N2 Fluxes Research Articles

Growth in the global N2 sink attributed to N fertilizer inputs over 1860 to 2000

Cropland expansion and fertilizer applications are among the most important substantial effects of human actions on the global nitrogen (N) cycle. However, questions remain over the fate of anthropogenic N inputs, particularly whether a significant fraction of N-based fertilizers have been lost to inert N2 or reactive N forms. Here, we combine natural N isotope constraints on the pre-industrial N cycle with global mass-balance modeling to investigate the role of cropland conversion on gaseous N emissions and hydrological N leaching fluxes. We estimate that cropland expansion has been accompanied by >9-fold increase in N input rates to cropping systems, roughly doubling the baseline N budget of the terrestrial biosphere. As a consequence, approximately 10 times more N is exported from modern croplands to the hydrosphere than in 1860, with a five-fold increase in cropland N gases emission to the atmosphere. Atmospheric NH3, NO, N2O and N2 fluxes increased from 8.6, 16.6, 11.7 and 31.9TgNyr−1, respectively, in 1860 to 17.7, 23.6, 15.2 and 39.7TgNyr−1, respectively, by 2000. Thus, the growth in N2 accounted for ~20% of cropland-driven N losses (dissolved plus gaseous pathways), with the remaining 80% exported as reactive N forms. Although the increase in N2 emissions has mitigated some of the unwanted side-effects of N fertilizer applications on human health, the economy, and climate change, this inert sink has been unable to keep pace with the increase in N inputs for enhanced food production. Our results imply that, unless new management steps are taken, an increasing fraction of N fertilizers will mobilize to reactive N forms in the global land, air and water systems, thus further accelerating the negative consequences of human modifications of the N cycle this century.

PBI-HFA Hollow Fiber Membrane for Selective Hydrogen Separation

We demonstrated for the development of PBI-HFA based asymmetric hollow fiber membrane for hydrogen separation. High molecular weight PBI-HFA was synthesized in-house by a solution polycondensation method [1]. Hollow fiber membranes were fabricated by conventional dry-jet wet spinning technique. Defect free asymmetric PBI-HFA hollow fiber membranes were successfully produced, which influenced of PBI-HFA concentration and LiCl as shown in Fig. 1. The effect of bore fluid chemistry was found to have a significant effect on the morphology as well as gas performance of the hollow fiber membranes. Development of PBI-HFA hollow fiber membranes with improved gas permeation properties their known excellent thermal and chemical properties depicts their potential for gas separation applications at harsh environment. As shown in Fig. 2, PBI-HFA hollow fiber showed good gas flux and selectivities for industrially important gas pairs such as H2/CO, H2/N2 with the ideal selectivities of 35.7 and 43.4, respectively, while the permeation fluxes of H2, N2, CO2 and CO were at 664, 15.3, 263.37 and 18.6 GPU at 1bar as given in Table. 1. Mixed gas results as given in Table. 2 discussed above indicate that PBI-HFA hollow fiber is an excellent H2 selective membrane material for syngas separations. This improvement in gas selectivities was attributed to their increased closer chain packing by high solubility. Pressure P (H2) a P(N2) a P(CO2) a P(CO) a 1bar 664 15.3 263.37 18.6 43.4 2.52 35.70 17.21 2bar 1219 31.02 471.26 35.82 39.3 2.59 34.03 15.19 3bar 1783 46.61 691.78 54.13 38.25 2.58 32.94 14.84 4bar 2353 63.58 978.76 71.83 37.13 2.4 32.76 15.44 a Expressed as permeance in GPU (1 GPU = 1´10-6cm3(STP) cm-2s-1 cmHg-1). Table 1. Gas separation performance of PDMS coated PBI hollow fiber at normalized temperature. (H2)% (N2)% (CO2)% (CO)% Mixture gas composition 51.53 21.2 7.63 17.86 1 bar 89.6 0 3.05 0.33 2bar 84.19 0 2.49 7.99 3bar 65 3.7 6.49 23.5 Table 2. Mixture Gas separation performance of PDMS coated PBI hollow fiber membrane at 21℃, 100℃, and 200℃ temperature [1] S. Kumbharkar, M. N. Islam, R. Potrekar, and U. Kharul, "Variation in acid moiety of polybenzimidazoles: investigation of physico-chemical properties towards their applicability as proton exchange and gas separation membrane materials," Polymer, vol. 50, pp. 1403-1413, 2009. Figure 1

A workflow for reproducing mean benthic gas fluxes

AbstractLong‐term data sets provide unique opportunities to examine temporal variability of key ecosystem processes. The need for such data sets is becoming increasingly important as we try to quantify the impact of human activities across various scales and in some cases, as we try to determine the success of management interventions. Unfortunately, long‐term benthic flux data sets for coastal ecosystems are rare and curating them is a challenge. If we wish to make our data available to others now and into the future, however, then we need to provide mechanisms that allow others to understand our methods, access the data, reproduce the results, and see updates as they become available. Here we use techniques, learned through the EarthCube Ontosoft Geoscience Paper of the Future project, to develop best practices to allow us to share a long‐term data set of directly measured net sediment N2 fluxes and sediment oxygen demand at two sites in Narragansett Bay, Rhode Island (USA). This technical report describes the process we used, the challenges we faced, and the steps we will take in the future to ensure transparency and reproducibility. By developing these data and software sharing tools we hope to help disseminate well‐curated data with provenance as well as products from these data, so that the community can better assess how this temperate estuary has changed over time. We also hope to provide a data sharing model for others to follow so that long‐term estuarine data are more easily shared and not lost over time.

Crystallochemistry and electrical properties of Al-doped Sr2SiO4 electrolytes

A new Sr2Si1−xAlxO4−x/2☐x/2 (x=0, 0.028, 0.06 and 0.075) silicate series has been prepared and the structural and electrical properties were investigated. These samples present three different polymorphs (β, α′L and α′H) and the increase of Al-content resulted in a shift of the polymorph content, from a monoclinic form to an orthorhombic one. Aluminium contents higher than x=0.06 did not lead to any further change in the ratio of polymorphs. High temperature X-ray diffraction studies showed a complete stabilisation of α′L, presenting a superstructure orthorhombic form, at temperatures between RT and 250°C. At temperatures higher than 500°C a basic orthorhombic cell is stabilised. Thermogravimetric analysis displayed an increase of the water uptake on doping, due to the generation of oxide vacancies in the lattice. Electrical characterisation in dry and wet atmospheres showed an important proton contribution to the conductivity. In addition, measurements in N2 and O2 fluxes displayed a significant p-type electronic contribution to the overall conductivity.

Fluxes of N2 and N2O and contributing processes in summer after grassland renewal and grassland conversion to maize cropping on a Plaggic Anthrosol and a Histic Gleysol

Grassland renewal and grassland conversion to arable land are common agricultural practices on intensively used grassland sites, especially in north-western Europe. However, grassland ploughing can cause a flush of soil organic nitrogen (N) mineralisation due to soil disturbance during tillage and decomposition of stubble and roots from the old grass sward. This is known to result in enhanced nitrous oxide (N2O) emissions, but information about the underlying microbial processes, especially the role of N2O reduction to N2 via denitrification, is scarce. Therefore we applied the 15N gas flux method in situ to grassland recently ploughed under for maize cropping, renewed grassland and permanent grassland on a Histic Gleysol and a Plaggic Anthrosol which differed in organic matter content and drainage. We used needle injection of 15N-labelled KNO3− at three different depths in the soil to achieve homogeneous label distribution. Fluxes of N2O and N2, mineral N concentration and 15N enrichment of these were measured for 44 days after label addition. Overall, no differences in N2O and N2 emissions were found between grassland conversion/renewal and permanent grassland. N2 emissions increased up to 9115 g N ha−1 day−1 on a single sampling day following grassland conversion to maize cropping on the Histic Gleysol, leading to a great contribution of denitrification when N2O/(N2O + N2) ratio was low. However, heterogeneity of 15N label distribution proved to be a major difficulty in the water-saturated Histic Gleysol and caused potential uncertainty in identification of various production pathways. Lower gaseous losses and higher nitrification potential were detected in the Plaggic Anthrosol, indicating a higher threat of possible leaching of excess mineral N following grassland conversion/renewal.

Sediment Nitrogen Fixation: a Call for Re-evaluating Coastal N Budgets

Coastal ocean primary productivity is often limited by nitrogen (N) availability, which is determined by the balance between N sources (e.g., N-fixation, groundwater, river inputs, etc.) and sinks (e.g., denitrification, sediment burial, etc.). Historically, heterotrophic N-fixation in sediments was excluded as a significant source of N in estuarine budgets, based on low, indirectly measured rates (e.g., acetylene reduction assay) and because it was unnecessary to achieve mass balance. Many recent studies using net N2 flux measurements have shown that sediment N-fixation can equal or exceed N2 loss. In an effort to quantify N2 production and consumption simultaneously, we measured N-fixation and denitrification directly in sediment cores from a temperate estuary (Waquoit Bay, MA). N-fixation, dissimilatory nitrate reduction to ammonium, and denitrification occurred simultaneously, and the net N2 flux shifted from uptake (N-fixation) to efflux (denitrification) over the 120-h incubation. Evidence for N-fixation included net 28N2 and 30N2 uptake, 15NH4 + production from 30N2 additions, 15Norganic matter production, and nifH expression. N-fixation from 30N2 was up to eight times higher than potential denitrification. However, N-fixation calculated from 15NO3 − was one half of the measured fixation from 30N2, indicating that 15NO3-isotope labeling calculations may underestimate N-fixation. These results highlight the dynamic nature of sediment N cycling and suggest that quantifying individual processes allows a greater understanding of what net N2 fluxes signify and how that balance varies over time.

Quantification of first metatarsal-phalangeal joint (MTPJ) kinematics using biplane fluoroscopy

A new Sr2Si1−xAlxO4−x/2☐x/2 (x=0, 0.028, 0.06 and 0.075) silicate series has been prepared and the structural and electrical properties were investigated. These samples present three different polymorphs (β, α′L and α′H) and the increase of Al-content resulted in a shift of the polymorph content, from a monoclinic form to an orthorhombic one. Aluminium contents higher than x=0.06 did not lead to any further change in the ratio of polymorphs. High temperature X-ray diffraction studies showed a complete stabilisation of α′L, presenting a superstructure orthorhombic form, at temperatures between RT and 250 °C. At temperatures higher than 500 °C a basic orthorhombic cell is stabilised. Thermogravimetric analysis displayed an increase of the water uptake on doping, due to the generation of oxide vacancies in the lattice. Electrical characterisation in dry and wet atmospheres showed an important proton contribution to the conductivity. In addition, measurements in N2 and O2 fluxes displayed a significant p-type electronic contribution to the overall conductivity.

Supplemental tests of gas trapping device for N2 flux measurement

The gas trapping device method (GTD) is a relatively new method to measure N2 flux from waters. However, the non-equilibrium diffusion error and the reliability of GTD method compared to other previously established N2 flux measurement methods has not been evaluated. In this study, the diffusive error of GTD, coming from non-equilibrium N2 partial pressure between the headspace inside the gas sample bottle and the air, was estimated using a sterilization experiment. Moreover, the GTD and MIMS methods were compared for measuring N2 flux from water under similar conditions. The results showed that there were maximum diffusion errors of 2.99% in the sample bottles prefilled with pure Helium, while only 1.09–1.76% diffusion errors in bottles prefilled with other N2 standard gas (15% or 75%), indicating minor non-equilibrium diffusion errors. N2 fluxes from water measured by GTD and MIMS methods are quite similar under all three concentrations of nitrate (5.30, 10.55 and 17.25mgL−1) and two levels of temperature (20 and 30°C). Therefore, the GTD method offers a reliable alternative method to estimate N2 flux rate in aquatic ecosystem.

Directly Measured Denitrification Reveals Oyster Aquaculture and Restored Oyster Reefs Remove Nitrogen at Comparable High Rates

Coastal systems are increasingly impacted by over-enrichment of nutrients, which has cascading effects for ecosystem functioning. Oyster restoration and aquaculture are both hypothesized to mitigate excessive nitrogen (N) loads via benthic denitrification. The degree to which these management activities perform similar functions for removing N, however, has not been extensively examined in New England, a place where nutrient runoff is high and increasing oyster (Crassostrea virginica) restoration and aquaculture activity is taking place. Here, we use a novel in situ methodology to directly measure net N2 and O2 fluxes across the sediment-water interface in a shallow (~1 m) coastal pond in southern Rhode Island. We collected data seasonally during 2013 and 2014 at restored oyster reefs, oyster aquaculture, oyster cultch (shell), and bare sediment. Restored oyster reefs and aquaculture had the highest mean (± SE) denitrification rates, 581.9 (± 164.2) and 346 (± 168.6) μmol N2-N m-2 h-1, respectively, and are among the highest recorded for oyster-dominated environments. Denitrification rates at sites with oyster cultch were 60.9 (± 44.3) μmol N2-N m-2 h-1, which is substantially less than the sites with active oysters but still more than 50% higher than denitrification rates measured in bare sediment (24.4 ± 10.1 μmol N2-N m-2 h-1). The increase in denitrification rates at treatments, however, varied by season and the greatest rates for restored reefs were in the fall. Overall, the greatest aggregate denitrification rates occurred in the fall. Sediment oxygen demand (SOD) followed similar patterns but with greater overall rates in the summer, and displayed a strong linear relationship with denitrification (R2 = 0.9273). Our results demonstrate that habitats associated with live oysters have higher net denitrification rates and that oyster reef restoration and oyster aquaculture may provide similar benefits to the ecosystem in terms of N removal. However, gas fluxes may also be affected where three-dimensional structure is introduced via oyster shell cultch and this appears to be seasonally-dependent. These data will be important for managers as they incorporate oysters into nutrient reduction strategies and consider system-level trade-offs in services provided by oyster reef restoration and aquaculture activities.

Soil Gas Diffusivity Controls N2O and N2 Emissions and their Ratio

Core Ideas Relative gas diffusivity controls both N2O and N2 emissions. Relative gas diffusivity integrates the effects of soil bulk density and matric potential. Nitrogen use efficiency is likely to be driven by soil physics. Knowledge of soil biological and physical interactions with respect to N2O and N2 fluxes is essential to ensure that agricultural land management is environmentally and economically sustainable. This study determined how varying soil relative gas diffusivity (Dp/Do) affected cumulative N2O and N2 fluxes under simulated ruminant urinary‐N deposition. Using repacked soil cores, the effects of varying soil bulk density (ρb; from 1.1 to 1.5 Mg m−3) and soil matric potential (ψ; −10 to −0.2 kPa) on Dp/Do were examined in a Templeton silt loam soil (Udic Haplustept) following the application of simulated ruminant urine (700 kg N ha−1). Fluxes of N2O and N2, soil inorganic N, pH, and dissolved organic C (DOC) dynamics were monitored over 35 d. Soil Dp/Do declined as soil bulk density and soil moisture increased. Soil N2O emissions increased exponentially as Dp/Do decreased until Dp/Do equaled 0.005, where upon N2O fluxes decreased rapidly due to complete denitrification, such that N2 fluxes reached a maximum of 60% of N applied at a Dp/Do of <0.005. Regression analysis showed that Dp/Do was better able to explain the variation in N2O and N2 fluxes than water‐filled pore space (WFPS) because it accounted for the interaction of soil ρb and ψ. This study demonstrates that soil Dp/Do can explain cumulative N2O and N2 emissions from agricultural soils. Under grazed pasture systems, potential exists to reduce the emissions of the greenhouse gas N2O and significant economic losses of N as N2 if soil management and irrigation can be maintained to maximize Dp/Do.

Effects of thiamphenicol on nitrate reduction and N2O release in estuarine and coastal sediments

Nitrate overload is an important driver of water pollution in most estuarine and coastal ecosystems, and thus nitrate reduction processes have attracted considerable attention. Antibiotics contamination is also an emerging environmental problem in estuarine and coastal regions as a result of growing production and usage of antibiotics. However, the effects of antibiotics on nitrate reduction remain unclear in these aquatic ecosystems. In this study, continuous-flow experiments were conducted to examine the effects of thiamphenicol (TAP, a common chloramphenicol antibiotic) on nitrate reduction and greenhouse gas N2O release. Functional genes involved in nitrogen transformation were also quantified to explore the microbial mechanisms of the TAP influence. Production of N2 were observed to be inhibited by TAP treatment, which implied the inhibition effect of TAP on nitrate reduction processes. As intermediate products of nitrogen transformation processes, nitrite and N2O were observed to accumulate during the incubation. Different TAP inhibition effects on related functional genes may be the microbial mechanism for the changes of nutrient fluxes, N2 fluxes and N2O release rates. These results indicate that the antibiotics residues in estuarine and coastal ecosystems may contribute to nitrate retention and N2O release, which could be a major factor responsible for eutrophication and greenhouse effects.

Application of the <sup>15</sup>N gas-flux method for measuring in situ N<sub>2</sub> and N<sub>2</sub>O fluxes due to denitrification in natural and semi-natural terrestrial ecosystems and comparison with the acetylene inhibition technique

Abstract. Soil denitrification is considered the most un-constrained process in the global N cycle due to uncertain in situ N2 flux measurements, particularly in natural and semi-natural terrestrial ecosystems. 15N tracer approaches can provide in situ measurements of both N2 and N2O simultaneously, but their use has been limited to fertilized agro-ecosystems due to the need for large 15N additions in order to detect 15N2 production against the high atmospheric N2. For 15N–N2 analyses, we have used an “in-house” laboratory designed and manufactured N2 preparation instrument which can be interfaced to any commercial continuous flow isotope ratio mass spectrometer (CF-IRMS). The N2 prep unit has gas purification steps and a copper-based reduction furnace, and allows the analysis of small gas injection volumes (4 µL) for 15N–N2 analysis. For the analysis of N2O, an automated Tracegas Preconcentrator (Isoprime Ltd) coupled to an IRMS was used to measure the 15N–N2O (4 mL gas injection volume). Consequently, the coefficient of variation for the determination of isotope ratios for N2 in air and in standard N2O (0.5 ppm) was better than 0.5 %. The 15N gas-flux method was adapted for application in natural and semi-natural land use types (peatlands, forests, and grasslands) by lowering the 15N tracer application rate to 0.04–0.5 kg 15N ha−1. The minimum detectable flux rates were 4 µg N m−2 h−1 and 0.2 ng N m−2 h−1 for the N2 and N2O fluxes respectively. Total denitrification rates measured by the acetylene inhibition technique in the same land use types correlated (r = 0.58) with the denitrification rates measured under the 15N gas-flux method, but were underestimated by a factor of 4, and this was partially attributed to the incomplete inhibition of N2O reduction to N2, under a relatively high soil moisture content, and/or the catalytic NO decomposition in the presence of acetylene. Even though relatively robust for in situ denitrification measurements, methodological uncertainties still exist in the estimation of N2 and N2O fluxes with the 15N gas-flux method due to issues related to non-homogenous distribution of the added tracer and subsoil gas diffusion using open-bottom chambers, particularly during longer incubation duration. Despite these uncertainties, the 15N gas-flux method constitutes a more reliable field technique for large-scale quantification of N2 and N2O fluxes in natural terrestrial ecosystems, thus significantly improving our ability to constrain ecosystem N budgets.

Depletion of oxygen, nitrate and nitrite in the Peruvian oxygen minimum zone cause an imbalance of benthic nitrogen fluxes

Oxygen minimum zones (OMZ) are key regions for fixed nitrogen loss in both the sediments and the water column. During this study, the benthic contribution to N cycling was investigated at ten sites along a depth transect (74–989m) across the Peruvian OMZ at 12°S. O2 levels were below detection limit down to ~500m. Benthic fluxes of N2, NO3–, NO2–, NH4+, H2S and O2 were measured using benthic landers. Flux measurements on the shelf were made under extreme geochemical conditions consisting of a lack of O2, NO3– and NO2– in the bottom water and elevated seafloor sulphide release. These particular conditions were associated with a large imbalance in the benthic nitrogen cycle. The sediments on the shelf were densely covered by filamentous sulphur bacteria Thioploca, and were identified as major recycling sites for DIN releasing high amounts of NH4+up to 21.2mmolm−2d−1 that were far in excess of NH4+ release by ammonification. This difference was attributed to dissimilatory nitrate (or nitrite) reduction to ammonium (DNRA) that was partly being sustained by NO3– stored within the sulphur oxidizing bacteria. Sediments within the core of the OMZ (ca. 200–400m) also displayed an excess flux of N of 3.5mmolm−2d−1 mainly as N2. Benthic nitrogen and sulphur cycling in the Peruvian OMZ appears to be particularly susceptible to bottom water fluctuations in O2, NO3− and NO2−, and may accelerate the onset of pelagic euxinia when NO3− and NO2− become depleted.

Denitrification in a subtropical, semi-arid North American savanna: field measurements and intact soil core incubations

Information on denitrification (particularly N2) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrification measurements for a northern hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N2, N2O and CO2 losses and compared these with field measurements of N2O, NOy, NH3 and CO2. Under field-realistic soil moisture and O2 conditions (average 17.5–20 % O2, minimum 15 %) incubated soils produced no measurable N2 flux (detection limit 52.2 µg N m−2 h−1). Only in a subset of grove soils were fluxes of 70–75 µg N m−2 h−1 recorded after 102 h of incubation at 5–10 % O2 following wetting of very dry soils. Making the assumption that potential N2 production falls just below the detection limit (likely an overestimate given the conditions needed to generate measurable fluxes), N2 flux rates would fall on the low end of that recorded for a tropical Australian savanna (45–110 µg N m−2 h−1) under comparable abiotic conditions. Assuming maximum possible production rates, N2 could comprise <32–76 % of total soil N gas flux following soil wetting in summer. Lack of flux response to soil wetting in winter suggests that cold-season N2 fluxes are negligible. N2O fluxes for core incubations were significantly higher than for field chambers; thus it is likely that incubations may overestimate N2O flux by reducing soil column consumption. Overall, results indicate that soil N2 fluxes are less dominant in this savanna than in other ecosystems investigated.

Denitrification, leaching, and river nitrogen export in the Community Earth System Model

AbstractRiver nitrogen export is simulated within the Community Earth System Model (CESM) by coupling nitrogen leaching and runoff fluxes from the Community Land Model (CLM) to the River Transport Model (RTM). The coupled CLM‐RTM prognostically simulates the downstream impact of human N cycle perturbation on coastal areas. It also provides a framework for estimating denitrification fluxes of N2 and associated trace gases like N2O in soils and river sediments. An important limitation of the current model is that it only simulates dissolved inorganic nitrogen (DIN) river export, due to the lack of dissolved organic nitrogen (DON) and particulate nitrogen (PN) leaching fluxes in CLM. In addition, the partitioning of soil N loss in CLM between the primary loss pathways of denitrification and N leaching/runoff appears heavily skewed toward denitrification compared to other literature estimates, especially in nonagricultural regions, and also varies considerably among the four model configurations presented here. River N export is generally well predicted in the model configurations that include midlatitude crops, but tends to be underpredicted in rivers that are less perturbed by human agriculture. This is especially true in the tropics, where CLM likely underestimates leaching and runoff of all forms of nitrogen. River export of DIN is overpredicted in some relatively unperturbed Arctic rivers, which may result from excessive N inputs to those regions in CLM. Better representation of N loss in CLM can improve confidence in model results with respect to the core model objective of simulating nitrogen limitation of the carbon cycle.

An initial investigation of a nano-composite silica ceramic membrane for hydrogen gas separation and purification

The effects of trans-membrane pressure difference on hydrogen recovery are investigated on a preliminary basis. The membrane has been developed for high temperature hydrogen separation from CH4, N2, and Ar gas molecules. Gas permeation and separation performance of the developed membrane was evaluated. The silica fabricated membrane exhibited high H2 gas flux and much lower fluxes for CH4, N2, and Ar and showed a rise in permeance with the inverse of the square root of temperature for H2. Plots obtained with respect to the effects of gauge pressure on H2, N2, CH4 and Ar component gas fluxes through the modified support (silica membrane) at a pressure of 1 bar showed that H2 recorded an almost four-fold high flux value of 0.76 mol/m2.s compared to that of 0.33 mol/m2.s (CH4), 0.25 mol/m2.s (N2) and 0.22 mol/m2.s (Ar). These results were nearly stable for the temperature range investigated (298 K, 373 K and 473 K). In addition, hydrogen maintained a relatively high permeance value of 1.62 × 10−5 mol m−2 s−1 Pa−1 at a low pressure of 0.1 bar under room temperature (298 K) compared to that of N2, CH4 and Ar which were much lower throughout the temperature range studied and in agreement with literature. H2 separation factor (experimental Knudsen) over CH4, N2 and Ar were quite close to the theoretical Knudsen and can therefore be experimentally applied through further optimization in the separation of H2 as a key constituent energy for the future.

Carbon amendment and soil depth affect the distribution and abundance of denitrifiers in agricultural soils.

The nitrite reductase (nirS and nirK) and nitrous oxide reductase-encoding (nosZ) genes of denitrifying populations present in an agricultural grassland soil were quantified using real-time polymerase chain reaction (PCR) assays. Samples from three separate pedological depths at the chosen site were investigated: horizon A (0-10cm), horizon B (45-55cm), and horizon C (120-130cm). The effect of carbon addition (treatment 1, control; treatment 2, glucose-C; treatment 3, dissolved organic carbon (DOC)) on denitrifier gene abundance and N2O and N2 fluxes was determined. In general, denitrifier abundance correlated well with flux measurements; nirS was positively correlated with N2O, and nosZ was positively correlated with N2 (P < 0.03). Denitrifier gene copy concentrations per gram of soil (GCC) varied in response to carbon type amendment (P < 0.01). Denitrifier GCCs were high (ca. 10(7)) and the bac:nirK, bac:nirS, bac:nir (T) , and bac:nosZ ratios were low (ca. 10(-1)/10) in horizon A in all three respective treatments. Glucose-C amendment favored partial denitrification, resulting in higher nir abundance and higher N2O fluxes compared to the control. DOC amendment, by contrast, resulted in relatively higher nosZ abundance and N2 emissions, thus favoring complete denitrification. We also noted soil depth directly affected bacterial, archaeal, and denitrifier abundance, possibly due to changes in soil carbon availability with depth.

Balancing watershed nitrogen budgets: accounting for biogenic gases in streams

Denitrification is critical for removal of reactive nitrogen (Nr) from ecosystems. However, measuring realistic, scalable rates and understanding the role of denitrification and other dissimilatory processes in watershed nitrogen (N) budgets remains a significant challenge in biogeochemistry. In this study, we focused on the stream reach and network scale in three Mid-Atlantic coastal plain watersheds. We applied open channel methods to measure biogenic N2 and N2O gas fluxes derived from both in-stream and terrestrial nitrogen processing. A large portion of biogenic N2 flux through streams (33–100 %, mean = 74 %) was a result of groundwater delivery of biogenic N2 with the remaining portion due to in-stream N2 production. In contrast, N2O was largely produced in-stream, with groundwater delivery contributing on average 12 % of the total biogenic N2O flux. We scaled these measurements across one stream network and compared them to hydrologic Nr export and net anthropogenic N inputs (NANI) to a 4.8 km2 watershed. The N budget revealed that, during the study period, the biogenic N2 flux through streams was comparable to the difference between NANI and hydrologic Nr export (i.e. the “missing” N). This study provides a methodological and conceptual framework for incorporating terrestrial and in-stream derived biogenic N gas fluxes into watershed N budgets and supports the hypothesis that denitrification is the primary fate of NANI that is not exported in streamflow.

Benthic remineralisation rates in shelf and slope sediments of the northern Benguela upwelling margin

The Benguela Upwelling System off Namibia is a region of intensive plankton production. Remineralisation of this biomass frequently causes the formation of an oxygen minimum zone. A part of the organic matter is further deposited on the broad shelf in form of an extensive mudbelt with high TOC concentrations. During February 2011 we retrieved sediment samples from shelf and slope sediment along the Namibian coast to establish fluxes of nutrients, oxygen, and N2 on the basis of pore water concentrations. In mudbelt sediment, fluxes were estimated as high as 8mmol NH4+ m−2d−1 and 0.9mmol PO43− m−2d−1, which is probably attributable to the activity of large sulphur bacteria. Especially phosphate is mobilised from sediment overlain by oxygen deficient bottom water when and where bottom water oxygen concentrations fall below 50µmoll−1. In comparison to nutrient transport by Southern Atlantic Central Water flowing onto the Namibian shelf, benthic nutrient fluxes of the mudbelt contribute less than 5% to the nutrient budget of the shelf.

Seasonal and diurnal dynamics of physicochemical parameters and gas production in vertical water column of a eutrophic pond

Small and shallow water bodies often exhibit high rates of biogeochemical activities, yet have not received much attention. Here we test the hypothesis that there is strong diurnal stratification of physicochemical characteristics in a hyper-eutrophic pond, which would lead to a heterogeneous distribution of gas production via biological processes along vertical profile of water column. Accordingly, we focused on quantifying the seasonal and diurnal dynamics of nutrients (NO3−, NH4+, total dissolved nitrogen TDN, PO43−, total dissolved phosphorus TDP), physicochemical environment factors (DO, pH, light intensity, Chlorophyll-a) and gas emission (N2, N2O, O2, CH4) in the vertical profile of an ultra-eutrophic pond located at the subtropical climate zone, China. The strong and persistent stratification of several parameters at the resolution of centimeters along vertical profile of water was observed in summer and autumn. Interestingly, the surface water and sediment–water interface produced much more gas than the middle layers of water column. The quantity and composition of gas collected from the surface water were greatly affected by O2 production, which followed the diurnal cycle of sunlight intensity. Other biological processes, e.g. nitrification and/or denitrification, may also have contributed to the heterogeneous gas production as high N2 and N2O fluxes were detected in the surface layer and sediment–water interface. CH4 production was mainly from the sediment–water interface, especially in summer when the bottom layer developed very low DO concentrations due to strong stratification. Our results demonstrate that besides the unexpectedly long and strong thermal and physicochemical stratification, the shallow eutrophic pond can undergo significant vertical heterogeneity of gas emission due to the strong diurnal stratification of these physicochemical parameters.