Differences In Denitrification Rates Research Articles

Comparing the effects of temperature and C/N ratios on denitrification between point bar and straight channel sediments

The riverbed and riparian zone are the hotspots of nitrogen removal in rivers. Denitrification plays an important role in the process of nitrogen removal. In this study, sediments from the straight channel riverbed and point bars were examined using indoor culture and acetylene inhibition techniques. The primary focus was to investigate the differences in denitrification rates between these sediments. The secondary focus was on the effects of temperature and carbon-nitrogen (C/N) ratios on the denitrification process. The results showed that denitrification rates were higher in point bar sediments compared to those in the straight channel bed. Denitrification rates were also higher in the near water area of the point bar than in the near bank area. Denitrification rates at 30 °C were significantly higher than at 10 °C for each sediment group as well as in samples having carbon and nitrogen amendments. Notably, when the C/N ratio was 2.5, the denitrification rate for samples with carbon and nitrogen additions was significantly higher than for samples without them. On average, denitrification rates increased by 2.56 times, 2.73 times and 2.87 times for sediments from the straight channel, inner part of the point bar, and near bank of the point bar, respectively. At the same time, the number of denitrifying bacteria was found to be higher in the point bar than in the straight channel. These findings highlight the beneficial role of point bars in accelerating sediment denitrification rates and emphasize the potential for increasing nitrogen removal capacity in rivers by optimizing temperature and C/N ratios. Changing the diversity of riverbed geomorphology, temperature and C/N can effectively enhance the nitrogen removal capacity of river ecosystems.

The co-regulation of nitrate and temperature on denitrification at the sediment-water interface in the algae-dominated ecosystem of Lake Taihu, China

Sediment denitrification is a dominant mechanism for nitrogen removal and can help to minimize lake eutrophication. However, the spatio-temporal variability of denitrification rates and its controlling factors in sediments of large shallow lakes are poorly understood. In this study, we investigated the controlling factors on the temporal and spatial variability of denitrification rates in Meiliang Bay in Taihu Lake, China, to determine the contribution of denitrification on the total lacustrine nitrogen budget. We collected 18 intact monthly sediment cores and an additional 36 seasonal sediment cores from January 2013 to January 2014. Cores were collected from the inner and outer sections of Meiliang Bay for analysis of denitrification rates and sediment properties. We also collected in situ surface water samples for water quality analysis. Denitrification rates at the sediment-water interface (SWI) were measured using acetylene inhibition techniques and intact sediment core incubation. We used a t test to determine the differences in water quality and sediment properties between the two sites and a one-way ANOVA to identify seasonal differences in denitrification rates, water quality, and sediment properties. We also applied Pearson’s correlation, distance-based redundancy analysis (db-RDA) and random forest model to identify the relationships between denitrification rates and environmental factors. Denitrification rates ranged from 0.76 to 40.94 μmol N m−2 h−1 and 0.13 to 52.55 μmol N m−2 h−1, with annual mean values of 19.97 and 17.15 μmol N m−2 h−1 for the Inner and Outer Bay, respectively. Sediment denitrification rates in the Inner and Outer Bay showed similar seasonal variability, with the highest values in spring and summer and the lowest values in autumn. Nitrate addition was shown to significantly increase denitrification rates in summer and autumn (P < 0.05); however, carbon addition showed no significant influence on denitrification rates in the four seasons. Our results imply that denitrification rates were nitrate-limited in summer and autumn. Distance-based redundancy analysis (db-RDA) and random forest model showed that denitrification rates were mainly determined by nitrate, temperature, and chlorophyll a (Chla), and that nitrate in the water column was the most important predictor of denitrification rates. In general, denitrification rates showed significant seasonal variability in Meiliang Bay due to the co-regulation of both water temperature and nitrate concentrations. The dominance of each controlling factor on denitrification rates varied in different seasons. Based on our calculations, nitrogen removal by denitrification accounted for approximately 10.7% of the total nitrogen input to Taihu Lake. Therefore, we suggest the need for effective measures to reduce external nitrogen inputs of to Lake Taihu to prevent on-going eutrophication.

Development and Evaluation of a Sustainable Long-release Carbon Material Applied for In-Situ Remediation of Groundwater Nitrogen Pollution

Due to slow flow rates and inter-substance reactions in groundwater, remediation requires the addition of materials with sustained release properties. This research uses agricultural waste and zero-valent iron (Fe0), coupling biology and chemistry, to research and develop a sustainable long-release carbon material with a synergistic physico-habitat, and to evaluate its performance, also taking into account the occurrence of nitrogen in groundwater. The material developed has a double-layer structure with an inner core and an outer shell. The core, consisting of agricultural waste, Fe0, and other raw materials, constitutes the repair layer. Agricultural waste provides a carbon source for microorganisms, and Fe0 can quickly remove nitrate via chemical reactions and reduce DO to develop an anaerobic environment in water. The shell provides a solute permeation layer and consists of primary minerals and other components. This can slow the release of TOC from the core and adsorb secondary contaminants. Physical properties testing showed that the materials core was uniformly cross-linked, and its shell exhibited a clear uniform pore structure (SEM). Favorable mechanical compression was recorded for particle strengths of up to 80-105 N. With a density of 1.1 g·cm-3, the material did not float in water. Experiments showed that the material had excellent sustained release. The amount[Max:21-25 mg·(g·L)-1] and rate[Max:0.185 mg·(g·L·d)-1] of TOC release exhibited a steady state trend, but fluctuated greatly in the case of agricultural wastes[(Max amount:53-75 mg·(g·L)-1, Max rate:0.455 mg·(g·L·d)-1]. In terms of further functional gene abundance, materials leachate was found to be conducive to denitrifying bacteria. In early denitrification and oxygen trapping experiments, the Fe0 chemical reaction was dominant for reduction of nitrogen and DO, facilitating denitrification. However, biological denitrification gradually dominated. Differences in denitrification rates between iron-free and iron-containing materials were smaller, as was the correlation between denitrification rate and iron content. These results indicate the formation of physico-habitat synergistic denitrification in the materials system.

Hydrologic and Biogeochemical Drivers of Riparian Denitrification in an Agricultural Watershed

This study investigated drivers of denitrification and overall NO3 − removal in an agricultural riparian area in central New York. Denitrification was measured using an in situ “push-pull” method with 15N–NO3 − as a tracer during summer and fall 2011 at a pair of riparian sites characterized by different hydrologic regimes. Median denitrification rates were 1347 and 703 μg N kg soil−1 day−1 for the two study sites. These rates are higher than those reported for other riparian areas, emphasizing the role of some riparian areas as hotspots of NO3 − removal. N2O production was significantly higher at one site, demonstrating that riparian areas can be a greenhouse gas source under certain conditions. Denitrification was negatively correlated with groundwater flux, suggesting that slower flushing of water, and thus longer residence time, promotes denitrification. A mass balance of NO3 − loss revealed that denitrification only accounted for 5–12 % of total NO3 − loss, and production of NH4 + indicated that dissimilatory NO3 − reduction to NH4 + (DNRA) may be occurring at both sites. While both sites were characterized by high NO3 − removal, differences in denitrification rates and NO3 − removal processes demonstrate the need to improve our ability to capture spatial and process heterogeneity in landscape biogeochemical models.

Denitrification in restored and unrestored Danish streams

Stream restoration often aims at mitigating nutrient pollution in aquatic ecosystems. However, despite recent research efforts, effects of restoration practices on in-stream nitrogen removal remain unclear. In this study, denitrification rates as well as factors controlling denitrification in unrestored and restored sections of two Danish streams (S1 and S2) were compared. The 15N isotope pairing technique was used to measure denitrification in situ. Denitrifier presence was analyzed by denaturing gradient gel electrophoresis (DGGE) and quantitative PCR of nitrite reductase (nirK and nirS) and nitrous oxide reductase (nosZ) genes. Denitrification rates were highly variable, with denitrification rates of 3106μmolNm−2h−1 in the unrestored section of S1, but no detectable denitrification in the restored section of S1, whereas in S2 restored and unrestored sections had similar denitrification rates of around 250μmolNm−2h−1. These large differences in denitrification rates were mainly due to differences in hydrologic conditions and sediment characteristics. High nitrate fluxes from upwelling groundwater created denitrification hotspots in the unrestored section of S1. Moreover, a lack of organic matter in the restored section of S1 likely caused a low abundance of denitrifiers and consequently no detectable denitrification. Our results indicate the importance of hydrology and sediment organic matter for stream nitrogen dynamics, which should be considered in restoration design.

Comparison of 15N2 flux and acetylene inhibition denitrification methods in Atchafalaya River basin sediments

Flooded swamps in the Atchafalaya River basin, Louisiana, USA, are thought to be a major mechanism for removing excess riverine nitrate that may be causing hypoxia in coastal Louisiana. Two denitrification methods, the 15N2 flux and acetylene inhibition technique, were compared and evaluated. Sediment samples were collected from three baldcypress swamp sites located within the Atchafalaya River basin. Highly 15N-labeled (61.7 at.%) and unlabeled nitrate (100 mg NO3–N L−1) were applied to the floodwater of separate sets of sediment cores. Denitrification potential was measured 2, 6, 10, and 16 days after nitrate addition. Two days after nitrate addition, no significant differences in denitrification rates were observed between methods. On days 6, 10, and 16, denitrification measured by the 15N2 method was higher compared to the acetylene inhibition procedure. When significance was observed, 15N2 denitrification was approximately 5.3 times greater.

Influence of Plant Communities on Denitrification in a Tidal Freshwater Marsh of the Potomac River, United States

We investigated whether marsh surface elevation, plant community composition (annuals vs. perennials), and organic matter quantity/quality were associated with differences in denitrification rates in an urban tidal freshwater marsh of the Potomac River, United States. We measured denitrification rates using both denitrification enzyme activity (DEA) with acetylene inhibition (June: n = 38, 3234 +/- 303; October: n = 38, 1557 +/- 368 ng N g dry soil(-1) h(-1)) and direct N(2) flux measurements with membrane inlet mass spectrometry (MIMS) (November: n = 6, 147 +/- 24 mumol m(-2) h(-1)). Organic carbon content and nitrate concentrations in soil, and plant community composition were correlated with elevation, but DEA rates did not differ across marsh surface elevation. Soil organic carbon was highest in plots dominated by perennial graminoids, but DEA rates did not differ across plant community types. The DEA rates increased with increasing soil ammonium concentrations and total N content, and DEA rates differed between summer and fall sampling. The MIMS rates did not differ across plant community types, but were correlated with soil organic N content. Denitrification rates suggest that potential N removal at the site could be substantial. In addition, denitrification rates measured in Dyke Marsh were higher than rates for sediments measured in the adjacent Potomac River. Tidal freshwater marshes can represent an important site for denitrification, and factors fostering denitrification should be considered when restoring urban tidal freshwater wetlands as they are faced with pressures from increasing land use change and sea level rise.

Quantification of denitrification potential in carbonaceous trickling filters

Biofilm samples from a carbonaceous trickling filter (TF) were evaluated in bench scale reactors to determine their maximum potential denitrification rates. Intact, undisturbed biofilms were placed into 0.6 L bench-scale reactors filled with sterilized, primary clarifier effluent spiked with nitrate to a final concentration of 16–18 mg/L as N. Dissolved oxygen concentrations were maintained between 2 and 4 mg/L in the bulk aqueous phase. Nitrate loss from the reactors was monitored over a 5 h period. Denitrification rates of 3.09–5.55 g-N/m 2 day were observed with no initial lag period. This suggests that the capacity for denitrification is inherent in the biofilm and that denitrification can take place even when oxygen is present in the bulk aqueous phase. There were no significant differences in denitrification rates per unit area of media (g–N/m 2 day) either between (a) experimental runs or (b) sampling locations over the trickling filter. This suggests that denitrification potentials are uniform over the entire volume of the full-scale TF. For wastewater treatment plants with TFs that currently nitrify downstream, this approach may be used to meet less stringent permitted discharge concentrations and may allow some facilities to postpone or eliminate construction of additional unit processes for denitrification.

Nitrogen loss through denitrification in a soil under pasture in New Zealand

Denitrification on several contrasting topographical sites in a New Zealand dairy-farm pasture was measured periodically over a year, using the acetylene inhibition technique, by incubating undisturbed soil cores in a closed system. The measured denitrification rates varied considerably both spatially and temporally. High coefficients of variation (CV) and log-normal distributions of denitrification rate were often observed. The spatial variance in denitrification rate changed temporally and was apparently related to soil moisture content and the grazing pattern. Denitrification rates followed a marked seasonal pattern, with highest rates being measured during the wet winter and lowest rates during the dry summer and early autumn. Differences in denitrification rates among sites were not consistent. However, slightly higher denitrification rates were usually detected in the floor of a gully and in a gateway area than on other sites. Mean denitrification rates from individual dates were positively correlated to soil moisture content. However, there was a negative correlation between denitrification rate and soil nitrate concentration, respiration rate and temperature. An annual nitrogen loss of 4.5 kg N ha −1 through denitrification was estimated in this legume-based dairy-farm pasture. Low soil moisture content was the primary factor limiting denitrification during the dry summer and early autumn. Low denitrification rates were also caused by lack of available soil NO 3 −-N.

Characterization of dissolved organic carbon in cleared forest soils converted to maize cultivation

The impact of cultivation on the nature of dissolved organic carbon (DOC) has been studied in order to understand differences in denitrification rates observed in situ. The quantitative aspect of DOC was studied by means of porous-cup vacuum samplers and water extraction. Qualitative characterization was achieved using δ 13C analysis and tangential ultrafiltration with nominal cut-offs at 10 000 and 100 000 Da. During the first stage, cultivation intensified mineralization of the initial organic matter. This led to an increase of 2- to 5-fold in the quantity of DOC, with an important spatial variability. This phenomenon generated mainly medium molecules (MM) and large molecules (LM). These molecules were supposed to be fulvic and humic acids, and large colloids or humic metal complexes. During the second stage, once the original organic matter is stabilized, DOC concentrations decreased with time of cultivation. Both soil organic carbon and DOC were enriched in carbon originating from maize with time of cultivation. DOC was mainly composed of small molecules (SM) and MM. These molecules might be organic or amino acids, polysaccharides, or fulvic acids. From a methodological point of view, it is noteworthy that DOC characteristics determined following extraction by means of porous cups differed from those for water extraction.

The relative role of denitrification and immobilization in the fate of inorganic nitrogen in mangrove sediments (Terminos Lagoon, Mexico)

Rates of direct and coupled nitrification‐denitrification were measured with 15N isotope techniques in intact sediment cores from fringe, basin, and riverine mangroves in Terrminos Lagoon, Mexico. We used intact sediment cores to evaluate differences in denitrification rates. The lack of 15N2 production in all cores amended with 200 (or 100) µmol core‐1 15NH4+ and the high recovery of 15N in the sediment in five of eight such experiments indicate that coupled nitrification‐denitrification was riot important to nitrogen transformation during the 8–10‐d incubations. However, when we added 450 µmol core−1 15NH4+ and 15NO3− to cores from the riverine mangrove, N2 production rates ranged from 28.9 to 221.1 µmol N m−2 h−1 for each nutrient. We compare coupled nitrification‐denitrification rates among different types of mangrove forests under natural conditions in the neotropics. Our results with nitrogen isotopes suggest that the uptake of inorganic N from tidal waters within mangrove ecosystems does not necessarily represent a nitrogen sink via denitrification, but rather a retention of nitrogen in mangrove sediments.

Heterotrophic denitrification potential as an adaptive response in groundwater bacteria

The denitrification rate of bacteria indigenous to aquifers with different nitrate concentrations was investigated. The objective was to test how high nitrate loads affect community composition and select for enhanced nitrate reduction rates among denitrifying strains. Groundwater bacteria were isolated from two aquifers with high in situ concentrations of nitrate (24.1 and 35.2 mg NO 3 −-N 1 −1) and one aquifer with a lower in situ concentration (6.3 mg NO 3 −-N 1 −1). The isolates were characterized with respect to 15 colony and cell morphological and 8 physiological features. The frequency of heterotrophic denitrifiers of all isolated strains was 45–50% in the nitrate contaminated aquifers compared to slightly more than 20% in the uncontaminated aquifer. Kinetics of nitrate reduction were measured by using progress curves ot nitrous oxide production. Bacterial respiration was measured to determine whether proposed differences in denitrification rates were due to differences in carbon utilization. Strains from the aquifers with enhanced nitrate concentrations showed shorter lag periods and higher growth rates than strains from the aquifer with low nitrate concentration. Denitrification rates for unamended incubations varied between 0.10 and 0.42 ml cells −1 day −1 and between 0.42 and 2.10 ml cells −1 day −1 for the glucose amended incubations. The average denitrification rate for the strains from the contaminated sites was 2-fold higher than for the strains from the uncontaminated site, and glucose amendment increased the difference to 2.5-fold. The differences in rates were not explained by differences in carbon utilization. The strains from the contaminated sites reduced more nitrate per unit of carbon dioxide produced than the strains from the uncontaminated aquifer. This suggests that strains in soils which have been heavily N-fertilized may have developed denitrification reductases with optimum activities at high in situ concentrations of nitrate.

Heterotrophic denitrification potential as an adaptive response in groundwater bacteria

The denitrification rate of bacteria indigenous to aquifers with different nitrate concentrations was investigated. The objective was to test how high nitrate loads affect community composition and select for enhanced nitrate reduction rates among denitrifying strains. Groundwater bacteria were isolated from two aquifers with high in situ concentrations of nitrate (24.1 and 35.2 mg NO3−-N 1−1) and one aquifer with a lower in situ concentration (6.3 mg NO3−-N 1−1). The isolates were characterized with respect to 15 colony and cell morphological and 8 physiological features. The frequency of heterotrophic denitrifiers of all isolated strains was 45–50% in the nitrate contaminated aquifers compared to slightly more than 20% in the uncontaminated aquifer. Kinetics of nitrate reduction were measured by using progress curves ot nitrous oxide production. Bacterial respiration was measured to determine whether proposed differences in denitrification rates were due to differences in carbon utilization. Strains from the aquifers with enhanced nitrate concentrations showed shorter lag periods and higher growth rates than strains from the aquifer with low nitrate concentration. Denitrification rates for unamended incubations varied between 0.10 and 0.42 ml cells−1 day−1 and between 0.42 and 2.10 ml cells−1 day−1 for the glucose amended incubations. The average denitrification rate for the strains from the contaminated sites was 2-fold higher than for the strains from the uncontaminated site, and glucose amendment increased the difference to 2.5-fold. The differences in rates were not explained by differences in carbon utilization. The strains from the contaminated sites reduced more nitrate per unit of carbon dioxide produced than the strains from the uncontaminated aquifer. This suggests that strains in soils which have been heavily N-fertilized may have developed denitrification reductases with optimum activities at high in situ concentrations of nitrate.

Soil denitrification rates in a subalpine watershed of the eastern Sierra Nevada

Denitrification rates in soils of six subalpine plant communities in an eastern Sierra Nevada watershed were determined by the acetylene inhibition method. Soil atmosphere samples were collected monthly from June 1986 through May 1987 in a riparian, wet meadow, dry meadow, north-facing forest, south-facing forest and barren site and analyzed for N 2O content using gas chromatography. Soil temperature, moisture, organic matter, C,N,C:N ratio, NO 3 N and pH were examined to assess their effects on denitrification rates. Mean denitrification rates for the year varied from 103.3 μg m −2h −1 in the north-facing forest to 120.2 μg m −2h −1 at the barren site, but did not differ significantly among any of the six plant communities. However, comparisons among months within individual communities revealed that the denitrification rates in each community varied significantly over the year, and in three of the six sites significant correlations between denitrification rates and other soil parameters were detected. Soil acidity was positively correlated with denitrification rate in the riparian and wet meadow communities, and in the dry meadow, soil moisture was positively correlated while soil temperature and organic matter were negatively correlated with denitrification rate. Comparisons among sites within individual months revealed significant differences in denitrification rates in June, September, October and January, but no single site consistently exhibited the highest or lowest rate in all 4 months, and only in October, when denitrification rates were positively correlated with soil temperature and moisture, was variation in denitrification rates among sites explained by other soil parameters. For the six plant communities overall, soil denitrification rates were highly variable from June to October, increased sharply from October to December, and then declined from December to May.

Denitrification under different cultivated plants: effects of soil moisture tension, nitrate concentration, and photosynthetic activity

Plant effects on the denitrification rate were investigated in pot experiments at different soil moisture tensions and nitrate concentrations. Nitrate concentrations and the soil moisture tension were regulated immediately before each measurement. The effects of the plants on denitrification rates were dependent on the soil moisture tension. At a low soil moisture tension (−7 cm H2O), there was a 10-fold increase in the denitrification rate (planted versus unplanted soil). At a medium moisture tension (−30 cm H2O) the plants had practically no effect, and at the highest tension (−60 cm H2O) the effect was slightly negative. Large differences in denitrification rates under different plant species were observed. At a low soil moisture tension, the average denitrification rate (μg N kg−1 soil h−1) was 39–42 under small grains (barley, wheat, and oats), 47–82 under the grasses (cocksfoot, meadow grass, meadow fescue, and timothy) and 18 under red clover. The differences between the monocots were attributable to differences in plant growth rates, rather than to any specific difference in stimulation or inhibition of denitrification, since the variations in photosynthetic activity fairly well predicted the differences in denitrification rates under different monocots. Clover, however, gave much lower denitrification rates than those predicted by the photosynthetic activity.