification accounts for about 50% of the biological NO₃ ^- decrease between 40 and 15 cm depth interval. Higher than average annual rainfall during 1983 and 1984 was reflected in an elevated water table as well as lower Bay salinities. Conversely, extremely low rainfall occurred in 1985 and 1986, and the water table reached an extreme low in Sep. 1986. Interestingly, the amounts of nitrate in the sediment column of our primary station varied directly with water table height and, presumably, the discharge rate of nitrate enriched groundwater. We suggest that this may be a result of the more efficient removal of advected nitrate by denitrification during low flow conditions.     

Geochemical composition of sediments in the Scheldt estuary with emphasis on trace metals

Superficial sediments of the Scheldt estuary were collected with a Van Veen grab at 57 stations between Temse and Vlissingen. They were analysed for major elements (Si, Al, Fe, Ca, Mg, Na, K, Cinorg and Corg) and trace metals (Cd, Pb, Cu, Zn, Cr, Ni, Co, Mn and Li). Factor analysis indicates that 44% of the variance can be explained by one factor which exhibits a high saturation for trace metals, organic matter, Al and Fe, all variables typical of fine mud. The high scores of this first factor are almost exclusively present in the upper estuary except for one area in front of Terneuzen. The second factor, which explains 23% of the variance, is typical of the carbonates and the third one (19% of the variance) is representative of the clay minerals. These two factors are more evenly distributed over the estuary. As usual, a strong influence of granulometry on the distribution of trace elements in the sediments was observed. Intercomparison of their composition within the Scheldt or with those of other aquatic systems requires thus a normalization procedure. This problem has been studied in detail by analysing various size fractions (63–16, 16–8, 8–4, <4 m) obtained by elutriation of the sample or by using a parameter characteristic of the fine fraction such as the concentration of a typical element (Al, Fe, Li, Corg). The normalization of trace metals allowed us to evaluate an enrichment factor of the trace elements in the estuarine deposits due to mans activities. In addition, it demonstrates the decrease of the anthropogenic impact on the composition of sediments by comparing the composition of sediments collected in 1976 and in 1994.     

河口潮间带沉积物重金属累积及生态风险评价

于2015年7月份在闽江口鳝鱼滩湿地布设两条样带(T1和T2),并分别选取5个采样点采集沉积物样品。因重金属Co和V对动物和人类会造成一定危害,且闽江河口潮间带Co和V的研究较少,则对其理化性质、Co和V的空间分布特征以及污染程度等进行了研究,旨在为Co和V的污染研究提供参考。结果显示,该沉积物中Co和V的平均浓度分别为15.19 mg/kg、102.94 mg/kg,均高于福建省背景浓度,与其他河口对比,Co和V浓度处于中等水平。Co和V的含量由陆向海方向略有下降。地累积指数法分析得出沉积物中Co含量呈现轻度污染状态,V呈现无污染状态。此外,Co和V的潜在生态危害系数E_rⁱ分别为7.24—14.85和1.47—4.17,表明潜在生态危害轻微,闽江河口潮间带沉积物中Co和V属于轻度污染。     

一株耐氧反硝化细菌的筛选及脱氮特性研究

从鱼塘中分离到1株高效的有氧反硝化菌,经初步鉴定,为芽孢杆菌。在溶解氧(OD)达到2mg/L时,除氮率达97%,OD达到4～5mg/L,仍有一定的反硝化作用,除氮率为85%以上。与典型的好氧反硝化菌Pseudomonasstutzeri[1]相比,有更强的耐溶解氧的优势。同时初步探讨了水体中不同溶解氧、碳源、pH、温度对该芽孢杆菌W2菌株反硝化作用的影响,水体中存在一定量有机碳源有利于反硝化,当以葡萄糖为碳源,pH为7.0～7.5,温度为32℃时,W2菌株具有最佳的降解人工废水反硝化能力。实验结果表明,在好氧条件下,菌体浓度为1000个/mL时,对自然水体中高达1mg/L亚硝酸浓度也能发挥高效的反硝化作用。     

Estimating denitrification rates in estuarine sediments: A comparison of stoichiometric and acetylene based methods

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrient stoichiometry in the subtidal sediments of Tomales Bay, California (USA). By amending whole cores with acetylene and saturating nitrate concentrations, we obtained potential denitrification rates, which ranged between 4 and 30 mmol N m^–2 d^–1. We determined the apparent Michaelis constant (K_app) and the maximum potential rate (V_mp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the K_app and V_mp of the denitrifying community exhibited significant variation over both depth in the sediment column and time of sampling.Estimates ofin situ denitrification obtained using our kinetic-fix adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m^–1 d^–1. Denitrification rates obtained using benthic flux stoichiometry ranged between 0.7 and 4.1 mmol N m^–2 d^–1. Average denitrification rates obtained using the kinetic-fix acetylene block approach exceeded those obtained from net benthic flux stoichiometry; however, these differences were not significant. We conclude that our kinetic-fix adaptation of the acetylene block technique provides realistic estimates of denitrification in sediments, even when pore water nitrate concentrations are low and nitrification and denitrification are closely coupled.     

Comparison between a chemical and a biological method to determine the N mineralization potential of temperate-humid region soils

The potentially mineralizable organic N of 33 different soils was estimated by a chemical test (hot extraction with 2N KCl) and the values compared with those previously obtained by a biological method (aerobic incubation in the laboratory). On average, the organic N solubilized by the chemical procedure was significantly lower than that mineralized by a two weeks aerobic incubation for all the soils as a whole. The same was true for soils developed over acid rocks and over sediments. However, the values obtained for the soils developed over limestone and basic rocks were similar by both methods. The values obtained by both methods were not significantly correlated neither when considering all soils together nor when considering different groups according to soil management or parent material. Significant correlations between both methods were only found when the soils were separated into two groups according to their organic N content: soils with less than 400 mg N 100 g^–1 soil and soils with more than 400 mg N 100 g^–1 soil. The organic N solubilized by the chemical procedure was significantly correlated with the hexosamine-N content; however, it was not correlated with the factors that control the biological mineralization of the organic N, except with the soluble Al content. Therefore, the chemical extraction did not seem to address the biologically active N pool in a selective way.     

Peaks of in situ N2O emissions are influenced by N2O‐producing and reducing microbial communities across arable soils

Agriculture is the main source of terrestrial N₂O emissions, a potent greenhouse gas and the main cause of ozone depletion. The reduction of N₂O into N₂ by microorganisms carrying the nitrous oxide reductase gene (nosZ) is the only known biological process eliminating this greenhouse gas. Recent studies showed that a previously unknown clade of N₂O‐reducers (nosZII) was related to the potential capacity of the soil to act as a N₂O sink. However, little is known about how this group responds to different agricultural practices. Here, we investigated how N₂O‐producers and N₂O‐reducers were affected by agricultural practices across a range of cropping systems in order to evaluate the consequences for N₂O emissions. The abundance of both ammonia‐oxidizers and denitrifiers was quantified by real‐time qPCR, and the diversity of nosZ clades was determined by 454 pyrosequencing. Denitrification and nitrification potential activities as well as in situ N₂O emissions were also assessed. Overall, greatest differences in microbial activity, diversity, and abundance were observed between sites rather than between agricultural practices at each site. To better understand the contribution of abiotic and biotic factors to the in situ N₂O emissions, we subdivided more than 59,000 field measurements into fractions from low to high rates. We found that the low N₂O emission rates were mainly explained by variation in soil properties (up to 59%), while the high rates were explained by variation in abundance and diversity of microbial communities (up to 68%). Notably, the diversity of the nosZII clade but not of the nosZI clade was important to explain the variation of in situ N₂O emissions. Altogether, these results lay the foundation for a better understanding of the response of N₂O‐reducing bacteria to agricultural practices and how it may ultimately affect N₂O emissions.     

Some aspects of redox trends in the bottom muds of a mesotrophic bayou estuary

Redox profiles, pH, bacterial numbers, and organic content of sediments in Bayou Texar, Pensacola, Florida, were examined in an effort to delineate spatial and temporal patterns. These data were accompanied by field determinations of dissolved oxygen, pH, temperature, and salinity. The results point to the importance of bacterial activity and bottom water dissolved oxygen upon the redox status of the sediments. It is suggested that, in certain systems, the redox status of deeper sediments may reveal long-term trends which are obscured at the sediment-water interface by transient effects.     

Denitrification by the sessile microbial community of a polluted river

Denitrification by the sessile microbial community of the River Tamagawa was studied in laboratory experiments. Inorganic nitrogen loss was observed when river water was incubated with sessile microbial community of the river in a continuously circulating system. It was confirmed by the ¹⁵N tracer technique that both sessile microbial communities of unpolluted and polluted areas had denitrifying activity, even though they were incubated in oxygenated river water. The denitrification rate of the sessile microbial community taken from a polluted area, measured by the ¹⁵N tracer technique, was 8–16 mg N/m²/day in October and December, 1977, and it was enhanced 10-fold by raising the water temperature from 14 to 30° C. Denitrification in the river was also suggested by determining the N₂: Ar ratio of gases evolved from the river bed.     

Distribution,speciation, and ecological risk assessment of heavy metals in surface sediments of Jiaozhou Bay,China

This paper analyzed the distribution and speciation of seven heavy metals in sediments in Jiaozhou Bay. The ecological risk was assessed using three index approaches (i.e., risk assessment code (RAC), contamination factor (CF), and potential ecological risk index (PERI)) and by a comparison with sediment quality guidelines (Chinese Marine Sediment Quality Standards (CMSQS), and threshold effect level (TEL) and probable effect level (PEL) from the USEPA). Pb, Cr, As, Cu, Zn, and Hg contents at most sites were above the corresponding TEL and Class I criteria (CMSQS) value. Particularly, high contents of Cu, detected at sites S₇ (124.5 mg kg^?1) and S₈ (118.3 mg kg^?1), exceeded the respective PEL value, indicating that harmful biological effects might occur. Speciation analysis, individual CF, and RAC calculations suggested that Cd had the highest bioavailable fraction and thus posed a very high risk to aquatic ecosystem; Cu and Zn showed a medium–high risk. Both global CF and PERI analysis indicated a high pollution risk at sites S₇, S₁, S₃, and S₂, but the assessments of specific sites were different. The incomplete consistency suggested that it is necessary to consider both total contents and chemical speciation for providing a more realistic appraisal for the risk of heavy metals in sediments.     

Greenhouse gas emissions from a Cu-contaminated soil remediated by in situ stabilization and phytomanaged by a mixed stand of poplar,willows, and false indigo-bush

Phytomanagement of trace element-contaminated soils can reduce soil toxicity and restore soil ecological functions, including the soil gas exchange with the atmosphere. We studied the emission rate of the greenhouse gases (GHGs) CO₂, CH₄, and N₂O; the potential CH₄ oxidation; denitrification enzyme activity (DEA), and glucose mineralization of a Cu-contaminated soil amended with dolomitic limestone and compost, alone or in combination, after a 2-year phytomanagement with a mixed stand of Populus nigra, Salix viminalis, S. caprea, and Amorpha fruticosa. Soil microbial biomass and microbial community composition after analysis of the phospholipid fatty acids (PLFA) profile were determined. Phytomanagement significantly reduced Cu availability and soil toxicity, increased soil microbial biomass and glucose mineralization capacity, changed the composition of soil microbial communities, and increased the CO₂ and N₂O emission rates and DEA. Despite such increases, microbial communities were evolving toward less GHG emission per unit of microbial biomass than in untreated soils. Overall, the aided phytostabilization option would allow methanotrophic populations to establish in the remediated soils due to decreased soil toxicity and increased nutrient availability.     

Spatial Variation in Denitrification and N2O Emission in Relation to Nitrate Removal Efficiency in a N-stressed Riparian Buffer Zone

Spatial variability in hydrological flowpaths and nitrate-removal processes complicates the overall assessment of riparian buffer zone functioning in terms of water quality improvement as well as enhancement of the greenhouse effect by N₂O emissions. In this study, we evaluated denitrification and nitrous oxide emission in winter and summer along two groundwater flowpaths in a nitrate-loaded forested riparian buffer zone and related the variability in these processes to controlling soil factors. Denitrification and emissions of N₂O were measured using flux chambers and incubation experiments. In winter, N₂O emissions were significantly higher (12.4 mg N m⁻² d⁻¹) along the flowpath with high nitrate removal compared with the flowpath with low nitrate removal (2.58 mg N m⁻² d⁻¹). In summer a reverse pattern was observed, with higher N₂O emissions (13.6 mg N m⁻² d⁻¹) from the flowpath with low nitrate-removal efficiencies. Distinct spatial patterns of denitrification and N₂O emission were observed along the high nitrate-removal transect compared to no clear pattern along the low nitrate-removal transect, where denitrification activity was very low. Results from this study indicate that spots with high nitrate-removal efficiency also contribute significantly to an increased N₂O emission from riparian zones. Furthermore, we conclude that high variability in N₂O:N₂ ratio and weak relationships with environmental conditions limit the value of this ratio as a proxy to evaluate the environmental consequences of riparian buffer zones.