Nitrous Oxide Emissions Increase Exponentially When Optimum Nitrogen Fertilizer Rates Are Exceeded in the North China Plain

The legacy effect of long-term nitrogen fertilization on nitrous oxide emissions

The primary driver of increasing atmospheric concentrations of nitrous oxide (N2O) is the use of organic and synthetic fertilizer to increase agricultural crop production. Current global estimates are based on IPCC N2O emission factor (EF) calculations, although there are shortcomings as many of the N2O EFs are derived from measurements during the cropping season. These neglect the fallow season, and do not adequately account for double or even triple cropping systems or legacy effects on soil N2O emissions in the following year. In this study, we assessed the legacy effect of fertilization on soil N2O fluxes using data from a long-term double-cropping field experiment with summer maize and winter wheat in rotation, in which no nitrogen (N; NN) and balanced manure with synthetic N (MN) fertilized treatments were switched to allow an assessment of legacy effects. Based on high-frequency measurements of N2O and previous data, we calculated that the historical N fertilization, or legacy effect, explained 23 % of the annual flux of 0.81 kg N ha-1 yr-1 in the first season of observation. In the following three seasons, the legacy effect of the previous N fertilization regime decreased to a negligible level, with N2O emissions mainly driven by in-season fertilization. Our data show that, on average, the seasonal EF for N2O was about 0.11 % higher in response to the previous N fertilization. Our study indicates that the current N2O EF may severely underestimate emissions because studies ignore legacy effects on N2O emissions from zero N plots and only compare zero N with N fertilization treatments for a given season or year to derive seasonal or annual N2O EF.

Modulation of soil nitrous oxide emissions and nitrogen leaching by hillslope hydrological processes

Soil nitrous oxide (N2O) emissions and nitrogen (N) leaching are key pathways for soil N loss in hillslope ecosystem, with potential implications for global warming and water body eutrophication. While soil N loss in hillslope ecosystem has been extensively studied, there is limited understanding of the spatiotemporal distribution patterns and factors driving soil N2O emissions and N leaching from a hillslope hydrology perspective. This study investigated N concentrations in leachate and soil N2O fluxes and their responses to soil hydrological factors on a tea plantation (TP) hillslope and a bamboo forest (BF) hillslope. Four distinct precipitation patterns-spring rainfall (SR), plum rain (PR), summer flood rain (SF), and drought period (DR)-were identified based on precipitation intensity, duration, and cumulative precipitation. Results showed that, soil N2O flux and leachate N concentrations were 8.2 times and 18.0 times higher On TP hillslope compared to the BF hillslope. The greatest soil N2O fluxes occurred during the PR period, while the lowest were observed during the DR period. Precipitation increased soil water content (SWC) and water-filled pore space, stimulating soil N cycling for N2O production. Fertilization activities and precipitation led to peak N concentration in leachate during the SR period. Additionally, soil wetness index (SWI) shaped spatial patterns of SWC, resulting in distinct spatial patterns of N2O emissions and nitrate leaching. Locations with higher SWI exhibited greater soil N2O flux and higher nitrate concentrations in leachate. This study emphasizes the significant effect of soil hydrological processes on soil N2O emissions and N leaching in hillslope ecosystems, providing valuable insights for N management in these environments.

IPCC Emission Factor Overestimates N2O Emissions from Agricultural Ditches

Agricultural ditches emit disproportionate amounts of nitrous oxide (N2O), but their contributions to regional or global N2O emissions remain unclear due to limited data. The Intergovernmental Panel on Climate Change (IPCC) recommends using emission factors (EFs) to estimate indirect N2O emission, but the EF for ditches (EF5g) is categorized as groundwater, which potentially introduces a significant bias. This study conducted a regional-scale campaign in the North China Plain, one of the world's most intensive agricultural regions, and calculated the EF5g values from agricultural ditches by the concentration method (N2O-N/NO3--N). The results found that the regional-scale mean EF5g value (0.0028) was less than half of the IPCC default value (0.006), illustrating that the current IPCC methodology significantly overestimates N2O emissions from agricultural ditches. Despite the relatively small EF5g values, agricultural ditches exhibited a high mean N2O concentration (3.36 μg L-1) and a large regional emission (1.14 ± 0.86 Gg N2O-N yr-1), which is equal to 3.8 ± 2.9% of direct N2O emission from the croplands in the North China Plain. Since ditches are ubiquitous in agricultural regions and are likely to expand under climate change, refining EF5g is crucial to accurately assess their contribution to global N2O budgets.

Reevaluating the Drivers of Fertilizer-Induced N2O Emission: Insights from Interpretable Machine Learning

Direct nitrous oxide (N2O) emissions from fertilizer application are the largest anthropogenic source of global N2O, but the factors influencing these emissions remain debated. Here, we compile 1134 observations of fertilizer-induced N2O emission factor (EF) from 229 publications, covering various regions and crops globally. We then employ an interpretable machine learning model to investigate the driving factors of fertilizer-induced N2O emissions. Our results reveal that pH, soil organic carbon, precipitation, and temperature are the most influential factors, overweighing the impacts of management practices. Nitrogen application rate has a positive impact on the EF, but the effect diminishes as nitrogen application rate increases, which has been overestimated in previous studies. Soil pH has three-stage influence on EF: positive when 7.3 ≤ pH ≤ 8.7, significantly negative between 6.8 and 7.3, and insignificant at lower pH levels (4.7 ≤ pH ≤ 6.8). Moreover, we confirm the nonlinear contributions of temperature and precipitation to EF, which may cause an unexpected increase in N2O emission under climate change. Our research provides crucial insights for global N2O modeling and mitigation strategies.

Partial substitution of manure increases N2O emissions in the alkaline soil but not acidic soils

The fertilization regimes of combining manure with synthetic fertilizer are benefits for crop yields and soil fertility in cropping systems as compared to sole synthetic fertilization, but the responses of nitrous oxide (N2O) emissions to these practices are inconsistent in the literatures. We hypothesized that it is caused by different proportions of nitrogen (N) applied as manure and various soil properties. Here, we conducted a microcosm experiment, and measured the N2O emissions from control (no N) and five manure substitution treatments (supplied 100 mg N kg-1 using the combination of urea with manure) with a range of proportions of N applied as manure (0, 25%, 50%, 75%, and 100%) in three different soil types (fluvo-aquic soil, black soil, and latosol) under aerobic condition. The stimulated effect on N2O emissions was more pronounced after manure application in an alkaline soil with high nitrification rate, due to relatively rapid soil DOC depletion and N mineralization of manure. N2O emissions from partial substitution of urea with manure were significantly higher than manure-only addition under high soil pH due to abundant labile C from manure. However, there was no difference between manure substitution treatments under acid soils. Nitrification inhibitor substantially decreased N2O emissions with increasing soil pH, but it was less effective in mitigating N2O emissions with larger proportion of manure. This is likely due to the slow nitrification under low soil pH, and denitrification derived N2O increased with increasing manure application rate. Collectively, our study shows that the application of manure substitution to alkaline soils requires careful consideration, which might have rapid nitrification potential and hence trigger significant N2O emissions. The knowledge gained in this work will help the decision-makers in optimizing a sound N fertilization regime interacted with soil properties for sustainable crop production and N2O mitigation.

Evidence that co-existing cadmium and microplastics have an antagonistic effect on greenhouse gas emissions from paddy field soils

Accumulation of microplastics (MPs) and cadmium (Cd) are ubiquitous in paddy soil. However, the combined effects of MPs and Cd on physiochemical and microbial mechanisms in soils and the attendant implications for greenhouse gas (GHG) emissions, remain largely unknown. Here, we evaluated the influence of polylactic acid (PLA) and polyethylene (PE) MPs on GHG emissions from Cd-contaminated paddy soil using a microcosm experiment under waterlogged and drained conditions. The results showed that PLA significantly increased CH4 and N2O emission fluxes and hence the global warming potential (GWP) of waterlogged soil. Soils treated with MPs+Cd showed significantly reduced GWP compared to those treated only with MPs suggesting that, irrespective of attendant consequences, Cd could alleviate N2O emissions in the presence of MPs. Conversely, the presence of MPs in Cd-contaminated soils tended to alleviate the bioavailability of Cd. Based on a structural equation model analysis, both the MPs-derived dissolved organic matter and the soil bioavailable Cd affected indirectly on soil GHG emissions through their direct influencing on microbial abundance (e.g., Firmicutes, Nitrospirota bacteria). These findings provide new insights into the assessment of GHG emissions and soil/cereal security in response to MPs and Cd coexistence that behaved antagonistically with respect to adverse ecological effects in paddy systems.

AOB Nitrosospira cluster 3a.2 (D11) dominates N2O emissions in fertilised agricultural soils

Ammonia-oxidation process directly contribute to soil nitrous oxide (N2O) emissions in agricultural soils. However, taxonomy of the key nitrifiers (within ammonia oxidising bacteria (AOB), archaea (AOA) and complete ammonia oxidisers (comammox Nitrospira)) responsible for substantial N2O emissions in agricultural soils is unknown, as is their regulation by soil biotic and abiotic factors. In this study, cumulative N2O emissions, nitrification rates, abundance and community structure of nitrifiers were investigated in 16 agricultural soils from major crop production regions of China using microcosm experiments with amended nitrogen (N) supplemented or not with a nitrification inhibitor (nitrapyrin). Key nitrifier groups involved in N2O emissions were identified by comparative analyses of the different treatments, combining sequencing and random forest analyses. Soil cumulative N2O emissions significantly increased with soil pH in all agricultural soils. However, they decreased with soil organic carbon (SOC) in alkaline soils. Nitrapyrin significantly inhibited soil cumulative N2O emissions and AOB growth, with a significant inhibition of the AOB Nitrosospira cluster 3a.2 (D11) abundance. One Nitrosospira multiformis-like OTU phylotype (OTU34), which was classified within the AOB Nitrosospira cluster 3a.2 (D11), had the greatest importance on cumulative N2O emissions and its growth significantly depended on soil pH and SOC contents, with higher growth at high pH and low SOC conditions. Collectively, our results demonstrate that alkaline soils with low SOC contents have high N2O emissions, which were mainly driven by AOB Nitrosospira cluster 3a.2 (D11). Nitrapyrin can efficiently reduce nitrification-related N2O emissions by inhibiting the activity of AOB Nitrosospira cluster 3a.2 (D11). This study advances our understanding of key nitrifiers responsible for high N2O emissions in agricultural soils and their controlling factors, and provides vital knowledge for N2O emission mitigation in agricultural ecosystems.

China's anthropogenic N2O emissions with analysis of economic costs and social benefits from reductions in 2022

We assess China's overall anthropogenic N2O emissions via the official guidebook published by Chinese government. Results show that China's overall anthropogenic N2O emissions in 2022 were around 1593.1 (1508.7-1680.7) GgN, about 47.0 %, 27.0 %, 13.4 %, 4.9 %, and 7.7 % of which were caused by agriculture, industry, energy utilization, wastewater, and indirect sources, respectively. Maximum reduction rate for N2O emissions from agriculture, industry, energy utilization, wastewater, and indirect sources can achieve 69 %, 99 %, 79 %, 86 %, and 48 %, respectively, in 2022. However, given current global scenarios with a rapidly changing population and geopolitical and energy tension, the emission reduction may not be fully fulfilled. Without compromising yields, China's theoretical minimum anthropogenic N2O emissions would be 600.6 (568.8-633.6) GgN. In terms of the economic costs for reducing one kg of N2O-N emissions, the price ranged from €12.9 to €81.1 for agriculture, from €0.08 to €0.16 for industry, and from €104.8 to €1571.5 for energy utilization. We acknowledge the emission reduction rates may not be completely realistic for large-scale application in China. The social benefits gained from reducing one kg of N2O-N emissions in China was about €5.2, indicating anthropogenic N2O emissions caused a loss 0.03 % of China's GDP, but only justifying reduction in industrial N2O emissions from the economic perspective. We perceive that the present monetized values will be trustworthy for at least three to five years, but later the numerical monetized values need to be considered in inflation and other currency-dependent conditions.

Quantifying synergies and trade-offs in the food-energy-soil-environment nexus under organic fertilization

Recycling livestock manure in agroecosystems can maintain crop production, improve soil fertility, and reduce environmental losses. However, there has been no comprehensive assessment of synergies and trade-offs in the food-energy-soil-environment nexus under manure application. Here, we evaluate the sustainability of maize production under four fertilization regimes (mineral, mineral and manure mixed, manure, and no fertilization) from the aspect of food security, energy output, soil quality, and environmental impact based on a five-year field experiment. Manure and mineral mixed fertilization maintained grain and straw quantity and quality compared with mineral fertilization. Manure and mineral mixed fertilization increased stem/leaf ratio and field residue index by 9.1-28.9% and 4.5-17.9%, respectively. Manure also maintained the theoretical ethanol yield but reduced the straw biomass quality index by increasing ash. Further, manure application increased the soil quality index by 40.5% and reduced N2O emissions by 55.0% compared with mineral fertilization. Manure application showed the highest sustainability performance index of 19, followed by mineral (15), mixed (13), and without fertilization (8). In conclusion, manure application maintains food production and energy output, enhances soil quality, and reduces environmental impact, thereby improving the sustainability of maize production.

Salinity change induces distinct climate feedbacks of nitrogen removal in saline lakes

Current estimations of nitrogen biogeochemical cycling and N2O emissions in global lakes as well as predictions of their future changes are overrepresented by freshwater datasets, while less consideration is given to widespread saline lakes with different salinity (representing salinization or desalinization). Here, we show that N2O production by denitrification is the main process of reactive nitrogen (Nr, the general abbreviations of NH4+-N, NO2--N and NO3--N) removal in hypersaline lake sediments (e.g. Lake Chaka). The integration of our field measurements and literature data shows that in response to natural salinity decrease, potential Nr removal increases while N2O production decreases. Furthermore, denitrification-induced N2 production exhibits higher salinity sensitivity than denitrification-induced N2O production, suggesting that the contribution of N2O to Nr removal decreases with decreasing salinity. This field-investigation-based salinity response model of Nr removal indicates that under global climate change, saline lakes in the process of salinization or desalination may have distinct Nr removal and climate feedback effects: salinized lakes tend to generate a positive climate feedback, while desalinated lakes show a negative feedback. Therefore, salinity change should be considered as an important factor in assessing future trend of N2O emissions from lakes under climate change.

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

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

Uncertainty in non-CO2 greenhouse gas mitigation contributes to ambiguity in global climate policy feasibility

Despite its projected crucial role in stringent, future global climate policy, non-CO2 greenhouse gas (NCGG) mitigation remains a large uncertain factor in climate research. A revision of the estimated mitigation potential has implications for the feasibility of global climate policy to reach the Paris Agreement climate goals. Here, we provide a systematic bottom-up estimate of the total uncertainty in NCGG mitigation, by developing 'optimistic', 'default' and 'pessimistic' long-term NCGG marginal abatement cost (MAC) curves, based on a comprehensive literature review of mitigation options. The global 1.5-degree climate target is found to be out of reach under pessimistic MAC assumptions, as is the 2-degree target under high emission assumptions. In a 2-degree scenario, MAC uncertainty translates into a large projected range in relative NCGG reduction (40-58%), carbon budget (±120 Gt CO2) and policy costs (±16%). Partly, the MAC uncertainty signifies a gap that could be bridged by human efforts, but largely it indicates uncertainty in technical limitations.

Combination of suitable planting density and nitrogen rate for high yield maize and their source-sink relationship in Northwest China

BACKGROUND

Increasing crop yield per unit area by increasing planting density is essential to ensure food security. However, the optimal combination of planting density and nitrogen (N) application for high-yielding maize and its source-sink characteristics need to be more clearly understood.

RESULTS

A 2-year field experiment was conducted combining three planting densities (D1: 70 000 plants ha^-1 ; D2: 100 000 plants ha^-1 ; D3: 130 000 plants ha^-1 ) and three nitrogen rates (N1: 150 kg hm^-2 ; N2: 350 kg hm^-2 ; N3: 450 kg hm^-2 ). The results showed that increasing planting density significantly increased leaf area index and grain yield but negatively affected ear traits. The Richards model was used to fit the dynamic changes of dry matter accumulation of maize under different treatments, and the fitting results were good. Increasing planting density increased population yield while limiting the development of individual plants, bringing the period of rapid dry matter accumulation to an early end and accelerating leaf senescence. An appropriate nitrogen rate could prolong the period of rapid accumulation of dry matter in maize, and increase the 100-kernel weight. Increasing planting density enhanced post-silking dry matter accumulation to a lesser extent, and the source-sink relationship of the maize population gradually developed from sink limitation to source limitation with increasing planting density.

CONCLUSION

The decrease in yield due to the insufficient source strength to meet the sink demand at too high densities was the reason that limited further improvement of the optimal planting density. An appropriate nitrogen rate facilitated the realization of yield potential at high density. © 2023 Society of Chemical Industry.

Optimizing Agronomic, Environmental, Health and Economic Performances in Summer Maize Production through Fertilizer Nitrogen Management Strategies

Although nitrogen (N) fertilizer application plays an essential role in improving crop productivity, an inappropriate management can result in negative impacts on environment and human health. To break this dilemma, a 12-year field experiment (2008-2019) with five N application rates was conducted on the North China Plain (NCP) to evaluate the integrated impacts of optimizing N management (Opt. N, 160 kg N ha-1 on average) on agronomic, environmental, health, and economic performances of summer maize production. Over the 12-year study, the Opt. N treatment achieved the maximal average grain yield (10.6 Mg ha-1) and grain protein yield (793 kg ha-1) among five N treatments. The life cycle assessment methodology was applied to determine the negative impacts on environmental and human health, and both of them increased with the N rate. Compared with the farmers' conventional N rate (250 kg N ha-1), the Opt. N treatment reduced acidification, eutrophication, global warming, and energy depletion potentials by 29%, 42%, 35%, and 18%, respectively, and reduced the health impact by 32% per Mg of grain yield or grain protein yield produced. Both the Opt. N and Opt. N*50-70% treatments resulted in high private profitability (2038 USD ha-1), ecosystem economic benefit (1811 USD ha-1), and integrated compensation benefit (17,548 USD ha-1). This study demonstrates the potential benefits of long-term optimizing of N management to maintain high maize yields and grain quality, to reduce various environmental impacts and health impacts, and to enhance economic benefits. These benefits can be further enhanced when Opt. N was combined with advanced agronomic management practices. The results also suggest that reducing the optimal N rate from 160 to 145 kg N ha-1 is achievable to further reduce the negative impacts while maintaining high crop productivity. In conclusion, optimizing the N management is essential to promote sustainable summer maize production on the NCP.

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

BACKGROUND

Arbuscular 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.

RESULTS

AMF 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.

CONCLUSIONS

The 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. Video Abstract.

Effects of periodic drying-wetting on microbial dynamics and activity of nitrite/nitrate-dependent anaerobic methane oxidizers in intertidal wetland sediments

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays an important role in methane (CH₄) consumption in intertidal wetlands. However, little is known about the responses of n-DAMO in intertidal wetlands to periodic drying-wetting caused by tidal cycling. Here, comparative experiments (waterlogged, desiccated, reflooded) with the Yangtze estuarine intertidal sediments were performed to examine the effects of periodic tidal changes on n-DAMO microbial communities, abundances, and potential activities. Functional gene sequencing indicated the coexistence of n-DAMO bacteria and archaea in the tide-fluctuating environments and generally higher biodiversity under reflooded conditions than consecutive inundation or emersion. The n-DAMO microbial abundance and associated activity varied significantly during alternative exposure and inundation, with higher abundance and activity under the waterlogged than desiccated conditions. Reflooding of intertidal wetlands might intensify n-DAMO activities, indicating the resilience of n-DAMO microbial metabolisms to the wetting-drying events. Structural equation modeling and correlation analysis showed that n-DAMO activity was highly related to n-DAMO microbial abundance and substrate availability under inundation, whereas salt accumulation in sediment was the primary factor restraining n-DAMO activity under the desiccation. Overall, this study reveals tidal-induced shifts of n-DAMO activity and associated contribution to mitigating CH₄, which may help accurately project CH₄ emission from intertidal wetlands under different tidal scenarios.

Quantifying nitrous oxide production rates from nitrification and denitrification under various moisture conditions in agricultural soils: Laboratory study and literature synthesis

Biogenic nitrous oxide (N2O) from nitrification and denitrification in agricultural soils is a major source of N2O in the atmosphere, and its flux changes significantly with soil moisture condition. However, the quantitative relationship between N2O production from different pathways (i.e., nitrification vs. denitrification) and soil moisture content remains elusive, limiting our ability of predicting future agricultural N2O emissions under changing environment. This study quantified N2O production rates from nitrification and denitrification under various soil moisture conditions using laboratory incubation combined with literature synthesis. 15N labeling approach was used to differentiate the N2O production from nitrification and denitrification under eight different soil moisture contents ranging from 40 to 120% water-filled pore space (WFPS) in the laboratory study, while 80 groups of data from 17 studies across global agricultural soils were collected in the literature synthesis. Results showed that as soil moisture increased, N2O production rates of nitrification and denitrification first increased and then decreased, with the peak rates occurring between 80 and 95% WFPS. By contrast, the dominant N2O production pathway switched from nitrification to denitrification between 60 and 70% WFPS. Furthermore, the synthetic data elucidated that moisture content was the major driver controlling the relative contributions of nitrification and denitrification to N2O production, while NH4 + and NO3 - concentrations mainly determined the N2O production rates from each pathway. The moisture treatments with broad contents and narrow gradient were required to capture the comprehensive response of soil N2O production rate to moisture change, and the response is essential for accurately predicting N2O emission from agricultural soils under climate change scenarios.

Sustainable water and nitrogen optimization to adapt to different temperature variations and rainfall patterns for a trade-off between winter wheat yield and N2O emissions

Optimizing irrigation and nitrogen (N) fertilizer applications is essential to ensure crop yields and lower environmental risks under climate change. The DeNitrification-DeComposition (DNDC) model was employed to investigate the impacts of irrigation regime (RF, rainfed; MI, minimum irrigation; CI, critical irrigation; FI, full irrigation) and N fertilizer rate (N60, N90, N120, N150, N180, N210, N240, N270, and N300 kg ha-1) on yield and nitrous oxide (N2O) emissions from winter wheat growing season under different temperature rise levels (+0, +0.5, +1.0, +1.5, and +2.0 °C scenarios) and precipitation year types (wet, normal, and dry seasons) in the North China Plain. Model evaluations demonstrated that simulated soil temperature, soil moisture, daily N2O flux, yield, and cumulative N2O emissions were generally in close agreement with measurements from field experiment over three growing seasons. By adopting simulation scenarios analysis, the model was then used to explore the effects of irrigation and N fertilizer inputs to balance yield and N2O emissions from winter wheat growing season. Based on reduced water and fertilizer inputs and N2O emissions with little yield penalty, recommended management practices included application of MI-N150 in wet season, CI-N120 in both normal and dry seasons, and CI-N150 for +0 to +2.0 °C scenarios. Recommended practices in different precipitation year types reduced irrigation amount by 75-150 mm, N rate by 75-105 kg N ha-1, yield by 0.16-0.86 t ha-1, cumulative N2O emissions by 0.13-0.64 kg ha-1, and yield-scaled N2O emissions by 15.5-85.0 mg kg-1 compared with current practices. The corresponding metrics for different elevated temperature levels decreased by 75 mm, 75 kg N ha-1, 0.09-0.50 t ha-1, 0.12-0.52 kg ha-1, and 13.7-72.3 mg kg-1, respectively. The proposed management practices can help to maintain high agronomic productivity and alleviate environmental pollution from agricultural ecosystems, thereby providing an important basis for mitigation strategies to adapt to climate change.

Agriculture-Induced N2O Emissions and Reduction Strategies in China

Greenhouse gases are one of the most important factors in climate change, their emissions reduction is a global problem. Clarifying the spatial patterns of N2O, as an important component of greenhouse gases, it is of great significance. Based on the planting and breeding data of China from 2000 to 2019, this paper measures the N2O emissions of agricultural systems, and uses kernel density to explore the spatial distribution differences between the eight major economic zones. Finally, the proposed emissions reduction countermeasures are provided. The research results show that the N2O emissions of China's agricultural system showed a trend of increasing first and then decreasing, and in 2019, the national N2O emissions were 710,300 tons, agricultural land emissions and animal husbandry emissions were the main sources of N2O emissions. The difference in N2O emissions by province was significant, the concentration trend was more prominent, and the differences of N2O emissions between provinces and regions were diverse. In order to achieve the reduction in N2O emissions, it is necessary to carry out low-carbon production of staple grains for different parts and economic zones, and focusing on low-carbon production in the Central Part and the West Part, as well as the Northeast and the Greater Southwest zones, is essential.

Appropriate N fertilizer addition mitigates N2O emissions from forage crop fields

Forage crops are widely cultivated as livestock feed to relieve grazing pressure in agro-pastoral regions with arid climates. However, gaseous losses of soil nitrogen (N) following N fertilizer application have been considerable in response to the pursuit of increased crop yield. A two-year experiment was carried out in a typical saline field under a temperate continental arid climate to investigate the effect of N application rate on N₂O emissions from barley (Hordeum vulgare L.), corngrass (Zea mays × Zea Mexicana), rye (Secale cereale L.), and sorghum-sudangrass hybrid (Sorghum bicolor × Sorghum sudanense). The dynamics of N₂O emissions, hay yield, and crude protein (CP) yield were measured under four N application rates (0, 150, 200, and 250 kg ha^-1) in 2016 and 2017. An N₂O emission peak was observed for all crop species five days after each N application. Cumulative N₂O fluxes in the growing season ranged from 0.66 to 2.40 kg ha^-1 and responded exponentially to N application rate. Emission factors of N₂O showed a linear increase with N application rate for all crop species, but the linear slopes significantly differed between barley or rye and corngrass and sorghum-sudangrass hybrid. The hay and CP yields of all forage grasses significantly increased with the increase of N application rate from 0 to 200 kg ha^-1. Barley and rye with lower hay and CP yields showed higher N₂O emission intensities. The increased level of N₂O emission intensity was higher from 200 to 250 kg ha^-1 than from 150 to 200 kg ha^-1. At N application rates of 200 and 250 kg ha^-1, CP yield had a significantly negative correlation with cumulative N₂O emission and explained 50.5% and 62.9% of the variation, respectively. In conclusion, ~200 kg ha^-1 is the optimal N rate for forage crops to minimize N₂O emission while maintaining yield in continental arid regions.

Towards a clean production by exploring the nexus between agricultural ecosystem and environmental degradation using novel dynamic ARDL simulations approach

Agriculture, which serves as a lifeline for us, is unequivocally vital for an agriculture-dependent economy like Bangladesh, not only for its food supply but also because of its significant contribution towards achieving Sustainable Development Goals (SDGs) 1 and 2. However, in a third-world nation like Bangladesh, where farming practices largely circumvent the environmental consequences, raised our concern. In this milieu, this study is a novel attempt to explore the association between agricultural ecosystem and environmental degradation in Bangladesh using a long time spanning from 1972 to 2018. We observed a long-run association between the agroecosystem and CO₂ emission. Further, findings from the dynamic autoregressive distributed lag (DARDL) simulation model revealed that the environmental quality of Bangladesh is heavily distorted by total cereal production, total livestock head, enteric methane emissions, N₂O emissions from manure application, and CO₂ equivalent N₂O emissions from synthetic fertilizers in the short and long run, whereas agricultural technology, pesticide use in agriculture, and burned biomass crop residue deteriorated the environmental quality only in the long run. The counterfactual diagram entailed from the DARDL model projected the trend of CO₂ emission in response to positive and negative changes in the analyzed variables. Lastly, this study established a causal relationship between the agroecosystem and environmental degradation using frequency domain causality. Indeed, our study will aid in reshaping agricultural practices in an eco-friendly manner to mitigate environmental degradation and help formulate pragmatic policy actions so that agro-lead nations can thrive in the race of achieving SDGs 1, 2, and 13.

Optimum fertilizer application rate to ensure yield and decrease greenhouse gas emissions in rain-fed agriculture system of the Loess Plateau

Application of nitrogen (N) can increase the supply of N in soil and, in turn, can lead to higher yield-but also to large increase in emissions of greenhouse gases (GHGs) if applied in excess. To determine the optimum dose of N for maize planting system, we analysed the relationship between yield and emissions of GHGs at seven levels of N, namely 50, 100, 150, 200, 250, 300, and 350 kg ha^-1, using the DNDC (denitrification decomposition) model and maize grown with and without mulching. The model simulated the following variables: maize production; emissions of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄); global warming potential (GWP); and GHG intensity (GHGI). We used data from 1980 to 2013 for a rain-fed region of the Loess Plateau in north-western China and validated the DNDC model against data from field experiments. The model performed well in simulating yield and GHG emissions (Adj.R² > 0.61). Under mulching, the average yield of maize was 3.6-12.2 t ha^-1 and the partial factor productivity was 73.1-35.0 kg kg^-1; and both of these were significantly higher 78%-236% than those in the crop without mulching. The emissions of CO₂, N₂O, and the GWP increased with the increase in the dose of N whereas CH₄ emissions remained unaffected by the dose. Mulching increased yields significantly in the north-western region, and the GWP and GHGI were higher mainly in the central and north-western regions. The optimum dose of N for maize grown with mulching ranged between 150 kg ha^-1 and 200 kg ha^-1 and offers the best balance between higher yield and lower emissions. The optimum dose may promote the development of mulched maize and provide a reference standard for dryland agriculture in zones with similar climates elsewhere in the world.

Optimizing organic amendment applications to enhance carbon sequestration and economic benefits in an infertile sandy soil

A thorough understanding of the agricultural, ecological, and economic benefits of organic amendment (OA) application in infertile soils is crucial for facilitating agricultural sustainability. We conducted a three-year field study to evaluate the effects of OA application on soil organic carbon (SOC) sequestration, crop yields, and the net ecosystem economic benefit (NEEB) in a typical infertile sandy soil (with an initial SOC content of 2.56 g kg^-1) of the ancient Yellow River alluvial plain. In addition to the control (CK; non-OA application), two types of OAs, namely, manure-based organic fertilizer (M) and spent mushroom residue (MR), were each applied at 12, 24, and 36 Mg ha^-1 yr^-1. Two scenarios of OA application practices, namely, conventional manual OA application (AMA) and mechanical OA application (AME), were considered in the economic evaluation. An increase of 1 g kg^-1 SOC content could improve the crop yield by 2.25 Mg ha^-1 yr^-1. Compared with the CK, the application of OAs enhanced the SOC content and SOC stock by 14.6%-39.8% and 8.5%-28.2%, respectively. However, the SOC sequestration efficiency of the OAs tended to decrease under high rates of OA application. MR was observed to have greater potential than M in sequestering SOC and promoting soil aggregates. OA-induced SOC sequestration could neutralize 36.6%-97.8% of greenhouse gas emissions, which resulted in a reduction in the global warming potential and its cost by 0.62-2.68 Mg CO₂-eq ha^-1 yr^-1 and 15.46-65.78 CNY ha^-1 yr^-1, respectively. Nevertheless, in terms of the NEEB, the benefits of OA application on crop yield and SOC sequestration were largely offset by the increased material and labor costs. Compared with AMA, AME could save 10%-27% of agricultural costs. The AME of MR at a rate of 24 Mg ha^-1 yr^-1 achieved the highest NEEB. The results of this study suggest that a strategy involving the appropriate OA, optimal application rate, and cheapest incorporation cost for a specific individual soil should be adopted to achieve a sustainable solution for promoting crop productivity, enhancing SOC sequestration, and ensuring farmer income in infertile farming regions.

Water environmental pressure assessment in agricultural systems in Central Asia based on an Integrated Excess Nitrogen Load Model

Agricultural runoff is the main source of water pollution in Central Asia. Excessive nitrogen (N) inputs from overuse of chemical fertilizers are threatening regional water resources. However, the scarcity of quantitative data and simplified empirical models limit the reliability of grey water footprint (GWF), particularly in undeveloped regions. In this study, we developed an Integrated Excess Nitrogen Load Model (IENLM) to calculate excess N load and evaluate its potential water environmental pressure in Central Asia. The model optimized the biological N fixation and atmospheric N deposition modules by involving more environmental variables and human activities. Results showed that N fertilizer application contributed over 60% to total N input and was mainly responsible for 42.9% increase of total GWF from 101.5 to 145.0 billion m³ during 1992 - 2018. Water pollution level (WPL) increased from 0.55 in 1992 to 2.41 in 2018 and the pollution assimilation capacity of water systems has been fully consumed just by N load from agriculture since 2005. GWF intensity and grey water pollution - efficiency types in all Central Asian countries have improved in recent years except for Turkmenistan. N fertilizer application and agricultural economy development were the main driving factors induced N pollution. Results were validated by riverine nitrate concentrations and the estimates from prior studies. In future, combining the N fertilizer reduction with other farm management practices were projected to effectively improve the WPL. The modeling framework is favorable for N pollution research in data-scarce regions and provides a scientific basis for decision-making for agriculture and water resource managements.

Soil oxygen depletion and corresponding nitrous oxide production at hot moments in an agricultural soil

Hot moments of nitrous oxide (N₂O) emissions induced by interactions between weather and management make a major contribution to annual N₂O budgets in agricultural soils. The causes of N₂O production during hot moments are not well understood under field conditions, but emerging evidence suggests that short-term fluctuations in soil oxygen (O₂) concentration can be critically important. We conducted high time-resolution field observations of O₂ and N₂O concentrations during hot moments in a dryland agricultural soil in Northern China. Three typical management and weather events, including irrigation (Irr.), fertilization coupled with irrigation (Fer.+Irr.) or with extreme precipitation (Fer.+Pre.), were observed. Soil O₂ and N₂O concentrations were measured hourly for 24 h immediately following events and measured daily for at least one week before and after the events. Soil moisture, temperature, and mineral N were simultaneously measured. Soil O₂ concentrations decreased rapidly within 4 h following irrigation in both the Irr. and Fer.+Irr. events. In the Fer.+Pre. event, soil O₂ depletion did not occur immediately following fertilization but began following subsequent continuous rainfall. The soil O₂ concentration dropped to as low as 0.2% (with the highest soil N₂O concentration of up to 180 ppmv) following the Fer.+Pre. event, but only fell to 11.7% and 13.6% after the Fer.+Irr. and Irr. events, which were associated with soil N₂O concentrations of 27 ppmv and 3 ppmv, respectively. During the hot moments of all three events, soil N₂O concentrations were negatively correlated with soil O₂ concentrations (r = -0.5, P < 0.01), showing a quadratic increase as soil O₂ concentrations declined. Our results provide new understanding of the rapid short response of N₂O production to O₂ dynamics driven by changes in soil environmental factors during hot moments. Such understanding helps improve soil management to avoid transitory O₂ depletion and reduce the risk of N₂O production.

Enrichment of nosZ-type denitrifiers by arbuscular mycorrhizal fungi mitigates N2 O emissions from soybean stubbles

Hotspots of N2 O emissions are generated from legume residues during decomposition. Arbuscular mycorrhizal fungi (AMF) from co-cultivated intercropped plants may proliferate into the microsites and interact with soil microbes to reduce N2 O emissions. Yet, the mechanisms by which or how mycorrhizal hyphae affect nitrifiers and denitrifiers in the legume residues remain ambiguous. Here, a split-microcosm experiment was conducted to assess hyphae of Rhizophagus aggregatus from neighbouring maize on overall N2 O emissions from stubbles of nodulated or non-nodulated soybean. Soil microbes from fields intercropped with maize/soybean amended with fertilizer nitrogen (SS-N1) or unamended (SS-N0) were added to the soybean chamber only. AMF hyphae consistently reduced N2 O emissions by 20.8%-61.5%. Generally, AMF hyphae promoted the abundance of N2 O-consuming (nosZ-type) denitrifiers and altered their community composition. The effects were partly associated with increasing MBC and DOC. By contrast, AMF reduced the abundance of nirK-type denitrifiers in the nodulated SS-N0 treatment only and that of AOB in the non-nodulated SS-N1 treatment. Taken together, our results show that AMF reduced N2 O emissions from soybean stubbles, mainly through the promotion of N2 O-consuming denitrifiers. This holds promise for mitigating N2 O emissions by manipulating the efficacious AMF and their associated microbes in cereal/legume intercropping systems.

Intensive vegetable production results in high nitrate accumulation in deep soil profiles in China

A comprehensive understanding of the patterns and controlling factors of nitrate accumulation in intensive vegetable production is essential to solve this problem. For the first time, the national patterns and controlling factors of nitrate accumulation in soil of vegetable systems in China were analysed by compiling 1262 observations from 117 published articles. The results revealed that the nitrate accumulation at 0-100 cm, 100-200 cm, 200-300 cm, and >300 cm were 504, 390, 349, and 244 kg N ha^-1, with accumulation rates of 62, 54, 19, and 16 kg N ha^-1 yr^-1 for plastic greenhouse vegetables (PG); for open field vegetables (OF), they were 264, 217, 228, and 242 kg N ha^-1 with accumulation rates of 26, 24, 18, and 10 kg N ha^-1 yr^-1, respectively. Nitrate accumulation at 0-100 cm, 0-200 cm, and 0-400 cm accounted for 5%, 11%, and 17% of accumulated nitrogen (N) inputs for PG, and represented 4%, 9%, and 13% of accumulated N inputs for OF. Nitrogen input rates and soil pH had positive effects and soil organic carbon, water input rate, and carbon to nitrogen ratio (C/N) had negative effects on nitrate accumulation in root zone (0-100 cm soil). Nitrate accumulation in deep vadose zone (>100 cm soil) was positively correlated with N and water input rates, and was negatively correlated with soil organic carbon, C/N, and the clay content. Thus, for a given vegetable soil with relatively stable soil pH and soil clay content, reducing N and water inputs, and increasing soil organic carbon and C/N are effective measures to control nitrate accumulation.

Nitrous oxide respiring bacteria in biogas digestates for reduced agricultural emissions

Inoculating agricultural soils with nitrous oxide respiring bacteria (NRB) can reduce N₂O-emission, but would be impractical as a standalone operation. Here we demonstrate that digestates obtained after biogas production are suitable substrates and vectors for NRB. We show that indigenous NRB in digestates grew to high abundance during anaerobic enrichment under N₂O. Gas-kinetics and meta-omic analyses showed that these NRB’s, recovered as metagenome-assembled genomes (MAGs), grew by harvesting fermentation intermediates of the methanogenic consortium. Three NRB’s were isolated, one of which matched the recovered MAG of a Dechloromonas, deemed by proteomics to be the dominant producer of N₂O-reductase in the enrichment. While the isolates harbored genes required for a full denitrification pathway and could thus both produce and sequester N₂O, their regulatory traits predicted that they act as N₂O sinks in soil, which was confirmed experimentally. The isolates were grown by aerobic respiration in digestates, and fertilization with these NRB-enriched digestates reduced N₂O emissions from soil. Our use of digestates for low-cost and large-scale inoculation with NRB in soil can be taken as a blueprint for future applications of this powerful instrument to engineer the soil microbiome, be it for enhancing plant growth, bioremediation, or any other desirable function.

Spatial patterns of net greenhouse gas balance and intensity in Chinese orchard system

Fruit production has been expanding due to the pursuit of healthier lifestyles in China. Determining the greenhouse gas (GHG) emissions status of the orchard system could contribute to adopting appropriate measures to alleviate climate change pressure from the growing fruit production. In this study, the net GHG balance and GHG intensity (GHGI) in the Chinese fruit production were estimated at the regional level using a meta-analysis based on databases compiled from relevant publications during 2000-2019, including soil nitrous oxide (N₂O) and methane (CH₄) emissions or uptake, upstream carbon dioxide (CO₂) emissions related to farm practices, and the change of soil organic carbon (SOC) storage from the life cycle perspective. Results showed that the net GHG balance and GHGI varied among six regions, with ranges of 6.4 ± 0.3 to 10.0 ± 0.6 Mg CO₂e ha^-1 yr^-1, and 2.2 ± 0.2 to 3.0 ± 0.2 kg CO₂e kg^-1, respectively. Synthetic nitrogen (N) fertilization was the largest source of overall GHG emissions from fruit production throughout China, accounting for 46% and ranging from 43% to 55% in the six fruit production regions. Fertilizer-induced N₂O emissions were responsible for 22-31% of the total GHG emissions, and the N₂O-N emission factor was identified as 0.7%. Also, power use for irrigation contributed a non-negligible 17% to the emissions on a national level, yet with large regional variations. In addition, fruit production in North, Northeast, Central, and East, and South China have relatively lower GHGIs than in Northwest and Southwest China. The estimated total GHG emissions from the Chinese fruit production were 102 Tg CO₂e, with the contribution of SOC change to a decrease by 11% for the year 2018. Our results highlight an urgency to lower fruit production-related carbon emissions by extending optimized N fertilization and irrigation modes in China's orchard system.

Cropping system design can improve nitrogen use efficiency in intensively managed agriculture

New agronomic and management approaches are urgently required to meet the challenges of improving resource use efficiency and crop yields in intensive agricultural systems. Here we report the fertilizer N use efficiency (FNUE), fate of fertilizer N and N budgets in newly designed cropping systems as compared with conventional winter wheat-summer maize double cropping (Con. W/M) in the North China Plain. A¹⁵N labelling approach was used to quantify FNUE by these new cropping systems which included optimized winter wheat-summer maize (Opt. W/M) with two harvests in one year; winter wheat/summer maize-spring maize (W/M-M) and winter wheat/summer soybean-spring maize (W/S-M) with three harvests in two years, and spring maize (M) with one harvest in one year. The results showed that only 18-20% of fertilizer N was recovered by crops in Con. W/M. Although Opt. W/M significantly increased FNUE to 33%-35% with increased crop yields, it consumed as much groundwater as Con. W/M. The W/M-M, W/S-M and M significantly increased FNUE to 27%-44% and reduced groundwater use and fertilizer N losses when compared to Con. W/M. The W/M-M achieved a comparable grain yield, but W/S-M and M had significantly lower grain yields when compared to Con. W/M. However, grain N harvest in W/S-M was comparable with Con. W/M due to higher grain N content in soybean. Post-anthesis fertilizer N uptake provided little contribution to total N uptake, and accounted for 5%, 12%, 7% and 2% of the average N uptake for winter wheat, spring maize, summer maize and summer soybean, respectively. When taking the second crop into account, Con. W/M recovered 27% of fertilizer N, while it increased to 36%-50% under the new cropping systems. We conclude that W/M-M and W/S-M will deliver significant improvements in the environmental footprints and sustainability of intensively managed cropping systems in the North China Plain.

Crop straw incorporation alleviates overall fertilizer-N losses and mitigates N2O emissions per unit applied N from intensively farmed soils: An in situ 15N tracing study

A thorough elucidation of the coupled effects of N fertilization and straw incorporation on N₂O emissions and N losses is crucial for alleviating negative environmental impacts in intensively farmed regions. Here, we conducted an in situ ¹⁵N tracing experiment to assess the source of N₂O emissions and fate of fertilizer-N in soil intensively farmed with summer maize (Zea mays L.). Four treatments, i.e., no N fertilization and no straw incorporation (N0S0), straw incorporation only (N0S1), N fertilization only (N1S0), and N fertilization plus straw incorporation (N1S1), were established in the study. Compared with straw removal, straw incorporation increased the seasonal N₂O emissions by 22.3% but reduced the N₂O emissions per unit of applied N by 6.22% (P > 0.05). The emission of fertilizer-derived N₂O occurred mainly in the 13-17 days after fertilization; thereafter, the ratio of fertilizer-derived N₂O fluxes would be less than 5%. N fertilization significantly stimulated non-fertilizer-derived N₂O emissions and soil CO₂ fluxes, especially when straw was incorporated (P < 0.05), indicating that N fertilization might have triggered the mineralization of straw-N and/or native soil organic N. The soil NO₃^--N concentration in straw-incorporated plots tended to be lower than that in straw-removed plots, especially after N fertilization events. Straw incorporation sequestered 52.5% (27.4 kg N ha^-1) more fertilizer-N in 1 m of soil than straw removal (P < 0.05) while significantly increasing the fertilizer-N harvest index and maintaining grain yield. Overall, compared with straw removal, straw incorporation significantly reduced total fertilizer-N losses (by 12.8%, i.e., 14.58 kg N ha^-1; P < 0.05). Our study highlights the benefits of straw incorporation for increasing in-season and multiseason fertilizer-N use efficiencies and alleviating fertilizer-N-induced environmental costs in intensively farmed regions.

Finding the optimal fertilizer type and rate to balance yield and soil GHG emissions under reclaimed water irrigation

Water and inorganic nitrogen fertilizer have a notable impact on crop yield and greenhouse gas (GHG) emissions from soil. Reclaimed water (RW) is widely used for irrigation when there are shortages of water resources. It is very important to control yield and greenhouse gas emissions by fertilization under reclaimed water irrigation (RWI). The study consisted of a continuous test that evaluated three types of fertilizer treatments (urea, amine, and slow-release fertilizer) and a no-fertilizer treatment under three-year RWI and four fertilizer levels (150, 200, 250 and 300 kg.N.ha^-1) under one-year RWI to determine the best fertilizer to support maize production and reduce GHG (CO₂ and N₂O) emissions from soil; further, the applicability of RWI in the DNDC model was verified. For many years, GHG emissions under RWI showed an increasing trend, but the effect was not significant. A strong correlation was found between the GHG emissions flux and fertilizer amount, and a threshold fertilization amount existed between 220 and 260 kg.N.ha^-1 that minimized yield-scaled N₂O emissions and the ratio of GHG cumulative emission to yield (GHG/Y). The results indicated that the optimal amounts of SF and UF under RWI were 240 and 225 kg.N.ha^-1 by second-order equation and the DNDC model, respectively, and the rate better balanced the yield and GHG emissions.

Calibration and validation of the DNDC model to estimate nitrous oxide emissions and crop productivity for a summer maize-winter wheat double cropping system in Hebei, China

The main aim of this paper was to calibrate and evaluate the DeNitrification-DeComposition (DNDC) model for estimating N₂O emissions and crop productivity for a summer maize-winter wheat double cropping system with different N fertilizer rates in Hebei, China. The model's performance was assessed before and after calibration and model sensitivity was investigated. The calibrated and validated DNDC performed effectively in estimating cumulative N₂O emissions (coefficient of determination (1:1 relationship; r²) = 0.91; relative deviation (RD) = -13 to 16%) and grain yields for both crops (r² = 0.91; RD = -21 to 7%) from all fertilized treatments, but poorly estimated daily N₂O patterns. Observed and simulated results showed that optimal N fertilizer treatment decreased cumulative N₂O flux, compared to conventional N fertilizer, without a significant impact on grain yields of the summer maize-winter wheat double cropping system. The high sensitivity of the DNDC model to rainfall, soil organic carbon and temperature resulted in significant overestimation of N₂O peaks during the warm wet season. The model also satisfactorily estimated daily patterns/average soil temperature (^o C; 0-5 cm depth) (r² = 0.88 to 0.89; root mean square error (RMSE) = 4 °C; normalized RMSE (nRMSE) = 25% and index of agreement (d) = 0.89-0.97) but under-predicted water filled pore space (WFPS; %; 0-20 cm depth) (r² = 0.3 to 0.4) and soil ammonium and nitrate (exchangeable NH₄⁺ & NO₃^-; kg N ha^-1; r² = 0.97). With reference to the control treatment (no N fertilizer), DNDC was weak in simulating both N₂O emissions and crop productivity. To be further improved for use under pedo-climatic conditions of the summer maize-winter wheat double cropping system we suggest future studies to identify and resolve the existing problems with the DNDC, especially with the control treatment.

Straw decreased N2O emissions from flooded paddy soils via altering denitrifying bacterial community compositions and soil organic carbon fractions

Straw return is widely applied to increase soil fertility and soil organic carbon storage. However, its effect on N2O emissions from paddy soil and the associated microbial mechanisms are still unclear. In this study, wheat straw was amended to two paddy soils (2% w/w) from Taizhou (TZ) and Yixing (YX), China, which were flooded and incubated for 30 d. Real-time PCR and Illumina sequencing were used to characterize changes in denitrifying functional gene abundance and denitrifying bacterial communities. Compared to unamended controls, straw addition significantly decreased accumulated N2O emissions in both TZ (5071 to 96 mg kg-1) and YX (1501 to 112 mg kg-1). This was mainly due to reduced N2O production with decreased abundance of major genera of nirK and nirS-bacterial communities and reduced nirK and nirS gene abundances. Further analyses showed that nirK-, nirS- and nosZ-bacterial community composition shifted mainly along the easily oxidizable carbon (EOC) arrows following straw amendment among four different soil organic carbon fractions, suggesting that increased EOC was the main driver of alerted denitrifying bacterial community composition. This study revealed straw return suppressed N2O emission via altering denitrifying bacterial community compositions and highlighted the importance of EOC in controlling denitrifying bacterial communities.

Oxygen Regulates Nitrous Oxide Production Directly in Agricultural Soils

Oxygen (O₂) plays a critical and yet poorly understood role in regulating nitrous oxide (N₂O) production in well-structured agricultural soils. We investigated the effects of in situ O₂ dynamics on N₂O production in a typical intensively managed Chinese cropping system under a range of environmental conditions (temperature, moisture, ammonium, nitrate, dissolved organic carbon, and so forth). Climate and management (fertilization, irrigation, precipitation, and temperature), and their interactions significantly affected soil O₂ and N₂O concentrations (P < 0.05). Soil O₂ concentration was the most significant factor correlating with soil N₂O concentration (r = -0.71) when compared with temperature, water-filled pore space, and ammonium concentration (r = 0.30, 0.25, and 0.26, respectively). Soil N₂O concentration increased exponentially with decreasing soil O₂ concentrations. The exponential model of N treatments and fertilization with irrigation/precipitation events predicted 74-90% and 58% of the variance in soil N₂O concentrations, respectively. Our results highlight that the soil O₂ status is the proximal, direct, and the most decisive environmental trigger for N₂O production, outweighing the effects of other factors and could be a key variable integrating the aggregated effects of various complex interacting variables. This study offers new opportunities for developing more sensitive approaches to predicting and through appropriate management interventions mitigating N₂O emissions from agricultural soils.

Weakened growth of cropland-N2 O emissions in China associated with nationwide policy interventions

China has experienced rapid agricultural development over recent decades, accompanied by increased fertilizer consumption in croplands; yet, the trend and drivers of the associated nitrous oxide (N₂ O) emissions remain uncertain. The primary sources of this uncertainty are the coarse spatial variation of activity data and the incomplete model representation of N₂ O emissions in response to agricultural management. Here, we provide new data-driven estimates of cropland-N₂ O emissions across China in 1990-2014, compiled using a global cropland-N₂ O flux observation dataset, nationwide survey-based reconstruction of N-fertilization and irrigation, and an updated nonlinear model. In addition, we have evaluated the drivers behind changing cropland-N₂ O patterns using an index decomposition analysis approach. We find that China's annual cropland-N₂ O emissions increased on average by 11.2 Gg N/year² (p < .001) from 1990 to 2003, after which emissions plateaued until 2014 (2.8 Gg N/year² , p = .02), consistent with the output from an ensemble of process-based terrestrial biosphere models. The slowdown of the increase in cropland-N₂ O emissions after 2003 was pervasive across two thirds of China's sowing areas. This change was mainly driven by the nationwide reduction in N-fertilizer applied per area, partially due to the prevalence of nationwide technological adoptions. This reduction has almost offset the N₂ O emissions induced by policy-driven expansion of sowing areas, particularly in the Northeast Plain and the lower Yangtze River Basin. Our results underline the importance of high-resolution activity data and adoption of nonlinear model of N₂ O emission for capturing cropland-N₂ O emission changes. Improving the representation of policy interventions is also recommended for future projections.

Impacts of six potential HONO sources on HOx budgets and SOA formation during a wintertime heavy haze period in the North China Plain

The Weather Research and Forecasting/Chemistry (WRF-Chem) model updated with six potential HONO sources (i.e., traffic, soil, biomass burning and indoor emissions, and heterogeneous reactions on aerosol and ground surfaces) was used to quantify the impact of the six potential HONO sources on the production and loss rates of OH and HO₂ radicals and the concentrations of secondary organic aerosol (SOA) in the Beijing-Tianjin-Heibei (BTH) region of China during a winter heavy haze period of Nov. 29-Dec. 3, 2017. The updated WRF-Chem model well simulated the observed HONO concentrations at the Wangdu site, especially in the daytime, and well reproduced the observed diurnal variations of regional-mean O₃ in the BTH region. The traffic emission source was an important HONO source during nighttime but not significant during daytime, heterogeneous reactions on ground/aerosol surfaces were important during nighttime and daytime. We found that the six potential HONO sources led to a significant enhancement in the dominant production and loss rates of HO_x on the wintertime heavy haze and nonhaze days (particularly on the heavy haze day), an enhancement of 5-25 μg m^-3 (75-200%) in the ground SOA in the studied heavy haze event, and an enhancement of 2-15 μg m^-3 in the meridional-mean SOA on the heavy haze day, demonstrating that the six potential HONO sources accelerate the HO_x cycles and aggravate haze events. HONO was the key precursor of primary OH in the BTH region in the studied wintertime period, and the photolysis of HONO produced a daytime mean OH production rate of 2.59 ppb h^-1 on the heavy haze day, much higher than that of 0.58 ppb h^-1 on the nonhaze day. Anthropogenic SOA dominated in the BTH region in the studied wintertime period, and its main precursors were xylenes (42%), BIGENE (31%) and toluene (21%).

Data-driven estimates of global nitrous oxide emissions from croplands

Croplands are the single largest anthropogenic source of nitrous oxide (N₂O) globally, yet their estimates remain difficult to verify when using Tier 1 and 3 methods of the Intergovernmental Panel on Climate Change (IPCC). Here, we re-evaluate global cropland-N₂O emissions in 1961–2014, using N-rate-dependent emission factors (EFs) upscaled from 1206 field observations in 180 global distributed sites and high-resolution N inputs disaggregated from sub-national surveys covering 15593 administrative units. Our results confirm IPCC Tier 1 default EFs for upland crops in 1990–2014, but give a ∼15% lower EF in 1961–1989 and a ∼67% larger EF for paddy rice over the full period. Associated emissions (0.82 ± 0.34 Tg N yr^–1) are probably one-quarter lower than IPCC Tier 1 global inventories but close to Tier 3 estimates. The use of survey-based gridded N-input data contributes 58% of this emission reduction, the rest being explained by the use of observation-based non-linear EFs. We conclude that upscaling N₂O emissions from site-level observations to global croplands provides a new benchmark for constraining IPCC Tier 1 and 3 methods. The detailed spatial distribution of emission data is expected to inform advancement towards more realistic and effective mitigation pathways.