Influence N2O Emissions Research Articles

Nutrient enrichment—but not warming—increases nitrous oxide emissions from shallow lake mesocosms

AbstractShallow lakes and ponds play a crucial role in the processing of carbon and other nutrients. However, many lakes and ponds worldwide are affected by climate change and nutrient pollution. How these pressures affect the emission of the greenhouse gas nitrous oxide (N2O) is unclear. Warming and eutrophication are expected to increase the production and emission of N2O in lakes and ponds, but changes in ecological structure and function may complicate these seemingly straightforward relationships. In this study, we used the world's longest running, mesocosm‐based, freshwater climate change experiment to disentangle the effect of nutrient enrichment and warming on N2O emissions. We gathered a large dataset on N2O concentrations and ancillary variables, comprising three sampling campaigns between 2011 and 2020 and a total of 687 individual mesocosm measurements. Our results demonstrated that nutrient enrichment increased N2O emissions, while warming (+2.5–4.0°C and +3.75–6.0°C) had no discernable effect. Our study indicates that curtailing nitrogen influxes into lakes and ponds is the most effective strategy to minimize N2O emissions, and while warming may influence N2O emissions, it does not appear to be a direct driver. These findings underscore the importance of prioritizing nitrogen mitigation efforts to curb N2O emissions from shallow lakes and ponds.

Machine learning reveals dynamic controls of soil nitrous oxide emissions from diverse long-term cropping systems.

Soil nitrous oxide (N2O) emissions exhibit high variability in intensively managed cropping systems, which challenges our ability to understand their complex interactions with controlling factors. We leveraged 17 years (2003-2019) of measurements at the Kellogg Biological Station Long-Term Ecological Research (LTER)/Long-Term Agroecosystem Research (LTAR) site to better understand the controls of N2O emissions in four corn-soybean-winter wheat rotations employing conventional, no-till, reduced input, and biologically based/organic inputs. We used a random forest machine learning model to predict daily N2O fluxes, trained separately for each system with 70% of observations, using variables such as crop species, daily air temperature, cumulative 2-day precipitation, water-filled pore space, and soil nitrate and ammonium concentrations. The model explained 29%-42% of daily N2O flux variability in the test data, with greater predictability for the corn phase in each system. The long-term rotations showed different controlling factors and threshold conditions influencing N2O emissions. In the conventional system, the model identified ammonium (>15 kg N ha-1) and daily air temperature (>23°C) as the most influential variables; in the no-till system, climate variables such as precipitation and air temperature were important variables. In low-input and organic systems, where red clover (Trifolium repens L.; before corn) and cereal rye (Secale cereale L.; before soybean) cover crops were integrated, nitrate was the predominant predictor of N2O emissions, followed by precipitation and air temperature. In low-input and biologically based systems, red clover residues increased soil nitrogen availability to influence N2O emissions. Long-term data facilitated machine learning for predicting N2O emissions in response to differential controls and threshold responses to management, environmental, and biogeochemical drivers.

Temperature differentially regulates estuarine microbial N2O production along a salinity gradient

Nitrous oxide (N2O) is atmospheric trace gas that contributes to climate change and affects stratospheric and ground-level ozone concentrations. Ammonia oxidizers and denitrifiers contribute to N2O emissions in estuarine waters. However, as an important climate factor, how temperature regulates microbial N2O production in estuarine water remains unclear. Here, we have employed stable isotope labeling techniques to demonstrate that the N2O production in estuarine waters exhibited differential thermal response patterns between nearshore and offshore regions. The optimal temperatures (Topt) for N2O production rates (N2OR) were higher at nearshore than offshore sites. 15N-labeled nitrite (15NO2-) experiments revealed that at the nearshore sites dominated by ammonia-oxidizing bacteria (AOB), the thermal tolerance of 15N-N2OR increases with increasing salinity, suggesting that N2O production by AOB-driven nitrifier denitrification may be co-regulated by temperature and salinity. Metatranscriptomic and metagenomic analyses of enriched water samples revealed that the denitrification pathway of AOB is the primary source of N2O, while clade II N2O-reducers dominated N2O consumption. Temperature regulated the expression patterns of nitrite reductase (nirK) and nitrous oxide reductase (nosZ) genes from different sources, thereby influencing N2O emissions in the system. Our findings contribute to understanding the sources of N2O in estuarine waters and their response to global warming.

Correlations between sewage sludge composting physicochemical parameters and emissions of greenhouse gases and ammonia: A statistical analysis

Mitigating the environmental impact of composting by the reduction of greenhouse gases (N2O, CH4) and ammonia (NH3) emissions is a major challenge. To meet this challenge, the understanding of the relationships between composted substrates initial physicochemical parameters and gas emissions is essential. From a long-term perspective, it will allow to guide the recipe formulation of the initial mixture to be composted, with a view to reducing gas emissions during composting. This study gathered literature data targeting sewage sludge composting and performed statistical correlation analyses between cumulative gas emissions and the following parameters: sewage sludge, bulking agent and composted mixture initial physicochemical parameters (pH, dry matter, total carbon, total nitrogen, C/N), the dry mass ratio of sewage sludge to bulking agent and the duration of composting. The average values of cumulative emissions show a large variability: 1.37 ± 2.71 gC.kg initial mix DM−1, 0.13 ± 0.17 gN.kg initial mix DM−1 and 2.23 ± 2.79 gN.kg initial mix DM−1 for CH4, N2O and NH3 emissions respectively. Although the correlation analysis highlighted some significant interesting correlations between initial physicochemical parameters and gas emissions (p.value < 0.05), reliable multiparametric model could not fit the data, meaning that the actual literature data are not sufficient to explain most part of gas emissions. Among the most interesting relationships, the study showed that the dry matter of the composted mixture is negatively correlated to N2O emissions, while the ratio of sewage sludge to bulking agent and the duration of composting are positively correlated to N2O emissions. It was also shown that the pH of the bulking agent is positively correlated to NH3 emissions. Considering the large part of the emission variability that is not explained and the difficulty to link the correlation with their causality, it will be interesting to improve composting gas emissions knowledge in future research by analyzing free air space, bulking agent adsorption capacity and available and biodegradable organic matter. These parameters are of particular interest in solving the main problems associated with sewage sludge composting, namely porosity and nitrogen retention. This study also highlighted the necessity to extend the duration of the composting studies over 40 days in order to measure possible N2O late release and better identify parameters influencing N2O emissions.

The impacts of film mulching and ridging on N2O emissions, relevant functional genes, and microbial communities in rain-fed potato fields

Rain-fed potato (Solanum tuberosum) fields in drylands significantly contribute to nitrous oxide (N2O) emissions, making them an important focus of agricultural greenhouse gas research. Film mulching and ridging are key agricultural methods in potato cultivation. Investigating the impact of these methods on N2O emissions, nitrifying/denitrifying functional genes, and microbial communities can provide a theoretical basis for soil emission reduction and more sustainable dryland agriculture. We examine the effects of flat tillage with mulching, ridge tillage with mulching, flat tillage without mulching, and ridge tillage without mulching, on potato fields under natural rainfall conditions in Wuchuan County, China. N2O emission fluxes were monitored using a static (dark) chamber and gas chromatography. Real-time quantitative PCR (q-PCR) was used to quantify abundances of nitrifying and denitrifying bacteria related to N2O emissions at various potato-growth stages. Illumina high-throughput sequencing was used to investigate microbial community structure by targeting 16S rRNA genes; related soil elements (soil temperatures and moisture) are analyzed. Mulching and ridging indirectly influence N2O emissions, nitrifying/denitrifying functional gene copy numbers, and microbial community structure by altering soil temperature and moisture. Cumulative N2O emissions and emission intensity were both consistently higher in ridge tillage with mulching during the potato-growing period. Ammonia-oxidizing archaea are the main microorganisms that control N2O emissions, with nitrification-coupled denitrification also being an important mechanism contributing to high N2O emissions during soil dry–wet cycles. Increased soil temperature and moisture elevated N2O emissions and functional gene copy numbers. The combination of mulching and ridging effectively uses the characteristics of both practices, making Nitrospira the dominant genus, and significantly increases N2O emissions.

Effects of Straw Addition on N2O and CO2 Emissions from Red Soil Subjected to Different Long-Term Fertilization

Straw return, as an important measure for soil fertility improvement in farmland, significantly affects the emissions of greenhouse gases N2O and CO2. Thus, the collected soil samples from five long-term (30-year) fertilization treatments (no fertilization, CK; recommended chemical fertilizer, F; 200 % of recommended chemical fertilizer, 2F; pig manure, M; and chemical fertilizer combined with pig manure, FM) were amended with and without straw and incubated under constant temperature and humidity conditions (25 ℃ and 65 % maximum field water holding capacity) for 20 days so as to investigate the key factors influencing N2O and CO2 emissions in response to straw addition in long-term fertilization treatments. The results showed that fertilization significantly increased N2O emissions. Compared to those under the unfertilized treatment[(22.05 ±2.09) μg·kg-1, calculated as nitrogen, the same as below], cumulative N2O emissions from the chemical fertilizer treatments significantly increased by 119 %-195 %[(48.38 ±20.81) μg·kg-1 and (65.13 ±12.55) μg·kg-1 from the F and 2F treatments, respectively], and those from the manure treatments increased by 275 %-399 %[(82.72 ±12.73) μg·kg-1 and (1 101.99 ±425.71) μg·kg-1 from the M and FM treatments, respectively]. Soil NO3--N, DOC, and DTN were the main factors influencing N2O emissions from fertilized treatments in the absence of straw addition. Straw addition significantly increased cumulative N2O emissions by 345 % and 247 % in the 2F and M treatments, respectively, compared to those in the corresponding fertilized treatments without straw addition, with no significant effect on N2O emissions in the CK, F, and FM treatments. Straw addition increased DOC content and microbial activity and decreased soil NO3--N and DTN contents, thereby increasing N2O emissions. Fertilization also significantly increased CO2 emissions. Compared to those from the unfertilized treatment, cumulative CO2 emissions from the manure treatments significantly increased by 120 %-130 %[(122.11 ±4.3) mg·kg-1 (calculated as carbon, the same as below) and (116.47 ±4.55) mg·kg-1 from the M and FM treatments, respectively], and those in the 2F treatment increased by 28 %[(65.13 ±12.55) mg·kg-1]. In the absence of straw addition, soil MBC, DOC, and DTN were the main factors influencing CO2 emissions. Compared to those in the treatments without straw addition, straw addition significantly increased cumulative CO2 emissions by 660 %-1132 % among fertilization treatments, due to increased DOC and MBC contents and enhanced microbial activity. In conclusion, straw addition significantly increased N2O and CO2 emissions through increased soil DTN consumption and DOC content among fertilization treatments. In soils treated with manure amendment, straw return should be rationally considered for the purpose of balancing the comprehensive trade-offs between fertility improvement and greenhouse gas emissions.

Insight into N2O emission and denitrifier communities under different aeration intensities in composting of cattle manure from perspective of multi-factor interaction analysis

Nitrous oxide (N2O) emission from composting is a significant contributor to greenhouse effect and ozone depletion, which poses a threat to environment. To address the challenge of mitigating N2O emission during composting, this study investigated the response of N2O emission and denitrifier communities (detected by metagenome sequencing) to aeration intensities of 6 L/min (C6), 12 L/min (C12), and 18 L/min (C18) in cattle manure composting using multi-factor interaction analysis. Results showed that N2O emission occurred mainly at mesophilic phase. Cumulative N2O emission (QN2O, 9.79 mg·kg−1 DW) and total nitrogen loss (TN loss, 16.40 %) in C12 composting treatment were significantly lower than those in the other two treatments. The lower activity of denitrifying enzymes and the more complex and balanced network of denitrifiers and environmental factors might be responsible for the lower N2O emission. Denitrification was confirmed to be the major pathway for N2O production. Moisture content (MC) and Luteimonas were the key factors affecting N2O emission, and nosZ-carrying denitrifier played a significant role in reducing N2O emission. Although relative abundance of nirS was lower than that of nirK significantly (P < 0.05), nirS was the key gene influencing N2O emission. Community composition of denitrifier varied significantly with different aeration treatments (R2 = 0.931, P = 0.001), and Achromobacter was unique to C12 at mesophilic phase. Physicochemical factors had higher effect on QN2O, whereas denitrifying genes, enzymes and NOX− had lower effect on QN2O in C12. The complex relationship between N2O emission and the related factors could be explained by multi-factor interaction analysis more comprehensively. This study provided a novel understanding of mechanism of N2O emission regulated by aeration intensity in composting.

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3−and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

Biochar produced at high temperature mitigates N2O emission and promotes nitrogen retention in subtropical forest soils

AbstractBiochar is produced by burning biomass under oxygen‐limited conditions, and it has been widely used as a soil amendment to improve soil functions such as nutrient retention. However, whether the impact of biochar application on soil nitrogen (N) transformation and N2O emission varies with the pyrolysis temperature remains unclear, especially in different forest types in subtropical regions. In this study, a 60‐day laboratory incubation experiment was conducted to evaluate the impact of biochar with different pyrolysis temperatures (300°C [BC300], 500°C [BC500], and 800°C [BC800]) on net N transformation rates and N2O emission in soils collected from Castanopsis kawakamii dominated natural forest (NF) and Chinese fir (Cunninghamia lanceolate, CF) plantation in subtropical China. The results showed that the application of biochar significantly increased soil ammonium (NH4+) content (p &lt; 0.001) but reduced nitrate (NO3−) content (p &lt; 0.001) compared with the control. The soil NH4+ content of the BC800 treatment was significantly higher than that of other treatments (p &lt; 0.001). Biochar application significantly reduced soil net N mineralization (NRmin) and nitrification (NRnit) rate (p &lt; 0.001), but increased net ammonification (NRamm) rate (p &lt; 0.001). The application of biochar led to a remarkable decrease in cumulative N2O emission compared to the control (p &lt; 0.001). In particular, soils treated with high‐temperature biochar emitted significantly lower N2O compared to other treatments (p &lt; 0.001). The partial least squares path model demonstrated that biochar influenced N2O emission through a direct effect in NF soil and an indirect effect in CF soil. This study highlights the distinct role of biochar, particularly that produced under high pyrolysis temperatures as a soil amendment to mitigate N2O emission and promote N retention in both subtropical natural and planted forests.

Maize residue input rather than cover cropping influenced N2O emissions and soil–crop N dynamics during the intercrop and cash crop periods

The well-known benefits of cover cropping for mitigating climate change and improving soil quality could be threatened by increased greenhouse gas emissions. Therefore, such practices could have negative consequences in the context of global climate change. The present year-long field experiment, carried out under irrigated semiarid conditions, evaluated the effect of returning maize crop residues at two topsoil input levels in combination with bare fallow, a cereal (barley, Hordeum vulgare L.) and a legume (vetch, Vicia sativa L.) as cover crops as part of an annual cover crop–cash crop (maize, Zea mays L.) rotation. In addition, control plots, without the addition of nitrogen fertilizer (either in the previous cropping season or during the experiment), were established following the same experimental design in order to assess the carry-over impact of residual nitrogen from prior crops. Nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) fluxes were measured together with soil mineral nitrogen and crop yields and nitrogen use efficiency. The significant effect of maize residue input on N2O emissions during the cover crop phase (this was 43% greater for the lower maize residue input than for the higher) was offset after subsequent maize fertilization (there was a 170% increase following the higher maize residue input compared to the lower). Although no differences in soil mineral nitrogen content were reported between treatments before maize fertilization, N2O emissions were 72% higher on the fertilized plots than on the control plots. Before maize fertilization, N2O emissions were increased by the incorporation of cover crop residues and soil rewetting after irrigation, with the highest N2O fluxes being reported for the cereal crop. Cumulative CO2 fluxes were higher in the plots with cover crops (cereal and legume) than in bare fallow plots, while CH4 sink was not affected by cover cropping or maize residue input. The grain yield penalty for maize production (a 7% decrease) with the legume cover crop on the nitrogen fertilized plots (the opposite of what was observed in the control area) highlights the need for an accurate estimation of the nitrogen supply from the mineralization of legume residues. Overall, cover crop and maize residue input did not have any significant effect on annual cumulative N2O emissions, so the positive effects of these practices should be considered to recommend them under semiarid conditions for climate change mitigation and adaptation.

Estimating Nitrous Oxide (N2O) emissions from managed soils at higher spatial resolution in the Republic of Ireland

This paper aims to define a high spatial resolution model for estimating nitrous oxide (N2O) emissions from agricultural soils in Ireland. In 2020, N2O emissions from the management of agricultural soils represented 10% of the total national agricultural Greenhouse Gas (GHG) emissions. The high spatial resolution model employed here takes account of environmental factors that influence N2O emissions at a more disaggregated spatial scale than the Intergovernmental Panel on Climate Change (IPCC) national inventory-reporting framework. Activity data from the EU Farm Accountancy Data Network (for Ireland) is used in conjunction with biophysical and climatic data in a geographic information system (GIS) framework to estimate N2O emissions at a sub-national level. Results indicate that N2O emissions are 5% lower by applying this high spatial resolution modelling approach at the farm level compared to the baseline model (Tier 2). Nevertheless, 25% of the farms in the sample analysed had an overestimation in their N2O emissions of 20%, and another 25% of the farm sample had an average underestimation in their N2O emissions by 19%, a consequence of the variation of environmental factors among farms. Using a data panel regression, results confirm that the environmental factors examined are statistically significant within the model proposed, and with a simulation, we assessed the switching of 20% of CAN fertiliser for protected urea. According to the high spatial model, this mitigation measure can reduce N2O emissions from inorganic fertilisers by up to 15%. However, the reduction is conditioned by local factors and environmental conditions.

Response of N2O emission and denitrifying genes to iron (II) supplement in root zone and bulk region during wetting-drying alternation in paddy soil

Ferrous (Fe (II)) plays a key role in the production or consumption of N2O through biochemical pathways in paddy soil. Drying-wetting alternations can affect Fe cycling and the associated N2O emissions, however, the underlying microbial mechanisms remain largely unknown, particularly in plant systems with the presence of roots. In this study, a laboratory experiment with four Fe (II) levels (0, 50, 150, and 300 mg Fe (II) kg−1 dry soil) was conducted to investigate the response of N2O emission and denitrifying genes to iron (II) supplement in root zone and bulk region during wetting-drying cycles in paddy soils. The results indicated that supplemented Fe (II) led to a reduction in soil N2O emissions, predominantly observed during the drying phase. The bulk soil exhibited higher N2O fluxes compared to root zone. However, the decrease in cumulative N2O (cumul-N2O) by Fe (II) was significantly greater in bulk region than in root-affected soil (P < 0.001), registering 85–91 % reduction for soils amended with low to medium rates (50 and 150 mg Fe (II) kg−1 dry soil) throughout the experiment. This might be attributed to the more sensitive response of N2O reductase (nosZ gene) to low or medium Fe (II) supplementation in bulk soil than in root zone. The highest Fe (II) addition (300 mg Fe (II) kg−1 dry soil) showed a negative impact on nosZ-containing gene during initial flooding phase, resulting in the lower reduction rate (40 %) in cumulative N2O emission from bulk soil. Cross-correlation analysis showed that the changes of nosZ-containing genes in root zone occurred before variations of cumulative N2O data, indicating that compared to bulk soil, root zone soil exhibited a more pronounced of N2O consumption capability, especially after addition of ferrous. In summary, judicious Fe (II) application into soil promotes N2O reduction during wetting-drying alternation. The response of nosZ abundance to Fe (II) supplementation is likely the pivotal microbial factor influencing N2O emission reduction during wetting-drying alternation in paddy soil.

Effect of different types of biochar on soil properties and functional microbial communities in rhizosphere and bulk soils and their relationship with CH4 and N2O emissions.

Biochar as an agricultural soil amendment plays vital roles in mediating methane (CH4) and nitrous oxide (N2O) emissions in soils. The link between different types of biochar, bulk soil, and rhizosphere microbial communities in relation to CH4 and N2O emissions is being investigated in this study. The rice pot experiment was conducted using biochar at two temperatures (300°C and 500°C) in combination with three biochar levels (0, 2, 10% w/w). Soil properties and the abundance of genes associated with CH4 and N2O emissions from both rhizosphere and bulk soils were investigated. The study also aimed to examine the structure of microbial communities (pmoA, nosZ) in rhizosphere and bulk soils whereas CH4 and N2O emissions were monitored while growing rice. Results showed that biochar at 300°C and 10% incorporation significantly increased the CH4 emissions by up to 59% rise compared to the control group. Random Forest analysis revealed that the ratio of mcrA/pmoA along with the abundance of mcrA from both rhizosphere and bulk soils, the abundance of AOA, TN, DOC, and the community composition of pmoA-harboring microorganisms from both bulk and rhizosphere soils were important predictors of CH4 emissions. Therefore, the ratio of mcrA/pmoA in rhizosphere soil and the abundance of AOA in bulk soil were the main factors influencing CH4 emissions. Variation Partitioning Analysis (VPA) results indicated that the effects of these factors on bulk soil were 9% of CH4 emissions variations in different treatments, which contributed more than rhizosphere soils' factors. Moreover, random forest analysis results indicated that the abundance of AOB in bulk soil was the most important predictor influencing N2O emissions. The VPA result revealed that the factors in rhizosphere soil could explain more than 28% of the variations in N2O emissions. Our study highlights that rhizosphere soil has a more significant effect than bulk soil on N2O production. Our findings further the understanding of the link between bulk and rhizosphere attributes, and their impact on CH4 and N2O emissions in paddy soils. In summary, we recommend the application of biochar at 500°C and 2% incorporation rate for agricultural production in the area.

Effect of co-application of straw and various nitrogen fertilizers on N2O emission in acid soil

In order to explore the alteration of N transformation and N2O emissions in acid soil with the co-application of straw and different types of nitrogen (N) fertilizers, an incubation experiment was carried out for 40 days. There are totally five treatments in the study: (a) without straw and N fertilizer (N0), (b) straw alone application (SN0), (c) straw with NH4Cl (SN1), (d) straw with NaNO3 (SN2), and (e) straw with NH4NO3 (SN3). N2O emissions, soil physicochemical properties, and abundance/activity of ammonia-oxidizing archaea (AOA) were measured. The results showed that the combined application of straw and N enhanced N2O emissions, particularly, SN2 and SN3 treatments. Moreover, the soil pH was lower in co-application treatments and the average decreasing rate was 9.69%. Specially, the pH was lowest in the SN1 treatment. The results of correlation analysis indicated a markedly negative relationship between pH and N2O, as well as a negative relationship between pH and net mineralization rate. These findings suggest that pH alteration can affect the N transformation process in soil and thus influence N2O emissions. In addition, the dominant AOA at the genus level in the SN2 treatment was Nitrosopumilus, and Candidatus nitrosocosmicus in the SN3 treatment. The reshaped AOA structure can serve as additional evidence of the changes in the N transformation process. In conclusion, as the return of straw, the cumulation of N2O from arable acid soil depends on the form of N fertilizer. It is also important to consider how N fertilizer is applied to reduce the possibility of N being lost in the soil as gas.

Investigating key drivers of N2O emissions in heterogeneous riparian sediments: Reactive transport modeling and statistical analysis

Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to ozone depletion. Recent studies have identified river corridors as significant sources of N2O emissions. Surface water-groundwater (hyporheic) interactions along river corridors induce flow and reactive nitrogen transport through riparian sediments, thereby generating N2O. Despite the prevalence of these processes, the controlling influence of physical and geochemical parameters on N2O emissions from coupled aerobic and anaerobic reactive transport processes in heterogeneous riparian sediments is not yet fully understood. This study presents an integrated framework that combines a flow and multi-component reactive transport model (RTM) with an uncertainty quantification and sensitivity analysis tool to determine which physical and geochemical parameters have the greatest impact on N2O emissions from riparian sediments. The framework involves the development of thousands of RTMs, followed by global sensitivity and responsive surface analyses. Results indicate that characterizing the denitrification reaction rate constant and permeability of intermediate-permeability sediments (e.g., sandy gravel) are crucial in describing coupled nitrification-denitrification reactions and the magnitude of N2O emissions. This study provides valuable insights into the factors that influence N2O emissions from riparian sediments and can help in developing strategies to control N2O emissions from river corridors.

Arbuscular mycorrhizal fungi reduce N2O emissions from degraded residue patches

Nitrous oxide (N2O) is a potent greenhouse gas, and agricultural soils represent a major anthropogenic source. Crop residues provide nutrients for plants but also act as hotspots of N2O production. The hyphae of arbuscular mycorrhizal fungi (AMF) could proliferate in organic patches, utilize released N from the organic patches, and potentially mitigate N2O emissions. However, the effect of AMF on N2O emissions in degraded residue patches and the possible microbial mechanism remain uncertain. Here, a mesocosm experiment was conducted to investigate the impact of AMF (Funneliformis mosseae) inoculation on N2O emissions, availabilities of carbon and nitrogen, extracellular enzyme activities, and the abundance of key N-cycling genes in degraded residue patches. Our results showed that AMF hyphae significantly reduced N2O emissions from degraded residue patches. Quantitative PCR analysis of key functional genes involved in N2O production (amoA, nirK, nirS) and consumption (nosZ) showed that AMF significantly reduced the abundance of the bacterial amoA and nirS genes. NH4+, NO3−, total dissolved nitrogen (TDN), total nitrogen (TN), and dissolved organic carbon (DOC) contents decreased drastically in the presence of AMF. In addition, the activities of all tested extracellular enzymes were significantly decreased by AMF and positively correlated with DOC content. Multiple stepwise regression analysis demonstrated that the abundance of the nirS gene primarily influenced N2O emissions and was positively correlated with DOC content in degraded residue patches. Our findings indicate that AMF suppressed N2O producers, particularly nirS-type denitrifiers, by slowing down the release of C and N from degraded residues, thereby leading to a cascade effect on the decrease of N2O emissions. This study provides a promising approach to mitigate N2O emissions by enhancing AMF in the agroecosystems.

Effects of hotter, drier conditions on gaseous losses from nitrogen fertilisers

Global warming is expected to cause hotter, drier summers and more extreme weather events including heat waves and droughts. A little understood aspect of this is its effects on the efficacy of fertilisers and related nutrient losses into the environment. We explored the effects of high soil temperature (>25 °C) and low soil moisture (<40% water filled pore space; WFPS) on emissions of ammonia (NH3) and nitrous oxide (N2O) following application of urea to soil and the efficacy of urease inhibitors (UI) in slowing N losses. We incubated soil columns at three temperatures (15, 25, 35 °C) and three soil moisture contents (20, 40, 60% WFPS) with urea applied on the soil surface with and without UIs, and measured NH3 and N2O emissions using chambers placed over the columns. Four fertiliser treatments were applied in triplicate in a randomised complete block design: (1) urea; (2) urea with a single UI (N-(n-butyl) thiophosphoric triamide (NBPT); (3) urea with two UI (NBPT and N-(n-propyl) thiophosphoric triamide; NPPT); and (4) a zero N control. Inclusion of UI with urea, relative to urea alone, delayed and reduced peak NH3 emissions. However, the efficacy of UI was reduced with increasing temperature and decreasing soil moisture. Cumulative NH3 emission did not differ between the two UI treatments for a given set of conditions and was reduced by 22–87% compared with urea alone. Maximum cumulative NH3 emission occurred at 35 °C and 20% WFPS, accounting for 31% of the applied N for the urea treatment and 25%, on average for the UI treatments. Urease inhibitors did not influence N2O emissions; however, there were interactive impacts of temperature and moisture, with higher cumulative emissions at 40% WFPS and 15 and 25 °C accounting for 1.85–2.62% of the applied N, whereas at 35 °C there was greater N2O emission at 60% WFPS. Our results suggest that inclusion of UI with urea effectively reduces NH3 losses at temperatures reaching 35 °C, although overall effectiveness decreases with increasing temperature, particularly under low soil moisture conditions.

Mycorrhiza-mediated recruitment of complete denitrifying Pseudomonas reduces N2O emissions from soil

BackgroundArbuscular mycorrhizal fungi (AMF) are key soil organisms and their extensive hyphae create a unique hyphosphere associated with microbes actively involved in N cycling. However, the underlying mechanisms how AMF and hyphae-associated microbes may cooperate to influence N2O emissions from “hot spot” residue patches remain unclear. Here we explored the key microbes in the hyphosphere involved in N2O production and consumption using amplicon and shotgun metagenomic sequencing. Chemotaxis, growth and N2O emissions of isolated N2O-reducing bacteria in response to hyphal exudates were tested using in vitro cultures and inoculation experiments.ResultsAMF hyphae reduced denitrification-derived N2O emission (max. 63%) in C- and N-rich residue patches. AMF consistently enhanced the abundance and expression of clade I nosZ gene, and inconsistently increased that of nirS and nirK genes. The reduction of N2O emissions in the hyphosphere was linked to N2O-reducing Pseudomonas specifically enriched by AMF, concurring with the increase in the relative abundance of the key genes involved in bacterial citrate cycle. Phenotypic characterization of the isolated complete denitrifying P. fluorescens strain JL1 (possessing clade I nosZ) indicated that the decline of net N2O emission was a result of upregulated nosZ expression in P. fluorescens following hyphal exudation (e.g. carboxylates). These findings were further validated by re-inoculating sterilized residue patches with P. fluorescens and by an 11-year-long field experiment showing significant positive correlation between hyphal length density with the abundance of clade I nosZ gene.ConclusionsThe cooperation between AMF and the N2O-reducing Pseudomonas residing on hyphae significantly reduce N2O emissions in the microsites. Carboxylates exuded by hyphae act as attractants in recruiting P. fluorescens and also as stimulants triggering nosZ gene expression. Our discovery indicates that reinforcing synergies between AMF and hyphosphere microbiome may provide unexplored opportunities to stimulate N2O consumption in nutrient-enriched microsites, and consequently reduce N2O emissions from soils. This knowledge opens novel avenues to exploit cross-kingdom microbial interactions for sustainable agriculture and for climate change mitigation.C1gn2zbPQXat79TnoeAsNQVideo

Technological avenues and market mechanisms to accelerate methane and nitrous oxide emissions reductions.

Strategies targeting methane (CH4) and nitrous oxide (N2O) emissions are critical to meeting global climate targets. Existing literature estimates the emissions of these gases from specific sectors, but this knowledge must be synthesized to prioritize and incentivize CH4 and N2O mitigation. Accordingly, we review emissions sources and mitigation strategies in all key sectors (fuel extraction and combustion, landfilling, agriculture, wastewater treatment, and chemical industry) and the role of carbon markets in reducing emissions. The most accessible reduction opportunities are in the hydrocarbon extraction and waste sectors, where half (>3 Gt-CO2e/year) of the emissions in these sectors could be mitigated at no net cost. In total, 60% of CH4 emissions can be mitigated at less than $50/t-CO2. Expanding the scope of carbon markets to include these emissions could provide cost-effective decarbonization through 2050. We provide recommendations for carbon markets to improve emissions reductions and set prices to appropriately incentivize mitigation.

Episodic N2O emissions following tillage of a legume–grass cover crop mixture

Abstract. Nitrogen (N) fertilizer inputs to agricultural soils are a leading cause of nitrous oxide (N2O) emissions. Legume cover crops are an alternative N source that can reduce agricultural N2O emissions compared to fertilizer N. However, our understanding of episodic N2O flux following cover crop incorporation by tillage is limited and has focused on single-species cover crops. Our study explores whether increasing cover crop functional diversity with a legume–grass mixture can reduce pulse emissions of N2O following tillage. In a field experiment, we planted crimson clover (Trifolium incarnatum L.), cereal rye (Secale cereal L.), a clover–rye mixture, and a no-cover control at two field sites with contrasting soil fertility properties in Michigan. We hypothesized that N2O flux following tillage of the cover crops would be lower in the mixture and rye compared to the clover treatment because rye litter can decrease N mineralization rates. We measured N2O for approximately 2 weeks following tillage to capture the first peak in N2O emissions in each site. Across cover crop treatments, the higher-fertility site, CF, had greater cover crop biomass, 2-fold-higher aboveground biomass N, and higher cumulative N2O emissions than the lower-fertility site, KBS (413.4±67.5 vs. 230.8±42.5 g N2O-N ha−1; P=0.004). There was a significant treatment effect on daily emissions at both sites. At CF, N2O fluxes were higher following clover than the control 6 d after tillage. At KBS, fluxes from the mixture were higher than rye 8 and 11 d after tillage. When controlling for soil fertility differences between sites, clover and mixture led to approximately 2-fold-higher N2O emissions compared to rye and fallow treatments. We found partial support for our hypothesis that N2O would be lower following incorporation of the mixture than clover. However, treatment patterns differed by site, suggesting that interactions between cover crop functional types and background soil fertility influence N2O emissions during cover crop decomposition.