Surface N2O Research Articles

Arctic Tundra Plant Dieback Can Alter Surface N2O Fluxes and Interact With Summer Warming to Increase Soil Nitrogen Retention.

In recent years, the arctic tundra has been subject to more frequent stochastic biotic or extreme weather events (causing plant dieback) and warmer summer air temperatures. However, the combined effects of these perturbations on the tundra ecosystem remain uninvestigated. We experimentally simulated plant dieback by cutting vegetation and increased summer air temperatures (ca. +2°C) by using open-top chambers (OTCs) in an arctic heath tundra, West Greenland. We quantified surface greenhouse gas fluxes, measured soil gross N transformation rates, and investigated all ecosystem compartments (plants, soils, microbial biomass) to utilize or retain nitrogen (N) upon application of stable N-15 isotope tracer. Measurements from three growing seasons showed an immediate increase in surface CH4 and N2O uptake after the plant dieback. With time, surface N2O fluxes alternated between emission and uptake, and rates in both directions were occasionally affected, which was primarily driven by soil temperatures and soil moisture conditions. Four years after plant dieback, deciduous shrubs recovered their biomass but retained significantly lower amounts of 15N, suggesting the reduced capacity of deciduous shrubs to utilize and retain N. Among four plant functional groups, summer warming only increased the biomass of deciduous shrubs and their 15N retention, while following plant dieback deciduous shrubs showed no response to warming. This suggests that deciduous shrubs may not always benefit from climate warming over other functional groups when considering plant dieback events. Soil gross N mineralization (~ -50%) and nitrification rates (~ -70%) significantly decreased under both ambient and warmed conditions, while only under warmed conditions immobilization of NO3 - significantly increased (~ +1900%). This explains that plant dieback enhanced N retention in microbial biomass and thus bulk soils under warmed conditions. This study underscores the need to consider plant dieback events alongside summer warming to better predict future ecosystem-climate feedback.

Integrating the Hydrogenation of Nitrobenzene over Cu/Mgo Catalysts with the 1, 4-Butanediol Dehydrogenation Reaction

Using a fixed bed reactor and Cu/MgO catalysts, the hydrogenation of nitrobenzene to aniline has been combined with the dehydrogenation of 1,4-butanediol to c-butyrolactone reaction in the vapour phase. These Cu/MgO catalysts were made using the co-precipitation method, and they were examined using techniques for BET surface area, XRD, TPR, and N2O pulse-chemisorption. It has been described how the hydrogenation of nitrobenzene over Cu/MgO catalysts coupled with the 1,4-butanediol dehydrogenation reaction has a positive effect.

Nitrous oxide emissions and soil profile responses to manure substitution in the North China Plain drylands

Substituting synthetic fertilizers with manures in agriculture enhances soil properties and crop yield. However, the impact on nitrous oxide (N2O) emissions, especially from the soil profile, remains poorly understood. This study examined emissions from 2017 to 2019 on a well-established (>10-year) maize field site in the North China Plain. Three treatments were compared: 100 % synthetic nitrogen (NPK), 50 % synthetic fertilizer N + 50 % manure N substitution (50%MNS), and 100 % manure N substitution (100%MNS). N2O emissions were monitored for three years, and in 2019, N2O concentrations at 20 cm and 40 cm soil depths were analyzed in relation to surface N2O fluxes and environmental factors. The results showed manure substitution resulted in about 13.8 %–25.2 % (50%MNS) and 40.3 %–72.2 % (100%MNS) reduction in N2O emissions over the 3-year period compared with the NPK treatment. Throughout the maize growing season, the top-dressing accompanied by rainfall was responsible for the N2O emissions. The difference in N2O concentrations between all the treatments at 20 cm depth was insignificant, but at 40 cm depth the N2O concentrations were significantly higher for the 50%MNS treatment than the other treatments. The N2O fluxes and N2O concentration were not synchronized especially in NPK. The decoupled relationship between the N2O fluxes and the N2O concentration in the soil profile depth suggested the contribution of N2O produced in the soil profile to the surface N2O fluxes is limited. This study highlights that manure substitution is an efficient measure to reduce N2O emissions.

Distributions of N2O concentration and sea-to-air flux in the western Tropical North Pacific: Influences of eddies and typhoons

Marine systems are active regions for producing and emitting nitrous oxide (N2O), a potent greenhouse gas. In October 2018, samples were collected in the western tropical North Pacific (WTNP) to study the distributions, emissions, and influencing factors of N2O. The N2O concentrations in surface seawater showed little variation, ranging from 6.2 to 7.9 nmol L−1 (corresponding to N2O saturation range of 104–125%), with an average of 6.7 ± 0.6 nmol L−1. The vertical N2O distribution is a mirror image of dissolved oxygen (DO), increasing with depth from the surface to a maximum in the vicinity of the DO minimum. The sea-to-air fluxes of N2O ranged from 0.1 to 7.5 μmol m−2 d−1, with an average of 1.7 ± 2.2 μmol m−2 d−1, indicating that the WTNP was a net source of N2O to the atmosphere. Nitrification is the main process for N2O production. The presence of the Mindanao Eddy noticeably changes the vertical profiles of N2O in the water column, allowing the N2O-rich deep water to reach the subsurface layers, but has little effect on the surface N2O concentration. After Typhoon “Yutu” passed through, N2O concentrations in surface and subsurface water increased dramatically. The surface concentration increased by about 20%, and the sea-to-air flux of N2O increased by about 56%. Despite the short duration of the typhoon, its effect on N2O distribution and sea-to-air flux was more pronounced than that of eddies.

Dissolved Nitrous Oxide and Hydroxylamine in the Northern South China Sea in Summer

AbstractNitrous oxide (N2O) is a potent greenhouse gas and a strong ozone‐depleting substance. Ocean is an important natural source of atmospheric N2O, which has attracted increasing attention recently as to understand the biogeochemical cycles of nitrogen in marine systems. Nitrification is one of the major N2O production pathways during which N2O is produced as a side product and hydroxylamine (NH2OH) is produced as an obligate intermediate. However, few studies have reported the NH2OH variability in natural seawater and its relationship between N2O remains ambiguous. In summer 2022, spatial distribution and influencing factors of N2O and NH2OH in the northern South China Sea (SCS) were investigated. Dissolved N2O varied from 5.79 to 34.80 nmol L−1 in water column and generally increased with water depth above 800 m. N2O was correlated with dissolved oxygen and nitrate, inferring that N2O was mainly produced via nitrification. NH2OH varied from below detection limit to 3.91 nmol L−1. The adoption of multiple standard additions can largely reduce the bias in NH2OH measurement. Structural equation modeling showed that NH2OH has a potential effect favoring excessive N2O. Surface N2O saturation ranged between 102% and 278%, indicating that the northern SCS was a net source of atmospheric N2O in summer.

Influences of shallow groundwater depth on N2O diffusion along the soil profile of summer maize fields in North China Plain

The emissions of nitrous oxide (N2O) from agricultural fields are a significant contribution to global warming. Understanding the mechanisms of N2O emissions from agricultural fields is essential for the development of N2O emission mitigation strategies. Currently, there are extensive studies on N2O emissions on the surface of agricultural soils, while studies on N2O fluxes at the interface between the saturated and unsaturated zones (ISU) are limited. Uncertainties exist regarding N2O emissions from the soil-shallow groundwater systems in agricultural fields. In this study, a three-year lysimeter experiment (2019–2020, 2022) was conducted to simulate the soil-shallow groundwater systems under four controlled shallow groundwater depth (SGD) (i.e., SGD = 40, 70, 110, and 150 cm) conditions in North China Plain (NCP). Weekly continuous monitoring of N2O emissions from soil surface, N2O concentration in the shallow groundwater and the upper 10 cm of pores at the ISU, and nitrogen cycling-related parameters in the soil and groundwater was conducted. The results showed that soil surface N2O emissions increased with decreased shallow groundwater depth, and the highest emissions of 96.44 kg ha−1 and 104.32 kg ha−1 were observed at G2 (SGD = 40 cm) in 2020 and 2022. During the observation period of one maize growing season, shallow groundwater acted as a sink for the unsaturated zone when the groundwater depth was 40 cm, 70 cm, and 110 cm. However, when SGD was 150 cm, shallow groundwater became a source for the unsaturated zone. After fertilization, the groundwater in all treatment plots behaved as a sink for the unsaturated zone, and the diffusion intensity decreased with increasing SGD. The results would provide a theoretical basis for cropland water management to reduce N2O emissions.

Impacts of vertical mixing and ice-melt on N2O and CH4 concentrations in the Canadian Arctic Ocean

The sources and sinks of methane (CH4) and nitrous oxide (N2O) in the Arctic Ocean are subject to significant uncertainty, due to a combination of high spatial and temporal variability, and limited observations over relevant scales. To address this, an underway ship-board system was developed to simultaneously and continuously measure surface water CH4 and N2O concentrations in the Canadian Arctic Ocean, providing high-resolution data over a range of hydrographic regimes. Across our survey region during late summer, 2021, CH4 and N2O saturation ranged from 94% to 231% and 96%–120%, respectively. Localized CH4 and N2O maxima were observed along the cruise track, including high concentrations over the continental shelf of eastern Baffin Island, and in some waters influenced by sea ice cover or recent glacier retreat. We use our high-resolution data to examine the relationship between N2O and CH4 concentrations and a variety of oceanographic variables, demonstrating both positive correlations with salinity, which reflects the likely influence of vertical mixing, and negative correlations with salinity, indicative of ice-melt and freshwater inputs. These results build upon previous discrete CH4 and N2O observations in the Canadian Arctic, providing new information on the fine-scale distribution of these gases, and the potential underlying biogeochemical factors driving their variability. Results from our work have implications for understanding the biogeochemical cycling of Arctic Ocean CH4 and N2O under a rapidly warming climate.

Soil pore structure mediates the effects of soil oxygen on the dynamics of greenhouse gases during wetting–drying phases

The timing and magnitude of greenhouse gas (GHG) production depend strongly on soil oxygen (O2) availability, and the soil pore geometry characteristics largely regulate O2 and moisture conditions relating to GHG biochemical processes. However, the interactions between O2 dynamics and the concentration and flux of GHGs during the soil moisture transitions under various soil pore conditions have not yet been clarified. In this study, a soil-column experiment was conducted under wetting–drying phases using three pore-structure treatments, FINE, MEDIUM, and COARSE, with 0 %, 30 %, and 50 % coarse quartz sand applied to soil, respectively. The concentrations of soil gases (O2, nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4)) were monitored at a depth of 15 cm hourly, and their surface fluxes were measured daily. Soil porosity, pore size distribution, and pore connectivity were quantified using X-ray computed microtomography. The soil O2 concentrations were found to decline sharply as soil moisture increased to the water holding capacities of 0.46, 0.41, and 0.32 cm cm−3 in the FINE, MEDIUM, and COARSE, respectively. The dynamic patterns of the O2 concentrations varied across the soil pore structures, decreasing to anaerobic in FINE (<0.01 %) and MEDIUM (0.02 %), and to hypoxic (4.42 %) in COARSE. Correspondingly, the soil N2O concentration was the highest in FINE (101 μL L−1) and the lowest in COARSE (10 μL L−1), whereas the highest surface N2O flux was observed in MEDIUM (131 μg N m−2 h−1). As soil CO2 concentrations declined, CO2 fluxes increased from FINE to MEDIUM to COARSE. Most pores of FINE, MEDIUM, and COARSE were 15–80 μm, 85–100 μm, and 105–125 μm, respectively, in terms of diameter. The X-ray CT visible (>15 μm) porosity in FINE, MEDIUM and COARSE were 0.09, 0.17, and 0.28 mm3 mm−3, respectively. The corresponding Euler-Poincaré numbers were 180,280, 76,705, and −10,604, respectively, indicating higher connectivity in COARSE than in MEDIUM or FINE. In soil dominated by small air-filled porosity which limits gas diffusion and result in low soil O2 concentration, N2O concentration was increased and CO2 flux was inhibited as the moisture content increased. The turning point in the sharp decrease in O2 concentration was found to correspond with a moisture content, and a pore diameter of 95–110 μm was associated with the critical turning point between holding water and O2 depletion in soil. These findings suggest that O2-regulated biochemical processes are key to the production and flux of GHGs, which in turn are dependent on the soil pore structure and a coupling relationship between N2O and CO2. Improved understanding of the intense effect of soil physical properties provided an empirical foundation for the future development of mechanistic prediction models for how pore-space scale processes with high temporal (hourly) resolution up to GHGs fluxes at larger spatial and temporal scales.

Marine N2O cycling from high spatial resolution concentration, stable isotopic and isotopomer measurements along a meridional transect in the eastern Pacific Ocean

Nitrous oxide (N2O) is a potent greenhouse gas and ozone depleting substance, with the ocean accounting for about one third of global emissions. In marine environments, a significant amount of N2O is produced by biological processes in Oxygen Deficient Zones (ODZs). While recent technological advances are making surface N2O concentration more available, high temporal and spatial resolution water-column N2O concentration data are relatively scarce, limiting global N2O ocean models’ predictive capability. We present a N2O concentration, stable isotopic composition and isotopomer dataset of unprecedently large spatial coverage and depth resolution in the broader Pacific, crossing both the eastern tropical South and North Pacific Ocean ODZs collected as part of the GO-SHIP P18 repeat hydrography program in 2016/2017. We complement these data with dissolved gases (nitrogen, oxygen, argon) and nitrate isotope data to investigate the pathways controlling N2O production in relation to apparent oxygen utilization and fixed nitrogen loss. N2O yield significantly increased under low oxygen conditions near the ODZs. Keeling plot analysis revealed different N2O sources above the ODZs under different oxygen regimes. Our stable isotopic data and relationships between the N2O added by microbial processes (ΔN2O) and dissolved inorganic nitrogen (DIN) deficit confirm increased N2O production by denitrification under low oxygen conditions near the oxycline where the largest N2O accumulations were observed. The slope for δ18O-N2O versus site preference (SP, the difference between the central (α) and outer (β) N atoms in the linear N2O molecule) in the eastern tropical North Pacific ODZ was lower than expected for pure N2O reduction, likely because of the observed decrease in δ15Nβ. This trend is consistent with prior ODZ studies and attributed to concurrent production of N2O from nitrite with a low δ15N or denitrification with a SP &amp;gt;0‰. We estimated apparent isotope effects for N2O consumption in the ETNP ODZ of 3.6‰ for 15Nbulk, 9.4‰ for 15Nα, -2.3‰ for 15Nβ, 12.0‰ for 18O, and 11.7‰ for SP. These values were generally within ranges previously reported for previous laboratory and field experiments.

Deepened snow in combination with summer warming increases growing season nitrous oxide emissions in dry tundra, but not in wet tundra

Impacts of increased winter snowfall and warmer summer air temperatures on nitrous oxide (N2O) dynamics in arctic tundra are uncertain. Here we evaluate surface N2O dynamics in both wet and dry tundra in West Greenland, subjected to field manipulations with deepened winter snow and summer warming. The potential denitrification activity (PDA) and potential net N2O production (N2Onet) were measured to assess denitrification and N2O consumption potential. The surface N2O fluxes averaged 0.49 ± 0.42 and 2.6 ± 0.84 μg N2O–N m−2 h−1, and total emissions were 212 ± 151 and 114 ± 63 g N2O–N scaled to the entire study area of 0.15 km2, at the dry and wet tundra, respectively. The experimental summer warming, and in combination with deepened snow, significantly increased N2O emissions at the dry tundra, but not at the wet tundra. The deepened snow increased winter soil temperatures and growing season soil N availability (DON, NH4+-N or NO3−-N), but no main effect of deepened snow on N2O fluxes was found at either tundra ecosystem. The mean PDA was 5- and 121-fold higher than the N2Onet at the dry and wet tundra, respectively, suggesting that N2O might be reduced and emitted as dinitrogen (N2). Overall, this study reveals modest but evident surface N2O fluxes from tundra ecosystems in Western Greenland, and suggests that projected increases in winter precipitation and summer air temperatures may increase N2O emissions, particularly at the dry tundra dominating in this region.

Effects of limed manure digestate application in sandy soil on plant nitrogen availability and soil N2O emissions

Anaerobically-digested manure is frequently applied to agricultural soil to enhance plant growth and reduce the need for chemical fertilizers. This practice also stimulates microbial nitrogen transformations and often results in N2O emissions. A single mesophilic anaerobic digestion is insufficient for pathogen removal or inactivation and therefore, a post treatment is required for its stabilization and hygienization. Here, we examined the effects of limed manure-digestate as a nitrogen source for plant growth and on N2O emission compared with compost. A plant growth experiment was conducted in a sandy soil and N2O emissions were monitored throughout the experiment. Plants were irrigated with freshwater or liquid-N fertilizer. The combination of compost application and liquid-N fertilizer resulted in surface N2O fluxes over 0.7 ​mg ​m−2 d−1, which were correlated with ammonium concentration in the soil. The presence of N2O in the rhizosphere was only detected in compost-amended soil 2–10 days after plantation. A significantly-lower surface N2O flux of 0.4 ​mg ​m−2 d−1 was recorded with application of limed-digestate, probably due to its effects on nitrogen-transforming microorganisms. Both compost and limed-digestate enhanced plant growth, with a more distinct effect in the freshwater treatment. Our observations demonstrate that limed-digestate can be an efficient substitute for compost as it effectively supports plant growth with substantially-lower N2O emissions.

3‐D Atmospheric Modeling of the Global Budget of N2O and Its Isotopologues for 1980–2019: The Impact of Anthropogenic Emissions

AbstractNitrous oxide (N2O) is the third most important anthropogenic greenhouse gas and a major ozone‐depleting substance. Its main sources include anthropogenic activities (mostly agriculture) and natural emissions from ocean and soils. However, emission estimates for individual sources are highly variable due to uncertainties in N2O lifetime estimates and partitioning among sources. We derive annual global N2O emissions for 1990–2019 using NOAA Global Monitoring Laboratory (GML) surface N2O observations and the N2O lifetime calculated in the NASA GEOS‐5 chemistry climate model. The inferred global mean N2O emissions has gradually increased from ∼15.8 TgN/yr in the early 1990s to ∼17.8 TgN/yr in the 2010s. This implies that anthropogenic N2O emissions have grown rapidly from ∼6.7 TgN/yr in the 1990s to about ∼8.7 TgN/yr in the 2010s, a ∼30% increase. With specially designed N2O isotopic tracers in 3‐D GEOSCCM, we estimate that, on global average, stratospheric enrichment contributes about +7.7‰/yr, +7.6‰/yr, +8.0‰/yr to tropospheric δ15Nα, δ15Nβ, and δ18O budget, respectively. To balance the global mean isotopic signature for pre‐industrial terrestrial sources of δ15Nα ∼ 6.7‰, δ15Nβ ∼ −12.6‰, δ18O ∼ 35.4‰, our 3‐dimensional isotopic budget simulation using the GEOSCCM suggests global mean anthropogenic isotopic signatures in the recent decades are δ15Nα ∼ −18‰, δ15Nβ ∼ −20‰, δ18O ∼ 19‰. These anthropogenic isotopic estimates are significantly lighter than results from one‐box atmospheric model‐based estimates with the largest difference seen for δ15Nβ. More surface isotopic measurements are needed to better quantify the N2O isotopic signatures.

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

Nitrous oxide (N2O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N2O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N2O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N2O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N2O (ΔN2O) was positively correlated with the apparent oxygen utilization (AOU) (R2 = 0.48), suggesting that surface N2O may be produced by nitrification. In the intermediate layer, ΔN2O was positively correlated with AOU and NO3− (R2 = 0.92 and R2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N*) (R2 = 0.60). Hence, the highest ΔN2O value in the oxygen minimum layer suggested N2O production through nitrification and potential denitrification (up to 51% and 25% of measured N2O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN2O and AOU (R2 = 0.92), suggesting that the N2O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N2O (mean air−sea flux 2.0 ± 0.3 μmol m-2 d-1), and that nitrification and potential denitrification may be the primary mechanisms of N2O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N2O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N2O dynamics and environmental changes in the STWNPO and the global ocean.

Nitrous oxide in the central Bay of Bengal during the summer monsoon

Although oceans contribute significantly to the global nitrous oxide (N2O) emissions, there are less than desired data related to N2O concentrations and fluxes in the global oceans and, in particular, the Indian Ocean. Within the Indian Ocean, the quantification and understanding of N2O dynamics in the open Bay of Bengal (BOB) is lacking, specifically during the southwest monsoon. The present study focused on quantifying the dissolved N2O concentrations in the water column along with water–atmosphere fluxes of N2O from the central Bay of Bengal during peak monsoon. For this purpose, a study was conducted during July–August 2018, where dissolved N2O concentrations were measured from the surface to 2000 m depth at eight stations. The concentrations of dissolved N2O in the surface waters ranged from 4.93 to 6.33 nM with saturation levels varying from 81 to 108%. Dissolved N2O showed a small variation with depth and appeared to be undersaturated at all depths except at the surface and at the depth of deep chlorophyll maximum. Flux calculations revealed that out of eight, four stations acted as a minor source, whereas three were minor sink for N2O. The low concentrations of surface N2O and small vertical gradients of N2O in the water column suggested either low production of N2O or significant consumption in the water column. However, the lack of comprehensive complementary data limits us to conclude the exact interplay of the mechanism leading to such observation. Comparison of the present data with available studies in the coastal BOB suggested significantly lower concentrations and fluxes in the open BOB. However, the range reported during the present study is closer to the only study carried out in the central BOB around twenty years ago; suggesting that N2O dynamics in the BOB have not changed drastically in recent times.

Evaluation of the DNDCv.CAN model for simulating greenhouse gas emissions under crop rotations that include winter cover crops

Context Process-based modelling studies can help inform conservation practices for mitigating soil surface CO2 and N2O fluxes. Aims We evaluated the ability of the DeNitrification-DeComposition (DNDC) model to predict field-measured soil surface CO2 and N2O emissions in crop rotations managed with cover crop (CC) and without cover crop (NC) under the 27-year no-till field experiment in South Dakota, USA. Methods Emissions were measured in a 2-year corn–soybean and a 4-year corn–soybean–oat–winter wheat rotation. The model was calibrated with 2-year NC treatment and evaluated against three treatments (2-year CC, 4-year NC and 4-year CC) during the growing season of corn (2017) and soybean (2018). Key results Across all treatments, the model simulated soil temperature (MBE, −0.73–0.29°C; RMSE, 1.47–4.03°C; NSE, 0.54–0.90; d, 0.89–0.98; R2, 0.64–0.93) and moisture [water-filled porosity (wfps)] (MBE, 0.03–0.06 wfps; RMSE, 0.09–40.13 wfps; NSE, −0.24–0.49; d, 0.78–0.87; R2, 0.45–0.69) that agreed well with field measurements. Predicted daily soil CO2 fluxes (kg C ha−1) provided ‘good’ agreement with MBE (range −0.58−4.67), RMSE (range 2.10−7.36), d (range 0.68–0.93), NSE (range −0.92–0.79), and R2 (range 0.49–0.85). Statistics showed ‘poor’ agreement between the simulated and measured daily N2O emissions because peak emissions events in the measured data were less than predicted. Cumulative CO2 and N2O emissions and crop yields were well estimated by the model. Conclusions DNDCv.CAN simulated the impacts of diverse crop rotations and cover crops on soil moisture, temperature and greenhouse gas emissions in the humid south-east of USA. Implications Nitrogen transformation routines and effect of rainfall interception on soil water content need further investigation to address the variations in daily N2O emissions.

Dissolved Nitrous Oxide and Hydroxylamine in the South Yellow Sea and the East China Sea During Early Spring: Distribution, Production, and Emissions

Coastal marine systems are active regions for the production and emission of nitrous oxide (N2O), a potent greenhouse gas. Due to the inherently high variability in different coastal biogeochemical cycles, the factors and mechanisms regulating coastal N2O cycling remain poorly understood. Hydroxylamine (NH2OH), a potential precursor of N2O, has received less attention than other compounds in the coastal areas. Here, we present the spatial distribution of N2O and the first reported NH2OH distribution in the South Yellow Sea (SYS) and the East China Sea (ECS) between March and April 2017. The surface N2O concentrations in the SYS and the ECS varied from 5.9 to 11.3 nmol L–1 (average of 8.4 ± 1.4 nmol L–1) and were characterized by offshore and north–south decreasing gradients. NH2OH showed patchy characteristics and was highly variable, fluctuating between undetectable to 16.4 nmol L–1. We found no apparent covariation between N2O and NH2OH, suggesting the NH2OH pathway, i.e., nitrification (ammonium oxidation), was not the only process affecting N2O production here. The high NH2OH values co-occurred with the greatest chlorophyll-a and oxygen levels in the nearshore region, along with the relationships between NO2–, NO3–, and NH2OH, indicating that a “fresh” nitrifying system, favoring the production and accumulation of NH2OH, was established during the phytoplankton bloom. The high N2O concentrations were not observed in the nearshore. Based on the correlations of the excess N2O (ΔN2O) and apparent oxygen utilization, as well as ΔN2O vs. NO3–, we concluded that the N2O on the continental shelf was mainly derived from nitrification and nitrifier denitrification. Sea-to-air fluxes of N2O varied from −12.4 to 6.6 μmol m–2 d–1 (−3.8 ± 3.7 μmol m–2 d–1) using the Nightingale et al. (2000) formula and −13.3 to 6.9 μmol m–2 d–1 (−3.9 ± 3.9 μmol m–2 d–1) using the Wanninkhof (2014) formula, which corresponds to 75–112% in saturation, suggesting that the SYS and the ECS acted overall as a sink of atmospheric N2O in early spring, with the strength weakening. Our results reveal the factors and potential mechanisms controlling the production and accumulation of NH2OH and N2O in the SYS and the ECS during early spring.

Effects of feeding a pine-based biochar to beef cattle on subsequent manure nutrients, organic matter composition and greenhouse gas emissions

Biochar in ruminant diets is being assessed as a method for simultaneously improving animal production and reducing enteric CH4 emissions, but little is known about subsequent biochar-manure interactions post-excretion. We examined chemical properties, greenhouse gas (GHG) emissions and organic matter (OM) composition during farm scale stockpiling (SP) or composting (CP) of manure from cattle that either received a pine-based biochar in their diet (BM) or did not (RM). Manure piles were monitored hourly for temperature and weekly for top surface CO2, N2O and CH4 fluxes over 90 d in a semiarid location near Lethbridge, AB, Canada. Results indicate that cumulative CO2, N2O and CH4 emissions were not affected by biochar, implying that BM was as labile as RM. The pH, total C (TC), NO3-N and Olsen P were also not influenced by biochar, although it was observed that NH4-N and OM extractability were both 13% lower in BM than RM. Solid-state 13C nuclear magnetic resonance (NMR) showed that biochar increased stockpile/compost aromaticity, yet it did not alter the bulk C speciation of manure OM. Further analysis by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that dissolved OM was enriched by strongly reduced chemical constituents, with BM providing more humic-like OM precursors than RM. Inclusion of a pine-based biochar in cattle diets to generate BM is consistent with current trends in the circular economy, “closing the loop” in agricultural supply chains by returning C-rich organic amendments to croplands. Stockpiling/composting the resulting BM, however, may not provide a clear advantage over directly mixing low levels of biochar with manure. Further research is required to validate BM as a tool to reduce the C footprint of livestock waste management.

Meridional and Cross‐Shelf Variability of N2O and CH4 in the Eastern‐South Atlantic

AbstractUpward transport and/or mixing of trace gas‐enriched subsurface waters fosters the exchange of nitrous oxide (N2O) and methane (CH4) with the atmosphere in the Eastern‐South Atlantic (ESA). To date, it is, however, unclear whether this source is maintained by local production or advection of trace gas‐enriched water masses. The meridional and zonal variability of N2O and CH4 in the ESA were investigated to identify the contributions of the major regional water masses to the overall budget of N2O and CH4. The maximal sea surface N2O and CH4 concentrations and the main ESA upwelling cells co‐occurred with a strong negative correlation with the sea surface temperature (SST) (p &lt; 0.05). The dominance of the central water masses in the winter and spring seasons and the interplay between shelf topography and wind regime are suggested to determine enhanced gas transfer toward the sea‐air interface or “capping” at midwater depth. These parameters are supposed to be critical in the local budget of N2O and CH4 in the ESA. Our findings also show that the shape of N2O and CH4 gradients is very similar both meridionally and zonally; however, the extent of the differences between the high‐end and low‐end members of the concentrations/saturations range is different. This suggests a more pronounced effect of local sources on CH4 than N2O distribution, in particular in the Walvis Bay area. With respect to N2O, however, low‐oxygen waters from the poleward undercurrent impinge in the shelf close to Cape Frio and often result in N2O concentrations significantly higher than off Lüderitz (p &lt; 0.05).

How does deep-band fertilizer placement reduce N2O emissions and increase maize yields?

It is unclear how deep-band fertilizer placement influences N2O emission and maize (Zea mays L.) yield, and the production and diffusion of N2O within the soil profile under deep-band fertilizer placement treatments. Thus, we conducted a field experiment in 2019–2020 with spring maize to study the effects of different fertilization depths of 5 cm (D5), 15 cm (D15), 25 cm (D25), and 35 cm (D35) on N2O emissions. The N2O concentration in the fertilized soil profile was measured to calculate the N2O diffusion and production rates, and the surface N2O emissions were measured simultaneously. The results showed that the availability of NO3–-N in the soil was the main factor related to N2O production and emissions, and N2O emissions were strongly related to the production and diffusion of N2O in the 0–20 cm soil layer. Deep-band fertilizer placement significantly reduced surface N2O emissions, and the N2O production and diffusion rates in the 0–20 cm soil profile. The cumulative N2O diffusion in the 0–20 cm layer accounted for 79.2%, 73.7%, 58.2%, and 48.6% of those in 0–40 cm soil depth under D5, D15, D25, and D35, respectively. Compared with D5, D25 and D35 significantly reduced the N2O emissions by 30.8% and 59.3%, respectively, N2O-N emission factors by 41.6% and 80.0%, and yield-scaled N2O-N by 39.3% and 58.2%. D25 also increased the maize yield by 13.8% and obtained the highest nitrogen use efficiency (NUE, 43.6%), whereas D35 had negative effects on the maize yield and NUE. Therefore, deep-band fertilizer placement reduced the N2O production rate and diffusion flux in the near-surface soil layer by decreasing the NO3–-N content in the 0–20 cm soil layer. Thus, the location where N2O was produced transferred to the deeper soil layer to reduce surface N2O emissions. The recommended fertilization depth of 25 cm can reduce N2O emissions and significantly enhance the maize yield, N uptake, biomass, and NUE.

Plants are a natural source of nitrous oxide even in field conditions as explained by 15N site preference

Plants are either recognized to produce nitrous oxide (N2O) or considered as a medium to transport soil-produced N2O. To date, it is not clear whether in their habitat plants conduit N2O produced in soil or are a natural source. We aimed to understand role of plants in N2O emissions in field conditions. Therefore, rubber plants (Ficus elastica) were planted in the field; then plant and soil chambers were deployed simultaneously to collect gas samples, and 15N site preference (SP) of N2O was evaluated. The mean SP values of plant and soil emitted N2O were −20.85 ± 2.8‰ and −8.85 ± 1.08‰, respectively, and were significantly different (p < 0.0001); while bulk 15N of plant and soil emitted N2O were −10.83 ± 3.33‰ and −22.56 ± 3.37‰, respectively and were similar (p = 0.06). In the current study, soil always acted as a source of N2O, while plants were both source and sink. Plant and soil N2O fluxes had significant positive exponential relationship with both soil and air temperature. Soil water-filled pore space (WFPS) had significant negative linear relationship with only soil N2O fluxes. Plant N2O fluxes had significant positive linear relationship with plant respiration rates and negative linear relationship with plant surface areas. Based on the relationship between plant respiration rates and N2O fluxes, we suggest that mitochondria are the possible sites of N2O formation in plant cells while the relationship between plant surface areas and N2O fluxes suggests that roots are the parts of its formation in natural and field conditions. Our results suggest that plants are a natural source of N2O even at field conditions and challenge a view that plants are a medium to transport soil-produced N2O into the atmosphere.