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Abstract The Pearl River Estuary (PRE) has long received tremendous amounts of anthropogenic nitrogen, and is facing severe environmental problems. Denitrification and anaerobic ammonium oxidation (anammox) are known to be two major nitrogen removal pathways in estuarine sediments. Through the use of slurry and intact sediment core incubations, we examined the nitrogen removal pathways and quantified the in situ denitrification and anammox with associated gaseous nitrogen production rates. Sedimentary nitrogen removal was predominated by denitrification (93–100%) relative to a minimal contribution (<7%) from anammox. Among the detected environmental factors, salinity, bottom water NO x − (nitrate and nitrite) concentration, sedimentary organic matter and dissolved oxygen consumption rates showed good correlations with denitrification and anammox rates. Sedimentary nitrogen loss was mainly supported by endogenic coupled nitrification-denitrification (6.0 ± 1.5 × 106 mol N d−1), with water-column-delivered NO x − (2.1 ± 0.6 × 106 mol N d−1) as the secondary source. Such results suggested that sedimentary nitrogen removal involved mainly particulate organic form (allochthonous or autochthonous) deposited onto sediments, rather than inorganic forms in overlying water. Meanwhile, total N 2 O production from sediments was estimated to be 7.3 ± 2.1 × 104 mol N d−1, equivalent to ~35% of the daily N 2 O emissions in the PRE. Graphical abstract Unlabelled Image Highlights • Sedimentary nitrogen removal was predominated by denitrification relative to a minimal contribution from anammox. • Sedimentary nitrogen removal involved mainly particulate organic form deposited onto sediments. • The sediments acted as an important N 2 O source, equivalent to ~35% of the daily N 2 O emission in the Pearl River Estuary. [ABSTRACT FROM AUTHOR]

In this study the impact of hydrocarbons on nitrogen cycling in intertidal sediments from a barrier island in the north central Gulf of Mexico was investigated. Initial total petroleum hydrocarbon (TPH) concentrations of the sediments averaged 49 ± 27 mg kg−1 in June and 75 ± 48 mg kg−1 in November. Following the addition of crude oil, TPH concentrations increased to 340 ± 73 and 296 ± 50 mg kg−1 in June and November, respectively. Sediment oxygen demand (SOD), inorganic nitrogen and phosphorus fluxes, and denitrification capacity, measured with the isotope pairing technique (IPT) within a week of oiling, were consistently higher in June compared to rates in November when temperatures were 10°C lower. Despite significantly higher TPH concentrations in the oiled treatments, SOD, nutrient fluxes, nitrogen fixation, denitrification, and anammox, did not differ between the treatments. Potential denitrification rates, measured in the presence of excess nitrate (100 μM), were similar between the control and oil treatments, but significantly higher than rates measured with the IPT. The elevated rates of potential denitrification suggest the presence of an active extant microbial community capable of significant attenuation of porewater nitrate concentrations. The insignificant changes in nitrogen cycling in response to petroleum hydrocarbon additions at a site with hydrocarbons present, albeit at low concentrations, suggests resiliency in nitrogen cycling in response to additional hydrocarbon inputs in ranges investigated in this study. [ABSTRACT FROM AUTHOR]

Understanding the magnitude of nitrogen (N) loss and recycling pathways is crucial for coastal N management efforts. However, quantification of denitrification and anammox by a widely-used method, the isotope pairing technique, is challenged when dissimilatory NO 3 − reduction to NH 4 + (DNRA) occurs. In this study, we describe a revised isotope pairing technique that accounts for the influence of DNRA on NO 3 − reduction (R-IPT-DNRA). The new calculation procedure improves on previous techniques by (1) accounting for N 2 O production, (2) distinguishing canonical anammox from coupled DNRA-anammox, and (3) including the production of 30 N 2 by anammox in the quantification of DNRA. This approach avoids the potential for substantial underestimates of anammox rates and overestimates of denitrification rates in systems where DNRA is a significant NO 3 − reduction pathway. We apply this technique to simultaneously quantify rates of anammox, denitrification, and DNRA in intact sediments adjacent to a seagrass bed in subtropical Australia. The effect of organic carbon lability on NO 3 − reduction was also addressed by adding detrital sources with differing C:N (phytoplankton- or seagrass-derived). DNRA was the predominant pathway, contributing 49–74% of total NO 3 − reduction (mean 0.42 µmol N m −2 h −1 ). In this high C:N system, DNRA outcompetes denitrification for NO 3 − , functioning to recycle rather than remove N. Anammox exceeded denitrification (mean 0.18 and 0.04 µmol N m −2 h −1 , respectively) and accounted for 64–86% of N loss, a rare high percentage in shallow coastal environments. Owing to low denitrification activity, N 2 O production was ∼100-fold lower than in other coastal sediments (mean 7.7 nmol N m −2 h −1 ). All NO 3 − reduction pathways were stimulated by seagrass detritus but not by phytoplankton detritus, suggesting this microbial community is adapted to process organic matter that is typically encountered. The R-IPT-DNRA is widely applicable in other environments where the characterization of co-existing NO 3 − reduction pathways is desirable. [ABSTRACT FROM AUTHOR]

Microbial nitrogen (N) removal pathways in planted ( Canna indica L.) and unplanted integrated vertical-flow constructed wetland systems (IVCWs) were investigated. Results of, molecular biological and isotope pairing experiments showed that nitrifying, anammox, and denitrifying bacteria were distributed in both down-flow and up-flow columns of the IVCWs. Further, the N transforming bacteria in the planted IVCWs were significantly higher than that in the unplanted ones ( p < 0.05). Moreover, the potential nitrification, anammox, and denitrification rates were highest (18.90, 11.75, and 7.84 nmol N g −1 h −1 , respectively) in the down-flow column of the planted IVCWs. Significant correlations between these potential rates and the absolute abundance of N transformation genes further confirmed the existence of simultaneous nitrification, anammox, and denitrification (SNAD) processes in the IVCWs. The anammox process was the major N removal pathway (55.6–60.0%) in the IVCWs. The results will further our understanding of the microbial N removal mechanisms in IVCWs. [ABSTRACT FROM AUTHOR]

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification-denitrification, net denitrification, DNRA, and biogeochemical fluxes (O, NO, NH, CO, NO, and CH) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO concentrations (using N isotope tracing) interact to affect N cycling processes. At the lowest NO concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O, NO, NH fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems. [ABSTRACT FROM AUTHOR]

naerobic ammonium oxidation (anammox), the biochemical process oxidizing ammonium into dinitrogen gas using nitrite as an electron acceptor, has only been recognized for its significant role in the global nitrogen cycle not long ago, and its ubiquitous distribution in a wide range of environments has changed our knowledge about the contributors to the global nitrogen cycle. Currently, several groups of methods are used in detection of anammox bacteria based on their physiological and biochemical characteristics, cellular chemical composition, and both 16S rRNA gene and selective functional genes as biomarkers, including hydrazine oxidoreductase and nitrite reductase encoding genes hzo and nirS, respectively. Results from these methods coupling with advances in quantitative PCR, reverse transcription of mRNA genes and stable isotope labeling have improved our understanding on the distribution, diversity, and activity of anammox bacteria in different environments both natural and engineered ones. In this review, we summarize these methods used in detection of anammox bacteria from various environments, highlight the strengths and weakness of these methods, and also discuss the new development potentials on the existing and new techniques in the future. [ABSTRACT FROM AUTHOR]

The isotope pairing technique was used to obtain the first sedimentary denitrification rates for the St. Lawrence Estuary/Gulf system (Que´bec, Canada). Denitrification rates were measured on intact sediment cores collected at three stations down the slope of the Laurentian Trough at 103, 200 and 320 m depth. For samples collected in June 1997, total denitrification rates ranged from 1.8 to 3.3 μmol N m−2 h−1 among the three stations. Only 3–5% of the NO3− consumed by the sediments was subsequently denitrified. Nitrification was the major source of NO3− for denitrification, representing 56–79% of total denitrification rates. Sediment consumption of water column O2 (102–187 μmol O2 m−2 h−1) and NO3− (13.9–25.5 NO3− m−2 h−1) increased significantly downslope, as did total denitrification rates. Denitrification made a negligible (<0.2%) contribution to carbon mineralization in our cores, but may have a greater impact on the nitrogen cycle. In addition, data suggest the presence of a major dissimilatory nitrate sink in these sediments that far exceeds the requirements of denitrification. The latter observation is considered in terms of proposed interactions between the nitrogen cycle and metal reduction/oxidation. We conclude by considering limitations to the extrapolation of these data to annual and regional scale nitrogen budgets for the deep waters of the St. Lawrence Estuary. [Copyright &y& Elsevier]