Nutrient and microbiological characteristics of fine benthic organic matter in sediment settling ponds

1. Fine benthic organic matter (FBOM, particles <1 mm) was collected eight times in 1995 and 1996 from settling ponds located at the base of five catchments, and assayed for total C, N and P, extractable ammonium, mineralisable N, organic P, labile polysaccharides, denitrification potential, acetylene reduction and respiration rates, and β‐glucosidase and phosphatase activities. The five catchments (10–101 ha in size) are located in the Pacific North‐west of the United States. They contain either old‐growth forests dominated by Douglas‐fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) or stands that were harvested 30 years ago and replanted with Douglas‐fir, with riparian zones dominated by red alder (Alnus rubra), bigleaf and vine maple (Acer macrophyllum; A. circinatum) and understory herbaceous plants. 2. C : N ratios were significantly higher, and mineralisable N, extractable ammonium and labile polysaccharides were all significantly lower, in FBOM from old‐growth catchment sediment than in FBOM from catchments containing harvested stands, showing that the chemical characteristics of FBOM were influenced by forest harvest. C and N concentrations were greatest in sediment from old‐growth catchments, but microbial activities (respiration, denitrification potential, phosphatase and β‐glucosidase) tended to be greater in sediment from the harvested catchments. 3. Levels of certain chemical components of harvested‐catchment FBOM were elevated relative to those found in old growth; specifically, organic and total P, extractable ammonium, mineralisable N and labile polysaccharides, suggesting that stream FBOM from harvested basins is more biodegradable than stream FBOM from old‐growth basins. 4. In addition to effects of past timber harvest on FBOM characteristics, there were also significant seasonal differences in both logged and unlogged catchments in all variables except mineralisable N, labile polysaccharides and acetylene reduction rates. 5. The results indicate that past timber harvest in five river basins influenced both composition of and seasonal fluctuations in fine benthic organic matter (FBOM) collected from stream sediments in settling ponds, suggesting a linkage between forest harvest and stream productivity. 6. Comparisons between seasonal patterns in stream and settling pond sediment FBOM characteristics suggested that the readily decomposable organic matter entering sediments during a storm event are rapidly transported and decomposed during their movement through the catchment basin. It also showed the validity of studying settling pond sediments as a surrogate for mountain stream sediments.     

丹麦森林土壤反硝化作用的动力学分析

本项研究将乙炔和氯霉素抑制技术结合起来 ,对丹麦一森林土壤的反硝化作用进行了研究 ,并考察温度对其还原酶活性的影响 .反硝化还原酶活性和合成过程受O2 的抑制 ,厌氧培养时 ,需要一定时间消耗系统中残余的O2 来解除这种抑制作用 .在无抗生素抑制蛋白质合成时 ,硝酸还原酶只有少量合成 ,而N2 O还原酶却显著地诱导产生 .这一结果对土壤吸收N2 O能力的研究具有重要意义 .在各处理下 ,系统中未发生亚硝酸盐的明显积累 ,表明亚硝酸还原酶活性大于硝酸还原酶 .外加葡萄糖加速了反硝化作用 ,并能促进酶的合成和消除还原过程中的电子竞争 .供试土壤表现出很强的厌氧呼吸作用 ,并受外加C源的促进 .反硝化作用的活化能低于土壤厌氧呼吸的活化能 ,因此反硝化作用的Q1 0值较低 ,CO2 和N2 O的产生比例随温度升高而加大 .     

Denitrification in Marine Sediment: Measurement of Capacity and Estimate of In Situ Rate

The acetylene block technique was used to assay denitrification in undisturbed sediment cores of an intertidal mud flat. Nitrogen loss due to this process was estimated at 1 to 41 kg of N/hectare (ha) per year. Anaerobic nitrate-saturated slurry of the same sediment had a denitrification capacity of 1,026 kg of N/ha per year. The acetylene block technique failed at low nitrate concentrations, so that denitrification at average in situ nitrate concentrations could not be determined. Denitrification followed zero-order kinetics at nitrate concentrations high enough to allow successful blockage of N₂O reduction. Thus, an estimate of in situ rates based on kinetic parameters and in situ nitrate concentrations was impossible. No denitrification was observed in a slurry of the top 1.5 cm under aerated conditions and nitrate saturation. In undisturbed sediment, significant denitrification occurred in few discrete sites within a matrix of undetectable or low activity. Despite numerous errors contributing to the uncertainty of the estimate of in situ rates, the result obtained by this method was considered more valuable than the determination of denitrification capacities. Methods which include severe changes of physical and chemical parameters may frequently lead to overestimates of denitrification rates.     

Simultaneous occurrence of denitrification and nitrate ammonification in sediments of the French Mediterranean Coast

Dissimilatory nitrate reductions in coastal marine sediment of Carteau Cove (French Mediterranean Coast) were studied between April 1993 and July 1994. Simultaneous determination of denitrification and dissimilatory nitrate reduction to ammonium was achieved by using a combination of acetylene blockage and 15N techniques. After short incubations (maximum 5 h), a part of 15N labelled nitrate added to the sediment was recovered as ammonium without incorporation in organic matter. The result indicate that a fraction of nitrate was reduced to ammonium by a dissimilatory mechanism instead of denitrifying. Denitrifying and nitrate ammonifying activities ranged from 0 to 19.8 μmol l-1 d-1 and from 2.3 to 83.2 μmol l-1 d-1, respectively. Denitrification rates were highest in early spring whereas nitrate ammonification were highest in fall. The recovery of nitrate reduced as N2O-N plus ammonium was between 40 and 100%, the highest nitrogen losses were recorded in July. Depending on the station and time of year denitrification accounted for between 0 and 43% of the total nitrate reduction whereas dissimilatory nitrate reduction to ammonium (DNRA) accounted for between 18 and 100%. The reduction rate data suggest that the pathway of nitrate reduction to ammonium may be important in coastal sediments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Benthic organic carbon influences denitrification in streams with high nitrate concentration

1. Anthropogenic activities have increased reactive nitrogen availability, and now many streams carry large nitrate loads to coastal ecosystems. Denitrification is potentially an important nitrogen sink, but few studies have investigated the influence of benthic organic carbon on denitrification in nitrate‐rich streams. 2. Using the acetylene‐block assay, we measured denitrification rates associated with benthic substrata having different proportions of organic matter in agricultural streams in two states in the mid‐west of the U.S.A., Illinois and Michigan. 3. In Illinois, benthic organic matter varied little between seasons (5.9–7.0% of stream sediment), but nitrate concentrations were high in summer (>10 mg N L⁻¹) and low (<0.5 mg N L⁻¹) in autumn. Across all seasons and streams, the rate of denitrification ranged from 0.01 to 4.77 μg N g⁻¹ DM h⁻¹ and was positively related to stream‐water nitrate concentration. Within each stream, denitrification was positively related to benthic organic matter only when nitrate concentration exceeded published half‐saturation constants. 4. In Michigan, streams had high nitrate concentrations and diverse benthic substrata which varied from 0.7 to 72.7% organic matter. Denitrification rate ranged from 0.12 to 11.06 μg N g⁻¹ DM h⁻¹ and was positively related to the proportion of organic matter in each substratum. 5. Taken together, these results indicate that benthic organic carbon may play an important role in stream nitrogen cycling by stimulating denitrification when nitrate concentrations are high.     

Denitrification and dinitrogen fixation in two quaking fens in the Vechtplassen area,The Netherlands

This paper reports laboratory experiments on dinitrogen fixation and denitrification for two small quaking fens (discharge fen and recharge fen) using the acetylene reduction assay and the acetylene inhibition technique, respectively.Nitrogenase activity was detected in peat muck and associated with Alnus glutinosa saplings throughout the study period (May–October 1987), whereas no activity was observed with Sphagnum species. The annual amount of dinitrogen fixed was estimated at 2.1 and 12.7 kg N/ha/y for the recharge fen and the discharge fen, respectively.Denitrification at ambient nitrate levels (0.1 ppm NO₃) was absent in the discharge fen and very low in the recharge fen (0.1 g N/g/d, or 0.3 kg N/ha/y). In nitrate-amended soil samples denitrification rates were 2 to 3 orders of magnitude higher. It is argued that in situ denitrification rates in the fens studied will depend almost entirely on the nitrate supply by precipitation. Denitrification rates associated with precipitation are estimated at 1.1 kg N/ha/y for both fens.     

Linkages between organic matter mineralization and denitrification in eight riparian wetlands

Denitrification (N₂ production) and oxygen consumption rates were measured at ambient field nitrate concentrations during summer in sediments from eight wetlands (mixed hardwood swamps, cedar swamps, heath dominated shrub wetland, herbaceous peatland, and a wetland lacking live vegetation) and two streams. The study sites included wetlands in undisturbed watersheds and in watersheds with considerable agricultural and/or sewage treatment effluent input. Denitrification rates measured in intact cores of water-saturated sediment ranged from 20 to 260 mol N m^-2 h^-1 among the three undisturbed wetlands and were less variable (180 to 260 mol N M^-2 h^-1) among the four disturbed wetlands. Denitrification rates increased when nitrate concentrations in the overlying water were increased experimentally (1 up to 770 M), indicating that nitrate was an important factor controlling denitrification rates. However, rates of nitrate uptake from the overlying water were not a good predictor of denitrification rates because nitrification in the sediments also supplied nitrate for denitrification. Regardless of the dominant vegetation, pH, or degree of disturbance, denitrification rates were best correlated with sediment oxygen consumption rates (r ² = 0.912) indicating a relationship between denitrification and organic matter mineralization and/or sediment nitrification rates. Rates of denitrification in the wetland sediments were similar to those in adjacent stream sediments. Rates of denitrification in these wetlands were within the range of rates previously reported for water-saturated wetland sediments and flooded soils using whole core¹⁵N techniques that quantify coupled nitrification/denitrification, and were higher than rates reported from aerobic (non-saturated) wetland sediments using acetylene block methods.     

Nitrification and denitrification in a midwestern stream containing high nitrate: in situ assessment using tracers in dome-shaped incubation chambers

The extent to which in-stream processes alter or remove nutrient loads in agriculturally impacted streams is critically important to watershed function and the delivery of those loads to coastal waters. In this study, patch-scale rates of in-stream benthic processes were determined using large volume, open-bottom benthic incubation chambers in a nitrate-rich, first to third order stream draining an area dominated by tile-drained row-crop fields. The chambers were fitted with sampling/mixing ports, a volume compensation bladder, and porewater samplers. Incubations were conducted with added tracers (NaBr and either ¹⁵N[NO₃ ^?], ¹⁵N[NO₂ ^?], or ¹⁵N[NH₄ ⁺]) for 24–44 h intervals and reaction rates were determined from changes in concentrations and isotopic compositions of nitrate, nitrite, ammonium and nitrogen gas. Overall, nitrate loss rates (220–3,560 μmol N m^?2 h^?1) greatly exceeded corresponding denitrification rates (34–212 μmol N m^?2 h^?1) and both of these rates were correlated with nitrate concentrations (90–1,330 μM), which could be readily manipulated with addition experiments. Chamber estimates closely matched whole-stream rates of denitrification and nitrate loss using ¹⁵N. Chamber incubations with acetylene indicated that coupled nitrification/denitrification was not a major source of N₂ production at ambient nitrate concentrations (175 μM), but acetylene was not effective for assessing denitrification at higher nitrate concentrations (1,330 μM). Ammonium uptake rates greatly exceeded nitrification rates, which were relatively low even with added ammonium (3.5 μmol N m^?2 h^?1), though incubations with nitrite demonstrated that oxidation to nitrate exceeded reduction to nitrogen gas in the surface sediments by fivefold to tenfold. The chamber results confirmed earlier studies that denitrification was a substantial nitrate sink in this stream, but they also indicated that dissolved inorganic nitrogen (DIN) turnover rates greatly exceeded the rates of permanent nitrogen removal via denitrification.     

Co-occurrence of in-stream nitrogen fixation and denitrification across a nitrogen gradient in a western U.S. watershed

It is frequently assumed that nitrogen (N₂) fixation and denitrification do not co-occur in streams because each process should be favored under different concentrations of dissolved inorganic nitrogen (DIN), and therefore these processes are rarely quantified together. We asked if these processes could co-exist by conducting a spatial survey of N₂ fixation using acetylene reduction and denitrification using acetylene block [with and without amendments of carbon (C) as glucose and nitrogen (N) as nitrate]. Rates were measured on rocks and sediment in 8 southeastern Idaho streams encompassing a DIN gradient of 26–615 µg L^?1. Sampling at each site was repeated in summer 2015 and 2016. We found that both denitrification and N₂ fixation occurred across the gradient of DIN concentrations, with N₂ fixation occurring primarily on rocks and denitrification occurring in sediment. N₂ fixation rates on rocks significantly decreased 100× across the DIN gradient in 1 year of the study, and amended (with N and C) denitrification rates increased 10× across the DIN gradient in both years. Multiple linear regression and partial least squares models with environmental characteristics measured at the scale of entire stream reaches showed that C and phosphorus were positive predictors of amended and unamended denitrification rates, but no significant model could explain N₂ fixation rates across all streams and years. This, coupled with the observation that detectable rates of N₂ fixation occurred primarily on rocks and denitrification occurred primarily on sediment, suggests that microhabitat scale factors may better predict the co-occurrence of these processes within stream reaches. Overlooking the potential co-occurrence of N₂ fixation and denitrification in stream ecosystems will impede understanding by oversimplifying the contribution of each process to the N cycle.     

Denitrification in a Sand and Gravel Aquifer

Denitrification was assayed by the acetylene blockage technique in slurried core material obtained from a freshwater sand and gravel aquifer. The aquifer, which has been contaminated with treated sewage for more than 50 years, had a contaminant plume greater than 3.5-km long. Near the contaminant source, groundwater nitrate concentrations were greater than 1 mM, whereas 0.25 km downgradient the central portion of the contaminant plume was anoxic and contained no detectable nitrate. Samples were obtained along the longitudinal axis of the plume (0 to 0.25 km) at several depths from four sites. Denitrification was evident at in situ nitrate concentrations at all sites tested; rates ranged from 2.3 to 260 pmol of N₂O produced (g of wet sediment)⁻¹ h⁻¹. Rates were highest nearest the contaminant source and decreased with increasing distance downgradient. Denitrification was the predominant nitrate-reducing activity; no evidence was found for nitrate reduction to ammonium at any site. Denitrifying activity was carbon limited and not nitrate limited, except when the ambient nitrate level was less than the detection limit, in which case, even when amended with high concentrations of glucose and nitrate, the capacity to denitrify on a short-term basis was lacking. These results demonstrate that denitrification can occur in groundwater systems and, thereby, serve as a mechanism for nitrate removal from groundwater.     

Denitrification in a nitrogen-limited stream ecosystem

Denitrification was measured in hyporheic, parafluvial, and bank sediments of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. We used three variations of the acetylene block technique to estimate denitrification rates, and compared these estimates to rates of nitrate production through nitrification. Subsurface sediments of Sycamore Creek are typically well-oxygenated, relatively low in nitrate, and low in organic carbon, and therefore are seemingly unlikely sites of denitrification. However, we found that denitrification potential (C & N amended, anaerobic incubations) was substantial, and even by our conservative estimates (unamended, oxic incubations and field chamber nitrous oxide accumulation), denitrification consumed 5–40% of nitrate produced by nitrification. We expected that denitrification would increase along hyporheic and parafluvial flowpaths as dissolved oxygen declined and nitrate increased. To the contrary, we found that denitrification was generally highest at the upstream ends of subsurface flowpaths where surface water had just entered the subsurface zone. This suggests that denitrifiers may be dependent on the import of surface-derived organic matter, resulting in highest denitrification rate at locations of surface-subsurface hydrologic exchange. Laboratory experiments showed that denitrification in Sycamore Creek sediments was primarily nitrogen limited and secondarily carbon limited, and was temperature dependent. Overall, the quantity of nitrate removed from the Sycamore Creek ecosystem via denitrification is significant given the nitrogen-limited status of this stream.     

Denitrification and nitric oxide reduction in an aerobic toluene-treating biofilter

The presence of significant denitrification activity in an aerobic toluene-treating biofilter was demonstrated under batch and flow-through conditions. N2O concentrations of 9.2 ppmv were produced by denitrifying bacteria in the presence of 15% acetylene, in a flow-through system with a bulk gas phase O2 concentration of >17%. The carbon source for denitrification was not toluene but a byproduct or metabolite of toluene catabolism. Denitrification conditions were successfully used for the reduction of 60 ppmv nitric oxide to 15 ppmv at a flow rate of 3 L min-1 (EBRT of 3 min) in a fully aerated, 17% v/v O2 (superficially aerobic) biofilter. Higher NO removal efficiency (97%) was obtained by increasing the toluene supply to the biofilter.     

Seasonal dynamics of denitrification activity in two water meadows

Seasonal variation in denitrification activity was measured in twoflooded water meadows, one on peaty and one on sandy soil, over a three-yearperiod. Measurements were taken during flooded and drained periods, usingthe acetylene-blockage technique, and the rates were compared to massbalance estimates of nitrate removal in the percolating water.Denitrification activity was higher in sandy soil than in peaty soil. Higherwater infiltration rate and thereby higher nitrate load was considered to bethe cause of the higher denitrification in the sandy soil. Floodingsignificantly increased denitrification, and the rates were higher in autumnand winter than in spring. This was considered to be a result of highernitrogen concentration in inflowing stream water during winter. Annualdenitrification was estimated to 430–460 kg N ha^-1yr^-1 in the sandy soil meadow, and 220 kg N ha^-1yr^-1 in the peaty soil meadow. In the sandy soil there was alarge discrepancy between nitrate removal rates and denitrification rates,which can be explained by nitrification of ammonium released from the soil.In the peaty soil nitrate disappearance and denitrification correspondedfairly well.     

Nutrients and temperature additively increase stream microbial respiration

Rising temperatures and nutrient enrichment are co‐occurring global‐change drivers that stimulate microbial respiration of detrital carbon, but nutrient effects on the temperature dependence of respiration in aquatic ecosystems remain uncertain. We measured respiration rates associated with leaf litter, wood, and fine benthic organic matter (FBOM) across seasonal temperature gradients before (PRE) and after (ENR1, ENR2) experimental nutrient (nitrogen [N] and phosphorus [P]) additions to five forest streams. Nitrogen and phosphorus were added at different N:P ratios using increasing concentrations of N (~80–650 μg/L) and corresponding decreasing concentrations of P (~90–11 μg/L). We assessed the temperature dependence, and microbial (i.e., fungal) drivers of detrital mass‐specific respiration rates using the metabolic theory of ecology, before vs. after nutrient enrichment, and across N and P concentrations. Detrital mass‐specific respiration rates increased with temperature, exhibiting comparable activation energies (E, electronvolts [eV]) for all substrates (FBOM E = 0.43 [95% CI = 0.18–0.69] eV, leaf litter E = 0.30 [95% CI = 0.072–0.54] eV, wood E = 0.41 [95% CI = 0.18–0.64] eV) close to predicted MTE values. There was evidence that temperature‐driven increased respiration occurred via increased fungal biomass (wood) or increased fungal biomass‐specific respiration (leaf litter). Respiration rates increased under nutrient‐enriched conditions on leaves (1.32×) and wood (1.38×), but not FBOM. Respiration rates responded weakly to gradients in N or P concentrations, except for positive effects of P on wood respiration. The temperature dependence of respiration was comparable among years and across N or P concentration for all substrates. Responses of leaf litter and wood respiration to temperature and the combined effects of N and P were similar in magnitude. Our data suggest that the temperature dependence of stream microbial respiration is unchanged by nutrient enrichment, and that increased temperature and N + P availability have additive and comparable effects on microbial respiration rates.     

Denitrification in Marl and Peat Sediments in the Florida Everglades

The potential for denitrification in marl and peat sediments in the Shark River Slough in the Everglades National Park was determined by the acetylene blockage assay. The influence of nitrate concentration on denitrification rate and N₂O yield from added nitrate was examined. The effects of added glucose and phosphate and of temperature on the denitrification potential were determined. The sediments readily denitrified added nitrate. N₂O was released from the sediments both with and without added acetylene. The marl sediments had higher rates than the peat on every date sampled. Denitrification was nitrate limited; however, the yields of N₂O amounted to only 10 to 34% of the added nitrate when 100 μM nitrate was added. On the basis of measured increases in ammonium concentration, it appears that the balance of added nitrate may be converted to ammonium in the marl sediment. The sediment temperature at the time of sampling greatly influenced the denitrification potential (15-fold rate change) at the marl site, indicating that either the number or the specific activity of the denitrifiers changed in response to temperature fluctuations (9 to 25°C) in the sediment. It is apparent from this study that denitrification in Everglades sediments is not an effective means of removing excess nitrogen which may be introduced as nitrate into the ecosystem with supply water from the South Florida watershed and that sporadic addition of nitrate-rich water may lead to nitrous oxide release from these wetlands.     

Warming can boost denitrification disproportionately due to altered oxygen dynamics

Background

Global warming and the alteration of the global nitrogen cycle are major anthropogenic threats to the environment. Denitrification, the biological conversion of nitrate to gaseous nitrogen, removes a substantial fraction of the nitrogen from aquatic ecosystems, and can therefore help to reduce eutrophication effects. However, potential responses of denitrification to warming are poorly understood. Although several studies have reported increased denitrification rates with rising temperature, the impact of temperature on denitrification seems to vary widely between systems.

Methodology/Principal Findings

We explored the effects of warming on denitrification rates using microcosm experiments, field measurements and a simple model approach. Our results suggest that a three degree temperature rise will double denitrification rates. By performing experiments at fixed oxygen concentrations as well as with oxygen concentrations varying freely with temperature, we demonstrate that this strong temperature dependence of denitrification can be explained by a systematic decrease of oxygen concentrations with rising temperature. Warming decreases oxygen concentrations due to reduced solubility, and more importantly, because respiration rates rise more steeply with temperature than photosynthesis.

Conclusions/Significance

Our results show that denitrification rates in aquatic ecosystems are strongly temperature dependent, and that this is amplified by the temperature dependencies of photosynthesis and respiration. Our results illustrate the broader phenomenon that coupling of temperature dependent reactions may in some situations strongly alter overall effects of temperature on ecological processes.     

Effects of specific inhibitors on anammox and denitrification in marine sediments

The effects of three metabolic inhibitors (acetylene, methanol, and allylthiourea [ATU]) on the pathways of N2 production were investigated by using short anoxic incubations of marine sediment with a 15N isotope technique. Acetylene inhibited ammonium oxidation through the anammox pathway as the oxidation rate decreased exponentially with increasing acetylene concentration; the rate decay constant was 0.10+/-0.02 microM-1, and there was 95% inhibition at approximately 30 microM. Nitrous oxide reduction, the final step of denitrification, was not sensitive to acetylene concentrations below 10 microM. However, nitrous oxide reduction was inhibited by higher concentrations, and the sensitivity was approximately one-half the sensitivity of anammox (decay constant, 0.049+/-0.004 microM-1; 95% inhibition at approximately 70 microM). Methanol specifically inhibited anammox with a decay constant of 0.79+/-0.12 mM-1, and thus 3 to 4 mM methanol was required for nearly complete inhibition. This level of methanol stimulated denitrification by approximately 50%. ATU did not have marked effects on the rates of anammox and denitrification. The profile of inhibitor effects on anammox agreed with the results of studies of the process in wastewater bioreactors, which confirmed the similarity between the anammox bacteria in bioreactors and natural environments. Acetylene and methanol can be used to separate anammox and denitrification, but the effects of these compounds on nitrification limits their use in studies of these processes in systems where nitrification is an important source of nitrate. The observed differential effects of acetylene and methanol on anammox and denitrification support our current understanding of the two main pathways of N2 production in marine sediments and the use of 15N isotope methods for their quantification.     

Nitrification and denitrification response to varying periods of desiccation and inundation in a western Kansas stream

Changing environmental conditions and increased water consumption have transformed many historically perennial stream systems into intermittent systems. Multiple drying and wetting events throughout the year might impact many stream processes including nitrification and denitrification, key components of the nitrogen (N) cycle. During summer 2007, an experimental stream was used to dry and then rewet stream sediments to determine the effects of desiccation and rewetting of stream sediment on nitrification and denitrification potentials. Mean (±SE) nitrification and denitrification rates in sediment not dried (controls) were 0.431 ± 0.017 μg NO₃ ⁻–N/cm²/h and 0.016 ± 0.002 μg N₂O–N/cm²/h, respectively. As sediment samples dried, nitrification rates decreased. Rates in sediments dried less than 7 d recovered to levels equal or greater than those in the controls within 1 d of being rewetted. Denitrification rates were not affected by 1 d of drying, but samples dried greater than 1 d experienced reduced rates of denitrification. Denitrification in sediments dried 7 d or less recovered by day seven of being rewetted. Nitrification and denitrification processes failed to fully recover in sediments dried more than 7 d. These results demonstrate that alterations in stream’s hydrology can significantly affect N-cycle processes.     

Denitrification in Agriculturally Impacted Streams: Seasonal Changes in Structure and Function of the Bacterial Community

Denitrifiers remove fixed nitrogen from aquatic environments and hydrologic conditions are one potential driver of denitrification rate and denitrifier community composition. In this study, two agriculturally impacted streams in the Sugar Creek watershed in Indiana, USA with different hydrologic regimes were examined; one stream is seasonally ephemeral because of its source (tile drainage), whereas the other stream has permanent flow. Additionally, a simulated flooding experiment was performed on the riparian benches of the ephemeral stream during a dry period. Denitrification activity was assayed using the chloramphenicol amended acetylene block method and bacterial communities were examined based on quantitative PCR and terminal restriction length polymorphisms of the nitrous oxide reductase (nosZ) and 16S rRNA genes. In the stream channel, hydrology had a substantial impact on denitrification rates, likely by significantly lowering water potential in sediments. Clear patterns in denitrification rates were observed among pre-drying, dry, and post-drying dates; however, a less clear scenario was apparent when analyzing bacterial community structure suggesting that denitrifier community structure and denitrification rate were not strongly coupled. This implies that the nature of the response to short-term hydrologic changes was physiological rather than increases in abundance of denitrifiers or changes in composition of the denitrifier community. Flooding of riparian bench soils had a short-term, transient effect on denitrification rate. Our results imply that brief flooding of riparian zones is unlikely to contribute substantially to removal of nitrate (NO₃ ^-) and that seasonal drying of stream channels has a negative impact on NO₃ ^- removal, particularly because of the time lag required for denitrification to rebound. This time lag is presumably attributable to the time required for the denitrifiers to respond physiologically rather than a change in abundance or community composition.     

Denitrification potential of epilithic communities in a lotic environment

Denitrification potentials of epilithic microbial populations were assessed using the acetylene inhibition method, in which acetylene is used to block the reduction of nitrous oxide (N₂O) to nitrogen (N₂). Samples of the epilithic community were incubated in filtered river water containing modified Bushnell-Haas salts, glycerol, and yeast extract—under aerobic (0.2 atm O₂) and anaerobic (0.2 atm He) acetylene atmospheres. N₂O was produced under both atmospheres only if exogenous nitrate of nitrite was added. Denitrification potentials were typically higher when nitrite was the added electron acceptor. The rates of denitrification were temperature-and carbon-dependent and the maximum rate, 8.53 g N₂O–N per cm² per day occurred at 23°C when nitrite was the electron acceptor.