Importance Of Denitrification Research Articles

The Contribution of Anammox to Nitrogen Removal Is Greater in Bulk Soils Than in Rhizosphere Soils in Riparian Wetlands Along the Yangtze River

AbstractThe importance of denitrification and anaerobic ammonium oxidation (anammox) in removing nitrogen (N) from upland runoff has been well documented in riparian wetlands. However, the relative contributions of denitrification and anammox to N removal in the rhizosphere and non‐rhizosphere soils of riparian zones remain unclear. Here, we explored the denitrification and anammox rates in the rhizosphere and bulk soils (0–5 and 10–15 cm) of 39 riparian wetlands along the Yangtze River using the 15N isotope pairing technique. Additionally, we used a quantitative polymerase chain reaction assay to determine the abundance of soil denitrifying and anammox bacteria using nosZ and hzsB genes, respectively. The results showed that both denitrification and anammox rates were significantly higher in rhizosphere soils than in bulk soils, suggesting that the rhizosphere environment is favorable for N removal. The contribution of anammox constituted over half (62.53% ± 1.49%) of the N loss and was greater in bulk soils (68.57% ± 1.42%) than in rhizosphere soils (55.64% ± 2.42%). Higher nosZ and hzsB gene abundances were also observed in rhizosphere soils than in bulk soils. Denitrification and anammox rates were significantly regulated by edaphic properties, microbial abundance, and plant biomass. The structural equation model further revealed that soil pH and N availability could affect denitrification and anammox rates both directly and indirectly by altering nosZ and hzsB gene abundance. Overall, this study highlights that the rhizosphere is a control point for N removal and harbors more functional microbes than bulk soils. Therefore, revegetation may effectively enhance the N removal function of riparian wetlands along the Yangtze River.

Organic matter diagenesis in hadal setting: Insights from the pore-water geochemistry of the Mariana Trench sediments

The diagenetic processes of organic matter (OM) in hadal sedimentary environments (>6000 m water depth) are much less studied than those in other accessible environments. In this study, we quantify OM oxidation processes using pore-water geochemical measurements, fluxes, and diagenetic rate calculations in sediments along a depth transect from abyssal to hadal sites (5500–10257 m) in the Mariana Trench. The total benthic O2 consumption, and depth-integrated rates of nitrification and denitrification are positively correlated with water depth, indicating that OM diagenesis is enhanced in the deep sites of the hadal zone. The negative linear correlation between water depth and oxygen/nitrate penetration depths further supports this conclusion. In the abyssal sites, aerobic respiration dominates OM degradation, and anaerobic processes are negligible. In contrast, denitrification plays an important role in anaerobic OM degradation, and accounts for approximately 5% of the depth-integrated total OM mineralization at the deepest hadal site. Moreover, our results suggest that direct coupling between nitrification and denitrification is significant, and may define the turnover of N in hadal sediments. Although they are not without uncertainties, our results shed new light on the importance of denitrification in sedimentary diagenesis in the deepest part of the Earth's ocean, and could have important implications for understanding the current state of biogeochemical cycles in the hadal zone.

Multiyear dual nitrate isotope signatures suggest that N-saturated subtropical forested catchments can act as robust N sinks.

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH4+ ), causes significant nitrate (NO3- ) leaching from well-drained acid forest soils on hill slopes (HS), whereas significant retention of NO3- occurs in near-stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ15 N and δ18 O) of nitrate (NO3- ) in soil pore water along a hydrological continuum in the N-saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH4+ and production of NO3- in soils on HS were associated with a significant decrease in δ15 N-NO3- , suggesting efficient nitrification despite low soil pH. The concentration of NO3- declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ15 N and δ18 O of residual NO3- , providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent 15 N enrichment factor (ε) of NO3- of about -5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ15 N and δ18 O-NO3- in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ15 N-NO3- values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N-saturated, subtropical forests with important implications for regional N budgets.

Dual nitrate isotopes clarify the role of biological processing and hydrologic flow paths on nitrogen cycling in subtropical low‐gradient watersheds

AbstractNitrogen (N) is an important nutrient as it often limits productivity but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low‐relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low‐relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flow paths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in two of the three watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flow paths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification and denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, and mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low‐gradient watersheds and variability in N delivery flow paths among adjacent watersheds with similar physical characteristics.

Spatial distribution and environmental determinants of denitrification enzyme activity in reed-dominated raised fields

Denitrification is an important process of nitrogen removal in lake ecosystems. However, the importance of denitrification across the entire soil-depth gradients including subsurface layers remains poorly understood. This study aims to determine the spatial pattern of soil denitrification enzyme activity (DEA) and its environmental determinants across the entire soil depth gradients in the raised fields in Baiyang Lake, North China. In two different zones of the raised fields (i.e., water boundary vs. main body of the raised fields), the soil samples from −1.0 m to 1.1 m depth were collected, and the DEA and following environmental determinants were quantified: soil moisture, pH, total nitrogen (TN), ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), total organic carbon (TOC), and rhizome biomass of Phragmites australis. The results showed that the soil DEA and environmental factors had a striking zonal distribution across the entire soil depth gradients. The soil DEA reached two peak values in the upper and middle soil layers, indicating that denitrification are important in both topsoil and subsurface of the raised fields. The correlation analysis showed that the DEA is negatively correlated with the soil depth (p < 0.05). However, this phenomenon did not occur in the distance to the water edge, except in the upper layers (from 0.2 m to 0.7 m) of the boundary zone of the raised fields. In the main body of the raised fields, the DEA level remained high; however, it showed no significant relationship with the distance to the water edge. The linear regression analysis showed significant positive correlation of the DEA with the soil TN, NO3−-N, NH4+-N, and TOC; whereas it showed negative correlation with soil pH. No significant correlations with soil moisture and temperature were observed. A positive correlation was also found between the DEA and rhizome biomass of P. australis.

Advances in understanding nitrogen flows and transformations: gaps and research pathways

SUMMARYThe nitrogen (N) cycle is one of the best studied elemental cycles. However, the N flows and transformations, in particular in aggregated soils, at small scales and in plant–soil systems are not yet fully understood. Analytical and molecular techniques are now available to address N dynamics at small scales. The methodological advances should go hand in hand with the development of suitable mathematical models addressing the small scale and the full complexity of the many interacting effects. The importance of denitrification within the N cycle is highlighted and used as an example of the progress achieved in recent times. Research gaps and possible research pathways are outlined.

Towards closing the watershed nitrogen budget: Spatial and temporal scaling of denitrification

AbstractEnhanced consideration of the hydrogeomorphic template of watersheds is critical to understanding watershed nitrogen budgets. We developed a framework to estimate the spatial distribution and temporal dynamics of soil moisture and soil oxygen in surficial soils to scale nitrogen transformations for a forested watershed (Pond Branch) in Maryland, USA. We sampled soil cores in upland, hillslope hollow, riparian hollow, and riparian hummock landscape positions in different seasons for biogeochemical fluxes including measurement of N2gas produced via denitrification. We extrapolated these rates in space and time with information derived from in situ soil oxygen and soil moisture probes to scale fluxes from plots to the catchment level. We addressed three questions: (1) How important are seasonal, daily, and storm event variations in soil oxygen for denitrification? (2) How is denitrification spatially distributed through the watershed? (3) How important is denitrification to the watershed nitrogen budget? We found that microtopography within the riparian zone is a significant influence on soil oxygen dynamics and therefore redox‐sensitive biogeochemical processes such as denitrification. Riparian zone hollows (lower topographic positions) represented 0.5%–1.0% of the catchment area, but accounted for &gt;99% of total denitrification. Interestingly, topography was a much stronger controller of oxygen than rainfall, which had little influence on temporal variation in soil oxygen levels. Spatial and temporal extrapolations of measured rates suggest that a minimum of 16%–27% of atmospheric nitrogen deposition is lost to denitrification. These results suggest that the importance of denitrification in the nitrogen budget of forested watersheds depends fundamentally on the presence of landscape elements, such as riparian hollows that function as “hot spots” of activity.

Plant–microbe interactions and nitrogen dynamics during wetland establishment in a desert stream

In late-successional steady state ecosystems, plants and microbes compete for nutrients and nutrient retention efficiency is expected to decline when inputs exceed biotic demand. In carbon (C)-poor environments typical of early primary succession, nitrogen (N) uptake by C-limited microbes may be limited by inputs of detritus and exudates derived from contemporaneous plant production. If plants are N-limited in these environments, then this differential limitation may lead to positive relationships between N inputs and N retention efficiency. Further, the mechanisms of N removal may vary as a function of inputs if plant-derived C promotes denitrification. These hypotheses were tested using field surveys and greenhouse microcosms simulating the colonization of desert stream channel sediments by herbaceous vegetation. In field surveys of wetland (cienega) and gravelbed habitat, plant biomass was positively correlated with nitrate (NO3 −) concentration. Manipulation of NO3 − in flow-through microcosms produced positive relationships among NO3 − supply, plant production, and tissue N content, and a negative relationship with root:shoot ratio. These results are consistent with N limitation of herbaceous vegetation in Sycamore Creek and suggest that N availability may influence transitions between and resilience of wetland and gravelbed stable states in desert streams. Increased biomass in high N treatments resulted in elevated rates of denitrification and shifts from co-limitation by C and NO3 − to limitation by NO3 − alone. Overall NO3 − retention efficiency and the relative importance of denitrification increased with increasing N inputs. Thus the coupling of plant growth and microbial processes in low C environments alters the relationship between N inputs and exports due to increased N removal under high input regimes that exceed assimilative demand.

Denitrification and patterns of electron donors and acceptors in eight riparian zones with contrasting hydrogeology

A better understanding of nitrate removal mechanisms is important for managing the water quality function of stream riparian zones. We examined the linkages between hydrologic flow paths, patterns of electron donors and acceptors and the importance of denitrification as a nitrate removal mechanism in eight riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Nitrate-N concentrations in shallow groundwater from adjacent cropland declined from levels that were often 10–30 mg L−1 near the field-riparian edge to < 1 mg L−1 in the riparian zones throughout the year. Chloride data suggest that dilution cannot account for most of this nitrate decline. Despite contrasting hydrogeologic settings, these riparian zones displayed a well-organized pattern of electron donors and acceptors that resulted from the transport of oxic nitrate-rich groundwater to portions of the riparian zones where low DO concentrations and an increase in DOC concentrations were encountered. The natural abundances of d15N and in situ acetylene injection to piezometers indicate that denitrification is the primary mechanism of nitrate removal in all of the riparian zones. Our data indicate that effective nitrate removal by denitrification occurs in riparian zones with hydric soils as well as in non-hydric riparian zones and that a shallow water table is not always necessary for efficient nitrate removal by denitrification. The location of ‘hot spots’ of denitrification within riparian areas can be explained by the influence of key landscape variables such as slope, sediment texture and depth of confining layers on hydrologic pathways that link supplies of electron donors and acceptors.

Reach‐scale isotope tracer experiment to quantify denitrification and related processes in a nitrate‐rich stream, midcontinent United States

We conducted an in‐stream tracer experiment with Br and 15N‐enriched NO3− to determine the rates of denitrification and related processes in a gaining NO3− ‐rich stream in an agricultural watershed in the upper Mississippi basin in September 2001. We determined reach‐averaged rates of N fluxes and reactions from isotopic analyses of NO3−, NO2−, N2, and suspended particulate N in conjunction with other data in a 1.2‐km reach by using a forward time‐stepping numerical simulation that included groundwater discharge, denitrification, nitrification, assimilation, and air‐water gas exchange with changing temperature. Denitrification was indicated by a systematic downstream increase in the δ15N values of dissolved N2. The reach‐averaged rate of denitrification of surface‐water NO3− indicated by the isotope tracer was approximately 120 ± 20 µmol m−2 h−1 (corresponding to zero‐ and first‐order rate constants of 0.63 µmol L−1 h−1 and 0.009 h−1, respectively). The overall rate of NO3− loss by processes other than denitrification (between 0 and about 200 µmol m−2 h−1) probably was less than the denitrification rate but had a large relative uncertainty because the NO3− load was large and was increasing through the reach. The rates of denitrification and other losses would have been sufficient to reduce the stream NO3− load substantially in the absence of NO3− sources, but the losses were more than offset by nitrification and groundwater NO3− inputs at a combined rate of about 500‐700 µmol m−2 h−1. Despite the importance of denitrification, the overall mass fluxes of N2 were dominated by discharge of denitrified groundwater and air‐water gas exchange in response to changing temperature, whereas the flux of N2 attributed to denitrification was relatively small. The in‐stream isotope tracer experiment provided a sensitive direct reach‐scale measurement of denitrification and related processes in a NO3− ‐rich stream where other mass‐balance methods were not suitable because of insufficient sensitivity or offsetting sources and sinks. Despite the increasing NO3− load in the experimental reach, the isotope tracer data indicate that denitrification was a substantial permanent sink for N leaving this agricultural watershed during low‐flow conditions.

Balance between denitrification and microalgal incorporation of nitrogen in microtidal sediments, NE Kattegat

The importance of denitrification and microphytobenthos for nitrogen removal and retention was studied over an annual cycle in 2 microtidal shallow-water sediments, 1 sandy and 1 silty, on the west coast of Sweden. Denitrification rates (isotope-pairing) and oxygen and inorganic nitrogen fluxes were measured in undisturbed sediment incubated in the laboratory at in situ temperature, at mean monthly light intensities and in the dark. Algal N assimilation rates were calculated from primary productivity and N retention in the microphytobenthic biomass from chlorophyll a concentrations. Denitrification ranged from ca 1 to 1360 μmol N m–2 d–1, being generally 1 order of magnitude higher in the silty sediment. The sandy sediment was net oxygen producing throughout the year and the silty sediment during most of the year. The rate of algal N assimilation calculated from daily net primary productivity was 30 to 7000 μmol N m–2 d–1, generally exceeding denitrification by 1 to 2 orders of magnitude. The low denitrification rates are explained by competition for external and internally circulated substrate by microalgae, and a general negative effect of microalgal photosynthesis on denitrifiers. The results suggest that, in northern climate microtidal sediments, microphytobenthic N incorporation is more important than N removal by denitrification.

Denitrification in coastal ecosystems: methods, environmental controls, and ecosystem level controls, a review

In this review of sediment denitrification in estuaries and coastal ecosystems, we examine current denitrification measurement methodologies and the dominant biogeochemical controls on denitrification rates in coastal sediments. Integrated estimates of denitrification in coastal ecosystems are confounded by methodological difficulties, a lack of systematic understanding of the effects of changing environmental conditions, and inadequate attention to spatial and temporal variability to provide both seasonal and annual rates. Recent improvements in measurement techniques involving 15 N techniques and direct N2 concentration changes appear to provide realistic rates of sediment denitrification. Controlling factors in coastal systems include concentrations of water column NO 3 − , overall rates of sediment carbon metabolism, overlying water oxygen concentrations, the depth of oxygen penetration, and the presence/absence of aquatic vegetation and macrofauna. In systems experiencing environmental change, either degradation or improvement, the importance of denitrification can change. With the eutrophication of the Chesapeake Bay, the overall rates of denitrification relative to N loading terms have decreased, with factors such as loss of benthic habitat via anoxia and loss of submerged aquatic vegetation driving such effects.

Nitrogen Removal in a Wastewater Reservoir: The Importance of Denitrification by Epiphytic Biofilms on Submersed Vegetation

The aim of this study was to examine the importance of epiphytic denitrifying bacteria on submersed vegetation in removing N from a shallow nutrient-enriched freshwater ecosystem. The investigation was conducted during the summer of 1994 in a surface flow reservoir receiving municipal tertiary-treated wastewater. The submersed vegetation in the reservoir was dominated by Potamogeton pectinatus L. and filamentous green algae (FGA). The N loading was 2300 mg N h -1 m -2 and the N removal, calculated as the mean difference between influent and effluent N, was 190 mg N h -1 m -2 (8%). The majority of influent N consisted of NH 4 + , but the main part of the N removal was due to the removal of NO 3 - whereas no net retention of NH 4 + was found. Mean total soluble solids and BOD 7 retention was 69 and 38%, respectively. Denitrification measurements were conducted in darkness at in situ temperature in microcosms with P. pectinatus, FGA, or intact sediment cores. Epiphytic denitrification ranged between 0.21 to 7.0 mg N h -1 m -2 reservoir surface area depending on the abundance of the submersed vegetation (S-140 g DW m -2 ). Sediment denitrification was 4.7 mg N h -1 m -2 reservoir surface area. The mean assimilative N uptake of the submersed vegetation and epiphyton was 3.4 and 1.6 mg N h -1 m -2 reservoir surface area, respectively. Measured N removal rates through plant uptake and denitrification could only account for a minor part of the N removal observed by mass balance. However, microcosm denitrification measurements underestimate actual denitrification. Thus, the major part of the N removal was most likely due to denitrification. In conclusion, this study indicates that denitrification in epiphytic microbial communities on submersed vegetation can be of significant importance for the N removal in nutrient-enriched freshwater ecosystems.

Spatial and temporal variation in C:N:P ratios, delta15N, and delta13C of eelgrass Zostera marina as indicators of ecosystem processes, Tomales Bay, California, USA

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 157:147-157 (1997) - doi:10.3354/meps157147 Spatial and temporal variation in C:N:P ratios, δ15N, and δ13C of eelgrass Zostera marina as indicators of ecosystem processes, Tomales Bay, California, USA James W. Fourqurean1,*, Thomas O. Moore2, Brian Fry1, James T. Hollibaugh3 1Department of Biological Sciences and Southeast Environmental Research Program, Florida International University, Miami, Florida 33199, USA 2California State Department of Fish and Game, Marine Resources Division, PO Box 1560, Bodega Bay, California 94923, USA 3Department of Marine Science, University of Georgia, Athens, Georgia 30602, USA *E-mail: fourqure@fiu.edu The nutrient content, δ15N, and δ13C of seagrasses in Tomales Bay (California, USA) vary in both space and time. The variations in these measurements were not random, but followed spatial and seasonal patterns that provide insight into the sources and processing of nutrients in the Tomales Bay ecosystem. Wide ranges of carbon, nitrogen, and phosphorus content of green leaves of Zostera marina were found in 72 samples collected over the 2 yr of sampling. Carbon content varied from 29.0 to 40.9% of dry weight, nitrogen from 1.13 to 3.79%, and phosphorus from 0.11 to 0.90%. Stable isotopic composition was variable as well: the mean δ15N for all samples collected was 9.7 ± 0.3o/oo with a range of 6.2o/oo. Carbon isotopic content had a range of 7.5o/oo, with a mean delta13C of -9.6 ± 0.2o/oo. There was a strong spatial trend in the N content, but not the P content, of Z. marina leaves from Tomales Bay. The C:N ratio was around 15 near the mouth of the bay and increased linearly with distance into the bay to ~25 near the head of the bay; there was a concomitant pattern of increasing δ15N of seagrass leaves, from 7o/oo near the mouth of the bay to near 12o/oo at the head of the bay. The spatial pattern in N content was only present during summer months, but the spatial pattern in stable isotopic composition was present in both summer and winter. The patterns in the N and δ15N content of the seagrasses indicate the importance of denitrification in the C and N cycles in Tomales Bay. The analysis of patterns in variation of elemental content and isotopic composition of seagrasses is a powerful tool for investigating ecosystem-scale processes in coastal marine systems. Similar analyses of seagrasses from less well studied ecosystems should provide data to generate hypotheses about spatial and temporal variation in processes in these other systems. Seagrass · Ecosystems · Nitrogen · Phosphorus · Stable isotopes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 157. Publication date: October 16, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Nitrogen cycling between sediment and the shallow-water column in the transition zone of the Potomac River and estuary. I. Nitrate and ammonium fluxes

A three-year study of seasonal variation in water-column and sediment nitrogen species was conducted in the transition zone of the Potomac River 35 m from the Virginia shore at a site with an average water-column depth of approximately 1 m over sandy sediment. A diffusion-controlled sampler was used to collect water samples from the water column, at the interface between the water column and sediment, and at several tens of centimeters into the sediment. Nitrate was the predominant dissolved nitrogen species in the water column. The importance of denitrification was inferred by nitrate fluxes which were directed into the sediment from the water column during approximately 75% of the sampling periods and ranged from 0·02 to 0·69 mmol m −2 day −1. Flux of nitrate from the sediment into the water column, ⩾0·1 mmol m −2 day −1, due possibly to nitrification in surficial sediment, occurred during one spring and two summer sampling periods. Ammonium fluxes were less than 0·1 mmol m −2 day −1 during 90% of the sampling periods. Of the ammonium fluxes that were >0·05 mmol m −2 day −1, all were fluxes into the sediment during sampling periods when sediment resuspension occurred, and all were into the water column during periods of calm. The mean value of ammonium flux (0·005 ± 0·05 mmol m −2 day −1) from the sandy, shallow-water sediments was two orders of magnitude less than the ammonium fluxes from the deeper, silty channel sediments in the same reach of the river. Diffusive flux calculations suggest that approximately one order of magnitude more nitrate than ammonium is cycled between the shallow-water column and the sandy sediment in the transition zone of the Potomac River.

The biogeochemistry of nitrogen in freshwater wetlands

The biogeochemistry of N in freshwater wetlands is complicated by vegetation characteristics that range from annual herbs to perennial woodlands; by hydrologic characteristics that range from closed, precipitation-driven to tidal, riverine wetlands; and by the diversity of the nitrogen cycle itself. It is clear that sediments are the single largest pool of nitrogen in wetland ecosystems (100's to 1000's g N m-2) followed in rough order-of-magnitude decreases by plants and available inorganic nitrogen. Precipitation inputs (< 1–2 g N m-2 yr-1) are well known but other atmospheric inputs, e.g. dry deposition, are essentially unknown and could be as large or larger than wet deposition. Nitrogen fixation (acetylene reduction) is an important supplementary input in some wetlands (< < 1–3 g N m-2 yr-1) but is probably limited by the excess of fixed nitrogen usually present in wetland sediments. Plant uptake normally ranges from a few g N m-2 yr-1 to ∼ 35 g N m-2 yr-1 with extreme values of up to ∼ 100g N m-2 yr-1 Results of translocation experiments done to date may be misleading and may call for a reassessment of the magnitude of both plant uptake and leaching rates. Interactions between plant litter and decomposer microorganisms tend, over the short-term, to conserve nitrogen within the system in immobile forms. Later, decomposers release this nitrogen in forms and at rates that plants can efficiently reassimilate. The NO3 formed by nitrification (< 0.1 to 10 g N m-2 yr-1 has several fates which may tend to either conserve nitrogen (uptake and dissimilatory reduction to ammonium) or lead to its loss (denitrification). Both nitrification and denitrification operate at rates far below their potential and under proper conditions (e.g. draining or fluctuating water levels) may accelerate. However, virtually all estimates of denitrification rates in freshwater wetlands are based on measurements of potential denitrification, not actual denitrification and, as a consequence, the importance of denitrification in these ecosystems may have been greatly over estimated. In general, larger amounts of nitrogen cycle within freshwater wetlands than flow in or out. Except for closed, ombrotrophic systems this might seem an unusual characteristic for ecosystems that are dominated by the flux of water, however, two factors limit the opportunity for N loss. At any given time the fraction of nitrogen in wetlands that could be lost by hydrologic export is probably a small fraction of the potentially mineralizable nitrogen and is certainly a negligible fraction of the total nitrogen in the system. Second, in some cases freshwater wetlands may be hydrologically isolated so that the bulk of upland water flow may pass under (in the case of floating mats) or by (in the case of riparian systems) the biotically active components of the wetland. This may explain the rather limited range of N loading rates real wetlands can accept in comparison to, for example, percolation columns or engineered marshes.

Nitrate Transformations in Rural Streams

This review considers the extent and nature of nitrate transformations in rural streams. Mass balance studies have revealed considerable evidence for nitrate depletion during stream transport in agricultural landscapes. A variety of processes may be responsible for these nitrate losses. Several studies have indicated that uptake by aquatic macrophytes can be important in nitrate depletion. However, denitrification which occurs in anaerobic stream sediments has been implicated as the major cause of nitrate losses in many rural streams. The importance of denitrification in streams has been confirmed by the use of labelled NO3–N and acetylene blockage experiments. A wide range of stream sediments which include acidic sands, organic muds and calcareous silts exhibit a denitrification potential. Denitrification has also been identified in algal mats and slime enmeshed microbial communities on rock and gravel surfaces in streams. Denitrification is affected by a number of environmental factors which include water temperature and stream NO3–N concentration. Several recent studies have shown that denitrification rates are influenced by chemical and physical properties of stream sediments. In particular, denitrification may exhibit a significant positive correlation with the water-soluble carbon content of sediments.

Importance of Denitrification and Nitrate Reduction in Sediments to the Nitrogen Budgets of Lakes

DENITRIFICATION (biological conversion of nitrate and nitrite to nitrous oxide and molecular nitrogen) and nitrate reduction (assimilation of nitrate–nitrogen in microbial protoplasm) in lake sediments can play an important role in the nitrogen budgets of lakes. The hydrology of many lakes is such that ground water seepage through the floor constitutes a significant source of water income1. In such cases, nitrate in the incoming ground water must be considered when a nitrogen budget is calculated for the lake system. An excellent example is the estimate for Lake Mendota, Wisconsin2, in which nitrate in seepage water constituted nearly 36% of the nitrogen added to the lake.