Increase In Denitrification Rate Research Articles

Denitrification in shallow, sublittoral Gulf of Mexico permeable sediments

We examined nitrogen cycling over a 1‐yr period in shallow, sandy sediments at two contrasting sites near a barrier island in the northeastern Gulf of Mexico, and we provide here the direct determination of dinitrogen gas (N2) production at ambient nitrate concentrations in permeable marine sediments. Nitrogen‐stable isotope tracer techniques were used to quantify N2‐production rates and pathways in sediment cores and slurries. To simulate pore‐water advection, the dominant transport process in the upper layer of the permeable sand beds, intact sediment cores were perfused with aerated seawater. This perfusion increased denitrification rates up to 2‐fold in Apalachicola Bay sands and up to 17‐fold in Gulf of Mexico sublittoral sands, respectively, relative to static cores. Seasonal N2‐production rates were highest in spring and fall. Denitrified nitrate originated almost entirely from benthic nitrification at the exposed Gulf site, whereas water‐column nitrate dominated sedimentary denitrification at the sheltered Bay site. Sediment incubations in stirred chambers were used to determine net fluxes of oxygen (O2), N2, nitrate, and ammonium across the sediment‐water interface during varied degrees of continuous pore‐water exchange. Rates of N2 efflux correlated with rates of pore‐water flow increasing from 0.12 mmol N m‐2 d‐1 under diffusion‐limited transport conditions up to 0.87 mmol N m‐2 d‐1 with pore‐water advection. Mineralized nitrogen was completely converted to N2 gas in Gulf of Mexico sediments. Our results demonstrate that advective pore‐water circulation will accelerate benthic N2 production by coupled nitrification‐ denitrification and that substantial nitrogen loss occurs from coastal permeable sediments.

Sediment size and nutrients regulate denitrification in a tropical stream

Landuse changes might alter N cycling in tropical aquatic ecosystems, but understanding of N cycling in tropical streams is limited. We measured actual and potential denitrification rates during the dry season in Río Las Marías, a 4th-order Andean piedmont stream in Venezuela. Our objectives were to describe spatial and temporal variation in denitrification, quantify the effects of nutrient availability and substratum particle size on denitrification, and explore potential effects of anthropogenic sedimentation on denitrification. In 4 experiments, actual and potential denitrification rates ranged from 0 to 160 and from 0 to 740 μg N2O-N m−2 h−1, respectively. Rates were distributed approximately log-normally because of spatial variation. During a 1-mo period, actual denitrification rates decreased exponentially from 37 ± 39 to 5 ± 7 μg N2O-N m−2 h−1 (mean ± SD), probably because of a decline in water-column NO3-N concentration from 41 ± 14 to 12 ± 3 μg NO3-N/L. The texture (particle size) of stream substrata markedly affected denitrification rates. Actual rates were low in cobble, gravel, and fine sediments (<5 mm), but in fine sediments, rates increased in response to addition of excess NO3-N and organic C. In a 3-km stream reach, actual (but not potential) denitrification rates increased with the proportion of fine sediments (<2 mm) in mixed substrata. This increase was nonlinear, and the threshold value occurred at 37% fine particles, above which actual denitrification rates were almost always high. An experiment simulating the effects of anthropogenic sedimentation showed that topsoil inputs resulted in denitrification rates ∼8× higher than rates in trials where excess NO3-N and organic C were supplied. Denitrification is a small but potentially significant sink for available N in this N-limited system. Anthropogenic sedimentation associated with landuse change might significantly increase denitrification rates in streams.

Effect of bulk liquid BOD concentration on activity and microbial community structure of a nitrifying, membrane-aerated biofilm

Membrane-aerated biofilms (MABs) are an effective means to achieve nitrification and denitrification of wastewater. In this research, microsensors, fluorescence in situ hybridization (FISH), and modeling were used to assess the impact of bulk liquid biological oxygen demand (BOD) concentrations on the activity and microbial community structure of nitrifying MABs. With 1 g m(-3) BOD in the bulk liquid, the nitrification rate was 1.3 g N m(-2) day(-1), slightly lower than the 1.5 g N m(-2) day(-1) reported for no bulk liquid BOD. With bulk liquid BOD concentrations of 3 and 10 g m(-3), the rates decreased to 1 and 0.4 g N m(-2) day(-1), respectively. The percent denitrification increased from 20% to 100% when the BOD increased from 1 to 10 g m(-3) BOD. FISH results indicated increasing abundance of heterotrophs with increasing bulk liquid BOD, consistent with the increased denitrification rates. Modeling was used to assess the effect of BOD on nitrification rates and to compare an MAB to a conventional biofilm. The model-predicted nitrification rates were consistent with the experimental results. Also, nitrification in the MAB was much less sensitive to BOD inhibition than the conventional biofilm. The MAB achieved concurrent nitrification and denitrification, whereas little denitrification occurred in the conventional biofilm.

EFFECTS OF STREAM RESTORATION ON DENITRIFICATION IN AN URBANIZING WATERSHED

Increased delivery of nitrogen due to urbanization and stream ecosystem degradation is contributing to eutrophication in coastal regions of the eastern United States. We tested whether geomorphic restoration involving hydrologic "reconnection" of a stream to its floodplain could increase rates of denitrification at the riparian-zone-stream interface of an urban stream in Baltimore, Maryland. Rates of denitrification measured using in situ 15N tracer additions were spatially variable across sites and years and ranged from undetectable to >200 microg N x (kg sediment)(-1) x d(-1). Mean rates of denitrification were significantly greater in the restored reach of the stream at 77.4 +/- 12.6 microg N x kg(-1) x d(-1) (mean +/- SE) as compared to the unrestored reach at 34.8 +/- 8.0 microg N x kg(-1) x d(-1). Concentrations of nitrate-N in groundwater and stream water in the restored reach were also significantly lower than in the unrestored reach, but this may have also been associated with differences in sources and hydrologic flow paths. Riparian areas with low, hydrologically "connected" streambanks designed to promote flooding and dissipation of erosive force for storm water management had substantially higher rates of denitrification than restored high "nonconnected" banks and both unrestored low and high banks. Coupled measurements of hyporheic groundwater flow and in situ denitrification rates indicated that up to 1.16 mg NO3(-)-N could be removed per liter of groundwater flow through one cubic meter of sediment at the riparian-zone-stream interface over a mean residence time of 4.97 d in the unrestored reach, and estimates of mass removal of nitrate-N in the restored reach were also considerable. Mass removal of nitrate-N appeared to be strongly influenced by hydrologic residence time in unrestored and restored reaches. Our results suggest that stream restoration designed to "reconnect" stream channels with floodplains can increase denitrification rates, that there can be substantial variability in the efficacy of stream restoration designs, and that more work is necessary to elucidate which designs can be effective in conjunction with watershed strategies to reduce nitrate-N sources to streams.

N FERTILIZATION EFFECTS ON DENITRIFICATION AND N CYCLING IN AN AGGRADING FOREST

We investigated N cycling and denitrification rates following five years of N and dolomite amendments to whole-tree harvested forest plots at the long-term soil productivity experiment in the Fernow Experimental Forest in West Virginia, USA. We hypothesized that changes in soil chemistry and nutrient cycling induced by N fertilization would increase denitrification rates and the N2O:N2 ratio. Soils from the fertilized plots had a lower pH (2.96) than control plots (3.22) and plots that received fertilizer and dolomite (3.41). There were no significant differences in soil %C or %N between treatments. Chloroform-labile microbial biomass carbon was lower in fertilized plots compared to control plots, though this trend was not significant. Extractable soil NO3- was elevated in fertilized plots on each sample date. Soil-extractable NH4+, NO3-, pH, microbial biomass carbon, and %C varied significantly by sample date suggesting important seasonal patterns in soil chemistry and N cycling. In particular, the steep decline in extractable NH4+ during the growing season is consistent with the high N demands of a regenerating forest. Net N mineralization and nitrification also varied by date but were not affected by the fertilization and dolomite treatments. In a laboratory experiment, denitrification was stimulated by NO3- additions in soils collected from all field plots, but this effect was stronger in soils from the unfertilized control plots, suggesting that chronic N fertilization has partially alleviated a NO3- limitation on denitrification rates. Dextrose stimulated denitrification only in the whole-tree-harvest soils. Denitrification enzyme activity varied by sample date and was elevated in fertilized plots for soil collected in July 2000 and June 2001. There were no detectable treatment effects on N2O or N2 flux from soils under anaerobic conditions, though there was strong temporal variation. These results suggest that whole-tree harvesting has altered the N status of these soils so they are less prone to N saturation than more mature forests. It is likely that N losses associated with the initial harvest and high N demand by aggrading vegetation is minimizing, at least temporarily, the amount of inorganic N available for nitrification and denitrification, even in the fertilized plots in this experiment.

Use of cotton gin trash to enhance denitrification in restored forested wetlands

Lower Mississippi Valley (LMV) has lost about 80% bottomland hardwood forests, mainly to agriculture. This landscape scale alteration of the LMV resulted in the loss of nitrate (NO 3) removal capacity of the valley, contributing to nitrogen (N)-enhanced eutrophication and potentially hypoxia in the northern Gulf of Mexico. Restoration of hardwood forests in the LMV is a highly recommended practice to reduce NO 3 load of the Mississippi River. However, restored bottomland forests take decades to develop characteristic ecological functions including denitrifier activity. One way to enhance denitrifier activity in restored wetland forests is to amend the soils with an available carbon (C) source. This research investigated the effects of cotton gin trash (CGT) amendment on denitrification rate and N 2O:N 2 emission ratio from a restored bottomland forest soils and compared it to those from an adjacent unamended natural forest soils. CGT amendment increased denitrification rates in the restored forest soils to the level of the natural forest soils. N 2O:N 2 emission ratios from the restored and natural forest soils were highly variable and were not significantly different from each other. These findings suggest that restoration of bottomland hardwood forests in the LMV will require organic carbon amendment to achieve enhanced denitrifier activity for NO 3 removal while the restored forest is developing into a mature state over time.

Investigation of an Acetate-Fed Denitrifying Microbial Community by Stable Isotope Probing, Full-Cycle rRNA Analysis, and Fluorescent In Situ Hybridization-Microautoradiography

The acetate-utilizing microbial consortium in a full-scale activated sludge process was investigated without prior enrichment using stable isotope probing (SIP). [13C]acetate was used in SIP to label the DNA of the denitrifiers. The [13C]DNA fraction that was extracted was subjected to a full-cycle rRNA analysis. The dominant 16S rRNA gene phylotypes in the 13C library were closely related to the bacterial families Comamonadaceae and Rhodocyclaceae in the class Betaproteobacteria. Seven oligonucleotide probes for use in fluorescent in situ hybridization (FISH) were designed to specifically target these clones. Application of these probes to the sludge of a continuously fed denitrifying sequencing batch reactor (CFDSBR) operated for 16 days revealed that there was a significant positive correlation between the CFDSBR denitrification rate and the relative abundance of all probe-targeted bacteria in the CFDSBR community. FISH-microautoradiography demonstrated that the DEN581 and DEN124 probe-targeted cells that dominated the CFDSBR were capable of taking up [14C]acetate under anoxic conditions. Initially, DEN444 and DEN1454 probe-targeted bacteria also dominated the CFDSBR biomass, but eventually DEN581 and DEN124 probe-targeted bacteria were the dominant bacterial groups. All probe-targeted bacteria assessed in this study were denitrifiers capable of utilizing acetate as a source of carbon. The rapid increase in the number of organisms positively correlated with the immediate increase in denitrification rates observed by plant operators when acetate is used as an external source of carbon to enhance denitrification. We suggest that the impact of bacteria on activated sludge subjected to intermittent acetate supplementation should be assessed prior to the widespread use of acetate in the wastewater industry to enhance denitrification.

Denitrification as a Nitrogen Removal Mechanism in a Vermont Peatland

Atmospheric deposition of nitrogenous compounds to ombrotrophic peatlands (i.e., those that have peat layers higher than their surroundings and receive nutrients and minerals exclusively by precipitation) has the potential to significantly alter ecosystem functioning. This study utilized the acetylene inhibition technique to estimate the relative importance of denitrification in nitrogen removal from a primarily ombrotrophic peatland, in an attempt to estimate the threat of increased nitrogen loadings to these areas. Estimates of mean rates of denitrification ranged from -2.76 to 84.0 ng N(2)O-N cm(-3) h(-1) (equivalent to -150 to 4800 microg N(2)O-N m(-2) h(-1)) using an ex situ core technique and from -8.30 to 5.98 microg N(2)O-N m(-2) h(-1) using an in situ chamber technique. Core rates may have been elevated over natural field levels due to effects of disturbance on substrate availability, and chamber rates may have been low due to diffusional constraints on acetylene and N(2)O. Net nitrification was also measured in an attempt to evaluate this process as a source of nitrate for denitrifiers. The low rates of net nitrification measured, in combination with the low rates of in situ denitrification and the very low amounts of free nitrate measured in this peatland, suggests that inorganic N turnover in this wetland is low. Results showed that nitrate was a limiting factor for denitrification in this peatland, with mean rates from nitrate-amended cores ranging from 13.1 to 260 ng N(2)O-N cm(-3) h(-1), and it is expected that increases in nitrogen loadings will increase denitrification rates in this ecosystem.

Impact of burrowing crabs on C and N sources, control, and transformations in sediments and food webs of SW Atlantic estuaries

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 293:155-164 (2005) - doi:10.3354/meps293155 Impact of burrowing crabs on C and N sources, control, and transformations in sediments andfood webs of SW Atlantic estuaries Florencia Botto1,*, Ivan Valiela2, Oscar Iribarne1, Paulina Martinetto1,2, Juan Alberti1 1Departamento de Biología, Universidad Nacional de Mar del Plata, CC 573 Correo Central B7600WAG,Mar del Plata, Argentina2Boston University Marine Program, Marine Biological Laboratory, 7 MBL St. Woods Hole, Massachusetts 02543, USA *Email: fbotto@mdp.edu.ar ABSTRACT: The intertidal burrowing crab Chasmagnathus granulatus is the dominant species in soft bare sediments and vegetated intertidal areas along the SW Atlantic estuaries (southern Brazil, 28°S, to northern Patagonia, 42°S). C. granulatus creates burrows that can reach densities of 60 burrows m–2, and its burrowing activities increase water and organic matter content of sediments. To evaluate the long-term effect of burrows on the origin and transformation of accumulated organic matter within sediments, we compared C and N stable isotope signatures of sediments, plants, and consumers within areas with and without crabs. 15N signatures of sediments and primary producers were enriched by 3 to 7‰ in areas with crabs. The enrichment was present in 4 different Argentine estuarine environments (Mar Chiquita coastal lagoon, 37°46’S, 57°19’W, Bahia Blanca, 38°50’S, 62°07’W, San Blas, 40°33’S, 62°14’W, San Antonio, 40°48’S, 64°52’W). Enrichment owing to crab activity appeared to overwhelm possible different N loads, anthropogenic influence, and other properties. Crab activity thus uncoupled the nitrogen dynamics in sediments from external controls. Enrichment of the heavier isotope of N could be the result of an increase in denitrification rates in areas with burrows. Crabs therefore forced faster transformation of available to unavailable nitrogen, making less inorganic nitrogen available to deeper waters. Food webs in areas with and without crabs were similar in shape, but less mobile benthic organisms (nematodes, fiddler crabs and the polychaete Laeonereis acuta) showed enriched N isotopic signatures. The benthic food web seemed separate from that of suspension feeders or water column consumers. Benthic microalgae were an important source for infauna, and marsh plants were particularly important for burrowing crabs. KEY WORDS: Estuaries · Food webs · Stable isotopes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 293. Online publication date: June 02, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Hydrological controls on denitrification in riparian ecosystems

Abstract. Nitrous oxide fluxes and denitrification rates were measured in situ over a year at a riparian site in the UK. An exponential relationship was found between denitrification rates and soil moisture, with a sharp increase in denitrification rate at a water-filled pore space of 60–80%. Similar relationships were found in other studies compiled for comparison. The present study is unique in measuring denitrification in an "intact" ecosystem in the field, rather than in cores in the field or the lab. The exponential relationship between denitrification rate and soil moisture, with a "threshold" at 60–80% water-filled pore space (20–40% gravimetric moisture), has proven to be comparable across a wide range of ecosystems, treatments and study conditions. Whereas moisture content determines the potential for denitrification, the absolute rate of denitrification is determined by available nitrate (NO3-), dissolved organic carbon and temperature. As a first approximation, denitrification rates can be simply modelled by using a general exponential relationship between denitrification potential and water-filled pore space (or volumetric/gravimetric water content) multiplied by a constant value determined by the nitrogen status of the site. As such, it is recommended that the current relationship used in INCA to relate denitrification to soil moisture be amended to an exponential form, with a threshold of approximately 70% for the onset of denitrification. Keywords: nitrous oxide, denitrification, soil moisture, nitrogen, eutrophication, riparian

Denitrification in a constructed wetland receiving agricultural runoff

Constructed wetlands are recognized as a means to improve water quality through nitrogen (N) removal. Water-quality concerns in the N-sensitive Neuse River Estuary, North Carolina, USA, have necessitated enactment of a 30% reduction, in nitrogen (N) loading accompanied by an N loading cap. Open Grounds Farm (OGF) is an 18,220-ha row-crop farm located in the lower Neuse River Estuary. In 1999, a wetland was constructed to remove nutrients (N and Phosphorus), sediment, and pathogens in surface water draining from a 971-ha area of OGF. The wetland site is 5.1 ha of alternating segments of emergent marsh and open water. Nitrogen removal from the wetland via denitrification was measured monthly by analysis of dissolved nitrogen, oxygen, and argon in laboratory incubated sediment chambers using a Membrane Inlet Mass Spectrometer (MIMS). Nitrate concentration appeared to be the primary variable controlling denitrification rates. Spatial and temporal variability in rates of denitrification were investigated, including pre- and post- N loading events. Following rainfall, there was a 400% increase in denitrification rates in response to increased inorganic N loading. Nutrient loads entering and leaving the wetland were determined from nutrient analysis (twice monthly), intensive precipitation event sampling, and continuous flow measurements at the entrance and exit of the, wetland. Results indicated that the wetland received variable N loading (1-1,720 kg N per month) and had variable N removal via denitrification (8-81 kg N per month). Denitrification was an important mechanism for N removal.

Effect of liquid manure on the mole fraction of nitrous oxide evolved from soil containing nitrate

The same emission factor is applied to fertiliser N and manure N when calculating national N 2O inventories. Manures and fertilisers are often applied together to meet the N needs of the crop, but little is known about potential interactions leading to an increase in denitrification rate or a change in the composition of the end-products of denitrification. We used the 15N gas-flux method in a laboratory experiment to quantify the effect of liquid manure (LM) application on the fluxes of N 2 and N 2O when the soil contained fertiliser 15 NO 3 − . LM increased the mole fraction of N 2O from 0.5 to 0.85 in the first 12 h after application. More than 94% of the N 2O was from the reduction of NO 3 −, probably due to aerobic nitrate respiration as well as respiratory denitrification.

Effects of Freeze–Thaw and Soil Structure on Nitrous Oxide Produced in a Clay Soil

Freezing and thawing have been shown to cause significant soil physical and biological changes. The increase in denitrification following thawing may be attributed to the diffusion of organic substrates newly available to denitrifiers from disrupted soil aggregates. The objective of this study was to evaluate the effect of freezing and thawing on N2O production in a clay soil under contrasting crop rotations and tillage practices. Laboratory experiments were conducted in soil slurries to favor substrate diffusion, in macroaggregate fractions separated by wet sieving to characterize the biologically active soil organic matter (SOM) pool, and in undisturbed soil cores to simulate field conditions. In slurries, a freezing and thawing cycle increased denitrification rates by 32%. Soil slurries from no‐tillage under rotation (NT–R) exhibited denitrification rates 92% higher than those from conventional till under continuous cereal (CT–C). Macroaggregates fractions (0.25–2 and 2–5 mm) from both management systems increased their rates of C mineralization and denitrification activity by 95% following freezing, but the increases tended to be greater (57%) in small than in large macroaggregates. Higher rates of denitrification (55%) found in both aggregate fractions of NT–R system were attributed to the higher mineralizable organic C content. Undisturbed soil cores sampled in November showed increased N2O production by 220% after thawing. This thawing effect was also significantly higher in cores from NT–R than in those from CT–C.

Denitrification in sediments of the freshwater tidal Yorkshire Ouse: Science of the Total Environment 210/211 (1998: pages 321–327) omitted the right-hand part of two Tables (1 and 3). These are given in full here, together with the Abstract.

Studies were made on denitrification rate and nitrous oxide production in the 21-km freshwater tidal stretch of the Yorkshire Ouse. Denitrification was not detected in the water column at the most downstream site under conditions likely to be the most favourable for this process. All but one sample of the sediment cores showed high activity per unit area, with values ranging up to 576 μmol N m −2 h −1. In contrast to all other measurements on the Swale–Ouse river system, values for denitrification rate at the most downstream site (Selby) showed on three occasions a decrease in comparison with the next site upstream (Cawood). It is suggested that this may be due to sediment instability at this site. Experimental studies with sediment slurries made on two different dates showed an increase in denitrification rate at all sites in response to added nitrate, but none due to added glucose. However, a combination of nitrate and glucose added to slurries from the most upstream site (Naburn Weir) and assayed under low O 2 conditions showed a significantly higher denitrification rate than with added nitrate alone.

Influence of Nitrate and Phosphorus Loading on Denitrifying Enzyme Activity in Everglades Wetland Soils

There has been recent concern about the impact of increased nutrient loading on the northern Everglades ecosystem. We investigated the spatial and temporal distribution of denitrifying enzyme activity (DEA) along a P‐enrichment gradient in the Water Conservation Area 2A (WCA‐2A) and determined the effects of added P and NO−3 on DEA. The DEA in soil and detritus layers was measured under anaerobic conditions four times during 2 yr, using the acetylene blockage technique. The DEA ranged from 0.004 to 7.75 mg N2O–N kg−1 h−1 Highest rates of DEA were found in the detritus and surface (0–10 cm) soils, and rates decreased exponentially with increasing distance from the surface‐water inflow point, where nutrients are loaded to the wetland. Nitrate was found to be limiting, while the addition of P had no effect on the distribution of DEA in these soils. There was a seasonal effect on DEA, with higher activity observed during the summer when temperatures and hydraulic and nutrient loading were highest. Soils from outside the impacted zone demonstrated denitrifying potentials, within 10 h when spiked with inflow concentrations of NO−3, similar to DEA of soils from within the impacted zone. This suggests that soils from outside the impacted zone can increase denitrification rates when exposed to higher NO−3 concentrations in a relatively short time. Agricultural drainage water discharge, and consequent NO−3 loading, has created a zone of elevated DEA proximal to the S‐10C surface‐water inflow point in WCA‐2A.

Factors regulating denitrification in a soil under pasture

Experiments were conducted to obtain insights into factors regulating denitrification rate in a silt loam soil under permanent pasture in New Zealand, by removing possible limitations to denitrification during the incubation for the denitrification measurement. Soil temperature in the field was found to limit denitrification rate in all seasons relative to the denitrification rate measured at 25°C in the laboratory. This temperature effect was greatest in the cool–wet season. Additions of nitrate solution to soil cores stimulated denitrification rates in all seasons. This increase in denitrification rate suggests the availability of NO 3 − may have limited denitrification in this pasture soil. Denitrification rates also increased when soluble-carbon was added to the soil cores, but the magnitude of the effect depended on other edaphic factors. A large increase in denitrification rate was obtained by saturating the soil cores collected in most seasons, but particularly during the warm–dry period. However, little enhanced effect on denitrification rate by anaerobic incubation of soil cores was observed. These results suggest that the observed effect of water addition on denitrification rate may have been due to the easier diffusive movement of NO 3 −, or possibly soluble-C, to the microsites where denitrification was occurring in this pasture, and the creation of anaerobic sites in the soil may not have been as important to the increase of denitrification rate.

Effect of addition of anaerobic fermented OFMSW (organic fraction of municipal solid waste) on biological nutrient removal (BNR) process: preliminary results

The paper presents results coming from experiments on pilot scale plants about the possibility to integrate the organic waste and wastewater treatment cycles, using the light organic fraction produced via anaerobic fermentation of OFMSW as RBCOD source for BNR processes. The effluent from the anaerobic fermentation process, with an average content of 20 g/l of VFA+ lactic acid was added to wastewater to be treated in order to increase RBCOD content of about 60-70 mg/l. The results obtained in the BNR process through the addition of the effluent from the fermentation unit are presented. Significant increase of denitrification rate was obtained: 0.06 KgN-NO3/KgVSS d were denitrified in the best operative conditions studied. -Vmax shows values close to those typical of the pure methanol addition (about 0.3 KgN-NO3/KgVSS d). A considerable P release (35%) was observed in the anaerobic step of the BNR process, even if not yet a completely developed P removal process.

Denitrification in sediments of the freshwater tidal Yorkshire Ouse

Studies were made on denitrification rate and nitrous oxide production in the 21-km freshwater tidal stretch of the Yorkshire Ouse. Denitrification was not detected in the water column at the most downstream site (Selby) under conditions likely to be the most favourable for this process. All but one sample of the sediment cores showed high activity per unit area, with values ranging up to 575 μmol N m −2 h −1. In contrast to all other measurements on the Swale–Ouse river system, values for denitrification rate at the most downstream site showed on three occasions a decrease in comparison with the next site upstream (Cawood). It is suggested that this may be due to sediment instability at this site. Experimental studies with sediment slurries made on two different dates showed an increase in denitrification rate at all sites in response to added nitrate, but none due to added glucose. However, a combination of nitrate and glucose added to slurries from the most upstream site (Naburn Weir) and assayed under low O 2 conditions showed a significantly higher denitrification rate than with added nitrate alone.

The potential for feedback effects induced by global warming on emissions of nitrous oxide by soils

About 65% of all emissions of nitrous oxide, N2O, are from soils, and are caused by aerobic nitrification and anaerobic denitrification. Tropical forest soils are probably the most important single source, followed by cultivated soils. Emission rates in natural systems are related to the rate of N mineralization from organic matter, and N deposition; in agricultural systems they are related to the quantities of N used as fertilizers and, where relevant, to recent land use change. The global budget for N2O is not well balanced, and sources may still be underestimated. Direct evidence of a positive feedback of global warming on N2O emissions comes from studies of air in ice cores. One of the projected effects of future global warming is a lowering of water tables in northern peatlands; experiments suggest that this would lead to increased emissions, but that the effect on total emissions would be small. The results of many experiments with non‐peatland soils indicate that the effect of temperature on soil emissions is generally positive, and that the rate of increase may be very steep when denitrification is the principal process involved. Process‐level modelling suggests that the reason is increased soil respiration, which causes an increase in anaerobic volume in which denitrification can take place, in addition to the increased denitrification rate per unit anaerobic volume brought about directly by the rise in temperature. These results imply that generally a positive feedback on emissions from soils is likely. However, in some environments, a large proportion of total annual emissions can occur during freeze–thaw cycles; such cycles may become more or less frequent, depending on the climatic zone, and this may result in either a positive or negative feedback effect due to global warming. Models of global and regional trends give very conflicting predictions of the direction and the magnitude of climatic impacts on fluxes, but the prediction of a positive feedback seems to be the more soundly based.

Microbial adaptation, process performance and a suggested control strategy in a pre-denitrifying system with ethanol dosage

Biological nitrogen removal in activated sludge processes is dependent on sufficient supplies of easily metabolized carbon compounds for the denitrifying bacterial population. An external carbon source can increase denitrification rates and compensate for deficiencies in the influent C/N ratio. Plant performance and microbial adaptation were studied in a pre-denitrifying pilot-scale activated sludge plant with and without ethanol. Total nitrogen removal efficiency was 67 and 35% for the ethanol and reference line, respectively. The process responded rapidly to ethanol but one sludge age was necessary for full bacterial adaptation. An initial rapid increase suggests enzyme induction rather than alterations in bacterial species composition. Increased enzyme activity was explained by an increase in turn-over rate of biomass. Low effluent nitrate concentration was a result of the simultaneous use of influent COD and ethanol. Fluctuations in influent COD did not affect denitrification capacity with ethanol. Sludge settling properties were moderately better in the process without ethanol addition. An automatic control strategy for carbon dosage using feedforward from influent carbon and nitrate in the recirculated flow was simulated. Simulations with an adaptive linear quadratic controller demonstrated that the desired nitrate concentration at the end of the anoxic zone could be maintained despite relatively large disturbances.