Resource effects on denitrification are mediated by community composition in tidal freshwater wetlands soils

Accurate prediction of denitrification rates remains difficult, potentially owing to complex uncharacterized interactions between resource conditions and denitrifier communities. To better understand how the availability of organic matter (OM) and nitrate (NO₃^–), two of the resources most fundamental to denitrifiers, affect these populations and their activity, we performed an in situ resource manipulation in tidal freshwater wetland soils. Soils were augmented with OM to double ambient concentrations, using either compost or plant litter, and fertilized with KNO₃ at two levels (low: ～ 5 mg l^–1 NO₃^––N and high: ～ 50 mg l^–1 NO₃^––N) in a full factorial design. Community composition of nirS‐denitrifers (assessed using terminal restriction fragment length polymorphism) was interactively regulated by both NO₃^– concentration and OM type, and the associated shifts in community composition were relatively consistent across sampling dates (6, 9 and 12 months of incubation). Denitrification potential (pDNF) rates were also strongly affected by NO₃^– fertilization and increased by ～ 10–100‐fold. Path analysis revealed that the influence of resource availability on pDNF rates was largely mediated through changes in nirS‐denitrifier community composition. These results suggest that a greater understanding of denitrifier community ecology may enable more accurate prediction of denitrification rates.     

Soil environmental conditions rather than denitrifier abundance and diversity drive potential denitrification after changes in land uses

Land‐use practices aiming at increasing agro‐ecosystem sustainability, e.g. no‐till systems and use of temporary grasslands, have been developed in cropping areas, but their environmental benefits could be counterbalanced by increased N₂O emissions produced, in particular during denitrification. Modelling denitrification in this context is thus of major importance. However, to what extent can changes in denitrification be predicted by representing the denitrifying community as a black box, i.e. without an adequate representation of the biological characteristics (abundance and composition) of this community, remains unclear. We analysed the effect of changes in land uses on denitrifiers for two different agricultural systems: (i) crop/grassland conversion and (ii) cessation/application of tillage. We surveyed potential denitrification (PD), the abundance and genetic structure of denitrifiers (nitrite reducers), and soil environmental conditions. N₂O emissions were also measured during periods of several days on control plots. Time‐integrated N₂O emissions and PD were well correlated among all control plots. Changes in PD were partly due to changes in denitrifier abundance but were not related to changes in the structure of the denitrifier community. Using multiple regression analysis, we showed that changes in PD were more related to changes in soil environmental conditions than in denitrifier abundance. Soil organic carbon explained 81% of the variance observed for PD at the crop/temporary grassland site, whereas soil organic carbon, water‐filled pore space and nitrate explained 92% of PD variance at the till/no‐till site, without any residual effect of denitrifier abundance. Soil environmental conditions influenced PD by modifying the specific activity of denitrifiers, and to a lesser extent by promoting a build‐up of denitrifiers. Our results show that an accurate simulation of carbon, oxygen and nitrate availability to denitrifiers is more important than an accurate simulation of denitrifier abundance and community structure to adequately understand and predict changes in PD in response to land‐use changes.     

Influence of base flow stream bank seepage on riparian zone nitrogen biogeochemistry

We examined the effect of sustained stream bank seepage during base flow conditions on the pore water nitrogen biogeochemistry of two riparian zones in lowland agricultural areas in southern Ontario, Canada. Nitrate, ammonium and dissolved oxygen concentrations in riparian subsurface water over a two-year period showed well-organized spatial patterns along stream bank seepage flow paths that extended seasonally up to 25 m inland. High levels of dissolved oxygen and NO₃⁻ in stream inflow were depleted rapidly at the stream bank interface suggesting the occurrence of aerobic microbial respiration followed by denitrification. A zone of NH₄⁺ accumulation persisted in more anaerobic sediments inland from the bank margin, although the magnitude and intensity of the pattern varied seasonally. A bromide tracer and NO₃⁻ co-injection at the stream bank interface indicated that bank seepage occurred along preferential flow paths in a poorly sorted gravel layer in the two riparian zones. Depletion of NO₃⁻ in relation to co-injected bromide confirmed that the bank margin was a hot spot of biogeochemical activity within the riparian zone. Conceptual models of humid temperate riparian zones have focused on nitrogen biogeochemistry in relation to hillslope to stream hydrologic flow paths. However, our results suggest that sustained stream bank inflow during low flow conditions can exert a dominant control on riparian nitrogen cycling in lowland landscapes where level riparian zones bounded by perennial streams receive limited subsurface inflows from adjacent slopes.     

Manipulation of the Dissolved Organic Carbon Pool in an Agricultural Stream: Responses in Microbial Community Structure, Denitrification, and Assimilatory Nitrogen Uptake

Carbon (C) and nitrogen (N) are strongly coupled across ecosystems due to stoichiometrically balanced assimilatory demand as well as dissimilatory processes such as denitrification. Microorganisms mediate these biogeochemical cycles, but how microbial communities respond to environmental changes, such as dissolved organic carbon (DOC) availability, and how those responses impact coupled biogeochemical cycles in streams is not clear. We enriched a stream in central Indiana with labile DOC for 5?days to investigate coupled C and N cycling. Before, and on day 5 of the enrichment, we examined assimilatory uptake and denitrification using whole-stream ¹⁵N-nitrate tracer additions and short-term nitrate releases. Concurrently, we measured bacterial and denitrifier abundance and community structure. We predicted N assimilation and denitrification would be stimulated by the addition of labile C and would be mediated by increases in bacterial activity, abundance, and a shift in community structure. In response to the twofold increase in DOC concentrations in the water column, N assimilation increased throughout the enrichment. Community respiration doubled during the enrichment and was associated with a change in bacterial community structure (based on terminal restriction fragment length polymorphisms of the 16S rRNA gene). In contrast, there was little response in denitrification or denitrifier community structure, likely because labile C was assimilated by heterotrophic communities on the stream bed prior to reaching denitrifiers within the sediments. Our results suggest that coupling between C and N in streams involves potentially complex interactions with sediment texture and organic matter, microbial community structure, and possibly indirect biogeochemical pathways.     

Abundance, diversity and functional gene expression of denitrifier communities in adjacent riparian and agricultural zones

Lands under riparian and agricultural management differ in soil properties, water content, plant species and nutrient content and are therefore expected to influence denitrifier communities, denitrification and nitrous oxide (N(2) O) emissions. Denitrifier community abundance, denitrifier community structure, denitrification gene expression and activity were quantified on three dates in a maize field and adjacent riparian zone. N(2) O emissions were greater in the agricultural zone, whereas complete denitrification to N(2) was greater in the riparian zone. In general, the targeted denitrifier community abundance did not change between agricultural and riparian zones. However, nosZ gene expression was greater in the riparian zone than the agricultural zone. The community structure of nirS-gene-bearing denitrifiers differed in June only, whereas the nirK-gene-bearing community structure differed significantly between the riparian and the agricultural zones at all dates. The nirK-gene-bearing community structure was correlated with soil pH, while no significant correlations were found between nirS-gene-bearing community structure and soil environmental variables or N(2) O emissions, denitrification or denitrifier enzyme activity. The results suggested for the nirK and nirS-gene-bearing communities different factors control abundance vs. community structure. The nirK-gene-bearing community structure was also more responsive than the nirS-gene-bearing community structure to change between the two ecosystems.     

Importance of denitrifiers lacking the genes encoding the nitrous oxide reductase for N2O emissions from soil

Analyses of the complete genomes of sequenced denitrifying bacteria revealed that approximately 1/3 have a truncated denitrification pathway, lacking the nosZ gene encoding the nitrous oxide reductase. We investigated whether the number of denitrifiers lacking the genetic ability to synthesize the nitrous oxide reductase in soils is important for the proportion of N₂O emitted by denitrification. Serial dilutions of the denitrifying strain Agrobacterium tumefaciens C58 lacking the nosZ gene were inoculated into three different soils to modify the proportion of denitrifiers having the nitrous oxide reductase genes. The potential denitrification and N₂O emissions increased when the size of inoculated C58 population in the soils was in the same range as the indigenous nosZ community. However, in two of the three soils, the increase in potential denitrification in inoculated microcosms compared with the noninoculated microcosms was higher than the increase in N₂O emissions. This suggests that the indigenous denitrifier community was capable of acting as a sink for the N₂O produced by A. tumefaciens. The relative amount of N₂O emitted also increased in two soils with the number of inoculated C58 cells, establishing a direct causal link between the denitrifier community composition and potential N₂O emissions by manipulating the proportion of denitrifiers having the nosZ gene. However, the number of denitrifiers which do not possess a nitrous oxide reductase might not be as important for N₂O emissions in soils having a high N₂O uptake capacity compared with those with lower. In conclusion, we provide a proof of principle that the inability of some denitrifiers to synthesize the nitrous oxide reductase can influence the nature of the denitrification end products, indicating that the extent of the reduction of N₂O to N₂ by the denitrifying community can have a genetic basis.     

Soil microbial diversity, community structure and denitrification in a temperate riparian zone

Nitrate (NO ₃ ^– ) removal in riparian zones bordering agricultural areas occurs via plant uptake, microbial immobilisation and bacterial denitrification. Denitrification is a desirable mechanism for removal because the bacterial conversion of NO ₃ ^– to N gases permanently removes NO ₃ ^– from the watershed. A field and laboratory study was conducted in riparian soils adjacent to Carroll Creek, Ontario, Canada, to assess the spatial distribution of denitrification relative to microbial community structure and microbial functional diversity. Soil samples were collected in March, June, and August 1997 at varying soil depths and distances from the stream. Denitrification measurements made using the acetylene block technique on intact soil cores were highly variable and did not show any trends with riparian zone location. Microbial community composition and functional diversity were determined using sole carbon source utilization (SCSU) on Biolog® GN microplates. Substrate richness, evenness and diversity (Shannon index) were greatest within the riparian zone and may also have been influenced by a rhizosphere effect. A threshold relationship between denitrification and measures of microbial community structure implied minimum levels of richness, evenness and diversity were required for denitrification.     

Development of associations between microalgae and denitrifying bacteria in streams of contrasting anthropogenic influence

We compared the development of microalgal and bacterial-denitrifier communities within biofilms over 28 days in a restored-prairie stream (RP) and a stream receiving treated wastewater effluent (DER). Inorganic nutrient concentrations were an order of magnitude greater in DER, and stream waters differed in the quality of dissolved organics (characterized via pyrolysis-GC/MS). Biofilm biomass and the densities of algae and bacteria increased over time in both systems; however, algal and denitrifier community composition and the patterns of development differed between systems. Specifically, algal and denitrifier taxonomic composition stabilized more quickly in DER than RP, whereas the rates of algal and denitrifier succession were more closely coupled in RP than DER. We hypothesize that, under unenriched conditions, successional changes in algal assemblages influence bacterial denitrifiers due to their dependence on algal exudates, while under enriched conditions, this relationship is decoupled. Between-system differences in organic signatures supported this, as RP biofilms contained more labile, aliphatic compounds than DER. In addition, potential denitrification rates (DNP) were negatively correlated with the percentage of aromatic compounds within the biofilm organic signatures, suggesting a significant relationship between algal exudate composition and denitrification. These results are significant because anthropogenic factors that affect biofilm community composition may alter their capacity to perform critical ecosystem services.     

Impacts of varying durations of passive oxygen exposure on freshwater denitrifier community structure and function

Fertilizer use has dramatically increased the availability of nitrate (NO₃ ^?) in aquatic systems. Microbe-mediated denitrification is one of the predominant means of NO₃ ^? removal from freshwaters, yet oxygenation (O₂)-induced disruptions—e.g., extreme precipitation events—can occur, resulting in a disproportional increase in nitrous oxide (N₂O) production and efflux as facultative anaerobic bacterial populations use of O₂ as a terminal electron acceptor increases. We examined the effects of 12- and 24-h passive O₂ exposure on previously anaerobic bacterial communities focusing on denitrification enzyme activity (DEA), N₂O production, and bacterial community 16S rRNA and nitrous oxide reductase gene (nosZ) profiles after 12, 24, and 48 h of anaerobic recovery. Treatments experiencing 24-h O₂ exposure had significantly higher DEA 12 h into anaerobic recovery than treatments undergoing 12-h O₂ exposure. Initial N₂O emissions were significantly lower in the 24-h O₂ exposure treatments although by 24 h a dramatic spike (tenfold relative to the 12-h O₂ exposure treatments) in N₂O concentrations was observed. However, within 6 h (30-h anaerobic recovery) these differences were gone. Community nosZ profiles experiencing 24-h O₂ exposure exhibited reduced diversity after 24-h recovery, which corresponded with an increase in N₂O emissions. However, after 48 h of anaerobic recovery, nosZ diversity had recovered. These observations highlight the effects of short-term aerobic disruption on denitrification, as well as the effects on the denitrifier community profile. Together, these data suggest that recovery to ambient N cycling is exacerbated by disturbance length due to increased lag time and subsequent loss of denitrifier community diversity.     

Denitrification activity of Bradyrhizobium sp. isolated from Argentine soybean cultivated soils

Two hundred and fifty strains, all of them representatives of native Bradyrhizobium sp., isolated from soils cultivated with soybean have been characterized by their denitrification activity. In addition, the denitrification potential of those soils was also measured by evaluating the most-probable-number (MPN) of denitrifying bacteria and the denitrification enzyme assay (DEA). Of the 250 isolates tested, 73 were scored as probable denitrifiers by a preliminary screening method. Only 41 were considered denitrifiers because they produced gas bubbles in Durham tubes, cultures reached an absorbance of more than 0.1 and NO³⁻ and NO²⁻ were not present. Ten of these 41 were selected to confirm denitrification and to study denitrification genes. According to N₂O production and cell protein concentration with NO³⁻, the isolates could be differentiated in three categories of denitrifiers. The presence of the napA, nirK, norC and nosZ genes was detected by production of a diagnostic PCR product using specific primers. RFLP from the 16S-23S rDNA spacer region (IGS) revealed that denitrifiers strains could be characterized as Bradyrhizobium japonicum and strains which were non-respiratory denitrifiers as B. elkanii.     

文峪河上游河岸带不同植被类型土壤nirS反硝化菌群结构及功能

以文峪河上游河岸带不同演替阶段的8种植被类型五花草甸(WH)、沙棘林(HR)、柳树林(SS)、山杨林(PC)、山杨白桦林(PQ)、山杨白桦落叶松林(PQL)、落叶松云杉林(LP)和云杉林(PM)土壤为研究对象,采用高通量测序技术测定nirS反硝化细菌群落组成及相对丰度,乙炔抑制法测定反硝化酶活性(DEA)。对其土壤理化性质及反硝化细菌群落组成及相对丰度进行方差分析,采用冗余分析(RDA)和Spearman相关性分析不同植被类型及土层反硝化细菌群落结构及功能及土壤理化因子的关联性。结果表明:1)不同植被类型及土层土壤理化因子存在显著差异,柳树林(SS)0—15 cm土层硝态氮(NO~+_3-N)含量显著高于其他植被类型各土层;2)土壤反硝化菌群多样性指数在五花草甸(WH)、山杨白桦林(PQ)和云杉林(PM)中较其他植被类型高;3)沙棘林(HR)及柳树林(SS)反硝化酶活性(DEA)显著高于其他植被类型;4)不同植被类型反硝化优势菌群分布存在显著差异及特异性,如浮霉菌门(Planctomycetes)仅在落叶松云杉混交林(LP)和云杉林(PM)植被类型15—30 cm土层中分布;5)土壤pH、土壤有机碳(SOC)、土壤铵态氮(NH~+_4-N)和硝态氮(NO~+_3-N)等是影响土壤反硝化细菌群落结构及组成的重要因子,其中土壤铵态氮和硝态氮含量变化是导致土壤反硝化菌群多样性和反硝化酶活性差异的关键因子。本研究揭示了文峪河上游河岸带不同植被类型土壤反硝化细菌群落结构及功能的变化和分布特征,为进一步研究该区域河岸带氮素循环及水体污染防治提供重要参考依据。     

Rapid Nitrate Loss and Denitrification in a Temperate River Floodplain

Nitrogen (N) pollution is a problem in many large temperate zone rivers, and N retention in river channels is often small in these systems. To determine the potential for floodplains to act as N sinks during overbank flooding, we combined monitoring, denitrification assays, and experimental nitrate (NO₃⁻ -N) additions to determine how the amount and form of N changed during flooding and the processes responsible for these changes in the Wisconsin River floodplain (USA). Spring flooding increased N concentrations in the floodplain to levels equal to the river. As discharge declined and connectivity between the river and floodplain was disrupted, total dissolved N decreased over 75% from 1.41 mg l⁻¹, equivalent to source water in the Wisconsin River on 14 April 2001, to 0.34 mg l⁻¹ on 22 April 2001. Simultaneously NO₃⁻ -N was attenuated almost 100% from 1.09 to <0.002 mg l⁻¹. Unamended sediment denitrification rates were moderate (0–483 μg m⁻² h⁻¹) and seasonally variable, and activity was limited by the availability of NO ₃⁻ -N on all dates. Two experimental NO₃⁻ -N pulse additions to floodplain water bodies confirmed rapid NO₃⁻ -N depletion. Over 80% of the observed NO ₃⁻ -N decline was caused by hydrologic export for addition #1 but only 22% in addition #2. During the second addition, a significant fraction (>60%) of NO₃⁻ -N mass loss was not attributable to hydrologic losses or conversion to other forms of N, suggesting that denitrification was likely responsible for most of the NO₃⁻ -N disappearance. Floodplain capacity to decrease the dominant fraction of river borne N within days of inundation demonstrates that the Wisconsin River floodplain was an active N sink, that denitrification often drives N losses, and that enhancing connections between rivers and their floodplains may enhance overall retention and reduce N exports from large basins.     

“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.     

Use of Stable-Isotope Probing, Full-Cycle rRNA Analysis, and Fluorescence In Situ Hybridization-Microautoradiography To Study a Methanol-Fed Denitrifying Microbial Community

A denitrifying microbial consortium was enriched in an anoxically operated, methanol-fed sequencing batch reactor (SBR) fed with a mineral salts medium containing methanol as the sole carbon source and nitrate as the electron acceptor. The SBR was inoculated with sludge from a biological nutrient removal activated sludge plant exhibiting good denitrification. The SBR denitrification rate improved from less than 0.02 mg of NO₃⁻-N mg of mixed-liquor volatile suspended solids (MLVSS)⁻¹ h⁻¹ to a steady-state value of 0.06 mg of NO₃⁻-N mg of MLVSS⁻¹ h⁻¹ over a 7-month operational period. At this time, the enriched microbial community was subjected to stable-isotope probing (SIP) with [¹³C]methanol to biomark the DNA of the denitrifiers. The extracted [¹³C]DNA and [¹²C]DNA from the SIP experiment were separately subjected to full-cycle rRNA analysis. The dominant 16S rRNA gene phylotype (group A clones) in the [¹³C]DNA clone library was closely related to those of the obligate methylotrophs Methylobacillus and Methylophilus in the order Methylophilales of the Betaproteobacteria (96 to 97% sequence identities), while the most abundant clone groups in the [¹²C]DNA clone library mostly belonged to the family Saprospiraceae in the Bacteroidetes phylum. Oligonucleotide probes for use in fluorescence in situ hybridization (FISH) were designed to specifically target the group A clones and Methylophilales (probes DEN67 and MET1216, respectively) and the Saprospiraceae clones (probe SAP553). Application of these probes to the SBR biomass over the enrichment period demonstrated a strong correlation between the level of SBR denitrification and relative abundance of DEN67-targeted bacteria in the SBR community. By contrast, there was no correlation between the denitrification rate and the relative abundances of the well-known denitrifying genera Hyphomicrobium and Paracoccus or the Saprospiraceae clones visualized by FISH in the SBR biomass. FISH combined with microautoradiography independently confirmed that the DEN67-targeted cells were the dominant bacterial group capable of anoxic [¹⁴C]methanol uptake in the enriched biomass. The well-known denitrification lag period in the methanol-fed SBR was shown to coincide with a lag phase in growth of the DEN67-targeted denitrifying population. We conclude that Methylophilales bacteria are the dominant denitrifiers in our SBR system and likely are important denitrifiers in full-scale methanol-fed denitrifying sludges.     

Algal Exudates and Stream Organic Matter Influence the Structure and Function of Denitrifying Bacterial Communities

Within aquatic ecosystems, periphytic biofilms can be hot spots of denitrification, and previous work has suggested that algal taxa within periphyton can influence the species composition and activity of resident denitrifying bacteria. This study tested the hypothesis that algal species composition within biofilms influences the structure and function of associated denitrifying bacterial communities through the composition of organic exudates. A mixed population of bacteria was incubated with organic carbon isolated from one of seven algal species or from one of two streams that differed in anthropogenic inputs. Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) revealed differences in the organic composition of algal exudates and stream waters, which, in turn, selected for distinct bacterial communities. Organic carbon source had a significant effect on potential denitrification rates (DNP) of the communities, with organics isolated from a stream with high anthropogenic inputs resulting in a bacterial community with the highest DNP. There was no correlation between DNP and numbers of denitrifiers (based on nirS copy numbers), but there was a strong relationship between the species composition of denitrifier communities (as indicated by tag pyrosequencing of nosZ genes) and DNP. Specifically, the relative abundance of Pseudomonas stutzeri-like nosZ sequences across treatments correlated significantly with DNP, and bacterial communities incubated with organic carbon from the stream with high anthropogenic inputs had the highest relative abundance of P. stutzeri-like nosZ sequences. These results demonstrate a significant relationship between bacterial community composition and function and provide evidence of the potential impacts of anthropogenic inputs on the structure and function of stream microbial communities.     

Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: subsurface solute patterns

Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian zones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN; NH₄ ⁺- and NO₃ ⁻-N) were low in stream water (< 110 ug/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO₃ ⁻-N (550 ug/L) and oxygen concentrations were high (5.2 mg/L). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH₄ ⁺-N (200–700 ug/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 ug/L) than at Icacos (600 ug/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH₄ ⁺ and NO₃ ⁻ were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appear to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes through the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zone. However, geomorphology appears to strongly modify the importance of specific biotic components.     

A Doubling of Microphytobenthos Biomass Coincides with a Tenfold Increase in Denitrifier and Total Bacterial Abundances in Intertidal Sediments of a Temperate Estuary

Surface sediments are important systems for the removal of anthropogenically derived inorganic nitrogen in estuaries. They are often characterized by the presence of a microphytobenthos (MPB) biofilm, which can impact bacterial communities in underlying sediments for example by secretion of extracellular polymeric substances (EPS) and competition for nutrients (including nitrogen). Pyrosequencing and qPCR was performed on two intertidal surface sediments of the Westerschelde estuary characterized by a two-fold difference in MPB biomass but no difference in MPB composition. Doubling of MPB biomass was accompanied by a disproportionately (ten-fold) increase in total bacterial abundances while, unexpectedly, no difference in general community structure was observed, despite significantly lower bacterial richness and distinct community membership, mostly for non-abundant taxa. Denitrifier abundances corresponded likewise while community structure, both for nirS and nirK denitrifiers, remained unchanged, suggesting that competition with diatoms for nitrate is negligible at concentrations in the investigated sediments (appr. 1 mg/l NO₃^-). This study indicates that MPB biomass increase has a general, significantly positive effect on total bacterial and denitrifier abundances, with stimulation or inhibition of specific bacterial groups that however do not result in a re-structured community.     

The effect of nitrate in stream water on the relationship between gentrification and nitrification in a stream-sediment microcosm

SUMMARY 1. Microcosm experiments were carried out to simulate, in the laboratory, the conditions occurring at the water-sediment interface of a stream draining agricultural land. Constant boundary conditions were attained by passing synthetic 'stream water', saturated with dissolved oxygen and containing 1 mmol NO₃?N dm^?3 (or 1 mmol Cl? dm^?3, control), once only over the sediment surface. 2. Measurements were made of inorganic-N (nitrate, nitrite, ammonium), redox potential, potential denitrification and nitrification activities, and readily mineralizable carbon sediment profiles at three incubation times up to 24 days. The peaks in denitrification and nitrification activity moved down the profile with time in the nitrate-treated sediment, but stayed relatively stationary in the control treatment. Although the zone of nitrification was restricted to the top 2–3 mm of sediment in the control treatment, high fluxes of both dissolved oxygen and NH₄?N maintained a high nitrifier activity within this zone for the duration of the experiment. 3. Increases in denitrifier activity immediately below the nitrifier activity peak indicated that a coupled nitrification-denitrification sequence was operating in both the control and nitrate-treated sediment. The greater depth of nitrification when nitrate was present in the ‘stream water’ was attributed to a feedback mechanism in which enhanced denitrification in the sediment reduced the local demand for oxygen and permitted dissolved oxygen to diffuse further into the sediment. The progressively greater depth to which oxygen penetrated caused the contiguous peaks of potential nitrifier and denitrifier enzyme activity to migrate farther from the interface. However, diffusion rates of the reactants limited the depth to which these coupled reactions could extend. 4. The possible effect of this feedback mechanism on the nitrate status of natural sediment-stream water systems is briefly discussed.