Denitrification Fluxes Research Articles

Soil properties following long-term application of stockpiled feedlot manure containing straw or wood-chip bedding under barley silage production

Miller, J. J., Beasley, B. W., Drury, C. F., Hao, X. and Larney, F. J. 2014. Soil properties following long-term application of stockpiled feedlot manure containing straw or wood-chip bedding under barley silage production. Can. J. Soil Sci. 94: 389–402. The influence of long-term land application of stockpiled feedlot manure (SM) containing either wood-chip (SM-WD) or straw (SM-ST) bedding on soil properties during the barley (Hordeum vulgare L.) silage growing season is unknown. The main objective of our study was determine the effect of bedding material in stockpiled manure (i.e., SM-WD vs. SM-ST) on certain soil properties. A secondary objective was to determine if organic amendments affected certain soil properties compared with unamended soil. Stockpiled feedlot manure with SM-WD or SM-ST bedding at 77 Mg (dry wt) ha−1 yr−1 was annually applied for 13 to 14 yr to a clay loam soil in a replicated field experiment in southern Alberta. There was also an unamended control. Soil properties were measured every 2 wk during the 2011 and 2012 growing season. Properties included water-filled pore space (WFPS), total organic C and total N, NH4-N and NO3-N, water-soluble non-purgeable organic C (NPOC), water-soluble total N (WSTN), denitrification (acetylene inhibition method), and CO2 flux. The most consistent and significant (P≤0.05) bedding effects on soil properties in both years occurred for total organic C, C:N ratio, and WSTN. Total organic C and C:N ratio were generally greater for SM-WD than SM-ST, and the reverse trend occurred for WSTN. Bedding effects on other soil properties (WFPS, NH4-N, NO3-N, NPOC) occurred in 2012, but not in 2011. Total N, daily denitrification, and daily CO2 flux were generally unaffected by bedding material. Mean daily denitrification fluxes ranged from 0.9 to 1078 g N2O-N ha−1 d−1 for SM-ST, 0.8 to 326 g N2O-N ha−1 d−1 for SM-WD, and 0.6 to 250 g N2O-N ha−1 d−1 for the CON. Mean daily CO2 fluxes ranged from 5.3 to 43.4 kg CO2-C ha−1 d−1 for SM-WD, 5.5 to 26.0 kg CO2-C ha−1 d−1 for SM-ST, and from 0.5 to 6.8 kg CO2-C ha−1 d−1 for the CON. The findings from our study suggest that bedding material in feedlot manure may be a possible method to manage certain soil properties.

Pulse increase of soil N2O emission in response to N addition in a temperate forest on Mt Changbai, northeast China.

Nitrogen (N) deposition has increased significantly globally since the industrial revolution. Previous studies on the response of gaseous emissions to N deposition have shown controversial results, pointing to the system-specific effect of N addition. Here we conducted an N addition experiment in a temperate natural forest in northeastern China to test how potential changes in N deposition alter soil N2O emission and its sources from nitrification and denitrification. Soil N2O emission was measured using closed chamber method and a separate incubation experiment using acetylene inhibition method was carried out to determine denitrification fluxes and the contribution of nitrification and denitrification to N2O emissions between Jul. and Oct. 2012. An NH4NO3 addition of 50 kg N/ha/yr significantly increased N2O and N2 emissions, but their “pulse emission” induced by N addition only lasted for two weeks. Mean nitrification-derived N2O to denitrification-derived N2O ratio was 0.56 in control plots, indicating higher contribution of denitrification to N2O emissions in the study area, and this ratio was not influenced by N addition. The N2O to (N2+N2O) ratio was 0.41–0.55 in control plots and was reduced by N addition at one sampling time point. Based on this short term experiment, we propose that N2O and denitrification rate might increase with increasing N deposition at least by the same fold in the future, which would deteriorate global warming problems.

Assessing denitrification from seasonally saturated soils in an agricultural landscape: A farm-scale mass-balance approach

Riparian zones have received considerable attention as potential hotspots of denitrification at the landscape- or watershed-scale. Conceptually, the conditions that promote denitrification in these zones are also found in other parts of the landscape, namely those areas that are prone to saturate. However, spatiotemporal characterization and quantification of these potential denitrification hotspots are lacking, despite their importance to land managers tasked with mitigation of nitrogen (N) pollution, particularly in human-dominated landscapes. We quantified denitrification fluxes from the shallow saturated zone of an agricultural landscape using a topographic index-denitrification model, which facilitates scaling of in situ denitrification rates across the landscape based on frequency and duration of saturated conditions. Denitrification in the shallow saturated zone (i.e., where the water table is at or within a few meters of the soil surface) resulted in a N flux that was nearly half of the total denitrification from the landscape—in about a third of the area—as determined from a well-constrained whole-farm N balance constructed from farm records and field measures. Denitrification flux rates from saturated riparian soils were among the highest in the landscape, however the contribution of riparian areas to total landscape denitrification was less than 10%.

Assessing Nitrogen Transformations in a Flooded Agroecosystem Using the Isotope Pairing Technique and Nitrogen Functional Gene Abundances

Predicting N losses in flooded agricultural systems is complex because of the large number of potential N transformation pathways. Although coupled nitrification/denitrification at the flooded sediment near-surface oxic/anoxic interface is a recognized pathway of fertilizer N loss, the role of the newly discovered anaerobic ammonium oxidation (anammox) pathway in agricultural systems is poorly understood. A survey study was implemented to characterize sediment denitrification rates, anammox activity, and fluxes of ammonium and nitrate, and the vertical distribution of N functional genes were measured in Hawaiian taro (Colocasia esculenta) fields under three fertilizer management regimens (conventional, organic, and hybrid) as a model for flooded agroecosystems. Potential denitrification rates and anammox activity were measured using a slurry-based isotope pairing technique, and fluxes were determined by porewater modeling. Although significant numbers of anammox 16S rRNA genes were detected, only negligible anammox activity was found, illustrating the need to quantify not only the presence but also the activity of these bacteria. Quantitative polymerase chain reaction was performed for bacterial amoA, nirS, nosZ, and the 16S rRNA gene at 1-cm soil depth increments. Slurry isotope pairing technique-based potential denitrification rates (4.3-12.3 mmol N 2 m -2 day -1 ) were on the high end of previous anaerobic incubation studies. Flux-derived denitrification rates were up to three orders of magnitude lower than slurry isotope pairing technique rates, suggesting strong nitrification regulation. Organically managed fields exhibited significantly lower slurry and flux-based denitrification rates than conventional and hybrid systems. This study supports the use of alternative fertilizer management techniques in flooded agroecosystems by the mitigation of N losses through denitrification and decreased overall N flux rates.

Multidimensional modeling of biofilm development and fluid dynamics in a hydrogen-based, membrane biofilm reactor (MBfR)

A two-dimensional, particle-based biofilm model coupled with mass transport and computational fluid dynamics was developed to simulate autotrophic denitrification in a spiral-wound membrane biofilm reactor (MBfR), where hydrogen is supplied via hollow-fiber membrane fabric. The spiral-wound configuration consists of alternating layers of plastic spacer net and membrane fabric that create rows of flow channels, with the top and bottom walls comprised of membranes. The transversal filaments of the spacer partially obstruct the channel flow, producing complex mixing and shear patterns that require multidimensional representation. This study investigated the effect of hydrogen and nitrate concentrations, as well as spacer configuration, on biofilm development and denitrification fluxes. The model results indicate that the cavity spacer filaments, which rest on the bottom membranes, cause uneven biofilm growth. Most biofilm resided on the bottom membranes, only in the wake of the filaments where low shear zones formed. In this way, filament configuration may help achieve a desired biofilm thickness. For the conditions tested in this study, the highest nitrate fluxes were attained by minimizing the filament diameter and maximizing the filament spacing. This lowered the shear stress at the top membranes, allowing for more biofilm growth. For the scenarios studied, biomass limitation at the top membranes hindered performance more significantly than diffusion limitation in the thick biofilms at the bottom membranes. The results also highlighted the importance of two-dimensional modeling to capture uneven biofilm growth on a substratum with geometrical complexity.

Simulating the long term impact of nitrate mitigation scenarios in a pilot study basin

The agro-hydrological model TNT2 was used to explore for a period of 14 years (1987–2001) the likely consequences of mitigation scenarios on nitrate contamination of the stream water in a small agricultural catchment. The Best Management Practices (BMPs) historically designed and implemented in 1992 and two devised agricultural scenarios (catch crop (CC) implementation and a global reduction of N fertilizer) are evaluated in term of nitrate contamination in the environment. Two of the BMPs consist in implementing natural strips of Poplar and rye-grass strips (5 meters large) along stream and ditches and the third is a delay in the burial of wheat straws (from August to October). Simulations indicated that natural strips implementation would lead to a slight decrease of Nitrate Fluxes (NF) in river by respectively 3.3% and 6.6% for rye-grass and poplar strips: a benefit associated to the non fertilization of strips area. Denitrification has not been particularly disrupted in such areas. The delay in the burial of wheat straw in autumn decreases annual mineralization rate and annual plant uptake (by respectively 9 and 13kgNha−1y−1) but increases denitrification fluxes by 6kgNha−1y−1. This would lead to a slight decrease by 6% of NF in stream (equivalent to 3.3mgNO3−l−1) and an average decrease of the following sunflower yield by 27%. The global reduction of fertilization by 10% would decreased NF in stream by 13.8% (equivalent to 8mgNO3−l−1), with a global decrease by 8kgNha−1y−1 of plant uptake. The cumulative effect of BMPs and CC would have together lead to a decrease of nitrate concentration from 57.5 to 46.6mgNO3−l−1 reaching the UE environmental quality objectives (below 50mgNO3−l−1). Spring crops yield following CC would have been penalized and the decrease of NF is balanced by an increase of denitrification fluxes in the environment contributing to release of N2O, a greenhouse gas, into the atmosphere.

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.

The impact of copper, nitrate and carbon status on the emission of nitrous oxide by two species of bacteria with biochemically distinct denitrification pathways

Denitrifying bacteria convert nitrate (NO(3) (-) ) to dinitrogen (N(2) ) gas through an anaerobic respiratory process in which the potent greenhouse gas nitrous oxide (N(2) O) is a free intermediate. These bacteria can be grouped into classes that synthesize a nitrite (NO(2) (-) ) reductase (Nir) that is solely dependent on haem-iron as a cofactor (e.g. Paracoccus denitrificans) or a Nir that is solely dependent on copper (Cu) as a cofactor (e.g. Achromobacter xylosoxidans). Regardless of which form of Nir these groups synthesize, they are both dependent on a Cu-containing nitrous oxide reductase (NosZ) for the conversion of N(2) O to N(2) . Agriculture makes a major contribution to N(2) O release and it is recognized that a number of agricultural lands are becoming Cu-limited but are N-rich because of fertilizer addition. Here we utilize continuous cultures to explore the denitrification phenotypes of P. denitrificans and A. xylosoxidans at a range of extracellular NO(3) (-) , organic carbon and Cu concentrations. Quite distinct phenotypes are observed between the two species. Notably, P. denitrificans emits approximately 40% of NO(3) (-) consumed as N(2) O under NO(3) (-) -rich Cu-deficient conditions, while under the same conditions A. xylosoxidans releases approximately 40% of the NO(3) (-) consumed as NO(2) (-) . However, the denitrification phenotypes are very similar under NO(3) (-) -limited conditions where denitrification intermediates do not accumulate significantly. The results have potential implications for understanding denitrification flux in a range of agricultural environments.

Modelling approach to analyse the effects of nitrification inhibition on primary production

Summary 1. Wet tropical savannas have high grass productivity despite the fact that nitrogen is generally limiting for primary production and soil nutrient content is typically very low. Nitrogen recycling, and especially nitrification, is supposed to be a strong determinant of the balance between conservation and loss of nutrients at the ecosystem level. The important primary production observed in wet tropical savannas might be due to a tight nutrient cycling and the fact that some grass species inhibit soil nitrification. 2. Using a general theoretical ecosystem model taking both nitrate and ammonium into account, we investigate analytically, using a four‐compartment-differential-equation system the general conditions under which nitrification inhibition enhances primary production. We then estimate the quantitative impact of such a mechanism on the dynamics and budget of nitrogen in a welldocumented ecosystem, the Lamto savanna (Ivory Coast). This ecosystem is dominated by the grass Hyparrhenia diplandra , which drastically reduces nitrification in the whole savanna except for a small zone. While this small zone supports a lower grass primary production, nitrification is higher, most likely due to the presence of another genotype of H. diplandra , which has no effect on nitrification processes. Ultimately, we test whether differences in nitrification fluxes can alone explain this variation in primary production. 3. Model analysis shows that nitrification inhibition enhances primary production only if the recycling efficiency ‐ that is, the fraction of nitrogen passing through a compartment that stays inside the ecosystem ‐ of ammonium is higher than the recycling efficiency of nitrate. This condition probably manifests itself in most soils as ammonium is less mobile than nitrate and is not touched by denitrification. It also depends partially on the relative affinity of plants for ammonium or nitrate. The numerical predictions for this model in the Lamto savanna show that variations in nitrification inhibition capacity may explain observed differences in primary production. 4. In conclusion we find that nitrification inhibition is a process which probably enhances ecosystem fertility in a sustainable way, particularly in situations of high nitrate leaching and denitrification fluxes. This mechanism could explain the ecological advantage exhibited by native African grasses over indigenous grasses in South-American pastures.

Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA

Denitrification is an important net sink for NO3 − in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO3 − concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO3 − concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U denit, in μmol N m−2 h−1), results of different methods for a given set of conditions commonly were in agreement within a factor of 2–3. At approximately constant temperature (~20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U denit (~0–4,000 μmol N m−2 h−1) and stream NO3 − concentration (~20–1,100 μmol L−1) representing seasonal variation from spring high flow (high NO3 −) to late summer low flow (low NO3 −). The temporal dependence of U denit on NO3 − was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, U denit ≈26 * [NO3 −]0.44 or U denit ≈640 * [NO3 −]/[180 + NO3 −]; for a partially weighted dataset, U denit ≈14 * [NO3 −]0.54 or U denit ≈700 * [NO3 −]/[320 + NO3 −]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO3 − concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO3 − concentration. Hypothetical models based on our results illustrate: (1) U denit was inversely related to denitrification rate constant (k1denit, in day−1) and vertical transfer velocity (v f,denit, in m day−1) at seasonal and possibly event time scales; (2) although k1denit was relatively large at low flow (low NO3 −), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U denit; and (3) although NO3 − assimilation and denitrification were linked through production of organic reactants, rates of NO3 − loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1denit and v f,denit are linked implicitly with stream depth, NO3 − concentration, and(or) NO3 − load, estimates of U denit may be related more directly to field factors (including NO3 − concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U denit and stream NO3 − concentration and accounting for temporal variation.

Modeling denitrification in a tile-drained, corn and soybean agroecosystem of Illinois, USA

Denitrification is known as an important pathway for nitrate loss in agroecosystems. It is important to estimate denitrification fluxes to close field and watershed N mass balances, determine greenhouse gas emissions (N2O), and help constrain estimates of other major N fluxes (e.g., nitrate leaching, mineralization, nitrification). We compared predicted denitrification estimates for a typical corn and soybean agroecosystem on a tile drained Mollisol from five models (DAYCENT, SWAT, EPIC, DRAINMOD-N II and two versions of DNDC, 82a and 82h), after first calibrating each model to crop yields, water flux, and nitrate leaching. Known annual crop yields and daily flux values (water, nitrate-N) for 1993–2006 were provided, along with daily environmental variables (air temperature, precipitation) and soil characteristics. Measured denitrification fluxes were not available. Model output for 1997–2006 was then compared for a range of annual, monthly and daily fluxes. Each model was able to estimate corn and soybean yields accurately, and most did well in estimating riverine water and nitrate-N fluxes (1997–2006 mean measured nitrate-N loss 28 kg N ha−1 year−1, model range 21–28 kg N ha−1 year−1). Monthly patterns in observed riverine nitrate-N flux were generally reflected in model output (r 2 values ranged from 0.51 to 0.76). Nitrogen fluxes that did not have corresponding measurements were quite variable across the models, including 10-year average denitrification estimates, ranging from 3.8 to 21 kg N ha−1 year−1 and substantial variability in simulated soybean N2 fixation, N harvest, and the change in soil organic N pools. DNDC82a and DAYCENT gave comparatively low estimates of total denitrification flux (3.8 and 5.6 kg N ha−1 year−1, respectively) with similar patterns controlled primarily by moisture. DNDC82h predicted similar fluxes until 2003, when estimates were abruptly much greater. SWAT and DRAINMOD predicted larger denitrification fluxes (about 17–18 kg N ha−1 year−1) with monthly values that were similar. EPIC denitrification was intermediate between all models (11 kg N ha−1 year−1). Predicted daily fluxes during a high precipitation year (2002) varied considerably among models regardless of whether the models had comparable annual fluxes for the years. Some models predicted large denitrification fluxes for a few days, whereas others predicted large fluxes persisting for several weeks to months. Modeled denitrification fluxes were controlled mainly by soil moisture status and nitrate available to be denitrified, and the way denitrification in each model responded to moisture status greatly determined the flux. Because denitrification is dependent on the amount of nitrate available at any given time, modeled differences in other components of the N cycle (e.g., N2 fixation, N harvest, change in soil N storage) no doubt led to differences in predicted denitrification. Model comparisons suggest our ability to accurately predict denitrification fluxes (without known values) from the dominant agroecosystem in the midwestern Illinois is quite uncertain at this time.

Riparian zone denitrification affects nitrogen flux through a tidal freshwater river

Tidal freshwater zones (TFZ) of coastal rivers link terrestrial watersheds to the ocean and are characterized by large, regularly inundated riparian zones. We investigated the effect of riparian denitrification on nitrogen flux in the TFZ Newport River, North Carolina (U.S.A.) by developing an empirical model of denitrification and parameterizing it using measured denitrification rates, sediment oxidation-reduction potential dynamics, and riparian topography. Denitrification rates were measured monthly in laboratory-incubated sediment cores by using a membrane inlet mass spectrometer to assess net water-borne N2 flux from the cores. Annual average rates of denitrification in three intertidal riparian habitats, emergent marsh, mudflat, and hardwood forest, were 1864, 1956, and 2018 μg m−2 h−1, respectively. Laboratory experiments and in-situ monitoring revealed that the temporal lag between tidal inundation and reduced, denitrifying conditions was 4–5 h. Field measurements and remotely sensed data showed that the inundated surface area during high tide was three times greater than that at low tide. By combining data on denitrification, oxidation-reduction potential, and topography, the model predicted that the daily denitrification flux constituted 2–15% of the daily riverine nitrate flux during most of the year and >100% during low discharge periods. Current regional and global nitrogen budgets thus may overestimate nitrogen delivery to the ocean by not accounting for the TFZ denitrification.

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.

Pig slurry treatment modifies slurry composition, N 2O, and CO 2 emissions after soil incorporation

The treatment of manures may improve their agricultural value and environmental quality, for instance with regards to greenhouse gases mitigation and enhancement of carbon (C) sequestration. The present study verified whether different pig slurry treatments (i.e. solid/liquid separation and anaerobic digestion) changed slurry composition. The effect of the slurry composition on N 2O and CO 2 emissions, denitrification and soil mineral nitrogen (N), after soil incorporation, was also examined during a 58-day mesocosm study. The treatments included a non-treated pig slurry (NT), the solid fraction (SF), and the liquid fraction (LF) of a pig slurry and the anaerobically digested liquid fraction (DG). Finally, a non-fertilized (N0) and a treatment with urea (UR) were also present. The N 2O emissions measured represented 4.8%, 2.6%, 1.8%, 1.0% and 0.9% of N supplied with slurry/fertilizer for NT, LF, DG, SF and UR, respectively. Cumulative CO 2 emissions ranged from 0.40 g CO 2-C kg −1 soil (0.38 Mg CO 2-C ha −1) to 0.80 g CO 2-C kg −1 soil (0.75 Mg CO 2-C ha −1). They were highest for SF (56% of C applied), followed by NT (189% of C applied), LF (337% of C applied) and DG (321% of C applied). Ammonium was detected in the soil for all treatments only at day one, while nitrate concentration increased linearly from day 15 to day 58, at a rate independent of the type of slurry/fertilizer applied. The nitrate recovery at day 58 was 39% of the N applied for NT, 19% for SF, 52% for LF, 67% for DG, and 41% for UR. The solid fraction generally produced higher potential denitrification fluxes (75.3 for SF, 56.7 for NT, 53.6 for LF, 47.7 for DG and 39.7 mg N 2O + N 2-N kg −1 soil for UR). The high variability of actual denitrification results obfuscated any treatment effect. We conclude that treatment strongly affects slurry composition (mainly its C, fibre and NH 4 + content), and hence N 2O and CO 2 emission patterns as well as denitrification processes and nitrate availability. In particular, the solid fraction obtained after mechanical separation produced the most pronounced difference, while the liquid fraction and the anaerobically digested liquid fraction did not show significant difference with respect to the original slurry for any of the measured parameters. Combining data from the different fractions we showed that separation of slurry leads to reduced N 2O emissions, irrespective of whether the liquid fraction is digested or not. Furthermore, our results suggested that the default emission factor for N 2O emissions inventory is too low for both the non-treated pig slurry and its liquid fraction (digested or not), and too high for the separated solid fraction and urea.

Denitrification in cypress swamp within the Atchafalaya River Basin, Louisiana

Nitrogen has been implicated as a major cause of hypoxia in shallow water along the Louisiana/Texas, USA coasts. Excess nitrogen (mainly nitrate) from Mississippi and Atchafalaya River drainage basins may drive the onset and duration of hypoxia in the northern Gulf of Mexico. Restoring and enhancing denitrification have been proposed to reduce and control coastal hypoxia and improve water quality in the Mississippi River Basin. Sediments were collected from six baldcypress restoration sites within the Atchafalaya River Basin, Louisiana, USA. The acetylene blockage technique was used to measure background and potential sediment denitrification rates. Denitrification fluxes were measured before nitrate addition (background rates) and after nitrate addition of 100 mg N l −1 (potential denitrification) at three seasonal temperatures. Background denitrification was low across all cypress swamp sites ranging from 0.9 to 8.8, 0.6 to 28.5 and 8.8 to 47.5 g N evolved ha −1 d −1 at water/sediment column temperatures of 8, 22 and 30 °C, respectively. After nitrate addition, temperature had a significant effect on sediment denitrification potential. Maximum rates measured at 8, 22 and 30 °C were approximately 250–260, 550 and 970 g N ha −1 d −1, respectively. Most of the added nitrate in water columns, incubated at 8 °C, was removed after 65 d compared to 32 d and 17 d at 22 and 30 °C, respectively. These results indicate cypress swamps have the potential to assimilate and process elevated levels of floodwater nitrate with denitrification being a major removal mechanism.

Nitrogen cycling in the Middle Atlantic Bight: Results from a three‐dimensional model and implications for the North Atlantic nitrogen budget

The biogeochemistry of continental shelf systems plays an important role in the global elemental cycling of nitrogen and carbon, but remains poorly quantified. We have developed a high‐resolution physical‐biological model for the U.S. east coast continental shelf and adjacent deep ocean that is nested within a basin‐wide North Atlantic circulation model in order to estimate nitrogen fluxes in the shelf area of the Middle Atlantic Bight (MAB). Our biological model is a relatively simple representation of nitrogen cycling processes in the water column and organic matter remineralization at the water‐sediment interface that explicitly accounts for sediment denitrification. Climatological and regionally integrated means of nitrate, ammonium, and surface chlorophyll are compared with its model equivalents and were found to agree within 1 standard deviation. We also present regional means of primary production and denitrification, and statistical measures of chlorophyll pattern variability. A nitrogen budget for the MAB shows that the sediment denitrification flux is quantitatively important in determining the availability of fixed nitrogen and shelf primary production (it was found to remove 90% of all the nitrogen entering the MAB). Extrapolation of nitrogen fluxes estimated for the MAB to the North Atlantic basin suggests that shelf denitrification removes 2.3 × 1012 mol N annually; this estimate exceeds estimates of N2 fixation by up to an order of magnitude. Our results emphasize the importance of representing shelf processes in biogeochemical models.

Determination of denitrification in the Chesapeake Bay from measurements of N2 accumulation in bottom water

This study demonstrates the feasibility of using direct N2 measurements in an estuary for determination of denitrification. High precision measurements of dinitrogen: argon ratios (N2∶Ar) were made by membrane inlet mass spectrometry on water samples taken along the length of the Chesapeake Bay in July and October 2004. The N2∶Ar ratio in low salinity surface water was elevated relative to air saturation by 0.3–0.5% with no systematic change along the length of the Bay. N2∶Ar in high salinity bottom water exhibited a linear increase in the landward direction along a 144-km longitudinal section. In this section of the Bay covering 20% of the main stem, the bottom water salinity was statistically uniform and the increase in N2∶Ar was in the direction of net residual current flow. The system was analyzed as a capped river with the assumption that N2 entered the water from the underlying sediment where denitrification is known to take place. The rate of denitrification needed to support the measured increase in N2 was calculated using an average residual current velocity and water column depth. The increase in N2 with distance (0.046μmol N l−1 km−1) equated to an average denitrification flux of 73 μmol N m−2 h−1. N2 fluxes determined on sediment cores taken from the source and terminus regions of the delineated water mass were 45±23 and 83±39 μmol N m−2 hr−1, respectively, which were not statistically different from the whole system estimate. The measured change in oxygen concentration within the bottom water was used to estimate nitrogen remineralization and the efficiency of denitrification. Denitrification efficiency (nitrogen denitrified/nitrogen remineralized) was estimated to be in the range of 22–28% for the bottom water sediment system and 30–37% considering the sediment zone alone.

How important is N 2O production in removing atmospherically deposited nitrogen from UK moorland catchments?

Nitrate (NO 3 −) leaching due to anthropogenic nitrogen (N) deposition is an environmental problem in many parts of the UK uplands, associated with surface water acidification and affecting lake nutrient balances. It is often assumed that gaseous return of deposited N to the atmosphere as N 2O through denitrification may provide an important sink for N. This assumption was tested for four moorland catchments (Allt a’Mharcaidh in the Cairngorms, Afon Gwy in mid-Wales, Scoat Tarn in the English Lake District and River Etherow in the southern Pennines), covering gradients of atmospheric N deposition and surface water NO 3 − leaching, through a combination of field and laboratory experiments. Field measurements of N 2O fluxes from static chambers with and without additions of NH 4NO 3 solution were carried out every 4 weeks over 1 yr. Wetted soil cores from the same field plots were used in experimental laboratory incubations at 5 and 15 °C with and without additions of NH 4NO 3 solution, followed by measurement of N 2O fluxes. Field measurements showed that significant N 2O fluxes occurred in only a very small number of plots with most showing zero values for much of the year. The maximum fluxes were 0.24 kg-N/ha/yr from unamended plots at the River Etherow and 0.49 kg-N/ha/yr from plots with NH 4NO 3 additions at the Allt a’Mharcaidh. Laboratory incubation experiments demonstrated that large N 2O fluxes could be induced by warming and NH 4NO 3 additions, with the top 5 cm of soil cores responsible for the largest fluxes, reaching 11.8 kg-N/ha/yr from a podsol at Scoat Tarn. Acetylene block experiments showed that while N 2 was not likely to be a significant denitrification product in these soils, reduced N 2O fluxes indicated that nitrification was an important source of N 2O in many cases. A simple model of denitrification suggesting that 10–80% of net N inputs may be denitrified from non-agricultural soils was found to greatly over-estimate fluxes in the UK uplands. The proportion of deposition denitrified was found to be much closer to the IPCC suggested value of 1% with an upper limit of 10%. Interception of N deposition by vegetation may greatly reduce the net supply of N from this source, while soil acidification or other factors limiting carbon supply to soil microbes may prevent large denitrification fluxes even where NO 3 − supply is not limiting.

Nitrogen Behaviour and Nitrous Oxide Emission in the Tidal Seine River Estuary (France) as Influenced by Human Activities in the Upstream Watershed

The Seine River estuary (France) is the receptacle of a drainage basin characterised by high population density, heavy industrial activity and intensive agriculture. Whereas nitrate concentrations are high due to diffuse sources in the upstream drainage basin, ammonium mainly originates from the effluents of the Acheres wastewater treatment plant (WWTP) downstream from Paris and its suburbs (6.5 million equivalent-inhabitants). Ammonium is mostly nitrified in the tidal freshwater estuary and nitrification causes a strong summer oxygen deficit. Average longitudinal summer profiles of oxygen and nitrogen concentrations for two periods, between 1993–1997 and 1998–2003 in dry hydrological conditions (excluding the wet years 2000 and 2001) clearly reflect the changes due to the improved treatment of wastewater from Paris and its suburbs. On the basis of daily water flux data and twice monthly nitrogen measurements at the boundaries of the upstream freshwater estuarine section (108 km), we calculated nitrification and denitrification fluxes, whose annual averages were 43 and 71 × 103 kg N d−1 respectively from 1993 to 2003, with summer values (July–September) representing 73 and 57% of the annual fluxes, respectively. The degree of denitrification in the upper estuary appears to be closely related to the nitrification, itself more loosely related to the amount of reduced nitrogen (Kjeldahl) brought by the treated effluents from the Acheres WWTP. We estimated the total N2O emissions to about 40 kg N d−1 (25–60 kg N d−1) in the same sector.

Comparison of three modeling approaches for simulating denitrification and nitrous oxide emissions from loam‐textured arable soils

Soil denitrification fluxes and nitrous oxide (N2O) emissions from the soil surface simulated by a Water and Nitrogen Management Model (WNMM), with three different gas modules, are compared to measurement data sets from two irrigated wheat‐maize systems at two locations in the North China Plain (NCP) (2 years of measurement at the Luancheng site and 1 year of measurement at the Fengqiu site). The three gas modules are the WNMM gas module, the DAYCENT gas module, and the DNDC gas module. The term gas module used in this paper refers to the model component which simulates N2O emission from the processes of soil nitrification and denitrification. Soil water, temperature, organic matter decomposition, other nitrogen (N) transformations, such as mineralization and immobilization, and crop growth are simulated by the WNMM platform. For the 2‐year data set from Luancheng, the three gas modules generate similar soil mineral N dynamics in the 0–20 cm topsoil. The daily time step, simply structured WNMM gas module consistently performs the best among the three gas modules for predicting soil denitrification fluxes (R2 = 0.28, n = 39, p = 0.0006) and N2O emissions (R2 = 0.45, n = 36, p < 0.0001). Up to 73, 43, and 22% of total N2O emissions are nitrification‐induced as simulated by the DNDC, DAYCENT, and WNMM gas modules respectively, in this well‐drained loam soil during the 2‐year simulation. When applied to the 1‐year data set at the Fengqiu site, the WNMM gas module consistently performs better in estimating N2O emissions (R2 = 0.54, n = 35, p < 0.0001) compared to the other two modules. Simulations using the DNDC and DAYCENT gas modules explain over 40% of the temporal variation of N2O emission from the soil. Further testing on different soils and different agroecosystems is needed to confirm the superior performance of the WNMM gas module observed in this simulation study.