Estimates Of Denitrification Research Articles

Denitrification from nitrogen-fixing biologically crusted soils in a cool desert environment, southeast Utah, USA

IntroductionNitrogen fixation by microorganisms within biological soil crust (“biocrust”) communities provides an important pathway for N inputs in cool desert environments where soil nutrients are low and symbiotic N-fixing plants may be rare. Estimates of N fixation in biocrusts often greatly exceed that of N accretion rates leading to uncertainty regarding N loss pathways.MethodsIn this study we examined nitrogen fixation and denitrification rates in biocrust communities that differed in N fixation potential (low N fixation = light cyanobacterial biocrust, high N fixation = dark cyanolichen crust) at four temperature levels (10, 20, 30, 40°C) and four simulated rainfall levels (0.05, 0.2, 0.6, 1 cm rain events) under controlled laboratory conditions.ResultsAcetylene reduction rates (AR, an index of N fixation activity) were over six-fold higher in dark crusts relative to light crusts. Dark biocrusts also exhibited eight-fold higher denitrification rates. There was no consistent effect of temperature on denitrification rates, but there was an interactive effect of water addition and crust type. In light crusts, denitrification rates increased with increasing water addition, whereas the highest denitrification rates in dark crusts were observed at the lowest level of water addition.ConclusionsThese results suggest that there are no clear and consistent environmental controls on short-term denitrification rates in these biologically crusted soils. Taken together, estimates of denitrification from light and dark biocrusts constituted 3 and 4% of N fixation rates, respectively suggesting that losses as denitrification are not significant relative to N inputs via fixation. This estimate is based on a previously published conversion ratio of ethylene produced to N fixed that is low (0.295), resulting in high estimates of N fixation. If future N fixation studies in biologically crusted soils show that these ratios are closer to the theoretical 3:1 ratio, denitrification may constitute a more significant loss pathway relative to N fixed.

Theoretical and practical limitations of the acetylene inhibition technique to determine total denitrification losses

Abstract. The loss of N2 from intensively managed agro-ecosystems is an important part of the N budget. Flux monitoring of N2 emissions at the field scale, e.g., by eddy correlation or aerodynamic gradient method, is impossible due to the large atmospheric N2 background (78%). The acetylene (C2H2) inhibition technique (AIT) is a rather simple and frequently used, albeit imperfect, method to determine N2 losses from intact soil cores. In principle, AIT allows an estimation of total denitrification at high temporal resolution and on small spatial scales, with limited workload and costs involved. To investigate its potential and limitations, a laboratory system with two different detection systems (photoacoustic IR spectroscopy and gas chromatography) is presented, which allowed simultaneous measurements of up to 7 intact soil cores in air-tight glass tubes in a temperature controlled cabinet (adjusted to field conditions) with automated C2H2 injection. A survey of total denitrification losses (N2 + N2O) over 1.5 yr in soil cores from an intensively managed, cut grassland system in central Switzerland supports previous reports on severe limitations of the AIT, which precluded reliable estimates of total denitrification losses. Further, the unavoidable sampling and transfer of soil samples to the laboratory causes unpredictable deviations from the denitrification activity in the field.

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

Abstract. Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N2 gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N2 is highly variable in space and inversely correlated with dissolved oxygen (O2). Negative relationships between O2 and δ15NNO3 across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong 15N:18O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32 % of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ15NNO3 based on denitrification progression suggest that isotopically-enriched nitrate (NO3–) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs.

Data‐based estimates of suboxia, denitrification, and N2O production in the ocean and their sensitivities to dissolved O2

Oxygen minimum zones (OMZs) are major sites of fixed nitrogen removal from the open ocean. However, commonly used gridded data sets such as the World Ocean Atlas (WOA) tend to overestimate the concentration of O2 compared to measurements in grids where O2 falls in the suboxic range (O2 < 2–10 mmol m−3), thereby underestimating the extent of O2 depletion in OMZs. We evaluate the distribution of the OMZs by (1) mapping high‐quality oxygen measurements from the WOCE program, and (2) by applying an empirical correction to the gridded WOA based on in situ observations. The resulting suboxic volumes are a factor 3 larger than in the uncorrected gridded WOA. We combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and N2O production. We obtain a removal of fixed nitrogen of 70 ± 50 Tg year−1 in the open ocean and 198 ± 64 Tg year−1 in the sediments, and a global N2O production of 6.2 ± 3.2 Tg year−1. Our results (1) reconcile water column denitrification rates based on global oxygen distributions with previous estimates based on nitrogen isotopes, (2) revise existing estimates of sediment denitrification down by 1/3d through the use of spatially explicit fluxes, and (3) provide independent evidence supporting the idea of a historically balanced oceanic nitrogen cycle. These estimates are most sensitive to uncertainties in the global export production, the oxygen threshold for suboxic processes, and the efficiency of particle respiration under suboxic conditions. Ocean deoxygenation, an expected response to anthropogenic climate change, could increase denitrification by 14 Tg year−1 of nitrogen per 1 mmol m−3 of oxygen reduction if uniformly distributed, while leaving N2O production relatively unchanged.

Estimation of denitrification in Baltic Sea deep water from gas tension measurements

Vertical profiles of dissolved N 2 concentrations were calculated from measurements of gas tension, O 2, CO 2, and H 2S concentrations, water temperature, and salinity at the Gotland Basin (Baltic Sea) central station in June 2008. A gas tension device (GTD) with a flow-through plenum was combined with a pump–CTD for continuous sampling. The maximum uncertainty of the N 2 concentration measurement was 0.40%. This was upgraded to 0.20% in anoxic deep-water layers. The data were used to determine the N excess with respect to saturation values. The results were attributed to the amount of total denitrification that had occurred in the water mass since its last contact with the atmosphere and interpreted in relation to the history of the water mass. In the water layer between 75 and 150 m depth, which is occasionally influenced by lateral inputs of oxygenated water, the denitrification signal was 27 μmol N L −1. At a depth below 150 m, the N excess in the water mass was 41 μmol N L −1, which is considered to be the upper limit. Based on the results of earlier investigations of the nitrogen budget of the same water mass, our findings indicated that about half of the N 2 excess was derived from denitrification in water masses located outside the Gotland Basin and subsequently transported to the basin with saltwater inflows. A comparison between N release due to mineralization processes (N 2 excess and dissolved inorganic nitrogen) and the corresponding release of total CO 2 yielded a mean C:N ratio of 7.7, which agrees reasonably with the elemental composition of particulate organic matter in the surface water.

Nitrogen Cycle of the Open Ocean: From Genes to Ecosystems

The marine nitrogen (N) cycle controls the productivity of the oceans. This cycle is driven by complex biogeochemical transformations, including nitrogen fixation, denitrification, and assimilation and anaerobic ammonia oxidation, mediated by microorganisms. New processes and organisms continue to be discovered, complicating the already complex picture of oceanic N cycling. Genomics research has uncovered the diversity of nitrogen metabolism strategies in phytoplankton and bacterioplankton. The elemental ratios of nutrients in biological material are more flexible than previously believed, with implications for vertical export of carbon and associated nutrients to the deep ocean. Estimates of nitrogen fixation and denitrification continue to be modified, and anaerobic ammonia oxidation has been identified as a new process involved in denitrification in oxygen minimum zones. The nitrogen cycle in the oceans is an integral feature of the function of ocean ecosystems and will be a central player in how oceans respond during global environmental change.

Open-channel estimation of denitrification

Open-channel estimation of denitrification

Quantifying Nitrogen Process Rates in a Constructed Wetland Using Natural Abundance Stable Isotope Signatures and Stable Isotope Amendment Experiments

This study describes the spatial variability in nitrogen (N) transformation within a constructed wetland (CW) treating domestic effluent. Nitrogen cycling within the CW was driven by settlement and mineralization of particulate organic nitrogen and uptake of NO3-. The concentration of NO3- was found to decrease, as the delta15N-NO3- signature increased, as water flowed through the CW, allowing denitrification rates to be estimated on the basis of the degree of fractionation of delta15N-NO3-. Estimates of denitrification hinged on the determination of a net isotope effect (eta), which was influenced byprocesses that enrich or deplete 15NO3- (e.g., nitrification), as well as the rate constants associated with the different processes involved in denitrification (i.e., diffusion and enzyme activity). The influence of nitrification on eta was quantified; however, it remained unclear how eta varied due to variability in denitrification rate constants. A series of stable isotope amendment experiments was used to further constrain the value of eta and calculate rates of denitrification, and nitrification, within the wetland. The maximum calculated rate of denitrification was 956 +/- 187 micromol N m(-2) h(-1), and the maximum rate of nitrification was 182 +/- 28.9 micromol N m(-2) h(-1). Uptake of NO3- was quantitatively more important than denitrification throughoutthe wetland. Rates of N cycling varied spatially within thewetland, with denitrification dominating in the downstream deoxygenated region of the wetland. Studies that use fractionation of N to derive rate estimates must exercise caution when interpreting the net isotope effect. We suggest a sampling procedure for future natural abundance studies that may help improve the accuracy of N cycling rate estimates.

Revisiting the application of open‐channel estimates of denitrification

Development of an open‐channel method for measurement of denitrification, without the use of expensive isotopic tracers, has generated considerable interest among researchers attempting to quantify N loss from lotic systems. Membrane inlet mass spectrometry allows measurement of small changes in N2 concentrations, facilitating calculation of whole reach denitrification rates using an N2 mass balance corrected for gas exchange. The method has been applied successfully within numerous rivers ranging widely in size and denitrification rate. Previous model‐based analyses suggest that the method can be applied in a broader suite of ecosystems, and specifically, that it is well suited to shallow streams where denitrification rates as low as 30–100 µmol N m−2 h−1 may be measurable. This coupled with increasing availability of necessary equipment, relatively low cost of measurements, and the ability to measure denitrification at environmentally relevant spatial scales suggests that broad adoption of the method is likely. In this paper, we revisit this model‐based analysis using alternate models of gas exchange and demonstrate that benthic turbulence‐induced gas exchange will restrict the suite of suitable study streams. Specifically, we note that within shallow streams and fast‐flowing systems denitrification may be measurable only at moderate or high rates. To help facilitate further application of the method, we extend our discussion beyond site selection to discuss assumptions of the open‐channel method, options for estimating the error in denitrification rates, and recommended practices for future studies.

The relationship between dissolved hydrogen and nitrogen fixation in ocean waters

Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N 2) fixation is estimated to supply 8×10 12 moles N y −1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N 2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N 2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H 2), but the extent to which it leads to supersaturation of H 2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H 2 concentration (nmol L −1), and rates of N 2 fixation (μmol N m −3 d −1), determined using 15N 2 tracer techniques (at 7 or 15 m), on a transect from Fiji to Hawaii. We find a significant correlation ( r=0.98) between dissolved H 2 and rates of N 2 fixation, with the greatest supersaturation of H 2 and highest rates of N 2 fixation being observed in the subtropical gyres at the southern (∼18°S) and northern (18°N) reaches of the transect. The lowest H 2 saturation and N 2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H 2 supersaturations and N 2 fixation measurements could be used to guide sampling for 15N fixation measurements or to aid the spatial interpolation of such measurements.

Modeling denitrification in aquatic sediments

Sediment denitrification is a major pathway of fixed nitrogen loss from aquatic systems. Due to technical difficulties in measuring this process and its spatial and temporal variability, estimates of local, regional and global denitrification have to rely on a combination of measurements and models. Here we review approaches to describing denitrification in aquatic sediments, ranging from mechanistic diagenetic models to empirical parameterizations of nitrogen fluxes across the sediment-water interface. We also present a compilation of denitrification measurements and ancillary data for different aquatic systems, ranging from freshwater to marine. Based on this data compilation we reevaluate published parameterizations of denitrification. We recommend that future models of denitrification use (1) a combination of mechanistic diagenetic models and measurements where bottom-waters are temporally hypoxic or anoxic, and (2) the much simpler correlations between denitrification and sediment oxygen consumption for oxic bottom waters. For our data set, inclusion of bottom water oxygen and nitrate concentrations in a multivariate regression did not improve the statistical fit.

Sedimentary denitrification rates across the Arabian Sea oxygen minimum zone

Abstract Sedimentary denitrification was measured at sites across the Pakistan Margin oxygen minimum zone (OMZ) during cruises before and after the 2003 Southwest (SW) Monsoon. Denitrification rates were calculated using nitrate fluxes and N 2 production rates in shipboard whole-core incubations and benthic lander deployments. We observed generally good agreement between the sedimentary denitrification rates obtained from nitrate consumption and N 2 production. Results of these investigations indicate that sedimentary denitrification is a persistent process within the Arabian Sea OMZ and the regions proximal to the permanent OMZ (e.g., 140 and 1200 m). Sedimentary denitrification in the region intensified during the SW Monsoon, as noted by the increase in associated rates between the inter-monsoon (April–May 2003) and post-monsoon (September–October 2003) assessments. Sedimentary denitrification was observed at all stations during the inter-monsoon period; nitrate consumption rates ranged from 0.40 to 3.17 mmol N m −2 d −1 . During the post-monsoon assessment, apparent sedimentary denitrification rates increased in the shallow sites (140 and 300 m), likely in response to a shoaling of the OMZ, but were not definitively observed at the deepest site (1850 m); where observed, post-monsoon sedimentary denitrification rates increased to 0.66–3.78 mmol N m −2 d −1 . These rates compare favorably to sedimentary denitrification rates obtained in other shelf environments and are higher than abyssal sedimentary denitrification rates. By extrapolating these rates to the entire Pakistan Margin OMZ, we estimate a net annual sedimentary denitrification flux of 1.1–10.5 Tg N yr −1 , moderately higher than current estimates of sedimentary denitrification in this region and in line with estimates of regional sedimentary denitrification in other sites.

Nitrate dynamics in artificially drained nested watersheds

There is concern that subsurface drainage, by destroying or by-passing active denitrification areas, may prevent nitrate retention processes and enhance nitrate contamination of surface water by agriculture. To address this question, we studied the flow and concentration signatures of drainage waters and their transformations in a series of 5 nested watersheds, from 1 to 100 km 2 area, in the Brie region near Paris (France). At all scales, nitrate concentrations are generally higher during the winter drainage season compared to the low flow periods (late spring to early fall). High nitrate concentrations characterizing drainage waters are visible at the 1st, 2nd and 3rd stream order but are “diluted” by surface runoff from forested zones and buffered by groundwater contributions. The analysis of nitrate chemographs and nitrate budgets established for the different nested watersheds show significant nitrogen retention. Isotopic measurements indicate that the nitrate pool is enriched in δ 15N– NO 3 - as its concentration decreases. Direct estimation of benthic denitrification with benthic chambers allowed concluding that benthic denitrification is not the only retention mechanism and that “underground” denitrification, affecting nitrate on its way from the base of the root zone down to the limit of the river bed, may in fact dominate nitrogen retention processes even in this intensively drained watershed.

Constraining denitrification in permeable wave-influenced marine sediment using linked hydrodynamic and biogeochemical modeling

Permeable marine sediments are ubiquitous complex environments, the biogeochemistry of which are strongly coupled to hydrodynamic process above and within the sediment. The biogeochemical processes in these settings have global scale implications but are poorly understood and challenging to quantify. We present the first simulation of linked turbulent-oscillatory flow of the water column, porous media flow, and solute transport in the sediment with oxygen consumption, nitrification, denitrification, and ammonification, informed by field- and/or experimentally-derived parameters. Nitrification and denitrification were significantly impacted by advective pore water exchange between the sediment and the water column. Denitrification rates showed a maximum at intermediate permeabilities, and were negligible at high permeabilities. Denitrification rates were low, with only ∼ 15% of total N mineralized being denitrified, although this may be increased temporarily following sediment resuspension events. Our model-estimated denitrification rates are about half of previous estimates which do not consider solute advection through the sediment. Given the critical role of sediment permeability, topography, and bottom currents in controlling denitrification rates, an improved knowledge of these factors is vital for obtaining better estimates of denitrification taking place on shelf sediment. Broad application of our approach to myriad conditions will lead to improved predictive capacity, better informed experimental and sampling design, and more holistic understanding of the biogeochemistry of permeable sediment.

Seasonal discrepancies in denitrification measured by isotope pairing and N2:Ar techniques

Isotope pairing (IP) and N2:Ar techniques for measuring sediment denitrification were compared in muddy sediments in a shallow sub-tropical embayment. At a concentration of 100 µM 15 NO 3 - (our experimental conditions), IP greatly underestimated denitrification, compared to N 2:Ar, in low-respiration sediments subject to bioturbation, where denitrification produced via coupled nitrifica- tion-denitrification (Dn) dominates overall denitrification. Consistent with this underestimation was the higher N2 flux measured using N2:Ar ratios, accounting for more of the stochimetrically predicted N mineralisation. We suggest that this underestimation is primarily due to non-homogenous mixing of the added 15 NO3 - with the endogenous 14 NO3 - in the sediment, associated with differentials in the rate of diffusion of 15 NO3 - across mucus linings in burrow structures versus rates of 14 NO3 - supply from nitri- fiers in the burrow walls. Strong support for this hypothesis was provided by the relationship between the methods discrepancy and polychaete biomass (r 2 = 0.99; p = 0.001), and the almost complete removal of the methods discrepancy when the upper heterogenous sediment layer that contained the burrow structures was experimentally removed. Due to a significant increase in potential denitrifica- tion sites in association with burrow structures, this effect can potentially cause a large underestima- tion of denitrification (up to 85%) using the IP technique. Our data show no benefit in longer pre- incubation times, suggesting that this effect may represent a fundamental limitation of the IP method in the sediments of oligotrophic (low carbon loading) systems. As such, the N2:Ar flux technique rep- resents a more realistic estimation of net denitrification in sediments with low organic carbon contents and low respiration rates and significant bioturbation. Most importantly, the present study also demon- strated that methodological comparisons need to be made over a range of environmental conditions.

METHODS FOR MEASURING DENITRIFICATION: DIVERSE APPROACHES TO A DIFFICULT PROBLEM

Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO3-) and nitrite (NO2-), to the gases nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2), is important to primary production, water quality, and the chemistry and physics of the atmosphere at ecosystem, landscape, regional, and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses, and future prospects for the different methods. Methodological approaches covered include (1) acetylene-based methods, (2) 15N tracers, (3) direct N2 quantification, (4) N2:Ar ratio quantification, (5) mass balance approaches, (6) stoichiometric approaches, (7) methods based on stable isotopes, (8) in situ gradients with atmospheric environmental tracers, and (9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well-defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.

Estimation of nitrogen losses via denitrification from a heavy clay soil under grass

Denitrification is an important pathway for nitrogen (N) loss from agricultural soils, but measured field data on denitrification in clay soils are scarce. The objective of the present research was to obtain and compare three different approaches to estimate the annual N loss via denitrification from a heavy clay soil. Using the acetylene inhibition technique we measured denitrification monthly at 20 °C in intact soil cores of a clay soil under intensively managed grass over a period of 2 years. This approach resulted in estimates of N loss via denitrification of 127 kg N ha −1 year −1 in 2003 and 143 kg N ha −1 year −1 in 2004. We also measured potential denitrification. These data were used in combination with independently calibrated ‘reduction functions’ to correct for suboptimal soil nitrate contents, water filled pore space or soil temperature. This approach resulted in estimates of 143 and 325 kg N ha −1 year −1 for 2003 and 2004, respectively. These measurements furthermore indicated that 75% of the denitrification occurred in the upper 20 cm of the soil. Estimates of denitrification based on field balances, including measurements of leaching losses to ground and surface waters, were 152 kg N ha −1 year −1 for 2003 and 5 kg N ha −1 year −1 for 2004, when storage of nitrogen was assumed negligible. While all three different estimation methods have considerable uncertainty, they invariably lead to the conclusion that N losses via denitrification in this intensively managed clay soil are high (more than 79 kg N ha −1 year −1 ). Measurement of the actual denitrification gave the most consistent and least uncertain estimates and accordingly we concluded that direct measurement of the actual denitrification should be preferred despite it's theoretical shortcomings. This implies that about 25% of the N applied to the field as fertilizer and manure is lost to the environment. In this heavy clay soil 90% of this loss proceeds via denitrification, and 10% proceeds via leaching and drainage.

Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

On the basis of the normalization to phosphate, a significant amount of nitrate is missing from the deep Bering Sea (BS). Benthic denitrification has been suggested previously to be the dominant cause for the BS nitrate deficit. We measured water column nitrate 15N/14N and 18O/16O as integrative tracers of microbial denitrification, together with pore water‐derived benthic nitrate fluxes in the deep BS basin, in order to gain new constraints on the mechanism of fixed nitrogen loss in the BS. The lack of any nitrate isotope enrichment into the deep part of the BS supports the benthic denitrification hypothesis. On the basis of the nitrate deficit in the water column with respect to the adjacent North Pacific and a radiocarbon‐derived ventilation age of ∼50 years, we calculate an average deep BS (>2000 m water depth) sedimentary denitrification rate of ∼230 μmol N m−2 d−1 (or 1.27 Tg N yr−1), more than 3 times higher than high‐end estimates of the average global sedimentary denitrification rate for the same depth interval. Pore water‐derived estimates of benthic denitrification were variable, and uncertainties in estimates were large. A very high denitrification rate measured from the base of the steep northern slope of the basin suggests that the elevated average sedimentary denitrification rate of the deep Bering calculated from the nitrate deficit is driven by organic matter supply to the base of the continental slope, owing to a combination of high primary productivity in the surface waters along the shelf break and efficient down‐slope sediment focusing along the steep continental slopes that characterize the BS.

Whole-system estimation of denitrification in a plains river: a comparison of two methods

Whole-system denitrification in the South Platte River was measured over a 13-month period using an open-channel N2 method and mass-balance measurements. Concentrations of dissolved N2 were measured with high precision by membrane-inlet mass spectrometry and estimates of denitrification were based on the mass flux of N2, after correction for reaeration and groundwater flux. Open-channel estimates of denitrification ranged from 0 to 3.08 g N m−2 d−1 and the mean annual rate was 1.62 g N m−2 d−1, which corresponds to removal of approximately 34% of the nitrate transported by the river over a distance of 18.5 km. Over the same period of time, estimates of denitrification based on mass-balance measurements ranged from 0.29 to 5.25 g N m−2 d−1 and the mean annual rate was 2.11 g N m−2 d−1. The two methods revealed similar seasonal patterns of denitrification the highest rates were measured from late April to August and the lowest rates were in winter. Both methods provide whole-system estimates of denitrification in running waters; where reaeration rate coefficients are low and flux of groundwater is well quantified, the open-channel method has fewer sources of uncertainty and is easier to implement.

Open‐channel estimation of denitrification

Estimates of denitrification based on standard methods often cannot easily be extrapolated to entire ecosystems because of high spatial heterogeneity in rates of denitrification. Diel changes in the concentration of dinitrogen (N2) in running waters provide the basis for estimating denitrification by an open‐channel technique in much the same way that ecosystem metabolism can be estimated from diel changes in the concentration of dissolved oxygen. The open‐channel N2 method was field tested at a site on the South Platte River downstream from Denver, CO; concentrations of N2 were measured by membrane inlet mass spectrometry. For a date in November 1998, the rate of denitrification was estimated to be 0.19 mol N2 m−2 d−1 and was similar to rates reported in previous studies based on mass‐balance analysis. The open‐channel N2 method is the first method to provide direct, whole‐system estimates of denitrification in flowing waters and may help to expand our understanding of nitrogen cycling in running waters.