Ground water flow paths in relation to nitrogen chemistry in the near-stream zone

Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L^–1, 1.2–1.8 mg L^–1 and 4.6–9.1 mg L^–1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L^–1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L^–1 and 0.4–2.2 mg L^–1 respectively. Ammonium — N concentrations (20–60 µg L^–1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.     

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.     

Nitrate depletion in the riparian zone and stream channel of a small headwater catchment

A mass balance procedure was used to determine rates of nitrate depletion in the riparian zone and stream channel of a small New Zealand headwater stream. In all 12 surveys the majority of nitrate loss (56–100%) occurred in riparian organic soils, despite these soils occupying only 12% of the stream's border. This disproportionate role of the organic soils in depleting nitrate was due to two factors. Firstly, they were located at the base of hollows and consequently a disproportionately high percentage (37–81%) of the groundwater flowed through them in its passage to the stream. Secondly, they were anoxic and high in both denitrifying enzyme concentration and available carbon. Direct estimates ofin situ denitrification rate for organic soils near the upslope edge (338 mg N m^–2 h^–1) were much higher than average values estimated for the organic soils as a whole (0.3–2.1 mg N m^–2 h^–1) and suggested that areas of these soils were limited in their denitrification activity by the supply of nitrate. The capacity of these soils to regulate nitrate flux was therefore under-utilized. The majority of stream channel nitrate depletion was apparently due to plant uptake, with estimates of thein situ denitrification rate of stream sediments being less than 15% of the stream channel nitrate depletion rate estimated by mass balance.This study has shown that catchment hydrology can interact in a variety of ways with the biological processes responsible for nitrate depletion in riparian and stream ecosystems thereby having a strong influence on nitrate flux. This reinforces the view that those seeking to understand the functioning of these ecosystems need to consider hydrological phenomena.     

Isotopic composition of nitrate-nitrogen as a marker of riparian and benthic denitrification at the scale of the whole Seine River system

Nitrogen budgets established for large river systems reveal that up to 60% of the nitrate exported from agricultural soils is eliminated, either when crossing riparian wetlands areas before even reaching surface waters, or within the rivers themselves through benthic denitrification. The study of nitrogen isotope ratios of riverine nitrates could offer an elegant means to assess the extent of denitrification and thus confirm these budgets, as it is known that denitrification results in a natural ¹⁵N enrichment of residual nitrates. The results reported here, for the Seine river system (France), demonstrate the feasibility of this isotopic approach at the scale of large watersheds. On the basis of in situ observations carried out in a large storage reservoir in the upstream Seine catchment (Der Lake), where intensive benthic denitrification occurs, as well as on the basis of laboratory experiments of denitrification under controlled conditions, it is shown that the isotopic discrimination associated with benthic denitrification is minimal ( of NO₃-N ranging from –1.5 to –3.6), probably because the rate-limiting step of the process consists of nitrate diffusion through the water-sediment interface. Riparian denitrification on the contrary, when it implies nitrate reduction during convective transfer through reducing environements, causes a much more significant isotopic enrichment of ¹⁵N of residual nitrate ( about –18). The authors report measurements of nitrogen isotopic composition of nitrate from rivers of various stream orders in the Seine river system under summer conditions. Anomalies in the data with respect to the values expected from the mixture of the various sources of nitrate are here attributed to riparian denitrification. However, the authors show that because of the patchy distribution of actively denitrifying riparian zones within the drainage network, the isotopic signature conferred to residual nitrate in river water intrinsically provides only a minimum estimate of the extent of denitrification.     

Water table elevation controls on soil nitrogen cycling in riparian wetlands along a European climatic gradient

Riparian zones have long been considered as nitrate sinks in landscapes. Yet, riparian zones are also known to be very productive ecosystems with a high rate of nitrogen cycling. A key factor regulating processes in the N cycle in these zones is groundwater table fluctuation, which controls aerobic/anaerobic conditions in the soil. Nitrification and denitrification, key processes regulating plant productivity and nitrogen buffering capacities are strictly aerobic and anaerobic processes, respectively. In this study we compared the effects of these factors on the nitrogen cycling in riparian zones under different climatic conditions and N loading at the European scale. No significant differences in nitrification and denitrification rates were found either between climatic regions or between vegetation types. On the other hand, water table elevation turned out to be the prime determinant of the N dynamics and its end product. Three consistent water table thresholds were identified. In sites where the water table level is within –10cm of the soil surface, ammonification is the main process and ammonium accumulates in the topsoils. Average water tables between –10 and –30cm favour denitrification and therefore reduce the nitrogen availability in soils. In drier sites, that is, water table level below –30cm, nitrate accumulates as a result of high net nitrification. At these latter sites, denitrification only occurs in fine textured soils probably triggered by rainfall events. Such a threshold could be used to provide a proxy to translate the consequences of stream flow regime change to nitrogen cycling in riparian zones and consequently, to potential changes in nitrogen mitigation.     

Nitrate removal in a first-order stream: reconciling laboratory and field measurements

A combination of laboratory and field experiments were carried out to evaluate nitrate(NO ₃ ^t- ) removal during stream transport in a first-order agricultural drainage stream. Intact stream sediment cores overlain with stream and NO ₃ ^– -amended stream water indicated NO ₃ ^– losses averaging 93 — 353 mg m^–2 day^–1, with NO ₃ ^– concentration exerting a primary control on loss rate. Isotopic data indicated enrichment of NO ₃ ^– - ¹⁵N over time as NO ₃ ^– concentrations decreased, indicating a denitrification loss. Field experiments were designed to evaluate dilution of streamwater with low-NO ₃ ^– groundwater in addition to other NO ₃ ^– removal processes during transport. A series of bromide tracer and NO ₃ ^– - addition experiments were carried out in the field; groundwater dilution dominated the downstream NO ₃ ^– concentration trends, accounting for all observed decreases in NO ₃ ^– concentration. Isotopic data did not point to denitrification downstream as a major NO ₃ ^– removal process. This apparent disparity between simulated laboratory and in-situ stream removal rates appears to be a function of the hydrological processes controlling exchanges between stream bottom sediments and the overlying water. These results suggest that caution must be exercised in extrapolating potentials for NO ₃ ^– removal measured in laboratory experiments to the field, as these rates could be overestimated in some watersheds.     

Physiology and kinetics of autotrophic denitrification by Thiobacillus denitrificans

Summary A strain of Thiobacillus denitrificans was isolated after enrichment under anaerobic conditions by the continuous culture technique using thiosulfate as energy source and nitrate as electron acceptor and nitrogen source. The isolate was an active denitrifyer, the optimal conditions being 30°C and pH 7.5–8.0. Denitrification was inhibited by sulfate (the reaction product) above 5 g SO ₄ ⁼ /l, whereas high concentrations of the substrates nitrate and thiosulfate were less harmful; nitrite affected denitrification above 0.2 g NO ₂ ^– /l. During the time course of denitrification in a batch culture growth and substrate consumption slowed down already after only half the substrate was utilized due to product inhibition. The following parameters were determined in continuous culture under nitrate limitation: _max=0.11 h^–1, K _S=0.2 mg NO ₃ ^– /l, maximum denitrification rate=0.78 g NO ₃ ^– /g cells·h, g cells/g NO ₃ ^– , g cells/g S₂O ₃ ⁼ . Nitrite did not accumulate during steady state denitrification; the denitrification gas was almost pure N₂. The concentrations of N₂O and NO were below 1 ppm.     

Denitrification in the water column of an intermittently anoxic fjord

Denitrification was studied in the water column in the Bunnefjord, inner part of the Oslofjord in southern Norway, using a ¹⁵N-technique (the isotope pairing method). The fjord is 150 m deep and during our surveys in September–December 1998 hydrogen sulphide was present in the deep water below 80 m. No significant denitrification was found in water samples from the surface layer (4 m depth), but high rates were observed within a deep density gradient between 62 and 78 m depth. Oxygen concentration within this layer was low (<21 mmol m^–3), and the concentration of NO₃ decreased from ca. 15 mmolm^–3 at 62 m depth to not detectable below 78 m. Pronounced peaks of NO₂ up to 4.4 mmol m^–3 were observed at 70–78 m depth. The maximum denitrification rate of 1.5 mmol N m^–3 d^–1 was observed at 70 m depth. Integrated for the whole layer, the denitrification rate was 13 mmol N m^–2 d^–1. A significant linear correlation was found between the denitrification rate and the ambient nitrate concentration which indicated that the rate was primarily controlled by the availability of nitrate in the O₂-poor water. Compared to rates reported for coastal water, denitrification in the water column in the Bunnefjord was high and the process appears to be a major sink of bioavailable nitrogen in the fjord.     

Factors influencing nitrate depletion in a rural stream

A mass balance procedure was used to analyze rates of nitrate depletion in three adjacent reaches of West Duffin Creek, Ontario, Canada. Daily nitrate losses in individual reaches were highly variable (0.5–24 kg N) during low and moderate stream flows in May–October, 1982–1985. Nitrate removal efficiency (nitrate loss as a % of nitrate input) showed a rapid exponential decline with increased nitrate inputs to each reach. Nitrate losses and nitrate removal efficiency also had a significant negative correlation with stream discharge. The association of large nitrate loads with high stream discharge reduced the nitrate removal capacity of the stream because of shorter residence times and a higher ratio of water volume to stream bed area. Water temperature exhibited a significant positive correlation with nitrate loss which may reflect increased denitrification at higher temperatures.Variations in nitrate losses and nitrate removal efficiency between the three reaches were highly influenced by differences in water residence time. Standarized nitrate losses with respect to water residence time revealed a longitudinal decline in nitrate depletion between the reaches which was associated with a downstream decrease in stream nitrate concentration and in the organic carbon content of fine textured sediments from pool habitats.     

Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content,and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to investigate whether differences in root growth among plant species are related to their ability to deplete the soil nitrate-N pool. Large differences were observed in root growth parameters. Monocot species had rooting depth penetration rates in the range of 1.0 to 1.2 mm d^–1 °C^–1, whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d^–1 °C^–1. Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum needed for the crops to reach a rooting depth of 1.0 m varied from 750 d °C for fodder radish to 1375 d °C for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m the non-legume dicot species generally had root intensities (number of root intersections m^–1 line on the minirhizotrons) 12 times as high as the monocot species.The amount of nitrate left in the topsoil (0–0.5 m) was only weakly correlated to a few of the measured plant and root parameters, whereas nitrate left in the subsoil (0.5–1.0 m) was clearly correlated to several root parameters. Subsoil nitrate residues were well correlated to root intensity, but showed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0–1.5 m), the soil water nitrate concentration was reduced from 119 g L^–1 without a catch crop to 61 g L^–1 under Italian ryegrass and to only 1.5 g L^–1 under fodder radish.The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In this study, none of the measurements made aboveground or in the upper soil layers were well related to subsoil nitrate depletion.     

Growth kinetics of Thermus thermophilus

Summary The nonsporulating extreme thermophile Thermus thermophilus was grown in continuous culture at dilution rates up to 2.65 h^–1 at 75°C and pH 6.9 on complex medium. Concomitantly very low yield (Y=0.12 g cell dry weight g^–1 utilized organic carbon) and incomplete substrate utilization (always less than 45%) were found. In batch cultures T. thermophilus could be grown with _max =h^–1, in shake flasks only with _max =h^–1 with the same low yield and incomplete substrate utilization. Stable steady states at 84C and 45°C were realized at a dilution rate of 0.3 h^–1 whereas at 86°C and 40°C no growth could be detected. Artefacts arising from wall growth (in bioreactors) or improper materials must be ruled out. Inhibition of growth by organic substrates was demonstrated at low concentrations: a decrease in the yield obtained was found when more than 0.7 gl^–1 of meat extract were supplied in the medium. The maintenance requirement for oxygen is potentially very high and was determined to be 10 to 15 mmol g^–1 h^–1.     

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.     

Denitrification potential of epilithic communities in a lotic environment

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

15N evidence for the origin and cycling of inorganic nitrogen in a small Amazonian catchment

The ¹⁵N composition of the dominant form of dissolved inorganic nitrogen (DIN) was determined in upland groundwater, riparian groundwater, and stream water of the Barro Branco catchment, Amazônas, Brazil. The ¹⁵N composition of organic nitrogen in riparian and upland leaf litter was also determined. The data for these waters could be divided into three groups: upland groundwater DIN predominately composed of NO₃ ^– with ¹⁵N values averaging 6.25 ± 0.9 riparian groundwater DIN primarily composed of NH₄ ⁺ with ¹⁵N values averaging 9.17 ± 1.0 and stream water DIN predominately composed of NO₃ ^– with ¹⁵N values averaging 4.52 ± 0.8 Nitrate samples taken from the stream source and from the stream adjacent to the groundwater transects showed a downstream increase in ¹⁵N from 1.0to 4.5 Leaf litter samples averaged 3.5 ± 1.2The observed patterns in isotopic composition, together with previously observed inorganic nitrogen species and concentration shifts between upland, riparian and stream waters, suggest that groundwater DIN is not the primary source of DIN to the stream. Instead, the isotopic data suggest that remineralization of organic nitrogen within the stream itself may be a major source of stream DIN, and that the majority of DIN entering the stream via groundwater flowpaths is removed at the riparian-stream interface.     

Estimating denitrification rates in estuarine sediments: A comparison of stoichiometric and acetylene based methods

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrient stoichiometry in the subtidal sediments of Tomales Bay, California (USA). By amending whole cores with acetylene and saturating nitrate concentrations, we obtained potential denitrification rates, which ranged between 4 and 30 mmol N m^–2 d^–1. We determined the apparent Michaelis constant (K_app) and the maximum potential rate (V_mp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the K_app and V_mp of the denitrifying community exhibited significant variation over both depth in the sediment column and time of sampling.Estimates ofin situ denitrification obtained using our kinetic-fix adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m^–1 d^–1. Denitrification rates obtained using benthic flux stoichiometry ranged between 0.7 and 4.1 mmol N m^–2 d^–1. Average denitrification rates obtained using the kinetic-fix acetylene block approach exceeded those obtained from net benthic flux stoichiometry; however, these differences were not significant. We conclude that our kinetic-fix adaptation of the acetylene block technique provides realistic estimates of denitrification in sediments, even when pore water nitrate concentrations are low and nitrification and denitrification are closely coupled.     

Swimming Performances of Four California Stream Fishes: Temperature Effects

The critical swimming velocity (U_crit) of four California stream fishes, hardhead, Mylopharodon conocephalus, hitch, Lavinia exilicauda, Sacramento pikeminnow, Ptychocheilus grandis, and Sacramento sucker, Catostomus occidentalis was measured at 10, 15, and 20°C. Hardhead, Sacramento sucker, and Sacramento pikeminnow swimming performances tended to be lowest at 10°C, higher at 15°C, and then decreased or remained constant at 20°C. Hitch swimming performance was lower at 10°C than at 20°C. There were no significant differences among species at 10 or 15°C, although pikeminnow and hitch were ca. 20% slower than hardhead or sucker. At 20°C hardhead, Sacramento sucker, and Sacramento pikeminnow had remarkably similar U_crit but hitch were significantly (by 11%) faster. We recommend that water diversion approach velocities should not exceed 0.3ms^–1 for hitch (20–30cm total length) and 0.4ms^–1 for hardhead, Sacramento pikeminnow, and Sacramento sucker (20–30cm TL).     

Nitrogen removal in subsurface water by narrow buffer strips in the intensive farming landscape of the Po River watershed, Italy

In many countries buffer strips have become an important management tool widely accepted for controlling the diffuse pollution and supporting the development of more sustainable agriculture. However, there is the need to investigate their role in intensive farming systems where a realistic and shareable proposal to realize buffer strips can only foresee the use of a limited space. We evaluated the nitrogen buffering capacities of two narrow riparian strips (5-8 m) along irrigation ditches located in a typical flat agricultural watershed of the alluvial plain of the River Po (Northern Italy). Subsurface water level and nutrient concentrations were monitored along transects of piezometers installed from crop fields to ditches in two different areas. Spatial and temporal variation in water chemistry and hydrology were investigated to individuate the main processes (biological or physical) leading to groundwater nitrate depletion related to fertilization, pluviometric regime and seasonal variation. The results obtained indicate an elevated nitrate removal efficiency in both riparian areas. Compared to the high mean concentrations measured at the exit of the crop fields (10-90 mg l⁻¹ N-NO₃⁻), nitrate levels within riparian sites can be very low, completely disappearing below the ditches. The patterns of some chemical species (O₂, SO₄²⁻ and HCO₃⁻) and the potential denitrification rates suggest that denitrification plays a predominant role in the N-NO₃⁻ depletion observed in the first few meters of the herbaceous strip. The key factors in the system are the elevated groundwater residence time and the effect of the evapotranspiration. The water uptake by woody vegetation affects the subsurface water to flow through the riparian zone and, at the same time, it contributes to completely remove the nitrate from the groundwater.Our findings also suggest the double role of riparian vegetation both in ecohydrological and biological terms. In fact the water uptake by trees affects the subsurface flow pattern and contributes to completely remove the nitrate in the riparian zone.     

Subsurface denitrification in a forest riparianzone: Interactions between hydrology and supplies ofnitrate and organic carbon

The influence of hydrology andpatterns of supply of electron donors and acceptors onsubsurface denitrification was studied in a forestriparian zone along the Boyne River in southernOntario that received high nitrogen inputs from a sandaquifer. Two hypotheses were tested: (1) subsurfacedenitrification is restricted to localized zones ofhigh activity; (2) denitrification zones occur atsites where groundwater flow paths transportNO₃ ^– to supplies of available organiccarbon. A plume of nitrate-rich groundwater withconcentrations of 10–30 mg N L^–1 flowed laterallyat depths of 1.5–5 m in sands beneath peat for ahorizontal distance of 100–140 m across the riparianzone to within 30–50 m of the river. In situ acetyleneinjections to piezometers revealed that significantdenitrification was restricted to a narrow zone ofsteep NO₃ ^– and N₂O decline at theplume margins. The location of these denitrificationsites in areas with steep gradients of groundwater DOCincrease supported hypothesis 2. Many of thesedenitrification hotspots occurred near interfacesbetween sands and either peats or buried river channeldeposits. Field experiments involving in situadditions of either glucose or NO₃ ^– topiezometers indicated that denitrification wasC-limited in a large subsurface area of the riparianzone, and became N-limited beyond the narrow zone ofNO₃ ^– consumption. These data suggest thatdenitrification may not effectively removeNO₃ ^– from groundwater transported at depththrough permeable riparian sediments unlessinteraction occurs with localized supplies of organicmatter.     

Net N input and riverine N export from Illinois agricultural watersheds with and without extensive tile drainage

Some of the largest riverine N fluxes in the continental USA have been observed in agricultural regions with extensive artificial subsurface drainage, commonly called tile drainage. The degree to which high riverine N fluxes in these settings are due to high net N inputs (NNI), greater transport efficiency caused by the drainage systems, or other factors is not known. The objective of this study was to evaluate the role of tile drainage by comparing NNI and riverine N fluxes in regions of Illinois with similar climate and crop production practices but with different intensities of tile drainage. Annual values of NNI between 1940 and 1999 were estimated from county level agricultural production statistics and census estimates of human population. During 1945–1961, riverine nitrate flux in the extensively tile drained region averaged 6.6kgNha^–1year^–1 compared to 1.3 to 3.1kgNha^–1 for the non-tile drained region, even though NNI was greater in the non-tile drained region. During 1977–1997, NNI to the tile-drained region had increased to 27kgNha^–1year^–1 and riverine N flux was approximately 100% of this value. In the non-tile-drained region, NNI was approximately 23kgNha^–1year^–1 and riverine N flux was between 25% and 37% of this value (5 to 9kgNha^–1year^–1). Denitrification is not included in NNI and, therefore, any denitrification losses from tile-drained watersheds must be balanced by other N sources, such as depletion of soil organic N or underestimation of biological N fixation. If denitrification and depletion of soil organic N are significant in these basins, marginal reductions in NNI may have little influence on riverine N flux. If tile drained cropland in Illinois is representative of the estimated 11 million ha of tile drained cropland throughout the Mississippi River Basin, this 16% of the drainage area contributed approximately 30% of the increased nitrate N flux in the Lower Mississippi River that occurred between 1955 and the 1990s.     

Seasonal denitrification in flooded and exposed sediments from the Amazon floodplain at Lago Camaleão

Denitrification processes were measured by the acetylene-blockage technique under changing flood conditions along the aquatic/terrestrial transition zone on the Amazon floodplain at Lago Camaleão, near Manaus, Brazil. In flooded sediments, denitrification was recorded after the amendment with NO ₃ ^– (100 mol liter^–1) throughout the whole study period from August 1992 to February 1993. It ranged from 192.3 to 640.7 mol N m^–2 h^–1 in the 0- to 5-cm sediment layer. Without substrate amendment, denitrification was detected only during low water in November and December 1992, when it occurred at a rate of up to 12.2 mol N m^–2 h^–1 Higher rates of denitrification at an average rate of 73.3 mol N m^–2 h^–1 were measured in sediments from the shallow lake basin that were exposed to air at low water. N₂O evolution was never detected in flooded sediments, but in exposed sediments, it was detected at an average rate of 28.3 mol N m^–2 h^–1 during the low-water period. The results indicate that under natural conditions there is denitrification and hence a loss in nitrogen from the Amazon floodplain to the atmosphere. Rates of denitrification in flooded sediments were one to two orders of magnitude smaller than in temperate regions. However, the nitrogen removal of exposed sediments exceeded that of undisturbed wetland soils of temperate regions, indicating a considerable impact of the flood pulse on the gaseous turnover of nitrogen in the Amazon floodplain.