N2O Release Research Articles

N2O dynamics in the hyporheic zone due to ripple migration

The migration rate of ripples along the streambed has a profound effect on the hyporheic exchange and nitrogen dynamics. However, how the denitrification intermediate N2O is released in mobile bedforms still remain unclear. In this study, the ripple domains are established based on the sediment stability criteria by using the mean channel flow velocity U and the median sediment grain size D50. A two-dimensional numerical model coupling hyporheic flow, ripple migration and N2O transformation is proposed to investigate how ripple celerity affects N2O dynamics in streambeds. The results show that there are different patterns for the release of N2O from migrating ripples with different medium grain sizes D50 (0.06–0.3 mm). N2O can be released via upwelling of anoxic pore water from the deeper area in immobile or slow-mobile ripples. However, rapid ripple migration and the resulting fast vertical flow reversal can suppress the net pore water displacement and lead to the formation of a redox seal, which separates the deeper anoxic area from the above oxic area and inhibits N2O emissions. The magnitude of the nutrients concentration gradient and the N2O flux between these two areas depend only on the dispersive and diffusive transport. Therefore, the controls of N2O emission flux hence shift from the pumping- dominated regime in stable ripples to a grain size dominated regime of the migrating ripples. The possible implications about the effects of hydrodynamic changes due to the migration of ripples are presented. Our results emphasize that knowledge of N2O production-consumption hotspots within streambeds is essential to understand the mechanisms of N2O emissions from the sediment–water interface.

Characteristics and preparation of oil-coated fertilizers: A review.

As the slow-release fertilizer, oil-coated fertilizer can not only slow down the nutrients loss, but also have outstanding advantages in controlling the nutrients release. Based on a large number of literature, this paper systematically investigated the composition, classification, properties and preparation of oil-coated fertilizers, summarizes the challenges faced by the oil-coated fertilizers and offers a few suggestions for the future research. Through literature research, some important conclusions were found: (1) Oil-coated fertilizers are generally composed of core fertilizers and coated oil layers, and some have active interlayers. (2) Vegetable oils has the characteristics of easy degradation, water resistance and impact resistance, and the nutrient release curves of vegetable oil coated fertilizer in soil and still water are "S" type. (3) The modified polyurethane exhibits good compatibility with urea, and can control the release of N in a long period of time, which is 30days longer than the N release life of ordinary polyurethane-coated fertilizers. (4) Oil-coated fertilizers can reduce the loss of N by slowing down the hydrolysis rate of urea and the nitrification from NH4+ to NO3-, which reduces the N2O release by 70-80% compared to the uncoated fertilizers. Moreover, the paper also proposes a new preparation method of oil-coated material.

Copper availability governs nitrous oxide accumulation in wetland soils and stream sediments

Denitrification is microbially-mediated through enzymes containing metal cofactors. Laboratory studies of pure cultures have highlighted that the availability of Cu, required for the multicopper enzyme nitrous oxide reductase, can limit N2O reduction. However, in natural aquatic systems, such as wetlands and hyporheic zones in stream beds, the role of Cu in controlling denitrification remains incompletely understood. In this study, we collected soils and sediments from three natural environments -- riparian wetlands, marsh wetlands, and a stream -- to investigate their nitrogen species transformation activity at background Cu levels and different supplemented Cu loadings. All of the systems contained solid-phase associated Cu below or around geological levels (40–280 nmol g−1) and exhibited low dissolved Cu (3–50 nM), which made them appropriate sites for evaluating the effect of limited Cu availability on denitrification. In laboratory incubation experiments, high concentrations of N2O accumulated in all microcosms lacking Cu amendment except for one stream sediment sample. With Cu added to provide dissolved concentrations at trace levels (10–300 nM), the reduction rate of N2O to N2 in the wetland soils and stream sediments was enhanced. A kinetic model could account for the trends in nitrogen species by combining the reactions for microbial reduction of NO3− to NO2−/N2O/N2 and abiotic reduction of NO2− to N2. The model revealed that the rate of N2O to N2 conversion increased significantly in the presence of Cu. For riparian wetland soils and stream sediments, the kinetic model also suggested that overall denitrification is driven by abiotic reduction of NO2− in the presence of inorganic electron donors. This study demonstrated that natural aquatic systems containing Cu at concentrations less than or equal to crustal abundances may display incomplete reduction of N2O to N2 that would cause N2O accumulation and release to the atmosphere.

Microbial functional assemblages predicted by the FAPROTAX analysis are impacted by physicochemical properties, but C, N and S cycling genes are not in mangrove soil in the Beibu Gulf, China

Mangroves and their global significance in climate change based on the storage of blue carbon and release of CO2, CH4, and N2O have attracted great attention. Microbial communities and their functions are essential for the biogeochemical cycle of carbon (C), nitrogen (N), and sulfur (S). Therefore, the assessment of microbial functions in mangrove habitats, which suffer from different external environments, is necessary to evaluate their unique ecological service functions. In the present work, the functional profiles of microbial communities in four mangrove reserves (one sandy and three muddy, in Beibu Gulf, China) were detected by 16 s RNA gene amplicon-based Functional Annotation of Procaryotic Taxa (FAPROTAX) analysis as well as C, N, and S gene q-PCR chips. The FAPROTAX analysis showed that the metabolic potential in C, N, and S cycling indicated a difference in the four mangrove regions. Moreover, some functional groups involved in N, C, and S cycling were significantly correlated with physicochemical characteristics. However, N, C, and S metabolic genes detected by q-PCR chip did not present the same changes following the variation of environmental conditions as those predicted by the FAPROTAX analysis. Most of the N, C, and S cycling genes were equally distributed in the four mangroves. Several genes involved in N cycling (amoA, amoB, nirK, and nosZ), carbon degradation (amyA, apu, mnp, lig, and chiA), carbon fixation (aclB), and oxidation of H2S (SoxY) were significantly higher in sampling site in Beihai City (Bei), which was the only site with sandy soils. We hypothesized that soil texture (sandy or muddy) rather than physicochemical elements contributes more significantly to the abundance of N, S, and C metabolic genes in mangrove sediment. Our study demonstrated that the change of physicochemical property in mangrove soil was easier to modify the functional microbial groups than functional genes. We should pay attention to the functional redundancy of soil microbiota when using microorganisms as ecological indicator to access the soil activity in mangrove habitats.

Identification of key water parameters and microbiological compositions triggering intensive N2O emissions during landfill leachate treatment process

Landfill leachate treatment processes tend to emit more N2O compared to domestic wastewater treatment. This discrepancy may be ascribed to leachate water characteristics such as high refractory COD, ammonium (NH4+) content, and salinity. In this work, the leachate influent was varied to examine the N2O emission scenarios. NH4+-N, COD, and Cl− concentrations ranged between 1000–2500, 1000–10,000, and 500–3000 mg L−1, respectively. Simultaneously, we attempted to combine statistical analysis with high-throughput sequencing to understand the microbial mechanism with regards to N2O emission. Results show that the strong N2O emissions occur in the nitrifying tank due to the intensive aeration. The system receiving the lowest COD shows the maximum N2O emission factor of 42.7% of the removed nitrogen. Both redundancy analysis and a structural equation model verify that insufficient degradable organics are the key water parameter triggering intensive N2O emission within the designed influent limits. Furthermore, two essential but non-abundant functional bacteria, Flavobacterium (acting as a denitrifier) and Nitrosomonas (acting as a nitrifier), are identified as the core functional species that dramatically influence N2O emissions. An increase in influent COD promotes the proliferation of Flavobacterium and inhibits Nitrosomonas, which in turn reduce N2O release. Meanwhile, two keystone species of Castellaniella and Saprospiraceae unclassified are recognized. They may supply a suitable niche and integrity of the microbial community for N-cycle functional bacteria. These findings reveal the essential role of non-abundant species in microbial community, and expand the current understanding of microbial interactions underlying N2O dynamics in leachate treatment systems.

Sedimentary processes dominate nitrous oxide production and emission in the hypoxic zone off the Changjiang River estuary

Coastal oceans, known as the major nitrous oxide (N2O) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for N2O production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified N2O production rates using a 15N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for N2O production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of N2O in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the N2O release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal N2O budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.

Nitrate removal and nitrous oxide production from hothouse effluent draining to a pipe bioreactor

Agricultural hothouses are intensive food production systems relying on high fertilization and irrigation. Runoff from hothouses can contain high levels of nitrogen (N) and drain into streams. Here, we investigate the effectiveness of buried, inline woodchip bioreactors constructed using PVC pipes in removing nitrate (NO3−-N) and the possibility of pollution swapping from nitrate to the greenhouse gas nitrous oxide (N2O). Mean dissolved NO3−-N removal and N2O gas production were 6.0 ± 5.2 g N m−3 h−1 (0.9–12.3 g N m−3 h−1), and 35.1 ± 31.4 mg N L−1 h−1 (14–83 mg N L−1 h−1) respectively, across five surveys. NO3−-N removal and N2O-N production occurred primarily in hypoxic to anoxic conditions. Overall, these inline pipe bioreactors achieved nitrate removal efficiencies (NRE) of 14.5 ± 6.8% (8.2%–25.0%) and N2O production equivalent to 0.7 ± 0.6% (0.3–1.4%) of nitrate removal. Comparisons to the literature indicate that our bioreactors have a low NRE but a very high nitrate removal rate (NRR) on a woodchip volume basis. The bioreactor was operating at 21% water capacity. Therefore, increasing the bioreactor water height would be needed to maximize nitrate removal. Our results imply that these bioreactors can efficiently remove NO3-N without major N2O release to the atmosphere.

Unexpectedly minor nitrous oxide emissions from fluvial networks draining permafrost catchments of the East Qinghai-Tibet Plateau

Streams and rivers emit substantial amounts of nitrous oxide (N2O) and are therefore an essential component of global nitrogen (N) cycle. Permafrost soils store a large reservoir of dormant N that, upon thawing, can enter fluvial networks and partly degrade to N2O, yet the role of waterborne release of N2O in permafrost regions is unclear. Here we report N2O concentrations and fluxes during different seasons between 2016 and 2018 in four watersheds on the East Qinghai-Tibet Plateau. Thawing permafrost soils are known to emit N2O at a high rate, but permafrost rivers draining the East Qinghai-Tibet Plateau behave as unexpectedly minor sources of atmospheric N2O. Such low N2O fluxes are associated with low riverine dissolved inorganic N (DIN) after terrestrial plant uptake, unfavorable conditions for N2O generation via denitrification, and low N2O yield due to a small ratio of nitrite reductase: nitrous oxide reductase in these rivers. We estimate fluvial N2O emissions of 0.432 − 0.463 Gg N2O-N yr−1 from permafrost landscapes on the entire Qinghai-Tibet Plateau, which is marginal (~0.15%) given their areal contribution to global streams and rivers (0.7%). However, we suggest that these permafrost-affected rivers can shift from minor sources to strong emitters in the warmer future, likely giving rise to the permafrost non-carbon feedback that intensifies warming.

Contrasting response of soil N2O release to ammonium, nitrate, and urea addition rates is determined by substrate availability and microbial community abundance and composition

Although nonlinear responses of soil nitrous oxide (N2O) release to exogenous nitrogen (N) input have been reported, the contrasting effects of oxidized and reduced N forms and their driving mechanisms have rarely been investigated. Here, a 91-day microcosm incubation experiment with three forms (ammonium, nitrate, and urea) and eleven rates (0, 1, 2, 4, 6, 8, 10, 12, 14, 28, and 56 g N m−2 yr−1) of N addition was conducted to determine the response dose, saturation threshold, and turning point of N2O release from the temperate forest soil. Simultaneously, soil biochemical properties, N-cycling functional gene abundances, and bacterial and fungal community composition were determined to assess the biotic and abiotic contributions to changes in soil N2O release. The responses of soil N2O release rate to ammonium, nitrate, and urea addition rates exhibited saturation, sigmoidal, and exponential curves, respectively. The response dose of soil N2O release was higher in urea addition than in ammonium and nitrate additions (28 g N m−2 yr−1 vs. 8 g N m−2 yr−1). The tipping point and saturation thresholds for ammonium and nitrate additions were estimated to be 14 g N m−2 yr−1 and 56 g N m−2 yr−1, respectively, whereas neither occurred in urea addition. Except high rate of ammonium addition (≥28 g N m−2 yr−1), N inputs significantly increased AOB abundance and promoted soil NO3−-N accumulation. Also, N addition, especially high-rate ammonium, led to a shift in bacterial community from oligotrophic to copiotrophic groups. Oxidized nitrate input increased soil NO3−-N content and changed bacterial and fungal community structure, whereas reduced ammonium and urea additions increased soil NH4+-N content and AOB abundance, as well as altered bacterial community. These findings highlight that N forms contrastingly affect soil N2O release. Substrate availability, functional gene abundance, and microbial community composition equally contribute to soil N2O release, which should be considered in terrestrial ecosystem process models.

Application of a Partial Nitrogen Lab-Scale Sequencing Batch Reactor for the Treatment of Organic Wastewater and Its N2O Production Pathways, and the Microbial Mechanism

Partial nitrification (PN) is a widely used wastewater treatment process. Here a lab-scale sequencing batch reactor for PN (PN-SBR) was constructed and run with artificial organic wastewater for 225 days. Results showed that the SBR reached a stable PN state after 174 days of operation and &gt;98% of NH4+-N was removed and &gt;60% was converted to NO2−-N with low effluent NO3−-N content. In a PN-SBR cycle at stage IV, the release of N2O was accompanied by the production of hydroxylamine, occurring mainly in the conversion from anaerobic to aerobic phases, and the amount of N2O produced was about 6.3% of the total nitrogen. The N2O isotopic signature results suggested that hydroxylamine oxidation was the main pathway for N2O production. Illumina MiSeq sequencing results showed that Proteobacteria and Bacteroidetes were the dominant phyla throughout the operation period. Many heterotrophic nitrifiers were significantly enriched, leading to ammonia removal and nitrite accumulation, including Acidovorax, Paracoccus, Propionibacteriaceae_unclassified, Shinella, Comamonas and Brevundimonas. Representative strains were isolated from the reactor and they were capable of efficiently producing nitrite from ammonia. These results provide a guide for the direct running of PN reactors for treating organic wastewater and help to understand the microbial processes and N2O release pathways and the microbial mechanism of partial nitrification.

Effects of Silver Nanoparticles on Denitrification and Associated N2O Release in Estuarine and Marine Sediments

Effects of Silver Nanoparticles on Denitrification and Associated N2O Release in Estuarine and Marine Sediments

The impact of pristine and modified rice straw biochar on the emission of greenhouse gases from a red acidic soil

With the growing awareness of environmental impacts of land degradation, pressure is mounting to improve the health and productivity of degrading soils, which could be achieved through the use of raw and modified biochar materials. The primary objective of the current study was to investigate the efficiency of pristine and Mg-modified rice-straw biochar (RBC and MRBC) for the reduction of greenhouse gases (GHG) emissions and improvement of soil properties. A 90 days’ incubation experiment was conducted using treatments which included control (CK), two RBC dosages (1% and 2.5%), and two MRBC doses (1% and 2.5%). Soil physico-chemical and biological properties were monitored to assess the effects due to the treatments. Results showed that both biochars improved soil physicochemical properties as the rate of biochar increased. The higher rates of biochar (RBC2.5 and MRBC2.5) particularly increased enzymatic activities (Catalase, Invertase and Urease) in comparison to the control. Data obtained for phospholipid fatty acid (PLFA) concentration indicated an increase in the Gram-negative bacteria (G-), actinomycetes and total PLFA with the increased biochar rate, while Gram-positive bacteria (G+) showed no changes to either level of biochar. As regards fungi concentration, it decreased with the biochar addition, whereas arbuscular mycorrhizal fungi (AMF) showed non-significant changes. The release of CO2, CH4 and N2O showed a decreasing trend over the time. CO2 cumulative emission decreased for MRBC1 (5%) and MRBC2.5 (9%) over the pristine biochar treatments. The cumulative N2O emission decreased by 15–32% for RBC1 and RBC2.5 and by 22–33% for MRBC1 and MRBC2.5 as compared to the control, whereas CH4 emission showed non-significant changes. Overall, the present study provides for the first-time data that could facilitate the correct use of Mg-modified rice biochar as a soil additive for the mitigation of greenhouse gas emission and improvement of soil properties.

Short-Term and Long-Term Effects of Natural and Artificial Carbonaceous Substrates on Greenhouse Gas Fluxes

The emissions of two greenhouse gases (GHG), carbon dioxide (CO2) and nitrous oxide (N2O), from six substrates with different carbonaceous content were compared in short and long-term incubation experiments. Three natural soils and three artificial chars were mixed with carbon (C) poor soil (Cambisol) to simulate real conditions after application of char to farmland. The natural soils were a Cambisol, an Anthrosol and a Histosol with C contents of 1.3%, 4.4% and 13.2%, respectively. The three chars produced through thermal conversion of wood chips by hydrothermal carbonisation (HTC), fluidized bed gasification and pyrolysis had C contents of 56.9%, 75.4% and 79.9%, respectively. Emission rates of CO2 and N2O from the rewetted substrates were measured by gas chromatography over a short time of 72 h and over a long period of nearly two years. The short-term CO2 emissions from the natural soils showed a clear relationship to their C content. The emission rate for the Histosol/Cambisol mixture was three times higher than that for the pure Cambisol, 77.1 vs. 23.5 mg CO2-C kg−1 organic matter (OM) per hour. The C emission rates for the char/Cambisol mixtures were much lower, ranging between 3.0 and 9.1 mg CO2-C kg OM−1 h−1 , and did not correspond to their total C contents. Comparison between the two incubation lengths showed that the long-term CO2 emission rates were generally one order of magnitude lower than the short-term rates. The final emission rates for natural substrates over a period of two years were still twice those for artificial char substrates, between 2.2–3.5 mg CO2-C kg OM−1 h−1 and 1.3–1.8 mg CO2-C kg OM−1 h−1 , respectively. Although the contents of total nitrogen (Ntot) and extractable nitrogen (Nmin) were considerable in the chars under study, enhanced N2O release was not observed in the incubation experiments. Instead, N2O emission rates in the three mixtures of chars and Cambisol were lower by one to two orders of magnitude compared to the pure Cambisol in short-term incubations. Even long-term N2O emissions were 5 to 9 times lower. The highest degree of N2O reduction was found for the HTC char. Because of the high global warming potential of N2O, this positive effect of chars may play an important role in mitigating emissions of CO2 equivalents. Both CO2 and N2O must be taken into account when balancing GHG emitted after chars (biochar, gasifier char, HTC char) are applied to soil.

Food waste composting based on patented compost bins: Carbon dioxide and nitrous oxide emissions and the denitrifying community analysis

Mature compost and rice bran were used as bulking agents to perform Food Waste Rapid Composting (FWRC) in a patented composting bin. The characteristics of CO2 and N2O emission and the denitrifying community were investigated. The release of CO2 and N2O concentrated in the early composting stage and reduced greatly after 28 h, and the N2O emission peak of the treatment with mature compost was 8.5 times higher than that of rice bran. The high N2O generation resulted from massive denitrifying bacteria and NOx--N in the composting material. The relative abundances of denitrifiers, correspondingly genes of narG and nirK were much higher in the treatment with mature compost, which contributed to the N2O emission. Moreover, the correlation matrices revealed that N2O fluxes correlated well with moisture, pH, temperature, and the abundances of nirK and nosZ genes during FWRC.

Thawing Yedoma permafrost is a neglected nitrous oxide source

In contrast to the well-recognized permafrost carbon (C) feedback to climate change, the fate of permafrost nitrogen (N) after thaw is poorly understood. According to mounting evidence, part of the N liberated from permafrost may be released to the atmosphere as the strong greenhouse gas (GHG) nitrous oxide (N2O). Here, we report post-thaw N2O release from late Pleistocene permafrost deposits called Yedoma, which store a substantial part of permafrost C and N and are highly vulnerable to thaw. While freshly thawed, unvegetated Yedoma in disturbed areas emit little N2O, emissions increase within few years after stabilization, drying and revegetation with grasses to high rates (548 (133–6286) μg N m−2 day−1; median with (range)), exceeding by 1–2 orders of magnitude the typical rates from permafrost-affected soils. Using targeted metagenomics of key N cycling genes, we link the increase in in situ N2O emissions with structural changes of the microbial community responsible for N cycling. Our results highlight the importance of extra N availability from thawing Yedoma permafrost, causing a positive climate feedback from the Arctic in the form of N2O emissions.

A two-year measurement of methane and nitrous oxide emissions from freshwater aquaculture ponds: Affected by aquaculture species, stocking and water management

Aquaculture ponds are of increasing worldwide concerns as critical sources of atmospheric methane (CH4) and nitrous oxide (N2O), but little is known about these gases emissions as affected by aquaculture species, stocking and water management in aquaculture ponds. Here, a two-year study was carried out to quantify CH4 and N2O emissions from freshwater crab and fish aquaculture ponds in subtropical China. We further explored how the microbial functional genes [CH4: mcrA and pmoA; N2O: archaeal and bacterial amoA (AOA + AOB), nirS, nirK, nosZ] may drive CH4 and N2O release in the crab aquaculture pond typically undergoing flooding-to-drainage alteration. Over the two-year period, annual CH4 and N2O fluxes averaged 0.95 mg m−2 h−1 and 20.94 μg m−2 h−1 in the fish aquaculture, and 0.78 mg m−2 h−1and 28.48 μg m−2 h−1 in the crab aquaculture, respectively. The direct N2O emission factors were estimated to be 0.77% and 0.36% of the total N input by feed or 1.59 g N2O-N kg−1 and 1.06 g N2O-N kg−1 aquaculture yield in the crab and fish ponds, respectively. Among three functional stocking areas, CH4 and N2O emissions were consistently the highest at the feeding area (FA) in the both aquaculture ponds, followed by at the undisturbed area (UA) and aerated area (AA). The shift in sediment soil moisture from waterlogging to drainage conditions significantly increased the abundance of AOB relative to AOA and pmoA, decreased those of denitrifying functional genes (nirS, nirK, nosZ) and mcrA, while did not alter the functional group ratio of nirS + nirK relative to nosZ. Our results highlight that a better understanding of CH4 and N2O emissions from aquaculture ponds requires taking into consideration of data sourced from more diverse aquaculture systems with different management patterns. In addition, a deep analysis of the microbial processes that drive CH4 and N2O production and consumption from aquaculture ponds remains to be addressed in future studies.

Effect of dissolved oxygen on N2O release in the sewer system during controlling hydrogen sulfide by nitrate dosing

Nitrate dosing is commonly used for controlling hydrogen sulfide in sewer systems. However, it may potentially facilitate N2O emission due to the denitrification process promoted by nitrate addition. In this study, lab-scale sewer reactors were operated to investigate the impact of nitrate addition on N2O production in sewer systems. Results showed that the N2O flux even increased by six times with the addition of nitrate when dissolved oxygen (DO) in the wastewater exceeded 0.4 mg/L. Principal component analysis showed that the N2O concentration was notably affected by DO and oxidation-reduction potential (ORP) in the wastewater. Furthermore, it was founded that N2O flux had a strong linear relationship with the DO concentration in the batch test. The microbial analysis found that the nosZ possessing organisms decreased significantly in the micro-aerobic condition and the copy numbers of nosZ gene declined consequently. It indicated that the inhibition of N2O reduced to N2 was responsible for significant accumulation and emission of N2O in the micro-aerobic condition. Given the gravity sewers are not completely anaerobic, the DO concentration is ranged from 0.1 to 2.4 mg/L in gravity sewers with the partially filled flow. Therefore, more attention should be paid to the N2O production when nitrate dosing for hydrogen sulfide controlling in gravity sewers.

N2O hot moments were not driven by changes in nitrogen and carbon substrates or changes in N cycling functional genes

AbstractIf a dry soil is wetted it can produce a large flush of nitrous oxide (N2O), which can be 10 times above the background rate, this phenomenon is called a hot moment. However, there is uncertainty in the literature regarding the mechanisms that cause hot moments. Therefore, this study aimed to induce two different hot moments on soil cores to investigate changes in nitrogen (N) cycling functional genes, and correlate these changes with subsequent N2O emissions, and transformations in soil chemistry. We induced a ‘pre‐dry’ treatment which wetted soils from 50% to 90% water‐filled pore space (WFPS) and ‘pre‐wet’ treatment which wetted soils from 70% to 90% WFPS to analyse changes in gas fluxes (N2O, nitric oxide [NO] and carbon dioxide [CO2]), soil chemistry (pH, dissolved organic carbon [DOC], total oxidised nitrogen [TON], ammonium [NH4+]) and microbiology (bacterial 16S, fungal ITS and nitrogen cycling functional genes associated with denitrification). The soil produced emissions of N2O that were significantly higher for the pre‐dry treatment compared with the pre‐wet treatment (p = &lt;0.001; 9 times that of the pre‐wet). The pre‐dry treatments did not prime the soil any more than the pre‐wet treatments for CO2 and NO emissions, and the results showed that the soils TON and DOC concentrations were significantly higher in the pre‐wet conditions despite its lower N2O emissions. This suggests that the cause of the hot moment was not as dependant on the soil's substrate concentrations as previously thought. Moreover, the amount of denitrification genes in the pre‐dry soil was statistically similar to the pre‐wet (nirK, norB, nifH, nosZI, nxrA), apart from nirS which was marginally higher (p = 0.03). Contrary to other experiments, this study provides evidence that changes in soil substrates and functional gene abundance are not the cause of N2O hot moments. We therefore suggest that future studies should investigate changes in mRNA abundance related to N cycling genes.Highlights Rewetting following drought caused a significant release of N2O. The pre‐dry soil produced higher N2O fluxes but contained less key substrates (TOC, NH4+ and DOC) than the pre‐wet soil. The pre‐dry and pre‐wet soil had a similar quantity of denitrification functional genes. Differences in soil chemistry and denitrification functional genes between the pre‐dry and pre‐wet soil cannot explain the differences in N2O emissions.

Organic Matter Composition of Digestates Has a Stronger Influence on N2O Emissions than the Supply of Ammoniacal Nitrogen

Manures can be treated by solid–liquid separation and more sophisticated, subsequent approaches. These processes generate fertilizers, which may differ in composition and N2O release potential. The aim of the study was to investigate the influence of processing-related changes in digestate composition on soil-derived N2O emissions after application to soil. For that purpose, N2O emissions within the first 7 weeks after fertilization with two raw and eight processed digestates (derived from solid–liquid separation, drying and pelletizing of separated solid, and vacuum evaporation of separated liquid) were measured in the field in 2015 and 2016. Additionally, an incubation experiment was run for 51 days to further investigate the effect of subsequent solid and liquid processing on soil-derived N2O release. The results showed that, only in 2016, the separation of digestate into solid and liquid fractions led to a decrease in N2O emissions in the following order: raw digestate &gt; separated liquid &gt; separated solid. N removal during subsequent processing of separated solid and liquid did not significantly influence the N2O emissions after fertilization. In contrast, the concentrated application of the final products led to contradictory results. Within the solid processing chain, utilization of pellets considerably increased the N2O emissions by factors of 2.7 (field, 2015), 3.5 (field, 2016), and 7.3 (incubation) compared to separated solid. Fertilization with N-rich ammonium sulfate solution led to the lowest emissions within the liquid processing chain. It can be concluded that the input of less recalcitrant organic C into the soil plays a greater role in N2O release after fertilization than the input of ammoniacal N. Digestate processing did not generally reduce emissions but apparently has the potential to mitigate N2O emissions substantially if managed properly.

Stream Transport and Substrate Controls on Nitrous Oxide Yields From Hyporheic Zone Denitrification

AbstractRivers and streams act as globally significant sources of nitrous oxide (N2O) to the atmosphere, in part through denitrification reactions that will increase in response to ongoing anthropogenic nitrogen loading. While many factors that contribute to the release of N2O relative to inert dinitrogen (N2) are well described, the ability to predict N2O yields from streams remains a fundamental challenge. Here, I revisit results from the second Lotic Intersite Nitrogen eXperiments (LINX II) in the context of turbulent hyporheic exchange. Denitrification efficiency, or the fraction of nitrate delivered to the streambed by stream turbulence that is chemically reduced, emerges as the single best predictor of N2O yields and underpins the first statistically significant models of inter‐site N2O yields. This mechanistic connection is supported by reactive transport modeling of hyporheic zone denitrification representing advective flowpaths, flowpath mixing, and diffusion‐dominated anoxic microzones. Simulated N2O yields are inversely correlated with denitrification efficiency; however, advective models are unable to capture low LINX II N2O yields at low denitrification efficiencies. Hyporheic zone mixing exacerbates this inability to capture observed N2O yields via the promotion of N2O release from fast, oxic flowpaths. Instead, anoxic microzones are required to account for LINX II observations through consistently low N2O yields and the consumption of upstream‐produced N2O. Together, these results provide a framework for controls on stream N2O yields and suggest that stream corridor restoration designs aimed at increasing the capacity of hyporheic zones to remediate nitrate loading, as opposed to increasing hyporheic exchange, will also reduce proportional N2O emissions.