Nitrogen transformations at the sediment–water interface across redox gradients in the Laurentian Great Lakes

The capacity of a lake to remove reactive nitrogen (N) through denitrification has important implications both for the lake and for downstream ecosystems. In large oligotropic lakes such as Lake Superior, where nitrate (NO₃ ^?) concentrations have increased steadily over the past century, deep oxygen penetration into sediments may limit the denitrification rates. We tested the hypothesis that the position of the redox gradient in lake sediments affects denitrification by measuring net N-fluxes across the sediment–water interface for intact sediment cores collected across a range of sediment oxycline values from nearshore and offshore sites in Lake Superior, as well as sites in Lake Huron and Lake Erie. Across this redox gradient, as the thickness of the oxygenated sediment layer increased from Lake Erie to Lake Superior, fluxes of NH₄ ⁺ and N₂ out of the sediment decreased, and sediments shifted from a net sink to a net source of NO₃ ^?. Denitrification of NO₃ ^? from overlying water decreased with thickness of the oxygenated sediment layer. Our results indicate that, unlike sediments from Lake Erie and Lake Huron, Lake Superior sediments do not remove significant amounts of water column NO₃ ^? through denitrification, likely as a result of the thick oxygenated sediment layer.     

Annual Pattern of Denitrification and Nitrate Ammonification in Estuarine Sediment

The seasonal variation and depth distribution of the capacity for denitrification and dissimilatory NO₃⁻ reduction to NH₄⁺ (NO₃⁻ ammonification) were studied in the upper 4 cm of the sediment of Norsminde Fjord estuary, Denmark. A combination of C₂H₂ inhibition and ¹⁵N isotope techniques was used in intact sediment cores in short-term incubations (maximum, 4 h). The denitrification capacity exhibited two maxima, one in the spring and one in the fall, whereas the capacity for NO₃⁻ ammonification was maximal in the late summer, when sediments were progressively reduced. The denitrification capacity was always highest in the uppermost 1 cm of the sediment and declined with depth. The NO₃⁻ ammonification was usually higher with depth, but the maximum activity in late summer was observed within the upper 1 cm. The capacity for NO₃⁻ incorporation into organic material was investigated on two occasions in intact sediment cores and accounted for less than 5% of the total NO₃⁻ reduction. Denitrification accounted for between 13 and 51% of the total NO₃⁻ reduction, and NH₄⁺ production accounted for between 4 and 21%, depending on initial rates during the time courses. Changes of the rates during the incubation were observed in the late summer, which reflected synthesis of denitrifying enzymes. This time lag was eliminated in experiments with mixed sediment because of preincubation with NO₃⁻ and alterations of the near-environmental conditions. The initial rates obtained in intact sediment cores therefore reflect the preexisting enzyme content of the sediment.     

Nitrogen Metabolism Genes from Temperate Marine Sediments

In this study, we analysed metagenomes along with biogeochemical profiles from Skagerrak (SK) and Bothnian Bay (BB) sediments, to trace the prevailing nitrogen pathways. NO₃ ^? was present in the top 5 cm below the sediment-water interface at both sites. NH₄ ⁺ increased with depth below 5 cm where it overlapped with the NO₃ ^? zone. Steady-state modelling of NO₃ ^? and NH₄ ⁺ porewater profiles indicates zones of net nitrogen species transformations. Bacterial protease and hydratase genes appeared to make up the bulk of total ammonification genes. Genes involved in ammonia oxidation (amo, hao), denitrification (nir, nor), dissimilatory NO₃ ^? reduction to NH₄ ⁺ (nfr and otr) and in both of the latter two pathways (nar, nap) were also present. Results show ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) are similarly abundant in both sediments. Also, denitrification genes appeared more abundant than DNRA genes. 16S rRNA gene analysis showed that the relative abundance of the nitrifying group Nitrosopumilales and other groups involved in nitrification and denitrification (Nitrobacter, Nitrosomonas, Nitrospira, Nitrosococcus and Nitrosomonas) appeared less abundant in SK sediments compared to BB sediments. Beggiatoa and Thiothrix 16S rRNA genes were also present, suggesting chemolithoautotrophic NO₃ ^? reduction to NO₂ ^? or NH₄ ⁺ as a possible pathway. Our results show the metabolic potential for ammonification, nitrification, DNRA and denitrification activities in North Sea and Baltic Sea sediments.     

“Hotspots” and “Hot Moments” of Denitrification in Urban Brownfield Wetlands

The influence of hydrology and soil properties on disproportionately high (“hot”) rates of nitrate (NO₃ ^?) removal via denitrification has been relatively well established. It is poorly understood, however, how the unique soil characteristics of brownfield wetlands contribute to or hinder denitrification. In this study, we examined drivers of “hot” denitrification rates over time (“hot moments”) and space (“hotspots”) in a watershed located on an unrestored brownfield in New Jersey, USA. We carried out measurements of denitrification over 9-day sequences during three seasons in sites with the same vegetation (Phragmites australis) but different soils (fill material, remnant marsh soils, flooded organic-rich soils). Denitrification rates above the 3rd quartile value of the data distribution were defined as “hot” and the most important drivers of these rates were determined using mixed models. Porosity and NO₃ ^? availability were the strongest spatial and temporal predictors, respectively, of high denitrification rates, with coarse-textured, unflooded fill materials unexpectedly supporting the highest rates. These results suggest that pore-scale hydrology is a more complex controller of wetland denitrification than previously thought. Course-textured, unflooded soils have high fractions of air-filled pores relative to flooded soils, leading to more endogenous NO₃ ^? production, and less diffusion constraints than fine-textured soils, leading to higher NO₃ ^? availability to denitrifiers in suboxic pores. Laboratory studies confirmed denitrifiers were limited by NO₃ ^? availability. However, denitrification rates in all soils matched or exceeded atmospheric NO₃ ^? deposition and stormwater NO₃ ^? loading at the site, suggesting that brownfields may play an important role in NO₃ ^? removal from urban stormwater.     

The dark side of the hyporheic zone: depth profiles of nitrogen and its processing in stream sediments

1. Although it is well known that sediments can be hot spots for nitrogen transformation in streams, many previous studies have confined measurements of denitrification and nitrate retention to shallow sediments (<5 cm deep). We determined the extent of nitrate processing in deeper sediments of a sand plains stream (Emmons Creek) by measuring denitrification in core sections to a depth of 25 cm and by assessing vertical nitrate profiles, with peepers and piezometers, to a depth of 70 cm. 2. Denitrification rates of sediment slurries based on acetylene block were higher in shallower core sections. However, core sections deeper than 5 cm accounted for 68% of the mean depth‐integrated denitrification rate. 3. Vertical hydraulic gradient and vertical profiles of pore water chloride concentration suggested that deep ground water upwelled through shallow sediments before discharging to the stream channel. The results of a two‐source mixing model based on chloride concentrations suggested that the hyporheic zone was very shallow (<5 cm) in Emmons Creek. 4. Vertical profiles showed that nitrate concentration in shallow ground water was about 10–60% of the nitrate concentration of deep ground water. The mean nitrate concentrations of deep and shallow ground water were 2.17 and 0.73 mg NO₃‐N L^?1, respectively. 5. Deep ground water tended to be oxic (6.9 mg O₂ L^?1) but approached anoxia (0.8 mg O₂ L^?1) after passing through shallow, organic carbon‐rich sediments, which suggests that the decline in the nitrate concentrations of upwelling ground water was because of denitrification. 6. Collectively, our results suggest that there is substantial nitrate removal occurring in deep sediments, below the hyporheic zone, in Emmons Creek. Our findings suggest that not accounting for nitrate removal in deep sediments could lead to underestimates of nitrogen processing in streams and catchments.     

Denitrification kinetics and denitrifier abundances in sediments of lakes receiving atmospheric nitrogen deposition (Colorado, USA)

The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5–8?kg?N?ha^?1?y^?1) or low (<2?kg?N?ha^?1?y^?1) inputs of atmospheric N deposition. The nitrate (NO₃ ^?) concentration was significantly greater in high-deposition lakes (11.3?μmol?l^?1) compared to low-deposition lakes (3.3?μmol?l^?1). Background denitrification was positively related to NO₃ ^? concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose–response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis–Menten kinetics, the maximum denitrification rate and half-saturation NO₃ ^? concentration did not differ between deposition regions and were 765?μmol?N?m^?2?h^?1 and 293?μmol?l^?1?NO₃ ^?, respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification.     

The Synergistic Effects of High Nitrate Concentrations on Sediment Bioreduction

Groundwaters at nuclear sites can be characterized by low pH and high nitrate concentrations (10–100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction which can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of reduction through the terminal electron accepting cascade NO^? ₃ > NO^? ₂ > Mn(IV)/Fe(III) > SO^2? ₄ at low pH (～5.5) was slower than that in bicarbonate buffered systems (pH ～ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate-reduction. Under very high nitrate conditions (100 mM), bicarbonate buffering (pH ～ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities in this reduced sediment. In Fe(III)-reducing enrichment cultures from the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that under certain conditions and contrary to expectations, denitrification may support bioreduction via pH conditioning for optimal metal reduction and radionuclide immobilization.     

Influence of bioturbation on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in freshwater sediments

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO₃ ^?) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O₂, NO₃ ^?, NH₄ ⁺, CO₂, N₂O, and CH₄) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO₃ ^? concentrations (using N¹⁵ isotope tracing) interact to affect N cycling processes. At the lowest NO₃ ^? concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO₃ ^? concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O₂, NO₃ ^?, NH₄ ⁺ fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO₃ ^? concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.     

Impact of Bacterial NO3? Transport on Sediment Biogeochemistry

Experiments demonstrated that Beggiatoa could induce a H₂S-depleted suboxic zone of more than 10 mm in marine sediments and cause a divergence in sediment NO₃⁻ reduction from denitrification to dissimilatory NO₃⁻ reduction to ammonium. pH, O₂, and H₂S profiles indicated that the bacteria oxidized H₂S with NO₃⁻ and transported S⁰ to the sediment surface for aerobic oxidation.     

Denitrification in San Francisco Bay Intertidal Sediments

The acetylene block technique was employed to study denitrification in intertidal estuarine sediments. Addition of nitrate to sediment slurries stimulated denitrification. During the dry season, sediment-slurry denitrification rates displayed Michaelis-Menten kinetics, and ambient NO₃⁻ + NO₂⁻ concentrations (≤26 μM) were below the apparent K_m (50 μM) for nitrate. During the rainy season, when ambient NO₃⁻ + NO₂⁻ concentrations were higher (37 to 89 μM), an accurate estimate of the K_m could not be obtained. Endogenous denitrification activity was confined to the upper 3 cm of the sediment column. However, the addition of nitrate to deeper sediments demonstrated immediate N₂O production, and potential activity existed at all depths sampled (the deepest was 15 cm). Loss of N₂O in the presence of C₂H₂ was sometimes observed during these short-term sediment incubations. Experiments with sediment slurries and washed cell suspensions of a marine pseudomonad confirmed that this N₂O loss was caused by incomplete blockage of N₂O reductase by C₂H₂ at low nitrate concentrations. Areal estimates of denitrification (in the absence of added nitrate) ranged from 0.8 to 1.2 μmol of N₂ m⁻² h⁻¹ (for undisturbed sediments) to 17 to 280 μmol of N₂ m⁻² h⁻¹ (for shaken sediment slurries).     

Denitrification and nitrous oxide effluxes in boreal, eutrophic river sediments under increasing nitrate load: a laboratory microcosm study

Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N₂) and nitrous oxide (N₂O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 μM of ¹⁵N enriched nitrate (NO₃ ^?, ~98 at. %). Effluxes of both N₂ and N₂O from sediment to the overlying water increased with increasing NO₃ ^? load. Although the ratio of N₂O to N₂ increased with increasing NO₃ ^? load, it remained below 0.04, N₂ always being the main product. At the NO₃ ^? concentrations most frequently found in the studied river water (10–100 μM), up to 8% of the NO₃ ^? was removed in denitrification, whereas with the highest concentration (300 μM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO₃ ^? was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO₃ ^? load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO₃ ^? was assimilated by microbes during their growth. The ¹⁵N content in ammonium (NH₄ ⁺) in the efflux was low, suggesting that reduction of NO₃ ^? to NH₄ ⁺ was not the reason for the NO₃ ^? removal. This study provides the first published information on denitrification and N₂O fluxes and their regulation by NO₃ ^? load in eutrophic high latitude rivers.     

Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi‐site dual nitrate isotopes

Increasing nitrogen (N) deposition in subtropical forests in south China causes N saturation, associated with significant nitrate (NO₃^?) leaching. Strong N attenuation may occur in groundwater discharge zones hydrologically connected to well‐drained hillslopes, as has been shown for the subtropical headwater catchment “TieShanPing”, where dual NO₃^? isotopes indicated that groundwater discharge zones act as an important N sink and hotspot for denitrification. Here, we present a regional study reporting inorganic N fluxes over two years together with dual NO₃^? isotope signatures obtained in two summer campaigns from seven forested catchments in China, representing a gradient in climate and atmospheric N input. In all catchments, fluxes of dissolved inorganic N indicated efficient conversion of NH₄⁺ to NO₃^? on well‐drained hillslopes, and subsequent interflow of NO₃^? over the argic B‐horizons to groundwater discharge zones. Depletion of ¹⁵N‐ and ¹⁸O–NO₃^? on hillslopes suggested nitrification as the main source of NO₃^?. In all catchments, except one of the northern sites, which had low N deposition rates, NO₃^? attenuation by denitrification occurred in groundwater discharge zones, as indicated by simultaneous ¹⁵N and ¹⁸O enrichment in residual NO₃^?. By contrast to the southern sites, the northern catchments lack continuous and well‐developed groundwater discharge zones, explaining less efficient N removal. Using a model based on ¹⁵NO₃^? signatures, we estimated denitrification fluxes from 2.4 to 21.7 kg N ha^?1 year^?1 for the southern sites, accounting for more than half of the observed N removal. Across the southern catchments, estimated denitrification scaled proportionally with N deposition. Together, this indicates that N removal by denitrification is an important component of the N budget of southern Chinese forests and that natural NO₃^? attenuation may increase with increasing N input, thus partly counteracting further aggravation of N contamination of surface waters in the region.     

Capacity for Denitrification and Reduction of Nitrate to Ammonia in a Coastal Marine Sediment

The capacity for dissimilatory reduction of NO₃⁻ to N₂ (N₂O) and NH₄⁺ was measured in ¹⁵NO₃⁻-amended marine sediment. Incubation with acetylene (7 × 10⁻³ atmospheres [normal]) caused accumulation of N₂O in the sediment. The rate of N₂O production equaled the rate of N₂ production in samples without acetylene. Complete inhibition of the reduction of N₂O to N₂ suggests that the “acetylene blockage technique” is applicable to assays for denitrification in marine sediments. The capacity for reduction of NO₃⁻ by denitrification decreased rapidly with depth in the sediment, whereas the capacity for reduction of NO₃⁻ to NH₄⁺ was significant also in deeper layers. The data suggested that the latter process may be equally as significant as denitrification in the turnover of NO₃⁻ in marine sediments.     

The Fate of Nitrate in Intertidal Permeable Sediments

Coastal zones act as a sink for riverine and atmospheric nitrogen inputs and thereby buffer the open ocean from the effects of anthropogenic activity. Recently, microbial activity in sandy permeable sediments has been identified as a dominant source of N-loss in coastal zones, namely through denitrification. Some of the highest coastal denitrification rates measured so far occur within the intertidal permeable sediments of the eutrophied Wadden Sea. Still, denitrification alone can often account for only half of the substantial nitrate (NO₃ ⁻) consumption. Therefore, to investigate alternative NO₃ ⁻ sinks such as dissimilatory nitrate reduction to ammonium (DNRA), intracellular nitrate storage by eukaryotes and isotope equilibration effects we carried out ¹⁵NO₃ ⁻ amendment experiments. By considering all of these sinks in combination, we could quantify the fate of the ¹⁵NO₃ ⁻ added to the sediment. Denitrification was the dominant nitrate sink (50–75%), while DNRA, which recycles N to the environment accounted for 10–20% of NO₃ ⁻ consumption. Intriguingly, we also observed that between 20 and 40% of ¹⁵NO₃ ⁻ added to the incubations entered an intracellular pool of NO₃ ⁻ and was subsequently respired when nitrate became limiting. Eukaryotes were responsible for a large proportion of intracellular nitrate storage, and it could be shown through inhibition experiments that at least a third of the stored nitrate was subsequently also respired by eukaryotes. The environmental significance of the intracellular nitrate pool was confirmed by in situ measurements which revealed that intracellular storage can accumulate nitrate at concentrations six fold higher than the surrounding porewater. This intracellular pool is so far not considered when modeling N-loss from intertidal permeable sediments; however it can act as a reservoir for nitrate during low tide. Consequently, nitrate respiration supported by intracellular nitrate storage can add an additional 20% to previous nitrate reduction estimates in intertidal sediments, further increasing their contribution to N-loss.     

Seasonal oxygen,nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

The regulatory roles of temperature, eutrophication and oxygen availability on benthic nitrogen (N) cycling and the stoichiometry of regenerated nitrogen and phosphorus (P) were explored along a Baltic Sea estuary affected by treated sewage discharge. Rates of sediment denitrification, anammox, dissimilatory nitrate reduction to ammonium (DNRA), nutrient exchange, oxygen (O₂) uptake and penetration were measured seasonally. Sediments not affected by the nutrient plume released by the sewage treatment plant (STP) showed a strong seasonality in rates of O₂ uptake and coupled nitrification–denitrification, with anammox never accounting for more than 20 % of the total dinitrogen (N₂) production. N cycling in sediments close to the STP was highly dependent on oxygen availability, which masked temperature-related effects. These sediments switched from low N loss and high ammonium (NH₄ ⁺) efflux under hypoxic conditions in the fall, to a major N loss system in the winter when the sediment surface was oxidized. In the fall DNRA outcompeted denitrification as the main nitrate (NO₃ ^?) reduction pathway, resulting in N recycling and potential spreading of eutrophication. A comparison with historical records of nutrient discharge and denitrification indicated that the total N loss in the estuary has been tightly coupled to the total amount of nutrient discharge from the STP. Changes in dissolved inorganic nitrogen (DIN) released from the STP agreed well with variations in sedimentary N₂ removal. This indicates that denitrification and anammox efficiently counterbalance N loading in the estuary across the range of historical and present-day anthropogenic nutrient discharge. Overall low N/P ratios of the regenerated nutrient fluxes impose strong N limitation for the pelagic system and generate a high potential for nuisance cyanobacterial blooms.     

Factors Controlling Sediment Denitrification in Midwestern Streams of Varying Land Use

We investigated controls on stream sediment denitrification in nine headwater streams in the Kalamazoo River Watershed, Michigan, USA. Factors influencing denitrification were determined by using experimental assays based on the chloramphenicol-amended acetylene inhibition technique. Using a coring technique, we found that sediment denitrification was highest in the top 5 cm of the benthos and was positively related to sediment organic content. To determine the effect of overlying water quality on sediment denitrification, first-order stream sediments were assayed with water from second- and third-order downstream reaches, and often showed higher denitrification rates relative to assays using site-specific water from the first-order stream reach. Denitrification was positively related to nitrate (NO₃ ⁻) concentration, suggesting that sediments may have been nutrient-limited. Using stream-incubated inorganic substrata of varying size classes, we found that finer-grained sand showed higher rates of denitrification compared to large pebbles, likely due to increased surface area per volume of substratum. Denitrification was measurable on both inorganic substrata and fine particulate organic matter loosely associated with inorganic particles, and denitrification rates were related to organic content. Using nutrient-amended denitrification assays, we found that sediment denitrification was limited by NO₃ ⁻ or dissolved organic carbon (DOC, as dextrose) variably throughout the year. The frequency and type of limitation differed with land use in the watershed: forested streams were NO₃ ⁻-limited or co-limited by both NO₃ ⁻ and DOC 92% of the time, urban streams were more often NO₃ ⁻-limited than DOC-limited, whereas agricultural stream sediments were DOC-limited or co-limited but not frequently limited by NO₃ ⁻ alone.     

Reduction of nitrate in clayey subsoils controlled by geochemical and microbial barriers

Vertical distribution of redox zones, concentrations of redox‐sensitive constituents, numbers of aerobic heterotrophic bacteria, and potential denitrification activity were studied in 1‐m cores taken at the transition between the oxidized and reduced layers in two Danish clayey subsoils. Based on the matrix soil colors, a redox sequence of oxidized, suboxic, and reduced zones was identified at both sites. The geochemical composition of the oxidized brown colored zone (to depths of 2.6 and 3.2 m) was characterized by high concentrations of NO₃ ^? and low amounts of total organic carbon, exchangeable forms of NH₄ ⁺, Fe²⁺, and Mn²⁺, and structural Fe(II) in the clay minerals. In the underlying 20‐ to 30‐cm‐deep suboxic zone, decreasing NO₃ ^? concentrations were observed together with increasing amount of exchangeable forms of Fe²⁺ and Mn²⁺, and structural Fe(II). Finally, in the reduced grey zone, NO₃ ^? was no longer present and maximum concentrations of other redox sensitive constituents occurred. Throughout the subsoils, the distribution of exchangeable Fe²⁺ corresponded most closely to changes in the colors of redox zones. The low‐organic Havrebjerg site displayed geochemical profiles indicating that NO₃ ^? was chemically reduced by structural Fe(II) in the clay minerals of the suboxic zone, and that the Fe(II) formed a geochemical barrier for the downward progression of NO₃ ^?. Aerobic heterotrophic bacteria occurred only in low numbers at this site and potential denitrification activity was very low. In contrast, the Sparresholm site had a significant population of bacteria in the suboxic zone, which also contained a heterogeneous distribution of potential denitrification activity. Specific microsites with facilitated transport of soluble organic substrates are proposed to support the denitrification activity in a heterogeneous distribution, constituting a microbial barrier for downward progression of NO₃ ^? in this subsoil.     

Nitrogen and phosphorus interchange between sediments and overlying water of a wastewater retention pond

Nitrogen and P interchange between the sediments and the overlying water of a simulated retention pond used for wastewater treatment were evaluated under conditions of seasonal temperature fluctuations and varying physico-chemical conditions (exposing floodwater surface to daylight vs. dark and turbulent vs. quiscent floodwater). Natural sediment columns obtained from two types of field retention ponds were used. One type of retention pond consisted of calcareous clay loam sediment while the sediment of second retention pond contained organic soil. Nutrient interchange between sediments and the overlying water was measured once a month over a period of one year. Nitrogen removal rates from floodwater were controlled by the initial floodwater NH ₄ ⁺ and NO ₃ ^? concentration, rate of NH ₄ ⁺ diffusion from the sediments to the overlying water, ammonification in the sediments, NH₃ volatilization and nitrification at the sediment-water interface, and denitrification in the sediments. Under the conditions studied, NH ₄ ⁺ concentrations of the floodwater were in the range of 0.01 to 0.05 µg/ml, while NO ₃ ^? concentrations were in the range of 0.27 to 0.78 µg/ml. Sediments with organic soil were found to be less effective in the removal of floodwater organic N, organic C and P, compared to the sediments with calcareous clay loam. Phosphorus exchange rates were dependent on the capacity of the sediment to adsorb or desorb P. Total P exchange rates were in the range of ?1.04 to 0.34 mg P/m² day. Seasonal temperature fluctuations, turbulent vs. quiscent water conditions or exposing the floodwater surface to daylight or dark had very little effect on N and P exchange rates.