Measurement Of Denitrification Research Articles

Modelling denitrification at the catchment scale

The objective of this work was to evaluate the importance of heterotrophic denitrification in the fate of nitrogen surpluses at the catchment scale. For that purpose we modified the denitrification module of TNT2 model and calibrated the model on a small catchment where denitrification measurements had been performed in different locations. The main interest of the TNT2 model is its ability to simulate the dynamics of the zones where soil and shallow water table interact, making it possible to spatialize the denitrification process. Daily water and nitrogen flux at the outlet were relatively well simulated (Nash of 0.85 and 0.77). In average, the model correctly simulates the denitrification measurements ( R = 0.68). Nitrogen flux towards the atmosphere, at the catchment scale (4.70 g N m − 2 year − 1 ), is of the same order of magnitude as the soluble N flux in the stream. The model was able to reproduce the distribution of denitrification in the riparian (mean of 9.26 g N m − 2 year − 1 ) and hillslope (mean of 3.45 g N m − 2 year − 1 ) domains of the catchment. The results confirm the importance of riparian denitrification, but show also that hillslope soils contribute significantly (60%) to the whole catchment denitrification. The variations of denitrification rates, and also of nitrate concentrations in stream were not very well simulated by the model, highlighting the complexity of the spatial and temporal controls of nitrogen dynamics in areas with high inputs of nitrogen fertilizers, especially under organic forms.

Denitrification in the mainstem Neuse River and tributaries, USA

We conducted quarterly (spring, summer, fall and winter) determinations of denitrification at representative sites in the mainstem Neuse River and tributaries, USA. Denitrification measurements were made on intact sediment cores using the N 2 /Ar and isotope pairing techniques. We measured nitrate (NO 3 - -N) and ammonium (NH 4 + -N) concentrations in the water column at the time of core collection and evaluated rates of sediment-water exchange of dissolved oxygen (O 2 ) in conjunction with denitrification rate determinations. Nitrate concentrations in the tributary streams and river water varied over an order or magnitude, from about 10 to 110 λmol/L, while NH 4 + -N concentrations were lower, varying from 0.6 to 10.8 μmol/L. Sediments consistently consumed dissolved O 2 , with rates varying from 256 to 1418 pmol m -2 h -1 . Most cores actively denitrified with rates ranging to 222 μmol N 2 -N m -2 h -1 . On average, coupled nitrification-denitrification accounted for 66 % of total denitrification. Lowest rates of denitrification were generally observed during December, when the water temperature was lowest at 4 °C. No other relationship was identified between rates of denitrification and measured physicochemical variables within or among sites or dates as sampling frequency and replication were low. Our study suggests that denitrification represents a sink of about 5 % for the NO 3 - -N load to the mainstem Neuse River and tributary streams.

Soil nitrogen dynamics in irrigated maize systems as impacted on by nitrogen and stubble management

The soil nitrogen (N) dynamics of an irrigated maize system in which stubble retention and stubble burned treatments were superimposed over treatments of varying N fertiliser rate were studied. The field site was near Whitton, New South Wales, Australia, and the work described here is part a life cycle analysis of greenhouse gas emissions from maize project. The objective of this part of the work was to quantify the fate of fertiliser N applied at the site. Field measurements of denitrification, mineral N content and recovery of 15N-labelled urea from microplots with and without ammonium thiosulfate were complimented with laboratory studies of denitrification and nitrous oxide (N2O) flux. Significantly (P < 0.05) more fertiliser N was recovered in the grain from the stubble incorporated treatment than the stubble burned treatment and there was greater recovery of fertiliser N in the soil at the end of the experiment in the stubble burned treatment. This may indicate that fertiliser N applied to the stubble burned system may be more exposed to soil-N transformations. The reason for the difference in uptake and soil residual is not clear but may be related to soil structure differences leading to less plant accessibility of N in the burned treatment. This difference may lead to more nitrous oxide emission from soil in the stubble burned treatments. Short-term (1 h) static chamber measurements in the field found a strong N-rate dependence of N2O emission rate for fertiliser rates between 0 and 300 kg N/ha. Inclusion of ammonium thiosulfate in the fertiliser formulation did not appear to have a significant impact on fertiliser N recovery.

Comparing functional assessments of wetlands to measurements of soil characteristics and nitrogen processing

One beneficial service of wetland ecosystems is the improvement of water quality through nitrogen (N) removal. However, one important N-removal process, denitrification, can produce the atmospheric pollutant nitrous oxide (N2O). Wetland biogeochemical functions, such as N processing, can be assessed by the hydrogeomorphic (HGM) approach using a suite of simple field observations made in a single visit to a wetland. HGM assessments score functions on a scale of 0–1 where 1 equals the functionality of an undisturbed reference standard wetland and 0 equals the functionality of a completely degraded wetland. We compared seasonal measurements of potential denitrification, N2O emissions, and related soil characteristics to HGM assessments of nine non-tidal riverine wetlands and seven flats wetlands in the Nanticoke River watershed in Delaware and Maryland, USA. Denitrification potential, measured as denitrification enzyme activity (DEA), was higher in riverine wetlands than in flats. DEA increased with increases in percent water-filled pore space, pH, ammonium concentration, and the percentages of N and organic carbon. DEA decreased with increases in oxidation-reduction potential (Eh) and water-table depth. The difference in DEA between riverine and flats wetlands was attributable to the differences in the correlated soil characteristics. N2O emission rates were higher on average in riverine wetlands than in flats, but the difference was not statistically significant. N2O emission rates were generally less predictable than DEA and showed only weak correlations with pH, water-table depth, and the percentage of water-filled pore space when data from riverine wetlands and flats were combined. HGM biogeochemistry function scores ranged from 0.18 to 1 for the riverine wetlands and from 0.24 to 0.98 for the flats. The scores did not correlate with N2O emission or DEA, except for summer DEA in flats, which increased with increasing score. Wetland alterations that increase soil moisture relative to reference standard conditions decrease biogeochemistry and hydrology function scores but increase DEA. Biogeochemistry function scores would more closely reflect denitrification potential if the scoring incorporated measurements of soil characteristics that correlate with DEA.

Anaerobic ammonium oxidation (anammox) in sediments of the Gulf of Finland

Anaerobic ammonium oxidation (anammox) and denitrification were measured in the open sea and coastal accumulation basins of the Gulf of Finland. The different methods used gave conflicting results on the importance of the anammox process in the sediments. Anammox generally contributed less than 20% to the total N2 production, and no anammox was found in a shallow inner estuary basin. However, the discovery of the anammox process in the open sea sediments challenges the denitrification measurements made in the area, as the coexistence of anammox and denitrifica- tion compromises the central assumptions behind the method used in denitrification measurements and causes overestimates of the N2 production. The high 15 NO3 - incubation concentration used in Baltic Sea denitrification measurements exacerbates this overestimation, which is likely to have been substantial.

Direct measurement of anaerobic ammonium oxidation (anammox) and denitrification in intact sediment cores

Anammox, i.e. the anaerobic oxidation of NH4 + with NO2 - to N2, has redefined our understanding of nitrogen cycling in aquatic ecosystems. The isotope pairing technique (IPT) is the dominant tool for quantifying denitrification in intact sediments, but it cannot distinguish anammox from denitrification as sources of N2 and may, where anammox is significant, lead to large errors in the estimate of true N2 production. In a previous study, the IPT was revised in theory and a solution was proposed whereby the parameter r14, i.e. the ratio of 14 NO3 - to 15 NO3 - in the NO3 - reduction zone is used to correct the IPT in the presence of anammox. We begin by exploring the limitations of the 2 indirect techniques previously proposed for estimating r14. The first, based on the contribution of anammox to N2 production (ra) in sediment slurry incubations, underestimates anammox and cannot fully correct the IPT. The second, derived from the production of 15 N-N2 gas as a function of 15 NO3 - concentration, although valid in sieved sediment, was ineffective in natural intact sediments. In con- trast, a newly developed direct technique based on the 15 N-labelling of N2O, corrects the IPT in the presence of significant anammox (48% of N2 formation) in natural sediment and can even distinguish between anammox and denitrification in estuarine sediment with a lower anammox contribution (21%). We contrast these findings with sediments where anammox is minimal (<1%). The 15 N-N2O technique allows denitrification and anammox to be quantified in sediment cores using techniques similar to those already established with the original IPT and, importantly, shows that the contribu- tion of anammox to N2 production is greater than previously measured using slurries.

Denitrification measurements of sediments using cores and chambers

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 326:49-59 (2006) - doi:10.3354/meps326049 Denitrification measurements of sediments using cores and chambers Peter I. Macreadie1,*, D. Jeff Ross1, Andrew R. Longmore2, Michael J. Keough1 1Department of Zoology, The University of Melbourne, Victoria 3010, Australia 2Marine and Freshwater Systems, Primary Industries Research Victoria, PO Box 114, Queenscliff, Victoria 3225, Australia *Email: petermacreadie@gmail.com ABSTRACT: Denitrification is commonly measured using in situ benthic chambers or laboratory incubations of sediment cores. These techniques are similar in principle but differ considerably in cost and practicality. Despite widespread use of both techniques, it is uncertain whether they give comparable results. We compared cores and chambers for measuring fluxes (dissolved oxygen [DO], N2, NH4+, NO3– and NO2–) and denitrification efficiency at 2 sites in Port Phillip Bay, Victoria, Australia. Overall, denitrification efficiency was not significantly different between cores and chambers, but fluxes of DO, NO3– and NO2– differed. Chambers demonstrated higher levels of oxygen consumption and net fluxes of NO3– and NO2– out of the sediment, suggesting that denitrification and nitrification were closely coupled. In contrast, there was a greater relative importance for uncoupled denitrification in cores as indicated by reduced oxygen consumption and net fluxes of NO3– into the sediment. We conclude that cores and chambers give different flux results and therefore are not comparable techniques for measuring denitrification. To ascertain the cause of this, we tested the hypothesis that cores failed to adequately incorporate the impacts of macrofauna on fluxes, due to the small size of cores relative to chambers. However, densities of macrofauna were not significantly different in cores and chambers. We then hypothesised that disturbance during core collection, transportation, and handling may account for differences, but cores deployed in situ and in the laboratory gave similar results. We suggest that compression of sediment during insertion of core cylinders into the sediment may account for differences between core and chamber fluxes. KEY WORDS: Denitrification · Benthic chambers · Sediment cores · Nutrient flux · Macrofauna · Disturbance · Port Phillip Bay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 326. Online publication date: November 17, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Quantification of denitrification in permeable sediments: Insights from a two‐dimensional simulation analysis and experimental data

Using a two‐dimensional simulation analysis, we investigated the effects of sediment flushing on denitrification and the implications for two methods commonly used to measure denitrification in intact sediment cores: the N2:Ar‐ratio method and isotope pairing technique (IPT). Our simulations of experimental chamber incubations showed that advective flushing of the sediment can significantly increase sediment denitrification driven by NO3− from the water column (up to a factor of 5), but that nitrification and coupled nitrification‐denitrification is reduced under conditions of sediment flushing (up to a factor of 6). N2 fluxes across SWI may differ significantly from actual rates of denitrification for periods lasting from 1 up to more than 5 d after changes in parameters such as sediment flushing rate and water column NO3− concentrations. Simulations of the isotope pairing technique, showed that the rate of labeled N2 production, after the addition of 15NO3− may take up to ~24 h to reach steady state, depending on NO3− concentrations in the water column and sediment flushing rate. Measurements of denitrification in sand using IPT confirmed that short term incubations (11 h) underestimated the actual denitrification. Furthermore, model simulations were able to give a good estimate of measured N2 fluxes across SWI at different flushing rates under non‐steady state conditions, confirming the ability of the model to realistically simulate experimental situations.

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.

In situ application of the 15NO3− isotope pairing technique to measure denitrification in sediments at the surface water‐groundwater interface

There is a need to understand the hydro‐ecological significance of surface‐subsurface interactions, including denitrification, in sediments in permeable catchments. Measurement of denitrification in such sediments is complicated by the NO3− reduction zone being relatively deep in the sediment (a few to tens of centimeters), there being 2 significant sources of NO3− (the overlying water and the underlying groundwater) and, in some environments, the impact of macrophytes such as Ranunculus. These factors negate the collection of sediment cores for measuring denitrification by traditional techniques. Instead, we have developed a minipore water probe system that can be used to identify NO3− reduction zones and measure in situ rates of denitrification based on spiking sediment pore waters with 15NO3− followed by a short incubation time (15 min) and subsequent quantification of 29N2 and 30N2. Given the short incubation time, we suggest that the technique can be applied to pore waters with significant advective flow, and present results for fine sand sediments from the River Frome (Dorset, England). The limitations of the technique, including a low depth resolution (2 cm) and intrusion of pore water from outside the sample target depth, are calculated and discussed. Accepting the disturbance caused by the initial insertion of the probes, which is transient, this approach enables denitrification to be quantified under close to in situ conditions without the use of inhibitors and without destruction or isolation of the sediment.

Denitrification in Agricultural Soils: Summarizing Published Data and Estimating Global Annual Rates

Information taken from 336 measurements of denitrification in agricultural soils was summarized to assess the influence on denitrification of several factors related to soil, climate, agricultural management and the measurement techniques. The data set is summarized by calculating means and medians and balanced median values (with correction for unbalanced features) for all factor classes in the data set, and by developing a summary model to calculate global denitrification rates for a 0.5 by 0.5 degree resolution. Our results suggest that agricultural fields with high nitrogen application rates and poor soil drainage show higher denitrification values than those with lower nitrogen application rate and good soil drainage. The data also indicate that conditions in wetland rice systems are more prone to denitrification than those in upland cropping and grassland systems. Large uncertainties in the results are caused by differences between the measurement techniques and lack of long-term measurements covering the range of environmental and management conditions found in global agricultural fields.

Maximum Rates of Nitrate Removal in a Denitrification Wall

Denitrification walls are constructed by mixing a carbon source such as sawdust into soils through which ground water passes. These systems can reduce nitrate inputs to receiving waters by enhancing denitrification. Maximum rates of nitrate removal by denitrification need to be determined for design purposes. To determine maximum rates of nitrate removal we added excess nitrate (50 mg N L(-1)) to a trench up-gradient of a denitrification wall during a 9-d trial. Bromide (100 g L(-1)) was also added as a conservative tracer. Movement of nitrate and bromide was measured from shallow wells and soil samples were removed for measurements of denitrification, carbon availability, nitrate, and other microbial parameters. Rates of nitrate removal, determined from the ratio of NO3-N to Br and ground water flow, averaged 1.4 g N m(-3) of wall d(-1) and were markedly greater than denitrification rates determined using the acetylene block technique (average: 0.11 g N m(-3) of wall d(-1)). These nitrate removal rates were generally lower than reported in other denitrification walls. Denitrification rates increased when nitrate was added to the laboratory incubations, indicating that despite large nitrate inputs in the field, denitrification remained limited by nitrate. This limitation was partially attributed to nitrate predominantly moving through zones of greater hydraulic conductivity or in the mobile fraction of the ground water and slow diffusion to the immobile fraction where denitrifiers were active.

Global change, nitrification, and denitrification: A review

We reviewed responses of nitrification, denitrification, and soil N2O efflux to elevated CO2, N availability, and temperature, based on published experimental results. We used meta‐analysis to estimate the magnitude of response of soil N2O emissions, nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and net and gross nitrification across experiments. We found no significant overall effect of elevated CO2 on N2O fluxes. DEA and NEA significantly decreased at elevated CO2; however, gross nitrification was not modified by elevated CO2, and net nitrification increased. The negative overall response of DEA to elevated CO2 was associated with decreased soil [NO3−], suggesting that reduced availability of electron acceptors may dominate the responses of denitrification to elevated CO2. N addition significantly increased field and laboratory N2O emissions, together with gross and net nitrification, but the effect of N addition on field N2O efflux was not correlated to the amount of N added. The effects of elevated temperature on DEA, NEA, and net nitrification were not significant: The small number of studies available stress the need for more warming experiments in the field. While N addition had large effects on measurements of nitrification and denitrification, the effects of elevated CO2 were less pronounced and more variable, suggesting that increased N deposition is likely to affect belowground N cycling with a magnitude of change that is much larger than that caused by elevated CO2.

Assay system for measurement of denitrification-N loss from ornamental plants potted in peat substrate

An assay system was evaluated for denitrification measurement with potted ornamental plants cultivated in peat substrate (Pelargonium zonale, Euphorbia pulcherrima). Flow-through chambers only enclosing the pot of the plants were considered best for denitrification measurement. Loss of N2O from the chambers by transport through the plant shoot was negligible with both species. To determine (N2O + N2)-N loss, C2H2 was applied to inhibit reduction of N2O. Experiments were conducted with unplanted substrate in closed incubation systems to determine optimum C2H2 concentration and pre-treatment duration. Complete inhibition of N2O reduction in peat substrate was achieved using 1 vol% C2H2. However, a concentration of 5 vol% C2H2 was chosen for further experiments because C2H2 concentrations in flow-through chambers varied. The duration of C2H2 pre-treatment (0, 2, 12, 24 h) showed no clear effect on (N2O + N2)-N accumulation. However, a pre-treatment duration of 2 h was chosen to guarantee immediate inhibition of N2O reductase at the start of experiments. Exposure to C2H2 gas proved to affect plants of both species. During C2H2 exposition in flow-through chambers, the leaves of P. zonale became chlorotic (48 h) and necrotic (72 h). E. pulcherrima showed no chlorosis but did exhibit leaf epinasty (24 h) and wilting (96 h). Transpiration of P. zonale and C availability in the growing medium of both species were not affected by 52 h and 24 h treatments with C2H2, respectively. As N emissions usually ended within 38 h of C2H2 treatment, it was concluded that side effects of C2H2 did not affect denitrification measurements.

Coupled nitrification-denitrification measured in situ in a Spartina alterniflora marsh with a 15NH4+ tracer

Measurements of N losses by denitrification from saltmarsh sediments have proved difficult because of the importance of plant metabolism and tidal cycles to sediment N cycling. In vitro approaches often do not measure the dominant coupled nitrification-denitrification pathway and/or alter in situ plant growth and redox conditions. We developed an in situ 15 NH4 + tracer approach to measure coupled nitrification-denitrification fluxes in an undisturbed New England Spartina alterniflora saltmarsh. The tracer was line-injected into sediments underlying natural S. alterniflora stands and in similar areas receiving long-term N amendment (up to 11.2 mol organic N m -2 yr -1 for 16 to 23 yr), and 15 N retention and loss routes were followed for 1 to 5 d. Denitrification losses in unfertilized grass stands ranged from 0.4 to 11.9 mmol N m -2 d -1 (0.77 ± 0.18 mol N m -2 yr -1 ). Denitrification in unfertilized sediments remained low until late summer, but underwent a ca. 4-fold increase in August and September, although sediment temperatures and respiration rates were high throughout the summer. Plant N uptake may limit the availability of N to support denitrification dur- ing the early summer, and denitrification may be released from competition with plant uptake in late summer, when plant growth slows. Denitrification rates in fertilized areas ranged from 22 to 77 mmol N m -2 d -1 (10.5 ± 4.9 mol N m -2 yr -1 ), and denitrification was likely controlled by the availability of fertilizer N rather than by competition with plants, since N was added in excess of plant demand. Our results emphasize the importance of in situ measurements of denitrification in understanding the dynamics of saltmarsh N cycling.

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.

Measurement of Denitrification in the Changjiang River

Environmental Context.Rivers are carrying an increased load of nitrogen-based matter (nitrates, nitrites) resulting from, among others, increased use of agricultural fertilizers. This nitrogen enrichment leads to a proliferation of plant life in the receiving water body, which in turn reduces the dissolved oxygen content and can cause the extinction of other organisms. Rivers can reduce their nitrogen levels through denitrification, the bacterially mediated transformation of dissolved nitrates and nitrites to gaseous N2 and N2O. This paper reports the first examination of denitrification in China’s largest river, the Changjiang (Yangtze) River, to understand the details of riverine denitrification and its role on controlling nitrogen export. Abstract.Rivers are an important link between terrestrial and aquatic ecosystems for nitrogen cycling, while denitrification plays a key role in riverine nitrogen removal. Denitrification was first examined in China’s largest river, the Changjiang River, by using a whole-reach method. The production rates of N2 by means of denitrification were 2.82 ± 1.18 and 5.74 ± 2.92 mmol(N) m−2 h−1 in October 2002 and March 2003, respectively, and the rates of N2O production were 1.98 ± 1.48 and 581 ± 1937 nmol(N) m−2 hr−1 in August and October 2002, respectively. Nitrogen removal through N2 and N2O emission accounted for 1–2% of NO3–-N flux through October 2002 to March 2003. Continued measurement throughout a whole year period after the construction of the Three Gorges Reservoir will provide more understanding of riverine denitrification and its role on controlling nitrogen export.

Emissions of N 2O and CO 2, denitrification measurements and soil properties in red clover and ryegrass stands

The relationships between the fluxes of nitrous oxide (N 2O) and carbon dioxide (CO 2), and their concentrations in the soil air, three different measures of potential denitrification, soil moisture, soil temperature and precipitation were investigated in soils from beneath ryegrass ( Lolium multiflorum Lam.), red clover ( Trifolium pratense L.) and mixture of ryegrass–red clover stands on a gleic cambisol. Investigations were carried out in order to test the hypothesis that the measure(s) of potential denitrification are good predictor(s) of N 2O fluxes and thus may be used in empirical models of N 2O emission. Potential denitrification characteristics used in this study involved (i) short-term denitrifying enzyme activity (DEA), (ii) long-term denitrification potential (DP), both determined in soils amended with nitrate and glucose, and (iii) denitrification rate (DR) measured using intact soil cores. Flux measurements were made using cylindrical chambers (internal diameter 31 cm, volume 0.015 m 3). The fluxes of N 2O and CO 2 and many other characteristics showed large spatial and temporal variability. Emissions of N 2O from the grass plots were closely related to N 2O concentrations in the soil atmosphere at 22.5 cm depth. Most soil properties did not correlate with N 2O fluxes. It was concluded that DP was not a good predictor for N 2O flux. DEA did not show significant relationship with N 2O flux, but it is suggested that if determined in representative, large soil samples, DEA could be a predictor of N 2O fluxes; this assumption needs, however, verification. The only potential denitrification characteristic which was significantly related to N 2O emission both in grass and clover treatments was DR, which was determined in soil cores.

Production of NO and N 2O in the presence and absence of C 2H 2 by soil slurries and batch cultures of denitrifying bacteria

We evaluated the potential of the C 2H 2-catalyzed NO oxidation reaction to influence N 2O production during denitrification. We measured the total amount of free NO and N 2O produced by slurries of sandy loam soil and by batch cultures of denitrifying bacteria under both anaerobic and low O 2 conditions. The maximum amount of free NO released by anaerobic Nos + (able to reduce N 2O to N 2) and Nos − (unable to reduce N 2O to N 2) batch cultures of Cytophaga johnsonae strains catalyzing denitrification of nitrate was 17–79 nmol NO per bottle. In all cases the maximum headspace concentrations of NO-N measured in anaerobic cultures in the absence of C 2H 2 was less than 0.21% of those of N 2O-N measured in the presence of 10 kPa C 2H 2. Peak NO production was delayed when between 2.0 and 4.5% O 2 was present. Less NO accumulated in cultures in the presence of both O 2 and C 2H 2, and the maximum amount of NO-N measured in the absence of C 2H 2 was less than 0.13% of the total amount of N 2O-N measured in the presence of C 2H 2. For an agricultural sandy loam soil, the maximum concentrations of free NO released from slurries were 598–897 ng NO-N g −1 of dry soil in the absence of C 2H 2 and 118–260 ng NO-N g −1 of dry soil in the presence of 10 kPa C 2H 2. The maximum concentration of NO-N released in the absence of C 2H 2 was between 0.32 and 8.1% of the maximum concentration of N 2O-N accumulated in the presence of 10 kPa C 2H 2. We conclude that scavenging of NO by the C 2H 2-catalyzed NO oxidation reaction in the presence of trace amounts of O 2 does not cause a serious underestimate of long-term measurements of active denitrification in anaerobic soils containing adequate carbon and nitrate sources.

Comparison of field and laboratory measurement of denitrification and N 2O production in the saturated zone of hydromorphic soils

We evaluated a new method to measure in situ denitrification under field conditions in a number of water-saturated subsoils that had a broad range of biogeochemical properties. A test solution containing 15NO 3 − and/or C 2H 2 was introduced to the subsoil and the subsequent production of dissolved denitrification products was measured to quantify denitrification activity. The method showed a clear production of denitrification products over time. Results were compared to laboratory-based measurements from the same soil incubated as anaerobic slurries with added 15NO 3 −. Rates of denitrification with the in situ and the laboratory methods ranged from 1–2800 and 1–1700 μg N kg −1 d −1, respectively. Generally the methods gave good agreement and we consider both to be valid. However, there were some significant deviations, which we attribute to spatial heterogeneity and laboratory effects. Because the laboratory method is so much easier to perform, we suggest it should be the preferred method for large-scale studies of denitrification from the soil types we investigated. However, the two methods showed poor agreement in determining the proportion of N 2O in the total denitrification output. This was because this proportion is subject to delicate and complex control. We conclude that neither method was suitable for quantifying N 2O emission from the denitrification measurements.