N2 Fluxes Research Articles

Effects of Straw Amendment in Combination with Synthetic N Fertilizer Addition on N2O, N2, and Their Stoichiometric Ratios in Three Different Agro-Ecosystems

Nitrogen (N) fertilizer and crop residue amendments are important agricultural practices that could increase soil health, fertility, and crop yield. Such practices may also change soil denitrification processes where contradictory observations have been reported on soil N2O emissions with fewer studies on N2 emissions due to its large atmospheric background concentrations limiting its soil-borne measurement. This study aims to investigate N2O production and reduction of N2 emissions under a conducive denitrifying environment (like anaerobic microsites, 80% WFPS, available N and C) after rice straw amendment and KNO3 application to three different soil types (fluvo-aquic, black, and paddy soils). In this regard, three treatments for three different soil types were set consisting of (a) a non-amended treatment (control), (b) a KNO3 treatment (KNO3, 20 mM KNO3), and (c) a straw plus KNO3 treatment (2.5 g rice straw kg−1 dry soil and 20 mM KNO3), which were incubated under 80% WFPS. Moreover, direct N2O and N2 fluxes were measured over 17 days in the current incubation experiment with a robotized incubation system using a helium atmosphere. Results showed that rice straw amendment combined with N fertilizer increased both N2O and N2 fluxes compared with control or KNO3 treatments in all three soil types. Overall, compared with the black and paddy soils, the N2O and N2 fluxes were higher in the fluvo-aquic soil, with a maximum of 234.2 ± 6.3 and 590.1 ± 27.3 g N ha−1 from F_SK treatment, respectively, during the incubation period. The general trends in three soil types of both N2O and N2 emissions were control < KNO3 < rice straw plus KNO3 treatments. Straw amendment in combination with KNO3 can stimulate a high denitrification rate (less N2O and higher N2), whereas their effect on stoichiometric ratios of N2O/(N2O + N2) highly depends on soil nitrate concentration, oxygen level, soil moisture content, and labile C. The current study underscores that the rice straw amendment in combination with N fertilizer can trigger denitrification with less increment on soil N2O but higher N2 emissions under conditions favoring denitrification.

Divergent drivers of the spatial variation in greenhouse gas concentrations and fluxes along the Rhine River and the Mittelland Canal in Germany

Lotic ecosystems are sources of greenhouse gases (GHGs) to the atmosphere, but their emissions are uncertain due to longitudinal GHG heterogeneities associated with point source pollution from anthropogenic activities. In this study, we quantified summer concentrations and fluxes of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and dinitrogen (N2), as well as several water quality parameters along the Rhine River and the Mittelland Canal, two critical inland waterways in Germany. Our main objectives were to compare GHG concentrations and fluxes along the two ecosystems and to determine the main driving factors responsible for their longitudinal GHG heterogeneities. The results indicated that the two ecosystems were sources of GHG fluxes to the atmosphere, with the Mittelland Canal being a hotspot for CH4 and N2O fluxes. We also found significant longitudinal GHG flux discontinuities along the mainstems of both ecosystems, which were mainly driven by divergent drivers. Along the Mittelland Canal, peak CO2 and CH4 fluxes coincided with point pollution sources such as a joining river tributary or the presence of harbors, while harbors and in-situ biogeochemical processes such as methanogenesis and respiration mainly explained CH4 and CO2 hotspots along the Rhine River. In contrast to CO2 and CH4 fluxes, N2O longitudinal trends along the two lotic ecosystems were better predicted by in-situ parameters such as chlorophyll-a concentrations and N2 fluxes. Based on a positive relationship with N2 fluxes, we hypothesized that in-situ denitrification was driving N2O hotspots in the Canal, while a negative relationship with N2 in the Rhine River suggested that coupled biological N2 fixation and nitrification accounted for N2O hotspots. These findings stress the need to include N2 flux estimates in GHG studies, as it can potentially improve our understanding of whether nitrogen is fixed through N2 fixation or lost through denitrification.

Impact of liming and maize residues on N2O and N2 fluxes in agricultural soils: an incubation study

AbstractSince it is known that nitrous oxide (N2O) production and consumption pathways are affected by soil pH, optimising the pH of agricultural soils can be an important approach to reduce N2O emissions. Because liming effects on N2O reduction had not been studied under ambient atmosphere and typical bulk density of arable soils, we conducted mesoscale incubation experiments with soils from two liming trials to investigate the impact of long-term pH management and fresh liming on N transformations and N2O production. Soils differed in texture and covered a range of pH levels (3.8–6.7), consisting of non-limed controls, long-term field-limed calcite and dolomite treatments, and freshly limed soils. Both soils were amended with 15N-labelled potassium nitrate (KNO3) and incubated with and without incorporated maize litter. Packed soil mesocosms were cycled through four phases of alternating temperatures and soil moistures for at least 40 days. Emissions of N2O and dinitrogen (N2) as well as the product ratio of denitrification N2O/(N2O + N2), referred to as N2Oi were measured with the 15N gas flux method in N2-reduced atmosphere. Emissions of N2O increased in response to typical denitrifying conditions (high moisture and presence of litter). Increased temperature and soil moisture stimulated microbial activity and triggered denitrification as judged from 15NO3− pool derived N2O + N2 emissions. Fresh liming increased denitrification in the sandy soil up to 3-fold but reduced denitrification in the loamy soil by 80%. N2Oi decreased throughout the incubation in response to fresh liming from 0.5–0.8 to 0.3–0.4, while field-limed soils had smaller N2Oi (0.1–0.3) than unlimed controls (0.9) irrespective of incubation conditions. Our study shows that the denitrification response (i.e., N2O + N2 production) to liming is soil dependent, whereas liming effects on N2Oi are consistent for both long- and short-term pH management. This extends previous results from anoxic slurry incubation studies by showing that soil pH management by liming has a good mitigation potential for agricultural N2O emissions from denitrification under wet conditions outside of cropping season.

Structural analysis of NbN thin films grown through rf magnetron sputtering

In this work NbN thin films have been grown through rf magnetron sputtering technique from an 8-NbN (99.99%) target. In particular, we have studied the influence of the additional N2 flux in the preparation chamber on crystallization and microstructure of the deposited films. The films have been characterized by X-Ray diffraction (XRD) in 0-20 configuration and at a grazing angle, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD results show that the films grown at different fluxes of N2 have high textured coefficients along the [200] direction. SEM results indicate the films have columnar growth with high homogeneity and an average thickness of 0.7 μm. TEM results reveal that the films have grown from crystalline nanoparticles of NbN with high texture along the (200) plane.

Short-term effect of liquid organic fertilisation and application methods on N2, N2O and CO2 fluxes from a silt loam arable soil

AbstractThe absence of N2 flux measurements in liquid manure-amended soils has resulted in a poor understanding of the effect of manure application on gaseous N losses. The aim of this study was to quantify N2, N2O, CO2, N2O reduction to N2, depth distribution of moisture, water-extractable organic C, NO3−, NH4+, pH, and diffusivity in a laboratory incubation experiment with an arable silt-loam soil. To quantify N processes and gaseous fluxes, 15N tracing was applied. An artificial livestock slurry-mixture was added to the soil in various treatments (control, surface or injected application; slurry-application rate: 42.9 kg N ha− 1; soil water content of either 40% or 60% water-filled pore space (WFPS)). The soil was incubated for 10 days. The depth distribution of the control parameters was measured twice during the experiment on days 5 and 10. The average increase in N2 and N2O fluxes from denitrification was about 900% in slurry-amended soils. The highest N2 and N2O fluxes from denitrification were measured in the slurry injection, 60% WFPS treatment (7.83 ± 3.50 and 11.22 ± 7.60 mg N m− 2 d− 1, respectively). The hypothesis that injected slurry at a higher water content enhances denitrification was confirmed. This study provides important insights into the formation, spatial and temporal variation of the manure-soil hotspot and its impact on the denitrification process. The results will form part of a dataset to develop, improve and test manure application submodules of biogeochemical models and will help to understand in detail the effect of hotspots on N-cycling in manure-treated soils.

Determining N2O and N2 fluxes in relation to winter wheat and sugar beet growth and development using the improved 15N gas flux method on the field scale

AbstractThe objectives of this field trial were to collect reliable measurement data on N2 emissions and N2O/(N2O + N2) ratios in typical German crops in relation to crop development and to provide a dataset to test and improve biogeochemical models. N2O and N2 emissions in winter wheat (WW, Triticum aestivum L.) and sugar beet (SB, Beta vulgaris subsp. vulgaris) were measured using the improved 15N gas flux method with helium–oxygen flushing (80:20) to reduce the atmospheric N2 background to < 2%. To estimate total N2O and N2 production in soil, production-diffusion modelling was applied. Soil samples were taken in regular intervals and analyzed for mineral N (NO3− and NH4+) and water-extractable Corg content. In addition, we monitored soil moisture, crop development, plant N uptake, N transformation processes in soil, and N translocation to deeper soil layers. Our best estimates for cumulative N2O + N2 losses were 860.4 ± 220.9 mg N m−2 and 553.1 ± 96.3 mg N m−2 over the experimental period of 189 and 161 days with total N2O/(N2O + N2) ratios of 0.12 and 0.15 for WW and SB, respectively. Growing plants affected all controlling factors of denitrification, and dynamics clearly differed between crop species. Overall, N2O and N2 emissions were highest when plant N and water uptake were low, i.e., during early growth stages, ripening, and after harvest. We present the first dataset of a plot-scale field study employing the improved 15N gas flux method over a growing season showing that drivers for N2O and N2O + N2 fluxes differ between crop species and change throughout the growing season.

A new incubation system to simultaneously measure N2 as well as N2O and CO2 fluxes from plant-soil mesocosms

AbstractThis study presents a novel plant-soil mesocosm system designed for cultivating plants over periods ranging from days to weeks while continuously measuring fluxes of N2, N2O and CO2. For proof of concept, we conducted a 33-day incubation experiment using six soil mesocosms, with three containing germinated wheat plants and three left plant-free. To validate the magnitude of N2 and N2O fluxes, we used 15N-enriched fertilizer and a 15N mass balance approach. The system inherent leakage rate was about 55 µg N m− 2 h− 1 for N2, while N2O leakage rates were below the detection limit (< 1 µg N m− 2 h− 1). In our experiment, we found higher cumulative gaseous N2 + N2O losses in sown soil (0.34 ± 0.02 g N m− 2) as compared to bare soil (0.23 ± 0.01 g N m− 2). N2 fluxes accounted for approximately 94–96% of total gaseous N losses in both planted and unplanted mesocosms. N losses, as determined by the 15N mass balance approach, were found to be 1.7 ± 0.5 g N m− 2 for the sown soil and 1.7 ± 0.6 g N m− 2 for the bare soil, indicating an inconsistency between the two assessment methods. Soil respiration rates were also higher in sown mesocosms, with cumulative soil and aboveground biomass CO2 respiration reaching 4.8 ± 0.1 and 4.0 ± 0.1 g C m− 2 over the 33-day incubation period, in sown and bare soil, respectively. Overall, this study measured the effect of wheat growth on soil denitrification, highlighting the sensitivity and utility of this advanced incubation system for such studies.

Grazing exclusion alters denitrification N2O/(N2O + N2) ratio in alpine meadow of Qinghai–Tibet Plateau

Grazing exclusion has been implemented worldwide as a nature-based solution for restoring degraded grassland ecosystems that arise from overgrazing. However, the effect of grazing exclusion on soil nitrogen cycle processes, subsequent greenhouse gas emissions and underlying mechanisms remain unclear. Here, we investigated the effect of four-year grazing exclusion on plant communities, soil properties, and soil nitrogen cycle-related functional gene abundance in an alpine meadow on the Qinghai–Tibet Plateau. Using an automated continuous-flow incubation system, we performed an incubation experiment and measured soil-borne N2O, N2, and CO2 fluxes to three successive “hot moment” events (precipitation, N deposition, and oxic-to-anoxic transition) between grazing-excluded and grazing soil. Higher soil N contents (total nitrogen, NH4+, NO3−) and extracellular enzyme activities (β-1,4-glucosidase, β-1,4-N-acetyl-glucosaminidase, cellobiohydrolase) are observed under grazing exclusion. The aboveground and litter biomass of plant community was significantly increased by grazing exclusion, but grazing exclusion decreased the average number of plant species and microbial diversity. The N2O + N2 fluxes observed under grazing exclusion were higher than those observed under free grazing. The N2 emissions and N2O/(N2O + N2) ratios observed under grazing exclusion were higher than those observed under free grazing in oxic conditions. Instead, higher N2O fluxes and lower denitrification functional gene abundances (nirS, nirK, nosZ, and nirK + nirS) under anoxia were found under grazing exclusion than under free grazing. The N2O site-preference value indicates that under grazing exclusion, bacterial denitrification contributes more to higher N2O production compared with under free grazing (81.6 % vs. 59.9 %). We conclude that grazing exclusion could improve soil fertility and plant biomass, nevertheless it may lower plant and microbial diversity and increase potential N2O emission risk via the alteration of the denitrification end-product ratio. This indicates that not all grassland management options result in a mutually beneficial situation among wider environmental goals such as greenhouse gas mitigation, biodiversity, and social welfare.

Biochar mitigates N2O emissions by promoting complete denitrification in acidic and alkaline paddy soils

AbstractBiochar is an efficacious amendment for mitigating nitrous oxide (N2O) emissions in soils. Nevertheless, the underlying mechanisms responsible for reduced N2O emissions by biochar in paddy soils remain inadequately elucidated. Here, using two typical paddy soils with contrasting pH values (5.40 and 7.56), the N2 and N2O fluxes and the associated functional genes were investigated in soil amended with varying amounts of biochar (0%, 0.5%, and 5%, weight/weight) via soil slurry incubation integrated with the N2/Ar technique and qPCR analysis. The results showed that N2O fluxes were significantly (p < 0.05) reduced by 0.65–3.64 times following biochar amendment, concomitant with a significant (p < 0.05) increase in N2 fluxes (5.47–46.14%) in both acidic and alkaline paddy soils. As a result, the N2O/(N2O + N2) ratios were significantly (p < 0.05) reduced by 1.53–4.65 fold in both soil types. In acidic paddy soils, the enhanced denitrification rates and the decreased N2O/(N2O + N2) ratios exhibited a strong correlation with increased pH values. In alkaline paddy soil, these changes were ascribed to the enhanced nosZ Clade I gene abundance and nosZ/(nirS + nirK) ratio. Our findings reveal that biochar primarily mitigates N2O emissions in paddy soils by promoting its reduction to N2.

Modeling Denitrification: Can We Report What We Don't Know?

AbstractBiogeochemical models simulate soil nitrogen (N) turnover and are often used to assess N losses through denitrification. Though models simulate a complete N budget, often only a subset of N pools/fluxes (i.e., N2O, , NH3, NOx) are published since the full budget cannot be validated with measured data. Field studies rarely include full N balances, especially N2 fluxes, which are difficult to quantify. Limiting publication of modeling results based on available field data represents a missed opportunity to improve the understanding of modeled processes. We propose that the modeler community support publication of all simulated N pools and processes in future studies.

Room temperature deposition of stable p-type ZnO:N thin films through chemical species modulation using reactive pulsed laser deposition

Low temperature processing as well as long-term electrical stability are required to match p-type ZnO thin films with flexible electronics applications. Herein, ZnO and ZnO:N thin films were deposited at room temperature by reactive pulsed laser deposition of a zinc metallic target. Several ZnO thin films were obtained by varying the oxygen working pressure from 1.2 × 10-5 Torr to 3.5 × 10-2 Torr. Chemical concentration was smoothly tuned as demonstrated by in situ X-ray Photoelectron Spectroscopy. Introducing and varying both N2 and O2 fluxes at a constant pressure, N-doped ZnO films from 0.3 at.% to 5 at.% were deposited. Adjusting the N2/O2 flux ratio, p-type ZnO:N films were successfully achieved, exhibiting a carrier concentration of 5.3 × 1018 cm−3, resistivity of 2 Ωcm, and mobility of 0.5 cm2V-1s−1. Four representative p-type samples were analyzed to confirm chemical and electrical reproducibility. Electrical properties of one of these films were measured again nine months after deposited, and p-type conductivity was still observed. Cathodoluminescence revealed green-dominant emissions in n-ZnO and UV-dominant emissions in p-ZnO:N films as nitrogen doping effect. Finally, a p-n diode was fabricated using photolithography, exhibiting rectification performance even nine months after deposited. P-type ZnO:N presented here are suitable for flexible electronics devices fabrication.

Do NO, N2O, N2 and CO2 fluxes differ in soils sourced from cropland and varying riparian buffer vegetation? An incubation study.

Riparian buffers are expedient interventions for water quality functions in agricultural landscapes. However, the choice of vegetation and management affects soil microbial communities, which in turn affect nutrient cycling and the production and emission of gases such as nitric oxide (NO), nitrous oxide (N2O), nitrogen gas (N2) and carbon dioxide (CO2). To investigate the potential fluxes of the above-mentioned gases, soil samples were collected from a cropland and downslope grass, willow and woodland riparian buffers from a replicated plot scale experimental facility. The soils were re-packed into cores and to investigate their potential to produce the aforementioned gases via potential denitrification, a potassium nitrate (KNO3 -) and glucose (labile carbon)-containing amendment, was added prior to incubation in a specialized laboratory DENItrification System (DENIS). The resulting NO, N2O, N2 and CO2 emissions were measured simultaneously, with the most NO (2.9 ± 0.31 mg NO m-2) and N2O (1413.4 ± 448.3 mg N2O m-2) generated by the grass riparian buffer and the most N2 (698.1 ± 270.3 mg N2 m-2) and CO2 (27,558.3 ± 128.9 mg CO2 m-2) produced by the willow riparian buffer. Thus, the results show that grass riparian buffer soils have a greater NO3 - removal capacity, evidenced by their large potential denitrification rates, while the willow riparian buffers may be an effective riparian buffer as its soils potentially promote complete denitrification to N2, especially in areas with similar conditions to the current study.

Invasive Spartina alterniflora accelerates the increase in microbial nitrogen fixation over nitrogen removal in coastal wetlands of China

Salt marsh plants play a vital role in mediating nitrogen (N) biogeochemical cycle in estuarine and coastal ecosystems. However, the effects of invasive Spartina alterniflora on N fixation and removal, as well as how these two processes balance to determine the N budget, remain unclear. Here, simultaneous quantifications of N fixation and removal via 15N tracing experiment with native Phragmites australis, invasive S. alterniflora, and bare flats as well as corresponding functional gene abundance by qPCR were carried out to explore the response of N dynamics to S. alterniflora invasion. Our results showed that N fixation and removal rates ranged from 0.77 ± 0.08 to 16.12 ± 1.13 nmol/(g·h) and from 1.42 ± 0.14 to 16.35 ± 1.10 nmol/(g·h), respectively, and invasive S. alterniflora generally facilitated the two processes rates. Based on the difference between N removal and fixation rates, net N2 fluxes were estimated in the range of −0.39 ± 0.14 to 8.24 ± 2.23 nmol/(g·h). Estimated net N2 fluxes in S. alterniflora stands were lower than those in bare flats and P. australis stands, indicating that the increase in N removal caused by S. alterniflora invasion may be more than offset by N fixation process. Random forest analysis revealed that functional microorganisms were the most important factor associated with the corresponding N transformation process. Overall, our results highlight the importance of N fixation in evaluating N budget of estuarine and coastal wetlands, providing valuable insights into the ecological effect of S. alterniflora invasion.

Small ponds have stronger potential for net nitrogen removal: Insight from direct dissolved N2 measurement

Growing demands for watershed nitrogen (N) removal have called attention to abundant small bodies of water such as ponds, which have long been heralded as efficient storage and processing systems. Although pond conservation, restoration, and creation have been widely implemented to mitigate N pollution, information is limited regarding the impact of size—that is, whether N removal potential and efficiency are dependent upon pond size. We investigated the dynamics of N removal rates in 56 ponds from a hilly watershed by studying their bimonthly N2 concentrations and fluxes. Our results showed that smaller ponds performed better in net N removal. This can be discerned from the areal N2 fluxes, which were the highest in small ponds (< 4, 000 m2). The corresponding N2 fluxes (4.73 ± 4.53 mmol N2 m−2 d−1) were 2 to 14 times greater than those observed in larger ponds. The N removal efficiency, a metric used to describe the portions of the substrates released as N2, was also significantly higher in the small ponds (∼8.7 %) than in the larger ponds (∼5.0 %). Further regression analysis showed that both areal N2 flux and N removal efficiency were negatively correlated with pond area. The underlying mechanisms behind the size effects of N removal could be attributed to small ponds having larger sediment contact area to water volume ratios. Thus, smaller ponds allow more opportunities for N to interact with bioactive sediments than larger ponds. Overall, our findings contribute to the understanding of the distal role of pond size in affecting N removal. This research also provides a strong rationale for considering the effects of system size when implementing management practices dedicated to maximizing N removal.

Modeling impacts of saltwater intrusion on methane and nitrous oxide emissions in tidal forested wetlands.

Emissions of methane (CH4 ) and nitrous oxide (N2 O) from soils to the atmosphere can offset the benefits of carbon sequestration for climate change mitigation. While past study has suggested that both CH4 and N2 O emissions from tidal freshwater forested wetlands (TFFW) are generally low, the impacts of coastal droughts and drought-induced saltwater intrusion on CH4 and N2 O emissions remain unclear. In this study, a process-driven biogeochemistry model, Tidal Freshwater Wetland DeNitrification-DeComposition (TFW-DNDC), was applied to examine the responses of CH4 and N2 O emissions to episodic drought-induced saltwater intrusion in TFFW along the Waccamaw River and Savannah River, USA. These sites encompass landscape gradients of both surface and porewater salinity as influenced by Atlantic Ocean tides superimposed on periodic droughts. Surprisingly, CH4 and N2 O emission responsiveness to coastal droughts and drought-induced saltwater intrusion varied greatly between river systems and among local geomorphologic settings. This reflected the complexity of wetland CH4 and N2 O emissions and suggests that simple linkages to salinity may not always be relevant, as non-linear relationships dominated our simulations. Along the Savannah River, N2 O emissions in the moderate-oligohaline tidal forest site tended to increase dramatically under the drought condition, while CH4 emission decreased. For the Waccamaw River, emissions of both CH4 and N2 O in the moderate-oligohaline tidal forest site tended to decrease under the drought condition, but the capacity of the moderate-oligohaline tidal forest to serve as a carbon sink was substantially reduced due to significant declines in net primary productivity and soil organic carbon sequestration rates as salinity killed the dominant freshwater vegetation. These changes in fluxes of CH4 and N2 O reflect crucial synergistic effects of soil salinity and water level on C and N dynamics in TFFW due to drought-induced seawater intrusion.

Plasma Enhanced High-Rate Deposition of Advanced Film Materials by Metal Reactive Evaporation in Organosilicon Vapors.

Dense homogeneous nanocomposite TiSiCN coatings with a thickness of up to 15 microns and a hardness of up to 42 GPa were obtained by the method of reactive titanium evaporation in a hollow cathode arc discharge in an Ar + C2H2 + N2-gas mixture with the addition of hexamethyldisilazane (HMDS). An analysis of the plasma composition showed that this method allowed for a wide range of changes in the activation degree of all components of the gas mixture, providing a high (up to 20 mA/cm2) ion current density. It is possible to widely change the chemical composition, microstructure, deposition rate, and properties of coatings obtained by this method, by changing the pressure, composition, and activation degree of the vapor-gas mixture. An increase in the fluxes of C2H2, N2, HMDS, and discharge current leads to an increase in the rate of coating formation. However, the optimal coatings from the point of view of microhardness were obtained at a low discharge current of 10 A and relatively low contents of C2H2 (1 sccm) and HMDS (0.3 g/h), exceeding which leads to a decrease in the hardness of the films and the deterioration of their quality, which can be explained by the excessive ionic exposure and the non-optimal chemical composition of the coatings.

Dinitrogen Emissions Dominate Nitrogen Gas Emissions From Soils With Low Oxygen Availability in a Moist Tropical Forest

AbstractLowland tropical forest soils are relatively N rich and are the largest global source of N2O (a powerful greenhouse gas) to the atmosphere. Despite the importance of tropical N cycling, there have been few direct measurements of N2 (an inert gas that can serve as an alternate fate for N2O) in tropical soils, limiting our ability to characterize N budgets, manage soils to reduce N2O production, or predict the future role that N limitation to primary productivity will play in buffering against climate change. We collected soils from across macro‐ and micro‐topographic gradients that have previously been shown to differ in O2 availability and trace gas emissions. We then incubated these soils under oxic and anoxic headspaces to explore the relative effect of soil location versus transient redox conditions. No matter where the soils came from, or what headspace O2 was used in the incubation, N2 emissions dominated the flux of N gas losses. In the macrotopography plots, production of N2 and N2O were higher in low O2 valleys than on more aerated ridges and slopes. In the microtopography plots, N2 emissions from plots with lower mean soil O2 (5%–10%) were greater than in plots with higher mean soil O2 (10%–20%). We estimate an N gas flux of ∼37 kg N/ha/yr from this forest, 99% as N2. These results suggest that N2 fluxes may have been systematically underestimated in these landscapes, and that the measurements we present call for a reevaluation of the N budgets in lowland tropical forest ecosystems.

Soil CH4 and N2O response diminishes during decadal soil warming in a temperate mountain forest

Global warming is considered to impact the fluxes of methane (CH4) and nitrous oxide (N2O) between forest soils and the atmosphere, but it is unclear whether the responses change over time. In this study the response of soil CH4 and N2O fluxes to field soil warming (+4 °C) were determined during years 2–5 and 14–16 in a soil warming experiment in a temperate forest. In the second and sixteenth year of soil warming, temperature sensitivities of CH4 and N2O fluxes were assessed in-situ by gradually rising field soil temperatures to ∼10 °C above ambient within a short period of three to four days. Production of dinitrogen (N2) was measured ex-situ in the sixteenth year of warming. Soil warming significantly reduced CH4 uptake (-19.5%) and increased N2O emissions (+41.6%) during the first years of warming, whereas no warming effects on soil CH4 and N2O fluxes were observed during the later years. Dinitrogen production was up to ten times higher than N2O production, though the high spatiotemporal variability masked any significant effects of soil warming on soil N2 fluxes. Temperature sensitivities (Q10) for CH4 uptake and N2O emissions were 2.07 and 4.06, respectively, in the second year of warming and 1.52 and 1.79, respectively, in the sixteenth year of soil warming. The diminishing warming response of the soil N2O fluxes likely were caused by longer-term changes in soil N availability and/or simultaneous acclimation of the soil microbial community to soil warming. Soil moisture was largely unaffected by soil warming, and soil temperature alone was only a weak predictor of soil CH4 fluxes. Methane fluxes therefore can be expected to be generally less affected than N2O fluxes. Overall, our results suggest that soil warming has only limited and transient effects on soil CH4 and N2O fluxes in this type of temperate forest.

Waterlogging effects on N2O and N2 emissions from a Stagnosol cultivated with Silphium perfoliatum and silage maize

European policy recommends that biomass production occur on marginal land, such as poorly draining Stagnosols. Compared to annual cropping, perennial crops may better mitigate N2O emissions at such sites, through more complete denitrification. To test that hypothesis, we compared N2 and N2O fluxes from the soils of a perennial crop (cup plant, Silphium perfoliatum L.) and an annual crop (silage maize, Zea mays L.). Intact soil columns (35 cm height, 14.4 cm diameter) were incubated for 37 days. The soils were fertilized with 60 or 120 kg N ha−1 and exposed to successive phases of waterlogging: free drainage, waterlogging of 1/3-, and waterlogging of 2/3- of the column. Source-specific N2O and N2 fluxes were measured using the 15 N gas flux method. Denitrification was higher in cup plant than maize soil and total N losses from denitrification were dominated by emissions from the third phase. Cup plant soil emitted 33.6 ± 78.1 mg N m−2 and 95.8 ± 64.4 mg N m−2 more N2O than maize soil in the low and high N treatments, respectively. The product ratio of denitrification (N2Oi = N2O/(N2 + N2O)) increased with waterlogging in maize soil, while remaining stable in cup plant soil. Emissions from the top 10 cm dominated the N2Oi rather than N2 fluxes from the saturated soil. This study did not show N2O mitigation in cup plant soil, instead highlighting the complexity of plant-soil effects on denitrification. We clearly showed that the application of a general N2Oi for agricultural soils across annual and perennial cropping is not recommended.

Characteristics of N2 and N2O Fluxes from a Cultivated Black Soil: A Case Study through In Situ Measurement Using the 15N Gas Flux Method

The magnitudes and source partitioning of soil dinitrogen (N2) and nitrous oxide (N2O) emissions are not well documented, yet. To address both issues for black soil subject to a typical cool temperate climate, soil N2O and N2 fluxes following the basal application event of an ammonium-based fertilizer (labeled by 15N) for maize were simultaneously measured in situ by using the 15N gas flux (15NFG) method. During the two-month field experiment, the measured N2 and N2O fluxes cumulated to 1.61 ± 0.47 and 0.12 ± 0.01 kg N ha−1, respectively, showing N2O to N2O plus N2 ratios (RN2O) of 0.02–0.31 (0.15 on average). Temperature was identified as a key factor regulating the total soil N2 fluxes (r2 = 0.27, p &lt; 0.01), despite the N2 fluxes originated from nitrate denitrification related to dissolved organic carbon concentrations (r2 = 0.39, p &lt; 0.01). Differently, both temperature and soil moisture jointly accounted for 85% and 74% of the variances in the N2O fluxes and the RN2O values, respectively (p &lt; 0.01). Moreover, the process(es) other than autotrophic nitrification and heterotrophic denitrification could be of substantial importance for the soil N2O emissions. Our findings emphasized the importance of temperature in regulating N2 emissions from black soil and the possible site- and/or time specificity of a soil factors-based parametrization of RN2O. In addition, this study implicates that labeling a nitrogen substrate of nitrification while using the 15N enrichment of N2O is necessary to more accurately quantify total soil N2 fluxes in situ by using the 15NFG approach even though further confirmation in future studies is still needed.