Loss Of NO Research Articles

Reactive nitrogen around the Arabian Peninsula and in the Mediterranean Sea during the 2017 AQABA ship campaign

Abstract. We present shipborne measurements of NOx (≡ NO + NO2) and NOy (≡ NOx+ gas- and particle-phase organic and inorganic oxides of nitrogen) in summer 2017 as part of the expedition “Air Quality and climate change in the Arabian BAsin” (AQABA). The NOx and NOz (≡ NOy-NOx) measurements, made with a thermal dissociation cavity ring-down spectrometer (TD-CRDS), were used to examine the chemical mechanisms involved in the processing of primary NOx emissions and their influence on the NOy budget in chemically distinct marine environments, including the Mediterranean Sea, the Red Sea, and the Arabian Gulf, which were influenced to varying extents by emissions from shipping and oil and gas production. Complementing the TD-CRDS measurements, NO and NO2 data sets from a chemiluminescence detector (CLD) were used in the analysis. In all regions, we find that NOx is strongly connected to ship emissions, both via direct emission of NO and via the formation of HONO and its subsequent photolytic conversion to NO. The role of HONO was assessed by calculating the NOx production rate from its photolysis. Mean NO2 lifetimes were 3.9 h in the Mediterranean Sea, 4.0 h in the Arabian Gulf, and 5.0 h in the Red Sea area. The cumulative loss of NO2 during the night (reaction with O3) was more important than daytime losses (reaction with OH) over the Arabian Gulf (by a factor 2.8) and over the Red Sea (factor 2.9), whereas over the Mediterranean Sea, where OH levels were high, daytime losses dominated (factor 2.5). Regional ozone production efficiencies (OPEs; calculated from the correlation between Ox and NOz, where Ox= O3+ NO2) ranged from 10.5 ± 0.9 to 19.1 ± 1.1. This metric quantifies the relative strength of photochemical O3 production from NOx compared to the competing sequestering into NOz species. The largest values were found over the Arabian Gulf, consistent with high levels of O3 found in that region (10–90 percentiles range: 23–108 ppbv). The fractional contribution of individual NOz species to NOy exhibited a large regional variability, with HNO3 generally the dominant component (on average 33 % of NOy) with significant contributions from organic nitrates (11 %) and particulate nitrates in the PM1 size range (8 %).

Irrigation Scheduling with Soil Gas Diffusivity as a Decision Tool to Mitigate N2O Emissions from a Urine-Affected Pasture

Pastures require year-round access to water and in some locations rely on irrigation during dry periods. Currently, there is a dearth of knowledge about the potential for using irrigation to mitigate N2O emissions. This study aimed to mitigate N2O losses from intensely managed pastures by adjusting irrigation frequency using soil gas diffusivity (Dp/Do) thresholds. Two irrigation regimes were compared; a standard irrigation treatment based on farmer practice (15 mm applied every 3 days) versus an optimised irrigation treatment where irrigation was applied when soil Dp/Do was ≈0.033 (equivalent to 50% of plant available water). Cow urine was applied at a rate of 700 kg N ha−1 to simulate a ruminant urine deposition event. In addition to N2O fluxes, soil moisture content was monitored hourly, Dp/Do was modelled, and pasture dry matter production was measured. Standard irrigation practices resulted in higher (p = 0.09) cumulative N2O emissions than the optimised irrigation treatment. Pasture growth rates under treatments did not differ. Denitrification during re-wetting events (irrigation and rain) contributed to soil N2O emissions. These results warrant further modelling of irrigation management as a mitigation option for N2O emissions from pasture soils, based on Dp/Do thresholds, rainfall, plant water demands and evapotranspiration.

Water management effect on soil oxidation, greenhouse gas emissions, and nitrogen leaching in drained peat soils

AbstractSoil subsidence of peatlands occurs worldwide due to drainage. The Everglades Agricultural Area (EAA), located in South Florida, has been drained for agriculture since 1914, with subsidence resulting in shallow soils in certain areas. The purpose of this study is to determine the impact of water management strategies on soil oxidation and N release as affected by differences in proximity to the bedrock. Oxidation rates (CO2 efflux), as well as CH4 and N2O emissions, were measured in lysimeters filled with shallow and deep peat subjected to four water treatments. Additionally, NO3–N, NH4–N, soluble organic N, and dissolved organic C were measured in leachate obtained from the collected soils. Average annual emissions from constantly drained soils were 298 g CO2–C m–2 yr–1, with most of the oxidation taking place between June and October. Short flood cycles increased annual oxidation rates compared with constantly drained soils, which had the second highest oxidation rate. Constantly flooded soils had the lowest annual oxidation rates, followed by summer flooded soils. Total N lost in leachate was highest for constantly drained soils, with NO3 being the dominant form. The deep soils had higher losses of soluble N and C, whereas NO3 losses from shallow soils were higher. Soil oxidation rates did not differ depending on proximity to the bedrock. We conclude that strategies that avoid short flooding cycles and include crop rotations that allow flooding during summer can reduce oxidation and N losses in leachate from EAA peats.

Type of pulse crop included in a 2-year rotation with wheat affects total N2O loss and intensity

Type of pulse crop included in a 2-year rotation with wheat affects total N2O loss and intensity

How Atmospheric Chemistry and Transport Drive Surface Variability of N2O and CFC‐11

AbstractNitrous oxide (N2O) is a long‐lived greenhouse gas that affects atmospheric chemistry and climate. In this work, we use satellite measurements of N2O, ozone (O3), and temperature from the Aura Microwave Limb Sounder (MLS) instrument to calculate stratospheric loss of N2O, and thus its atmospheric lifetime. Using three chemistry transport models simulating the Aura period 2005–2017, we verify the stratospheric sink using MLS data and follow that loss signal down to the surface and compare with surface observations. Stratospheric loss has a strong seasonal cycle and is further modulated by the Quasi‐Biennial Oscillation (QBO); these cycles are seen equally in both observations and the models. When filtered for interannual variability, the modeled surface signal is QBO‐caused, and it reproduces the observed pattern, highlighting the potential role of the QBO in tropospheric chemistry and composition, as well as in model evaluation. The observed annual surface signal in the northern hemisphere matches well with the models run without emissions, indicating the annual cycle is driven mostly by stratosphere‐troposphere exchange (STE) flux of N2O‐depleted air and not surface N2O emissions. In the southern hemisphere (SH), all three models disagree and thus provide no guidance, except for indicating that modeling annual STE in the SH remains a major model uncertainty. Parallel model simulations of CFCl3, which has greater stratospheric loss that N2O and possibly surreptitious emissions, show that its interannual variations parallel those of N2O, and thus the observed N2O variability can identify the stratospheric component of the observed CFCl3variability.

Microbial potential for denitrification in the hyperarid Atacama Desert soils

The hyperarid soils of the Atacama Desert, Chile, contain the largest known nitrate deposits in the world. They also represent one of the most hostile environments for microbial life anywhere in the terrestrial biosphere. Despite known for its extreme dryness, several heavy rainfall events causing localised flash flooding have struck Atacama Desert core regions during the last five years. It remains unclear, however, whether these soils can support microbial denitrification. To answer this, we sampled soils along a hyperaridity gradient in the Atacama Desert and conducted incubation experiments using a robotized continuous flow system under a He/O2 atmosphere. The impacts of four successive extreme weather events on soil-borne N2O and N2 emissions were investigated, i) water addition, ii) NO3− addition, iii) labile carbon (C) addition, and iv) oxygen depletion. The 15N–N2O site-preference (SP) approach was further used to examine the source of N2O produced. Extremely low N2O fluxes were detected shortly after water and NO3− addition, whereas pronounced N2O and N2 emissions were recorded after labile-C (glucose) amendment in all soils. Under anoxia, N2 emissions increased drastically while N2O emissions decreased concomitantly, indicating the potential for complete denitrification at all sites. Although increasing aridity significantly reduced soil bacterial richness, microbial potential for denitrification and associated gene abundance (i.e., napA, narG, nirS, nirK, cnorB, qnorB and nosZ) was not affected. The N2O15N site preference values based on two end-member model suggested that fungal and bacterial denitrification co-contributed to N2O production in less arid sites, whereas bacterial denitrification dominated with increasing aridity. We conclude that soil denitrification functionality is preserved even with lowered microbial richness in the extreme hyperarid Atacama Desert. Future changes in land-use or extreme climate events therefore have a potential to destabilize the immense reserves of nitrate and induce significant N2O losses in the region.

Nitrogen losses from hard red spring wheat fields in the northern plains

AbstractNitrogen dynamics varies spatially in response to complex interactions among weather, soil, and management practices. Twenty‐six spring wheat (Triticum aestivum L.) fields were studied for N losses, nitrous oxide (N2O) flux and ammonia (NH3) volatilization, grain yield and protein concentration in farmers fields in Minnesota and North Dakota of the Red River Valley during 2018– 2020 growing seasons. Spring wheat grain yield ranged from 2.8 to 9.7 Mg ha–1 with protein concentration ranging between 106 and 180 g kg–1. Average cumulative N2O and NH3 losses were 415 g N2O‐N ha–1 and 1.43 kg NH3 ha–1, respectively (n = 26). Spring wheat grain yield (r = .53, p = .01) had positive association with May rainfall. Grain protein concentration declined with delay in planting (r = –0.55, p = .01). Linear regression indicates a close association of daily N2O flux with soil moisture (r2 = .23, p < .01), soil inorganic N concentration (r2 = .02, p = .04), and days after planting (r2 = .20, p < .01). Soil inorganic N concentration (just after planting) had a positive association with cumulative N2O flux (r = .41, p = .03). This study showed rainfall, soil moisture, and inorganic N concentration play a dominant role in N dynamics across the region.

Mitigation of yield-scaled nitrous oxide emissions and global warming potential in an oilseed rape crop through N source management

Enhanced-efficiency nitrogen (N) fertilizers, such as those containing nitrification or urease inhibitors, can mitigate the carbon (C) footprint linked to the production of bioenergy crops through a reduction in direct nitrous oxide (N2O) emissions and indirect N2O losses. These indirect emissions are derived from ammonia (NH3) volatilization, which also have important environmental and health implications. The evaluation of the global warming potential (GWP) of different N sources using site-specific data of yield and direct and indirect emissions is needed for oilseed rape under rainfed semi-arid conditions, especially when meteorological variability is taken into account. Using urea as a N source, the N2O mitigation efficacy of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) alone or combined with the nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA) was evaluated under field conditions in a rainfed oilseed rape (Brassica napus L.) crop. Two additional N sources from calcium ammonium nitrate (CAN), with and without DMPSA, were included. The GWP of the treatments was estimated considering the emissions from inputs, operations and other direct and indirect emissions of greenhouse gases (GHGs), such as methane (CH4) and the volatilization of NH3. We also measured the abundance of key genes involved in nitrification and denitrification to improve the understanding of N2O emissions on a biochemical basis under the conditions of our study. The results show that due to the intense rainfall after fertilization and a rewetting event, N2O losses from fertilizers without inhibitors were greater than those previously reported under Mediterranean conditions, while NH3 losses were low and not affected by the urease inhibitor. The cumulative N2O emissions (which were greatly influenced by a rewetting peak three months after fertilization) from the urea fertilization were significantly higher than those from CAN. The presence of NBPT significantly reduced N2O losses by an average of 71%, with respect to urea. The use of DMPSA with CAN resulted in an abatement of N2O emissions (by 57%) and a significant increase in oil yield in comparison with CAN alone. All inhibitor-based treatments were effective in abating N2O emissions during the rewetting peak. The abundances of the nitrifier and denitrifier communities, especially ammonia-oxidizing bacteria (AOB), significantly decreased relative to the urea or CAN treatments as inhibitors were applied. Under the conditions of our study, the sustainability of a bioenergy crop such as oilseed rape can be improved by using inhibitors because they mitigated N2O emissions and/or enhanced the oil yield.

Are there memory effects on greenhouse gas emissions (CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub>) following grassland restoration?

Abstract. A 5-year greenhouse gas (GHG) exchange study of the three major gas species (CO2, CH4 and N2O) from an intensively managed permanent grassland in Switzerland is presented. Measurements comprise 2 years (2010 and 2011) of manual static chamber measurements of CH4 and N2O, 5 years of continuous eddy covariance (EC) measurements (CO2–H2O – 2010–2014), and 3 years (2012–2014) of EC measurement of CH4 and N2O. Intensive grassland management included both regular and sporadic management activities. Regular management practices encompassed mowing (three to five cuts per year) with subsequent organic fertilizer amendments and occasional grazing, whereas sporadic management activities comprised grazing or similar activities. The primary objective of our measurements was to compare pre-plowing to post-plowing GHG exchange and to identify potential memory effects of such a substantial disturbance on GHG exchange and carbon (C) and nitrogen (N) gains and losses. In order to include measurements carried out with different observation techniques, we tested two different measurement techniques jointly in 2013, namely the manual static chamber approach and the eddy covariance technique for N2O, to quantify the GHG exchange from the observed grassland site. Our results showed that there were no memory effects on N2O and CH4 emissions after plowing, whereas the CO2 uptake of the site considerably increased when compared to pre-restoration years. In detail, we observed large losses of CO2 and N2O during the year of restoration. In contrast, the grassland acted as a carbon sink under usual management, i.e., the time periods 2010–2011 and 2013–2014. Enhanced emissions and emission peaks of N2O (defined as exceeding background emissions 0.21 ± 0.55 nmol m−2 s−1 (SE = 0.02) for at least 2 sequential days and the 7 d moving average exceeding background emissions) were observed for almost 7 continuous months after restoration as well as following organic fertilizer applications during all years. Net ecosystem exchange of CO2 (NEECO2) showed a common pattern of increased uptake of CO2 in spring and reduced uptake in late fall. NEECO2 dropped to zero and became positive after each harvest event. Methane (CH4) exchange fluctuated around zero during all years. Overall, CH4 exchange was of negligible importance for both the GHG budget and the carbon budget of the site. Our results stress the inclusion of grassland restoration events when providing cumulative sums of C sequestration potential and/or global warming potential (GWP). Consequently, this study further highlights the need for continuous long-term GHG exchange observations as well as for the implementation of our findings into biogeochemical process models to track potential GHG mitigation objectives as well as to predict future GHG emission scenarios reliably.

Greenhouse gas emissions from an irrigated cropping rotation with dairy manure utilization in a semiarid climate

AbstractThis long‐term study was established to increase knowledge of greenhouse gas (GHG) emissions from irrigated cropping systems utilizing dairy manure solids and compost in semiarid southern Idaho. The objective of this field study was to determine the effect of synthetic N fertilizer (urea or SuperU [enhanced‐efficiency synthetic fertilizer]), composted dairy manure, dairy manure (fall or spring applied), and a control (no fertilizer or manure) on nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) emissions over the growing season. The fertilizer and manure treatments were not applied to alfalfa (Medicago sativa L.) (2017) but were applied to corn (Zea mays L.) (2018; except SuperU) and barley (Hordeum vulgare L.) (2019). Cumulative N2O losses over the 3 yr ranged from 2.8 to 5.2 kg N2O‐N ha–1, with the fall and spring manure emitting the greatest amounts of N2O. Emission factors indicated that up to 0.79% of the total N applied was lost as N2O‐N during the growing seasons. Cumulative losses of CO2 and CH4 across the rotation were on average 12,170 kg CO2–C ha–1 and –0.77 kg CH4–C ha–1, respectively, with no significant differences among the treatments. Major N2O pulses were associated with early‐season irrigation events and incorporation of fertilizer and manure, but overall fluxes tended to be the greatest when soil temperatures were higher. Dairy manure and compost applications were also found to cause rapid and significant increases in soil organic carbon (SOC) in the top 30 cm of soil under corn and barley. Despite the fact that manure does cause elevated soil N2O emissions, it should be considered as an alternative to synthetic fertilizer use due to its ability to increase SOC and potentially help reduce the global warming potential.

Differing effects of increasing calcium ammonium nitrate, urea and urea + NBPT fertiliser rates on nitrous oxide emission factors at six temperate grassland sites in Ireland

The present study evaluated the impact of three nitrogen (N) fertiliser formulations, applied at five N rates, on nitrous oxide (N2O) fluxes and annual direct N2O-N emission factors (EF) in temperate grassland. Closed static chambers were used to measure direct N2O fluxes at three geographically dispersed locations in Ireland over a two-year period, generating a total of 90 EFs across the six site-years and treatments. The three fertiliser formulations tested were calcium ammonium nitrate (CAN), urea, and urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) at 100, 200, 300, 400 and 500 kg N ha−1 yr−1. All treatments were applied in five equal split applications ranging from 20 to 100 kg N ha−1 split-1 over the growing season. The N2O-N EFs for CAN ranged from 0.39 − 4.68 with a mean of 1.62 (cv. 81 %), for urea from 0.04 – 1.7 with a mean of 0.46 (cv. 77 %) and for urea + NBPT from 0.18 – 1.7 with a mean of 0.60 (cv. 59 %). A significant positive relationship was found between the N rate and the annual N2O-N EFs in three (CAN), five (urea) and two (urea + NBPT) of six the site-years. For the remainder of the site-years EF was unaffected by N rate. These results indicate that fertiliser N choice and rate can be management factors that enable farmers to alter N2O losses in temperate grassland. Notably, the response of EF to increasing N rate was not consistent across the fertilisers, with the EF from urea being the most sensitive to the increasing N rate, urea + NBPT the least sensitive and CAN being intermediate. The accuracy of national greenhouse gas accounting could be improved by including N fertiliser formulation and its rate of application. Further research is also needed to understand the inconsistency in EF response to N rate across sites.

Evaluation of Tomato-Based Packing Material for Retention of Ammonia, Nitrous Oxide, Carbon Dioxide and Methane in Gas Phase Biofilters: A Laboratory Study

Biofilters are an effective air pollution control technology to break down gaseous contaminants and produce innocuous end products. This laboratory study aimed to evaluate a biofilter media, mainly composed by tomato waste, as packing material to reduce NH3, N2O, CO2 and CH4 losses from stored pig slurry. Three mixtures of packing materials, with and without oxalic acid, were arranged in treatments, namely: mixture of tomato waste, pine bark and agricultural compost; mixture of tomato waste and rice husk; tomato waste only. A control treatment (no biofilter) was also included. The experiments were conducted using a system of laboratory scale biofilters connected to jars filled with pig slurry and under a constant airflow rate. The gas concentrations were measured for 14 days and the physicochemical of the packing materials were assessed. Results showed that biofilter media mixtures had a potential for NH3 retention ranging from 51 to 77% and the addition of oxalic acid to these biofilters increased NH3 retention to 72–79%. Additionally, the biofilter media mixtures with and without oxalic acid showed a potential retention for CH4 (29–69%) but not for N2O, yet with no impact on the global warming potential. It can be concluded that tomato based biofilters had the potential to reduce gaseous emissions from slurry.

Nitrous oxide emissions, ammonia volatilization, and grain-heavy metal levels during the wheat season: Effect of partial organic substitution for chemical fertilizer

Wheat is one of the most important staple foods worldwide, and the organic substitution for chemical fertilizer has been increasingly receiving attention in wheat planting. However, its impact on nitrous oxide (N2O) emissions and ammonia (NH3) volatilization from the wheat fields, as well as on the heavy metal [Nickel (Ni), Copper (Cu), Zinc (Zn), Cadmium (Cd), and Lead (Pb)] contents in the wheat grain is poorly known. A two-year field experiment was performed with two organic manures (sewage sludge compost and pig manure compost) and two substitution ratios (25 % and 50 %) for chemical nitrogen (N) fertilizer during the wheat season. An increasing trend in wheat yield (0.33∼5.9 %) was observed and total N2O emission was significantly decreased by 33∼77 % with partial organic substitution (POS). Therefore, the fertilizer-induced N2O emission factor was highly decreased (0.12∼0.47 % VS. 0.77∼0.92 %). The POS relatively increased NH3 volatilization by 7.9∼17 % (p > 0.05) and as a whole, it played little role in the cumulative N losses of N2O and NH3. The concentrations of Ni, Cu, Zn, Cd, and Pb in wheat grain were 97.4∼335 μg kg−1, 4.51∼5.88 mg kg−1, 28.5∼47.5 mg kg−1, 24.7∼79.6 μg kg−1, and 16.6∼33.4 μg kg−1, respectively. Although POS relatively increased the contents of Cu and Zn, they were all lower than those of the food standard limit for wheat in China. Moreover, the values of metal pollution index (0.39∼0.71) in all treatments were less than one. Our health risk assessment further demonstrated that POS did not pose any health risks. The findings suggest that partial replacement of chemical fertilizer with sewage sludge compost and pig manure compost in the wheat season can significantly decrease N2O emissions while not increasing N losses or compromising yield and its quality.

Large differences in plant nitrogen supply in German and Swedish forests – Implications for management

In European forests, plant N supply varies from regions where N deposition is negligible and a low natural N supply limits production to regions where high N deposition adds to a high natural N supply. Here, we ask if the differences in N supply are too large to make one system of management for wood production, continuous–cover forestry or rotational forestry, optimal across these conditions.We analyzed the C/N ratio in c. 8400 samples of surficial soil layers along a 2400 km long transect through Sweden and Germany to obtain a quantitative description of differences in plant N supply. We discuss the differences in relation to forest management, especially evidence that soil C/N ratios below 25 are associated with higher N supply, risks of leaching of nitrate, and gaseous losses of N2O, whereas ratios above 25 are associated with a tighter N cycle and an N limitation to tree growth.The percent soil with C/N ratios above 25 declines from 91 in N. Norrland in Sweden to 26 in Germany. Simultaneously, mor soils (with a distinct organic layer on top of the mineral soil) decline from 95% to 16%, while mull soils (in which organic matter and mineral particles are mixed) increase from 1% to 40%. However, low C/N ratios also occur in the north, where we find the largest width in C/N ratios from 16 in mull soils to 36 in mor soils, which compares with a variation in Germany from 17 to 27. Soils under conifers generally have higher C/N ratios than soils under broadleaves, but our survey data cannot support that the trees are the sole cause of this pattern. Very low C/N ratios occur in conifer–dominated forests in the north.The high incidence (74%) of C/N ratios below 25 indicates that forest management in Germany should use methods, which minimize the risk of N losses. Continuous–cover forestry may fulfill that objective. In the north with 9% of the soils below this threshold, risks of N losses are small. There, rotational forestry involving clear–felling alleviates the competition for soil N from larger trees allowing successful regeneration of tree seedlings. From the perspective of interactions between plant N supply and management of forests for wood production, no single management system seems optimal along this large gradient. We propose that research on forest management systems should address the importance of N supply.

Nitrogen dynamics under corn fields in the Red River Valley of North Dakota

AbstractExcessive applications of fertilizer N in corn (Zea mays L.) has profound environmental and economic consequences. Corn grain yield, N losses, N2O denitrification, NH3 volatilization, and soil water NO3 concentration at 60‐cm depth were collected for eight corn fields in the Red River Valley of North Dakota. Relationships of soil and management factors with cumulative N2O and NH3 losses were studied. Grain yield varied from 3.58 to 5.66 Mg ha–1. Cumulative NH3 and N2O losses were ranged between 1.0–2.4 kg ha–1 and 24.7–150 g N2O‐N ha–1, respectively. Soil organic matter (SOM; r = .94, p = .001), clay (r = .94, p = .001), and initial inorganic N (0–60 cm) content (r = .83, p = .01) had a positive relationship with cumulative N2O losses. Cumulative NH3 had a negative relationship with soil pH (r2 = .63, p = .01). Results indicate that soil properties such as, SOM, clay concentration, and initial profile N had a control over N losses.

International comparison CCQM-K74.2018: Nitrogen dioxide, 10 μmol mol-1

Main textThe CCQM-K74.2018 comparison evaluated the level of compatibility of nitrogen dioxide (NO2) in nitrogen standards at nominal amount fractions of 10 μmol/mol provided by National Metrology Institutes. Fourteen laboratories participated in the comparison, submitting two standards each, with comparative measurements and comparison coordination undertaken by the BIPM. Measurements at the BIPM were performed from July 2018 to May 2019.The key comparison reference values (KCRV) for each submitted standard were established at three different times during the measurement period at the BIPM and based on the value and uncertainty provided by the BIPM's NO2 permeation system reference facility, operating with a standard measurement uncertainty of 0.04 μmol/mol, as demonstrated in the original CCQM-K74 comparison in 2009. Participants were required to report the values and measurement uncertainties assigned to their submitted standards before and after the measurement period at the BIPM, with all but one reported standard uncertainty ranging from 0.017 μmol/mol to 0.30 μmol/mol.The value of the participants' standards at the time of establishment of the KCRVs was determined from participants measured values. This was achieved by averaging all values when the measurements were consistent with each other in the two measurement periods. Alternatively, for standards that exhibited a decay in value, the values as a function of time were calculated using a model in which predicted the mid-point between the average value of participants' second series of values, and a linear decay from their first set of values. Relative standard uncertainties for participants' standards at the KCRV measurement times ranged from 0.03 μmol/mol to 0.25 μmol/mol.Values of standards from six of the fourteen participants agreed with the KCRV within their calculated uncertainties, with the remaining eight exhibiting biases ranging from 0.17 μmol/mol to 0.99 μmol/mol. The comparison has confirmed the challenge of controlling HNO3 levels in NO2 in N2 standards and accurately correcting for their presence as a function of time.The overall level of compatibility demonstrated in the comparison is not an improvement over that in the original comparison, the CCQM-K74 in 2009. The comparison protocol adopted for CCQM-K74.2018, was more challenging than the previous edition. In 2009, the travelling standards were prepared by one participant using the same cylinder type, surface treatment and preparation procedure. These standards were characterized for stability and with reference values provided by the BIPM. They exhibited a small decay rate that was calculated to have been no more than 0.1 nmol mol-1 per day loss of NO2, and accounted for by the addition of an uncertainty to the reference value. For CCQM-K74.2018, the standards were prepared by individual participants, with decay rates in different cylinders ranging from under 0.1 nmol mol-1 per day loss of NO2 to over 1 nmol mol-1 per day, and therefore on occasion an order of magnitude greater than in the original comparison. Whilst more challenging, the CCQM-K74.2018 study has provided an extended evaluation of the level of compatibility of NO2 in N2 standards.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Winter N2O accumulation and emission in sub-boreal grassland soil depend on clover proportion and soil pH

Inclusion of legume species into grass leys reduces nitrogen (N) fertilizer need but increases the risk of freeze-thaw induced N2O emissions. We investigated how liming and presence of clover affect N2O accumulation under snowpack and its emission during freeze-thaw cycles in autumn and spring under sub-boreal conditions. A field experiment was performed in southern Norway in limed and control plots containing grasses only (fertilized with 270 kg N ha−1 yr−1), a grass-red clover mixture (fertilized with 140 kg N ha−1 yr−1) and unfertilized pure red clover. Soil air samples were collected at 8, 24, and 40 cm depths and analyzed for gas concentrations including N2O, and N2O fluxes measured by a fast-chamber robot. Red clover produced more N2O than the grass-only plots during freeze-thaw cycles in autumn and spring and accumulated more N2O under snow cover (emissions were not measured during this period). Contrary to expectations, limed red clover plots emitted more N2O than control plots during freeze-thaw cycles. Liming reduced subnivean N2O accumulation in grass-only but not in grass-clover or pure clover plots. After spring fertilization, grass-only plots had larger N2O emissions than red clover plots. Our data suggest that winter-sensitive, N-rich clover biomass fuels decomposition and nitrification, thereby increasing NO3 - and depleting O2, resulting in increased N2O emissions from denitrification. Although liming of pure clover leys exacerbated the risk of high N2O emissions during freeze-thaw, this effect was not observed in grass-clover mixtures. Interestingly, grass-clover mixtures also emitted less N2O than expected from their proportions and the emissions recorded in pure grass and clover stands. This warrants further studies into off-season functional diversity effects on N cycling and N2O loss in temperate and boreal forage production.

Influence of organic particle addition on nitrification rates and ammonium oxidiser abundances in Baltic seawater

Nitrification is a key microbial process in the nitrogen cycle of the ocean that largely determines the pool sizes of nitrite, nitrate and ammonium but hardly affects the total pool of inorganic nitrogen (except through N2O losses to the atmosphere). This study explored the regulation of nitrification and its dependency on particle and ammonium concentrations. Incubation experiments were conducted in which particles of different origin were added to Baltic seawater, followed by 15NH4+-labelling and droplet digital PCR. Both nitrification rates according to particle size (>3 µm and 0.22-3 µm) and archaeal and betaproteobacterial ammonia monooxygenase α-subunit gene (amoA) abundances were determined in order to assess the impact of high particle concentrations found in coastal areas of the Baltic Sea. The data show that the addition of particulate organic matter enhanced the nitrification rates in bulk (~805 nmol l-1 d-1) but not in the <3 µm fraction (<100 nmol l-1 d-1), indicative of a strong influence of the added particles on nitrification rates. The regulation of nitrification rates by the substrate ammonium was suggested by the inverse correlation between these 2 variables. Measurements of amoA gene abundances showed the dominance of ammonium-oxidising bacteria over ammonium-oxidising archaea during most of the 41 d experiment. Abundances of amoA were highest in the particle-associated fraction of the bacteria (≤6.9 × 106 amoA copies l-1). These findings demonstrate the importance of particle-associated ammonium-oxidising bacteria in the nitrification processes of coastal and estuarine waters containing high particle densities.

Reduction in Nitrogen Exports from Stormflow after Conversion of a Dry Detention Basin to a Stormwater Wetland

Stormwater control measures such as dry detention basins and wetlands are often used to reduce the discharge of urban runoff and nutrients to streams, but differences in nutrient treatment may vary between practices. The goal of this study was to compare the nitrogen treatment efficiency of a dry detention basin before and after it was converted into a stormwater wetland. Inflow and outflow from a detention basin in Greenville, North Carolina was sampled during 13 storms and the stormwater wetland was sampled during 10 storms. Total dissolved nitrogen (TDN), NO3−, NH4+, chloride, dissolved organic carbon (DOC), and physicochemical properties were evaluated. Inflow and outflow from the detention basin had identical median concentrations of TDN (0.47 mg L−1). The median TDN concentration for wetland outflow (0.18 mg L−1) was 63% lower relative to inflow (0.49 mg L−1). The hydraulic residence time of stormwater in the wetland was more than 10 times greater relative to the dry basin. There was a significant (p < 0.001) reduction in dissolved oxygen and oxidation reduction potential and an increase in median DOC concentrations in wetland outflow relative to inflow. Most of the reduction in TDN within the wetland was attributed to loss of NO3− (80% reduction), possibly due to denitrification. Conversion of dry detention basins to wetlands may provide significant benefits with regards to reducing TDN transport associated with urban runoff.

Evaluation of nitrogen bank, a soil nitrogen management strategy for sustainably closing wheat yield gaps

Yield gaps of ∼50 % of water limited yield potential occur in wheat globally. Nitrogen (N) deficiency frequently contributes to the gap, but it is difficult to predict an appropriate rate of N to apply because of uncertainty about the forthcoming season and hence water limited potential yield and crop N requirements. We investigated closing the water limited yield gap attributable to N supply by maintaining a “bank” of soil mineral N that is nonlimiting for wheat yields in most seasons. The yield, loss of NO3−-N in leaching and denitrification, and gross margins of wheat crops were simulated at four Australian locations with contrasting soil types and potential yields, in response to N inputs calculated by reference to N banks ranging from 50 to 400 kg N ha-1, the Yield Prophet® decision support tool and the national average N rate. The N bank providing best yields at each location was close in amount to the long-term average rate calculated with Yield Prophet®. Compared to the national average rate of N fertiliser application (45 kg N ha-1), the highest-yielding N bank treatment on average increased the amount of N applied by 3–234 kg N ha-1, the fraction of water limited potential yield by 0.0 to 0.61, and gross margins by AUD 0–1,026 ha-1. Combined losses of N to the environment in NO3−leaching and through denitrification in these strategies also increased but by a disproportionately small amount compared to the increase in the amount applied (-3 to 8.5 kg N ha-1). These losses were relatively insensitive to the amount of N applied at three of the four locations; at the fourth location large episodic losses could occur from sandy soil despite low rainfall when the N bank exceeded water limited potential yield. The N bank approach to determining fertiliser rate has potential to sustainably reduce the yield gap where the target amount is matched to average site-specific yield potential.