Nitrification In Sediments Research Articles

Strong turbulence accelerates sediment nitrification-denitrification for nitrogen loss in shallow lakes

Due to energy dissipation, turbulent energy reaching bed sediment greatly differs in lakes with different depths, which potentially affects sediment denitrification and thereby nitrogen loss. In this study, we explored the impacts of turbulent energy reaching sediment on sediment nitrification rate using turbulence simulation experiments, and analyzed its role in determining sediment nitrogen loss in global lakes by investigating the relationship between denitrification rate with lake depth. Results demonstrated that sediment denitrification rate is affected by water depth in lakes with a depth of <~10 m, in which the rate was negatively correlated with lake depth, and maintained stably at low levels of <2.4 mg N m−2 day−1 in lakes with a depth of >~10 m. In shallow lakes, stronger turbulence reaching on sediment yielded higher nitrogen loss rate. Compare with the control, cumulated nitrogen loss from sediment increased by 10% at the turbulent velocity of 4.33 cm s−1 upon sediment. It is possibly because turbulence promoted faster transport of oxygen to surface sediment and enhanced the mineralization of buried organic matters to feed nitrification, which subsequently accelerated denitrification and thereby nitrogen loss. This study can add to our understanding of the role of lake morphology in nitrogen biogeochemical cycles.

Effect of Vermicompost on Soil and Runoff Properties in Northern Iran

This study aimed at measuring the effect of vermicompost on soil physical and chemical properties and runoff quality and quantity in agro-ecosystems in Northern Iran. Thus, runoff measurement plots of 1 × 5 m were set up in sloping lands under natural rainfall during 5 months. Runoff quality and quantity, soil properties, and plant growth were measured. This study shows that the amount of runoff water and its quality (sediment yields, nitrate content, pH, and conductivity) were not influenced by the fertilization treatments (vermicompost alone, chemical fertilization alone, or their mixture), except for the first month of the experiment where a lower runoff amount was measured when vermicompost was applied alone. Although no difference in plant growth was found, our results showed a significant improvement of soil properties when vermicompost was used as a fertilizer. Consequently, this study shows that vermicompost can be considered as an alternative application for reducing the use of chemical fertilizers and improving soil properties but its impact on plant growth and runoff is limited to the short term.

Sertraline inhibits top-down forces (predation) in microbial food web and promotes nitrification in sediment

Sertraline is a widely used antidepressant that becomes an aquatic pollutant through metabolic excretion and improper disposal. Determining the impact of sertraline on benthic microbial ecosystems is important for the transformation of river biogenic elements. However, the molecular initiating event induced by sertraline is more readily observed at higher levels, such as the individual or population level of larger organisms, and the effect is not pronounced in benthic organisms, which are directly involved in nitrogen transformation. Therefore, this study used DNA metabarcoding to analyze the effect of sertraline on the microbial ecosystem and material cycles in river sediment through the lens of a microbial food web. The presence of sertraline in the river sediment enhanced the mineralization capacity of nitrogen and increased the accumulation of nitrate in the sediment. Sertraline affected the structure of the microbial food web by stimulating different successions of bacteria and eukaryotes. A structural equation model revealed that sertraline affected the microbial food web model through top-down forces (predation) by reducing the trophic transfer efficiency from metazoans to protozoans. This effect resulted in decreases in the trophic transfer efficiency from protozoans to bacteria and increases in nitrogen mineralization capacity. This was followed by a gradual increase in the nitrification reaction under the action of nitrifying bacteria, increasing the threat to the ecological health of rivers. The results show that sertraline affects the material cycle of river ecosystems and emphasizes that the assessment of the ecological risks of sertraline needs to be considered from the perspective of the material cycle of ecosystems.

Effects of macrophytes on potential nitrification and denitrification in oligotrophic lake sediments

Macrophytes may either stimulate or depress nitrogen-related microbial processes via radial oxygen loss (ROL), production of exudates or uptake of inorganic N. ROL can favor aerobic processes as nitrification, exudates may stimulate denitrification, whereas N assimilation and competition with microbes may depress both processes. We measured rates of potential nitrification (PN) and denitrification (PD) in oligotrophic lacustrine sediments colonized by submersed and emergent macrophytes. Potential rates were also analyzed in adjacent control sediments devoid of vegetation. Aim of the work was to verify if the presence of macrophytes alters the potential activity of nitrifying or denitrifying bacteria. Vertical profiles (0−10 cm depth) of PN and PD rates were measured via oxic (nitrification) and anoxic (denitrification) slurries, where we measured the accumulation of NOx− from added NH4+ and the production of 30N2 from added 15NO3−, respectively. Results suggest that under oligotrophic settings macrophytes produced small effects on potential nitrification and denitrification activities. Despite elevated oxygen release demonstrated for most of the tested macrophytes, nitrification was likely constrained by significant plant-bacteria competition. Potential denitrification was comparatively more stimulated by macrophytes, but we address this result to a general increase of heterotrophic microbial activity in organic-richer vegetated sediments, due to dead root biomass or exudates. The highest PN and PD rates were measured in sediments colonized by Littorella uniflora, likely due to its large underground biomass (root:shoot ratio ∼3.5), root porosity and oxygen leakage.

Sediment nitrate reduction processes in response to environmental gradients along an urban river-estuary-sea continuum

Sediment denitrification (DEN), anaerobic ammonium oxidation (Anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are three important nitrate (NO3−) reduction pathways in aquatic ecosystems. These processes modify nitrogen (N) loadings from land to the ocean, with important implications on the management of coastal eutrophication. While NO3− reduction has been studied intensively for various types of habitats, studies on its distributions along river-estuary-sea continua remain scarce. In this study, we examined these three pathways along a N-laden urban river-estuary-sea continuum comprised of three types of habitats (urban river, estuary, and adjacent sea) in the densely populated Shanghai-East China Sea area. The potential DEN, Anammox, and DNRA rates decreased seaward both in summer and winter in response to decreasing sediment organic matter (OM, 20 to 7 to 7 mg C g−1), ferrous oxide (9 to 2.7 to 2.8 mg Fe g−1), and bottom water dissolved inorganic nitrogen (543 to 112 to 21 μM). Among these pathways, DEN remained a major component (~69.6%) across habitats, while Anammox (47.9%) rivaled DEN (48.3%) in the urban river in winter. N retention index (NIRI), the ratio between retained and removed NO3−, ranged from 0 to 0.5 and increased downstream. Together, these results suggest that the decreasing gradients of OM and inorganic matter shape the distribution of NO3− reduction along the continuum, reflecting the diminishing impact of the river and human inputs from the urban river to the ocean. Our results highlight the importance of taking a continuum perspective in N cycling studies and emphasize the role of urban rivers as N removal hotspots, which should be a focus of research and management.

Nano-biochar reduced soil erosion and nitrate loss in sloping fields on the Loess Plateau of China

The effects of biochar on soil erosion and nutrient loss on the Chinese Loess Plateau have been well reported, but the huge required application rate of biochar limits the biochar use. The specific nano-treatment can improve the distribution and properties of biochar (e.g., increase the specific surface area). To investigate the migration processes of runoff, sediment and nutrients on the nano-biochar (NB) applied sloping land during rainfall events, different mass content of NB (0.0%, 0.1%, 0.5%, 0.7%, and 1.0%) were used on the three types of vegetation covered sloping land (bare, caragana, and maize). The effects of NB and vegetation on runoff volume, sediment yield, and nitrate loss were studied under simulated rainfall in the northeastern region of the Loess Plateau. The main results were as follows: the application of NB effectively reduced runoff (39.7–74.4%), sediment yield (8.9–41.8%), and nitrate loss (13.6–59.8%) in all treatments. Losses of nitrate in runoff and sediment were negatively correlated with increased content of NB (when NB application rate was 1.0%, the maximum nitrate loss in runoff and sediment were 27.43 and 6.25 mg, respectively). The equivalent model of convection was more suitable for analyzing the nutrient transfer process with the application of NB. The effective mixing depth decreased with NB content and could be well described by a power function. NB might promote the nitrate migrate to deep soil and reduce nitrate loss in runoff and sediment. With NB application, the peak value of nitrate distribution in the profile moved downward, and the maximum value was obtained at 10–15 cm. Moreover, the most effective NB content for reducing nitrate loss were 1.0%. Comparing with bare and maize, caragana had the most significant effect on reducing runoff, sediment and nutrient loss. In addition, Synergistic effect of NB and caragana showed the great potential on soil and water loss control. The findings in this study provide an efficient and economic method for controlling soil erosion and nutrient loss on the Chinese Loess Plateau.

Analysis of 15N-NO3− Via Anoxic Slurries Coupled to MIMS Analysis: An Application to Estimate Nitrification by Burrowing Macrofauna

The increasing use of the stable isotope 15N-NO3− for the quantification of ecological processes requires analytical approaches able to distinguish between labelled and unlabeled N forms. We present a method coupling anoxic sediment slurries and membrane inlet mass spectrometry to quantify dissolved 15N-NO3− and 14N-NO3−. The approach is based on the microbial reduction of 14N-NO3− and 15N-NO3− mixed pool, the determination of the produced 29N2 and 30N2, and the calculation of the original 15N-NO3− and 14N-NO3− concentrations. The reduction is carried out in 12 mL exetainers containing 2 mL of sediment and 10 mL of water, under anoxia. To validate this approach, we prepared multiple standard solutions containing 15N-NO3− alone or in combinations with 14N-NO3−, with final concentrations varying from 0.5 to 3000 µM. We recovered nearly 90% of the initial 14N-NO3− or 15N-NO3−, over a wide range of concentrations and isotope ratios in the standards. We applied this method to a 15N-NO3− dilution experiment targeting the measurement of nitrification in sediments with and without the burrower Sparganophilus tamesis. The oligochaete did not stimulate nitrification, likely due to limited ventilation and unfavorable conditions for nitrifiers growth. The proposed method is reliable, fast, and could be applied to multiple ecological studies.

Bacterial Community Shifts Driven by Nitrogen Pollution in River Sediments of a Highly Urbanized City.

Effects of nitrogen pollution on bacterial community shifts in river sediments remain barely understood. Here, we investigated the bacterial communities in sediments of urban and suburban rivers in a highly urbanized city, Shanghai. Sediment nitrate (NO3−) and ammonia (NH4+) were highly accumulated in urban river. Operation Taxonomic Units (OTUs), Abundance-based Coverage Estimators (ACEs) and Chao 1 estimator in urban rivers were slightly lower than those in suburban rivers, while Shannon and Simpson indices were higher in urban rivers than those in suburban rivers. Proteobacteria, Firmicutes, and Bacteroidetes were the dominant bacterial phylum communities, accounting for 68.5–84.9% of all communities. In particular, the relative abundances of Firmicutes and Nitrospirae were significantly higher in suburban rivers than in urban rivers, while relative abundances of Bacteroidetes, Verrucomicrobia, and Spirochaetes were significantly lower in suburban rivers than in urban rivers. NH4+ was significantly and negatively correlated with abundances of Firmicutes, Nitrospirae, and Actinobacteria. Importantly, the significant and negative effects of sediment NH4+ on bacterial richness and diversity suggested that nitrogen pollution likely contribute to the decrease in the bacterial richness and diversity. The results highlight that nitrogen enrichment could drive the shifts of bacterial abundance and diversity in the urban river sediments where are strongly influenced by human activities under the rapid urbanization stress.

Effects of shrimp-aquaculture reclamation on sediment nitrate dissimilatory reduction processes in a coastal wetland of southeastern China

The conversion of natural saltmarshes to shrimp aquaculture ponds can potentially influence the biogeochemical cycling of nutrients in coastal wetlands, but its impact on the dynamics of sediment dissimilatory nitrate (NO3−) reduction remains poorly understood. In this study, three sediment NO3− reduction processes including denitrification (DNF), anaerobic ammonium oxidation (ANAMMOX), and dissimilatory NO3− reduction to ammonium (DNRA) were examined simultaneously in a natural saltmarsh and two shrimp culture ponds (5- and 18-year-old) in July and November, using nitrogen (N) isotope-tracing experiments. Our results showed that sediment potential DNF, ANAMMOX and DNRA rates were generally higher in the shrimp culture ponds than the natural saltmarsh in the two seasons. The rates of all three processes generally increased with the age of shrimp ponds, with the magnitude of increase being less pronounced for DNF and ANAMMOX than DNRA. The contribution of DNRA to total NO3− reduction increased significantly following saltmarsh conversion to shrimp ponds, suggesting that DNRA became an increasingly important biogeochemical process under shrimp culture. DNRA competed with DNF and limited reactive N loss to some extent after natural saltmarshes converted to shrimp culture ponds. The results of redundancy analysis revealed that the availability of substrates and sulfides in sediments, rather than the bacteria gene abundance, were the most important factor influencing the NO3− reduction processes. Overall, our findings highlighted that shrimp-aquaculture reclamation may aggravate nitrogen loading in coastal wetlands by promoting the production of bioavailable ammonium.

The differentiation of iron-reducing bacterial community and iron-reduction activity between riverine and marine sediments in the Yellow River estuary

Rivers are the primary contributors of iron and other elements to the global oceans. Iron-reducing bacteria play an important biogeochemical role in coupling the iron and carbon redox cycles. However, the extent of changes in community structures and iron-reduction activities of iron-reducing bacteria in riverine and coastal marine sediments remains unclear. This study presents information on the spatial patterns and relative abundance of iron-reducing bacteria in sediments of the Yellow River estuary and the adjacent Bohai Sea. High-throughput sequencing of bacterial 16S rRNA found that the highest relative abundances and diversities were from the estuary (Yellow River–Bohai Sea mixing zone). Pseudomonas, Thiobacillus, Geobacter, Rhodoferax, and Clostridium were the most abundant putative iron-reducing bacteria genera in the sediments of the Yellow River. Vibrio, Shewanella, and Thiobacillus were the most abundant in the sediments of the Bohai Sea. The putative iron-reducing bacterial community was positively correlated with the concentrations of total nitrogen and ammonium in coastal marine sediments, and was significantly correlated with the concentration of nitrate in river sediments. The riverine sediments, with a more diverse iron-reducing bacterial community, exhibited increased activity of Fe(III) reduction in enrichment cultures. The estuary-wide high abundance of putative iron-reducing bacteria suggests that the effect of river–sea interaction on bacterial distribution patterns is high. The results of this study will help the understanding of the biogeochemical cycling of iron in riverine and coastal marine environments.

Evaluation of Monochloramine and Free Chlorine Penetration in a Drinking Water Storage Tank Sediment Using Microelectrodes.

Sediment accumulation in water storage tanks may protect microorganisms from disinfectant exposure, causing the degradation of water quality. However, microbial activity and disinfectant penetration within water storage sediment remain largely uncharacterized. This study evaluated the penetration of monochloramine and free chlorine into a 2 cm (20000 μm) deep drinking water storage tank sediment using microelectrodes. The sediment was successively exposed to monochloramine for 4 months, free chlorine for 2 months, and monochloramine for 2 months. Temporal monochloramine, free chlorine, dissolved oxygen (DO), pH, ammonium, nitrite, and nitrate profiles were acquired using microelectrodes. The results showed that complete monochloramine or free chlorine penetration was not observed. Likewise, DO never fully penetrated the sediment, progressing inward with time to a maximum depth of 10000 μm and indicating microbial activity persisted over the entire 8 months. Decreasing ammonium and increasing nitrate concentrations, with minimal nitrite accumulation, further demonstrated microbial activity and indicated complete sediment nitrification. There were measurable levels of ammonium, nitrite, and nitrate during free chlorine application, and nitrification activity gradually resumed upon a switch back to monochloramine. These findings suggest that the periodic removal of sediment from drinking water storage facilities is desirable to remove potentially protected environments for microorganisms.

Distribution Analysis of Nitrate and Phosphate in Coastal Area: Evidence from Pangkep River, South Sulawesi

Nitrate and phosphate nutrient and the other oceanographical parameter are part of an indicator to evaluate the quality of the water environment. Our research aims to determine the nitrate and phosphate levels in the water and sediment along the Pangkep coastal area. The research was conducted in February 2018 in coastal Pangkep using five sampling stations. Physico-chemical parameters of the water, such as temperature, pH, salinity, and dissolved oxygen were measured directly, whereas nitrate and phosphate parameters were analyzed using a spectrophotometer. In addition, nitrate and phosphate parameters from the sediment were measured using an atomic absorption spectrophotometer. The sediment nitrate values ranged from 0.026-0.442 mg/L, and the water nitrate values ranged from 0.018-0.063 mg/L. Moreover, the phosphate values from sediment ranged from 0.019-0.306 mg/L, and the phosphate values from the water ranged from 0.13-0.056 mg/L. The results show decreasing values of nitrate and phosphate concentration in the water and sediment starting from the estuary to the sea. The highest values were indicated in the estuary, which represents a eutrophic condition in the environment. Nutrient enrichment in coral reefs occurs and mixes with oceanographic conditions resulting in oxygen depletion and changes in benthic community structure.

Coupling of denitrification and anaerobic ammonium oxidation with nitrification in sediments of the Yangtze Estuary: Importance and controlling factors

Coupling of denitrification and anaerobic ammonium oxidation (anammox) with nitrification is an important mechanism for controlling nitrogen removal from aquatic ecosystems. In this work, we determined ambient rates of denitrification and anammox and their coupling links to nitrification in sediments of the Yangtze Estuary along a salinity gradient, using continuous-flow experiments combined with 15N isotope pairing technique. The rates of total denitrification and coupled nitrification-denitrification ranged from 0.44 to 7.03 μmol N m−2 h−1 and from below the detection limit to 4.26 μmol N m−2 h−1, respectively. The rates of total anammox and coupled nitrification-anammox varied from 0.09 to 1.21 μmol N m−2 h−1 and from below the detection limit to 0.73 μmol N m−2 h−1, respectively. It is shown that high availability of NO3− in bottom water as well as ferrous iron and organic carbon in sediments was beneficial to the uncoupled nitrogen removal processes, while low NO3− concentration in bottom water promoted the coupled nitrification-nitrogen removal processes. Along the salinity gradient, the coupled nitrification-nitrogen removal processes were the main nitrogen removal pathway in the Yangtze Estuary, and their contribution to total nitrogen removal was affected by NO3− concentrations, dissolved oxygen and salinity of sampling sites. Overall, this study improves the understanding of nitrate transformation processes in the estuarine and coastal environments.

Nitrification Rates and Pollution Characteristics of Sediments with Different Geomorphic Features in the Shiwuli Stream, Chaohu Lake Basin

Sediment and overlying water samples were collected seasonally from five different geomorphic structures (i.e., pools, riffles, gravel bars, point bars, and runs) from two urban reaches of the Shiwulihe River in the Chaohu Lake Basin dominated by high ammonia concentration between July 2017 and March 2018. Both the sediment potential and areal nitrification rates were measured and their seasonal and geomorphological variability were evaluated. The specific differences between every two geomorphic structures were determined using the Mann-Whitney U test and the relationship between the overlying water environment or benthic sediments and sediment nitrification was explored based on regression analysis. The results show that:① The studied reaches are seriously polluted by nitrogen and phosphorus and most of the oxidation-reduction potential (ORP) values in the overlying water were are below 0 mV, suggesting strong reducing conditions of the water column. ② The potential nitrification rates (PNRs) across the five geomorphic structures range from 0.002 to 0.079 μmol·(g·h)-1, with a mean value of 0.023 μmol·(g·h)-1. The ranking order of PNRs is pools > point bars > riffles > gravel bars > runs, with a seasonal change pattern of summer > spring > autumn > winter. ③ The areal nitrification rates (ANRs) across the five geomorphic structures range between 0.140 and 13.543 μmol·(m2·h)-1, with an average of 3.658 μmol·(m2·h)-1. In general, the highest mean value was observed in riffles, followed by runs, and gravel bars and point bars; the smallest value was observed in pools. In addition, ANRs appear to have seasonal change patterns similar to that of the PNRs. ④ According to the difference analysis, there are significant differences between pools or riffles and other features of the PNRs. Extremely significant ANR differences were observed between more than half of the geomorphic structures. ⑤ Regression analysis shows a stronger correlation between sediment nitrification and the overlying water environment, compared with the surface sediment properties.

Short exposure to oxygen and sulfide alter nitrification, denitrification, and DNRA activity in seasonally hypoxic estuarine sediments.

Increased organic loading to sediments from eutrophication often results in hypoxia, reduced nitrification and increased production of hydrogen sulfide, altering the balance between nitrogen removal and retention. We examined the effect of short-term exposure to various oxygen and sulfide concentrations on sediment nitrification, denitrification and DNRA from a chronically hypoxic basin in Roskilde Fjord, Denmark. Surprisingly, nitrification rates were highest in the hypoxic and anoxic treatments (about 5 μmol cm−3 d−1) and the high sulfide treatment was not significantly different than the oxic treatment. Denitrification in the hypoxic treatment was highest at 1.4 μmol cm−3 d−1 and significantly higher than the high sulfide treatment. For DNRA, the rate in high sulfide treatment was 2 μmol cm−3 d−1. This was significantly higher than all oxygen treatments that were near zero. In this system, nitrifiers rapidly recovered from conditions typically considered inhibiting, while denitrifiers had a more muted response. DNRA bacteria appear to depend on sulfide for nitrate reduction. Anammox was insignificant. Thus, in estuaries and coastal systems that experience short-term variations in oxygen and sulfide, capabilities of microbial communities are more diverse and tolerant of suboptimal conditions than some paradigms suggest.

Polychaete mucopolysaccharide alters sediment microbial diversity and stimulates ammonia-oxidising functional groups.

Sediment nitrogen cycling is a network of microbially mediated biogeochemical processes that are vital in regulating ecosystem functioning. Mucopolysaccharides (mucus) are produced by many invertebrates and have the potential to be an important source of organic carbon and nitrogen to sediment microorganisms. At present, we have limited understanding of how mucopolysaccharide moderates total sediment microbial communities and specific microbial functional groups that drive nitrogen cycling processes. To start addressing this knowledge gap, sediment slurries were incubated with and without Hediste diversicolor mucus. Changes in dissolved inorganic nitrogen (ammonia, nitrite and nitrate) concentrations and bacterial and archaeal community diversity were assessed. Our results showed that mucopolysaccharide addition supported a more abundant and distinct microbial community. Moreover, mucus stimulated the growth of bacterial and archaeal ammonia oxidisers, with a concomitant increase in nitrite and nitrate. Hediste diversicolor mucopolysaccharide appears to enhance sediment nitrification rates by stimulating and fuelling nitrifying microbial groups. We propose that invertebrate mucopolysaccharide secretion should be considered as a distinct functional trait when assessing invertebrate contributions to sediment ecosystem function. By including this additional trait, we can improve our mechanistic understanding of invertebrate-microbe interactions in nitrogen transformation processes and provide opportunity to generate more accurate models of global nitrogen cycling.

Evidence for complete nitrification in enrichment culture of tidal sediments and diversity analysis of clade a comammox Nitrospira in natural environments.

Complete ammonia oxidizers (comammox), as novel microbial communities, are predicted to play an important role in the nitrogen cycle. Here we reported the presence of complete nitrification in tidal sediments and examined the diversity and abundance of comammox in natural ecosystems. Metagenome and metatranscriptome of the enrichment culture from tidal sediments harbored the genes of comammox. Near-complete comammox AmoA/B/C- and Hao-like sequences showed close relationships to the known comammox (with sequence identity from 79 to 99%) rather than classical betaproteobacterial ammonia-oxidizing bacteria (β-AOB) (57 to 66%) and ammonia-oxidizing archaea (AOA) (24 to 38%). To analyze the diversity of comammox in natural environments, a new primer set targeting clade A comammox Nitrospira (COM-A) amoA genes was designed based on sequences obtained in this study and sequences from published database. In silico evaluation of the primers showed the high coverage of 89 and 100% in the COM-A amoA database. Application of the primers in six different ecosystems proved their strong availability. Community composition of COM-A suggested a relatively higher diversity than β-AOB in similar environments. Quantification results showed that COM-A amoA genes accounted for about 0.4-5.6% in total amoA genes. These results provide novel insight into our perception of the enigmatic comammox and have significant implications for profound understanding of complex nitrification process.

The distribution of sediment and water column nitrification potential in the hyper-turbid Ems estuary

Estuaries, being transition zones between land and ocean, act as sink or source of nitrate and thus influence the conditions in adjacent coastal waters. Hence, nitrification, which is the process oxidizing ammonium via nitrite to nitrate and simultaneously consuming oxygen, is important in estuaries. The process has been studied in sediment and water column of many estuaries, but seldom in both estuarine compartments at the same time. In August 2014, we collected water and sediment samples during a sampling trip along the salinity gradient of the hyper-turbid Ems estuary, which ends up in the North Sea. We conducted nitrification incubations in microcosms to determine nitrification potentials and we measured a suite of abiotic factors like oxygen saturation, salinity, and dissolved inorganic nitrogen (DIN). Two approaches were used, one isotope dilution method for net (NNP) and gross (GNP) nitrification potentials and one method with substrate addition for substrate induced nitrification potentials (SNP). The long-term incubation set-ups of several days include inseparably nitrification-coupled processes like remineralization and nitrate consumption, as well as cell growth, and hence they do not represent in-situ rates of nitrification. DNA was also isolated and used for quantitative PCR of the archaeal and bacterial amoA genes, which encode for the ammonia-oxidizing enzyme ammonia monooxygenase (AMO). Nitrification varied over the salinity gradient of the estuary. GNP in water and sediments decreased with increasing salinity. No NNP could be measured in the sediments of the oligohaline part of the estuary, while SNP was four-fold higher than GNP in this part of the estuary. Generally, the gene abundance of the amoA gene was higher in the oligohaline/mesohaline area than in the polyhaline area, and archaea dominated the ammonia-oxidizing communities in all samples. The local similarity in partitioning of archaeal and bacterial amoA genes over the water column and sediment at each sampling station along the estuarine gradient implied a link between the archaeal and bacterial ammonia oxidizers in both compartments, which is likely due to resuspension of sediment particles in the water column of this hyper-turbid estuary.

Arbuscular mycorrhizal (AM) status in urban wetland plants and its impact factors

Urban wetlands play an important role in improving urban environment and microclimate, while they are suffering degradation and destruction during urbanization process. Arbuscular mycorrhizae may enhance plant tolerance to environmental stresses and increase vegetation restoration in wetlands. However, presently little is known how about arbuscular mycorrhizal (AM) symbiosis in urban wetland plants and the factors affecting AM formation. Here, a survey for AM status in urban wetland plants was done in Beijing area, and the factors affecting AM formation were also discussed, such as water and sediment characteristics. Results showed that 87.5% of plants (49 of 56 species) were colonized by AM fungi. Mycorrhizal colonization rates (MCRs) ranged from 2% to 72%, while most of them were low level (<25%). The highest mycorrhizal colonization rate (MCR) was observed in Tephroseris palustris. Relationships between MCRs and water properties as well as sediment properties were analyzed by Pearson’s correlation analysis. MCRs in Phragmites australi were negatively correlated with water ammonium nitrogen and a total dissolved phosphorus, while were positively correlated with nitrate nitrogen, nitrite nitrogen and total organic carbon (TOC) in sediment. MCRs in Typha orientalis were negatively correlated with water oxidation-reduction potential while were positively correlated with sediment TOC. MCRs in Glyceria maxima were positively correlated with sediment nitrate nitrogen. MCRs showed seasonal and temporal shift, while the variation was related to plant species. This study indicates that AM symbiosis widely exists in urban wetland plants, while AM formation in various plants is affected by different water and sediment properties.

Intracellular nitrate in sediments of an oxygen-deficient marine basin is linked to pelagic diatoms.

Intracellular nitrate is an important electron acceptor in oxygen-deficient aquatic environments, either for the nitrate-storing microbes themselves, or for ambient microbial communities through nitrate leakage. This study links the spatial distribution of intracellular nitrate with the abundance and identity of nitrate-storing microbes in sediments of the Bornholm Basin, an environmental showcase for severe hypoxia. Intracellular nitrate (up to 270 nmol cm-3 sediment) was detected at all 18 stations along a 35-km transect through the basin and typically extended as deep as 1.6 cm into the sediment. Intracellular nitrate contents were particularly high at stations where chlorophyll contents suggested high settling rates of pelagic primary production. The depth distribution of intracellular nitrate matched that of the diatom-specific photopigment fucoxanthin in the upper 1.6 cm and calculations support that diatoms are the major nitrate-storing microbes in these sediments. In contrast, other known nitrate-storing microbes, such as sulfide-oxidizing bacteria and foraminifers, played only a minor role, if any. Strikingly, 18S rRNA gene sequencing revealed that the majority of the diatoms in the sediment were pelagic species. We conclude that intracellular nitrate stored by pelagic diatoms is transported to the seafloor by settling phytoplankton blooms, implying a so far overlooked 'biological nitrate pump'.