Data-driven estimates of fertilizer-induced soil NH3 , NO and N2 O emissions from croplands in China and their climate change impacts

Shallow groundwater table fluctuations weaken nitrogen accumulation in the thin layer vadose zone of cropland around plateau lakes, Southwest China

Excessive accumulation of nitrogen (N) in the soil profile in the intensive agricultural region will seriously threaten groundwater quality and safety. However, the impact of shallow groundwater table (SGWT) fluctuations driven by seasonal variations on the N accumulation characterizations in the soil profiles has not been well quantified, particularly in the regions with thin layer vadose zone. Through in-situ monitoring and simulation experiments, the changes in the SGWT and N accumulation of soil profile in intensive cropland around 7 plateau lakes in Yunnan were studied during the rainy season (RS) and dry season (DS), and the N loss in soil profile of cropland driven by SGWT fluctuations was estimated. The results showed that the SGWT and N accumulation in soil profile of cropland around the plateau lakes had obvious seasonal variation characteristics. The proportion of N storage in different forms in 60-100 cm soil layer in the RS was greater than that in the DS, particularly the proportion of NH4+-N storage was as high as 55 %, while N accumulation in surface soil was obvious in the DS. Compared with the DS, due to the rising SGWT in the RS, the maximum storages of TN and NO3--N in the 0-100 cm soil layer decreased by17% and 36 %, respectively. The TN loss intensities from the 0-100 cm soil profiles of cropland around Fuxian Lake, Yilong Lake, Qilu Lake, Dianchi Lake, Yangzong Lake, Erhai Lake, and Xingyun Lake were 74, 54, 127, 105, 93, 72 and 207 kg/ha, respectively. Moreover, if the SGWT was <30 cm, the average TN loss intensity and amount could reach 177 kg/ha and 1250 t, respectively. Therefore, the SGWT regulation was one of the key measures to reducing soil N loss from the thin layer vadose zone of cropland around plateau lakes and improving groundwater quality.

Global net climate effects of anthropogenic reactive nitrogen

Anthropogenic activities have substantially enhanced the loadings of reactive nitrogen (Nr) in the Earth system since pre-industrial times1,2, contributing to widespread eutrophication and air pollution3-6. Increased Nr can also influence global climate through a variety of effects on atmospheric and land processes but the cumulative net climate effect is yet to be unravelled. Here we show that anthropogenic Nr causes a net negative direct radiative forcing of -0.34 [-0.20, -0.50] W m-2 in the year 2019 relative to the year 1850. This net cooling effect is the result of increased aerosol loading, reduced methane lifetime and increased terrestrial carbon sequestration associated with increases in anthropogenic Nr, which are not offset by the warming effects of enhanced atmospheric nitrous oxide and ozone. Future predictions using three representative scenarios show that this cooling effect may be weakened primarily as a result of reduced aerosol loading and increased lifetime of methane, whereas in particular N2O-induced warming will probably continue to increase under all scenarios. Our results indicate that future reductions in anthropogenic Nr to achieve environmental protection goals need to be accompanied by enhanced efforts to reduce anthropogenic greenhouse gas emissions to achieve climate change mitigation in line with the Paris Agreement.

Agricultural management practices in China enhance nitrogen sustainability and benefit human health

The potential of enhanced agricultural management practices to drive sustainability is rarely quantified at grassroots level. Here we analyse nitrogen use and loss in Chinese cropland, drawing from data collected in 2,238,550 sites in two national agricultural pollution source censuses from 2007 to 2017. We find an upswing of 10% in crop yields and an 8% reduction in nitrogen pollution during this period, owing to the promotion and adoption of various management practices (including the combination of organic and chemical fertilizers, straw recycling and deep placement of fertilizer). These practices have collectively contributed to an 18% increase in nitrogen use efficiency in the country. By fully embracing them, we project that annual cropland pollution could be further reduced by up to 1.4 Mt of nitrogen without compromising crop yields. Environmental and human health benefits are projected to consistently outweigh implementation costs in the future, with total benefits reaching US$15 billion.

Updated loss factors and high-resolution spatial variations for reactive nitrogen losses from Chinese rice paddies

Anthropogenic reactive nitrogen (Nr) loss has been a critical environmental issue. However, due to the limitations of data availability and appropriate methods, the estimation of Nr loss from rice paddies and associated spatial patterns at a fine scale remain unclear. Here, we estimated the background Nr loss (BNL, i.e., Nr loss from soils without fertilization) and the loss factors (the percentage of Nr loss from synthetic fertilizer, LFs) for five loss pathways in rice paddies and identified the national 1 × 1 km spatial variations using data-driven models combined with multi-source data. Based on established machine learning models, an average of 23.4% (15.3-34.6%, 95% confidence interval) of the synthetic N fertilizer was lost to the environment, in the forms of NH3 (17.4%, 10.9-26.7%), N2O (0.5%, 0.3-0.8%), NO (0.2%, 0.1-0.4%), N leaching (3.1%, 0.8-5.7%), and runoff (2.3%, 0.6-4.5%). The total Nr loss from Chinese rice paddies was estimated to be 1.92 ± 0.52 Tg N yr-1 in 2021, in which synthetic fertilizer-induced Nr loss accounted for 69% and BNL accounted for the other 31%. The hotspots of Nr loss were concentrated in the middle and lower regions of the Yangtze River, an area with extensive rice cultivation. This study improved the estimation accuracy of Nr losses and identified the hotspots, which could provide updated insights for policymakers to set the priorities and strategies for Nr loss mitigation.

Indirect influence of soil enzymes and their stoichiometry on soil organic carbon response to warming and nitrogen deposition in the Tibetan Plateau alpine meadow

Despite extensive research on the impact of warming and nitrogen deposition on soil organic carbon components, the response mechanisms of microbial community composition and enzyme activity to soil organic carbon remain poorly understood. This study investigated the effects of warming and nitrogen deposition on soil organic carbon components in the Tibetan Plateau alpine meadow and elucidated the regulatory mechanisms of microbial characteristics, including soil microbial community, enzyme activity, and stoichiometry, on organic carbon components. Results indicated that both warming and nitrogen deposition significantly increased soil organic carbon, readily oxidizable carbon, dissolved organic carbon, and microbial biomass carbon. The interaction between warming and nitrogen deposition influenced soil carbon components, with soil organic carbon, readily oxidizable carbon, and dissolved organic carbon reaching maximum values in the W0N32 treatment, while microbial biomass carbon peaked in the W3N32 treatment. Warming and nitrogen deposition also significantly increased soil Cellobiohydrolase, β-1,4-N-acetylglucosaminidase, leucine aminopeptidase, and alkaline phosphatase. Warming decreased the soil enzyme C: N ratio and C:P ratio but increased the soil enzyme N:P ratio, while nitrogen deposition had the opposite effect. The bacterial Chao1 index and Shannon index increased significantly under warming conditions, particularly in the N32 treatment, whereas there were no significant changes in the fungal Chao1 index and Shannon index with warming and nitrogen addition. Structural equation modeling revealed that soil organic carbon components were directly influenced by the negative impact of warming and the positive impact of nitrogen deposition. Furthermore, warming and nitrogen deposition altered soil bacterial community composition, specifically Gemmatimonadota and Nitrospirota, resulting in a positive impact on soil enzyme activity, particularly soil alkaline phosphatase and β-xylosidase, and enzyme stoichiometry, including N:P and C:P ratios. In summary, changes in soil organic carbon components under warming and nitrogen deposition in the alpine meadows of the Tibetan Plateau primarily depend on the composition of soil bacterial communities, soil enzyme activity, and stoichiometric characteristics.

Toward Low-Carbon Rice Production in China: Historical Changes, Driving Factors, and Mitigation Potential

Under the "Double Carbon" target, the development of low-carbon agriculture requires a holistic comprehension of spatially and temporally explicit greenhouse gas (GHG) emissions associated with agricultural products. However, the lack of systematic evaluation at a fine scale presents considerable challenges in guiding localized strategies for mitigating GHG emissions from crop production. Here, we analyzed the county-level carbon footprint (CF) of China's rice production from 2007 to 2018 by coupling life cycle assessment and the DNDC model. Results revealed a significant annual increase of 74.3 kg CO2-eq ha-1 in the average farm-based CF (FCF), while it remained stable for the product-based CF (PCF). The CF exhibited considerable variations among counties, ranging from 2324 to 20,768 kg CO2-eq ha-1 for FCF and from 0.36 to 3.81 kg CO2-eq kg-1 for PCF in 2018. The spatiotemporal heterogeneities of FCF were predominantly influenced by field CH4 emissions, followed by diesel consumption and soil organic carbon sequestration. Scenario analysis elucidates that the national total GHG emissions from rice production could be significantly reduced through optimized irrigation (48.5%) and straw-based biogas production (18.0%). Moreover, integrating additional strategies (e.g., advanced crop management, optimized fertilization, and biodiesel application) could amplify the overall emission reduction to 76.7% while concurrently boosting the rice yield by 11.8%. Our county-level research provides valuable insights for the formulation of targeted GHG mitigation policies in rice production, thereby advancing the pursuit of carbon-neutral agricultural practices.

Reducing greenhouse gas intensity using a mixture of controlled-release urea and common urea combining suitable maize varieties in a summer maize system

The one-time application of common urea blended with controlled-release urea (CRU) is considered effective for improving nitrogen use efficiency and grain yield and reducing the greenhouse gas emissions of summer maize in intensive agricultural systems. However, the trade-off between the economic and environmental performances of different blended fertilizer treatments for different maize varieties remains unclear. Therefore, a consecutive two-year field experiment was conducted in the North China Plain to study the effects of different ratios of CRU and common urea on the yield, nitrous oxide (N2O) emissions, yield-scaled total N2O emissions, greenhouse gas intensity (GHGI), and net ecosystem economic benefit (NEEB) in 2021 and 2022. Four N fertilizer treatments with equal rate at 180 kg N ha-1 were applied as N180U (all Urea), N180C1(1/3CRU), N180C2(2/3CRU), and N180C (all CRU), and two maize varieties (JNK728-yellow ripe variety and ZD958-green ripe variety) were used. The N180C1 and N180C2 treatments produced the highest grain yield in varieties JNK728 and ZD958 (9.4-11.5 t ha-1 and 9.0-11.0 t ha-1), respectively. Compared to the N180U treatment (conventional method), the N180C1 treatment reduced the GHGI (24.8 %-25.9 %) and increased the NEEB (33.1 %-33.4 %) in the JNK728 variety, whereas the N180C2 treatment reduced the GHGI (16.9 %-28.8 %) and increased the NEEB (27.2 %-48.1 %) in the ZD958 variety. The study concludes that a one-time application of blended nitrogen fertilizer in suitable varieties can minimize the GHGI and maximize the NEEB, which is an effective strategy for balancing yield and nitrogen efficiency in the summer maize system in the North China Plain.

Extreme rainfall erosivity: Research advances and future perspectives

Extreme rainfall erosivity, the capacity of intense rainfall to induce soil erosion, is vital for anticipating future impacts on soil conservation. Despite extensive research, significant differences persist in terms of understanding influencing mechanisms, potential impacts, estimation models and future trends of extreme rainfall erosivity. Quantitatively describing extreme rainfall erosivity remains a key issue in existing research. In this study, we comprehensively reviewed the literature to assess the relationships between extreme rainfall characteristics and rainfall erosivity, between extreme rainfall erosivity and soil erosion, estimation models and trend prediction. The aim was to summarize previous related research and achievements, providing a better understanding of the generation, impacts and future trends of extreme rainfall erosivity. Future research directions should include identifying the thresholds of extreme rainfall events, increasing research attention on tropical cyclones in terms of rainfall erosivity, considering on the impact of extreme rainfall erosivity on soil erosion, and improving rainfall erosivity estimation and simulation prediction methods. This study could contribute to adapting to global climate change and aiding in formulating soil erosion prevention and environmental protection recommendations.

Major drivers of soil acidification over 30 years differ in paddy and upland soils in China

Elevated nitrogen (N) fertilization has largely increased crop production in China, but also increased acidification risks, thereby threatening crop yields. However, natural soil acidification due to bicarbonate (HCO3) leaching and base cation (BC) removal by crop harvest also affect soil acidity whereas the input of HCO3 and BC via fertilizers and manure counteract soil acidification. Insights in rates and drivers of soil acidification in different land use types is too limited to support crop- and site-specific mitigation strategies. In this study, we assessed the historical changes in cropland acidification rates and their drivers for the period 1985-2019 at 151 sites in a typical Chinese county with the combined nutrient and soil acidification model VSD+. VSD+ could well reproduce long-term changes in pH and in the BC concentrations of calcium, magnesium and potassium between 1985 and 2019 in non-calcareous soils. In paddy soils, the acidity production rate decreased from 1985 onwards, mainly driven by a pH-induced reduction in HCO3 leaching and N transformations. In upland soils, however, acidity production was mainly driven by N transformations and hardly changed over time. Crop BC removal by harvesting played a minor role in both paddy and upland soils, but its relative importance increased in paddy soils. The acidity input was partly neutralized by HCO3 input from fertilizers and manure, which decreased over time due to a change from ammonia bicarbonate to urea. Soil buffering by both BC and aluminium release decreased in paddy soils due to a reduction in net acidity production, while it stayed relatively constant in upland soils. We conclude that acidification management in paddy soils requires a focus on avoiding high HCO3 leaching whereas the management in upland soils should focus on balancing N with recycling organic manure and crop residues.

Machine learning-based estimation and mitigation of nitric oxide emissions from Chinese vegetable fields

High fertilizer input and nitric oxide (NO) emissions characterize the intensive vegetable production system. However, the amount, geographic distribution, and effective mitigation strategies of NO emissions over Chinese vegetable fields remain largely uncertain. In this study, we developed a data-driven estimate of NO emissions and their spatial pattern in Chinese vegetable fields based on the Random Forest (RF) model. Additionally, we conducted a field experiment in a subtropical vegetable field to investigate the effect of climate-smart practices on NO emissions. The RF model results showed that soil NO emissions from Chinese vegetable fields were sensitive to nitrogen application amount, soil clay content, and pH. The total NO emission from Chinese vegetable fields in 2018 was estimated to be 75.9 Gg NO-N. The urgency to reduce NO emissions in vegetable fields was higher in northern than in southern China. Our meta-analysis and field experiment results suggested that biochar amendment and replacing chemical fertilizers with bio-organic fertilizers were win-win climate-smart management practices for mitigating NO emissions while improving vegetable production. Overall, our study provided new insights into NO emissions in vegetable soil ecosystems and can facilitate the development of regional NO emission inventories and effective mitigation strategies. These findings highlight the importance of adopting sustainable and climate-smart agricultural practices to reduce NO emissions and mitigate their adverse environmental impacts.

Suitable biochar application practices simultaneously alleviate N2O and NH3 emissions from arable soils: A meta-analysis study

Nitrogen (N) fertilization profoundly improves crop agronomic yield but triggers reactive N (Nr) loss into the environment. Nitrous (N2O) and ammonia (NH3) emissions are the main Nr species that affect climate change and eco-environmental health. Biochar is considered a promising soil amendment, and its efficacy on individual Nr gas emission reduction has been widely reported. However, the interactions and trade-offs between these two Nr species after biochar addition have not been comprehensively analysed. The influencing factors, such as biochar characteristics, environmental conditions, and management measures, remain uncertain. Therefore, 35 publications (145 paired observations) were selected for a meta-analysis to explore the simultaneous mitigation potential of biochar on N2O and NH3 emissions after its application on arable soil. The results showed that biochar application significantly reduced N2O emission by 7.09% while having no significant effect on NH3 volatilisation. Using biochar with a low pH, moderate BET, or pyrolyzed under moderate temperatures could jointly mitigate N2O and NH3 emissions. Additionally, applying biochar to soils with moderate soil organic carbon, high soil total nitrogen, or low cation exchange capacity showed similar responses. The machine-learning model suggested that biochar pH is a dominating moderator of its efficacy in mitigating N2O and NH3 emissions simultaneously. The findings of this study have major implications for biochar application management and aid the further realisation of the multifunctionality of biochar application in agriculture, which could boost agronomic production while lowering environmental costs.

A global meta-analysis of yield-scaled N2 O emissions and its mitigation efforts for maize, wheat, and rice

Maintaining or even increasing crop yields while reducing nitrous oxide (N2 O) emissions is necessary to reconcile food security and climate change, while the metric of yield-scaled N2 O emission (i.e., N2 O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta-analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield-scaled N2 O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield-scaled N2 O emissions across all available data followed the order wheat (322 g N Mg-1 , with the 95% confidence interval [CI]: 301-346) > maize (211 g N Mg-1 , CI: 198-225) > rice (153 g N Mg-1 , CI: 144-163). Yield-scaled N2 O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield-scaled N2 O emissions with increasing N use efficiency or in N2 O emissions for production systems with cereal yields >10 Mg ha-1 (maize), 6.6 Mg ha-1 (wheat) or 6.8 Mg ha-1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade-offs between crop production and N2 O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced-efficiency N fertilizers significantly reduced yield-scaled N2 O emissions at similar or even higher cereal yields. Our data-driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.

Arsenic species in soil profiles from chemical weapons (CWs) burial sites of China: Contamination characteristics, degradation process and migration mechanism

In this study, soil profiles and pore water from Japanese abandoned arsenic-containing chemical weapons (CWs) burial sites in Dunhua, China were analyzed to understand the distribution of arsenic (As) contamination, degradation, and migration processes. Results of As species analysis showed that the As-containing agents underwent degradation with an average rate of 87.55 ± 0.13%, producing inorganic pentavalent arsenic (As5+) and organic arsenic such as 2-chlorovinylarsonic acid (CVAOA), triphenylarsenic (TPA), and phenylarsine oxide (PAO). Organic arsenic pollutants accounted for 1.27-18.20% of soil As. In the vertical profiles, total As concentrations peaked at about 40-60 cm burial depth, and the surface agricultural soil exhibited moderate to heavy contamination level, whereas the contamination level was insignificant below 1 m, reflecting As migration was relatively limited throughout the soil profile. Sequential extraction showed Fe/Al-bound As was the predominant fraction, and poorly-crystalline Fe minerals adsorbed 33.23-73.13% of soil As. Oxygen-susceptible surface soil formed poorly-crystalline Fe3+ minerals, greatly reducing downward migration of arsenic. However, the reduction of oxidizing conditions below 2 m soil depth may promote As activity and require attention.

City-level decoupling between livestock and crop production and its effect on fertilizer usage: Evidence from China

China is one of the largest producers of livestock production and also with tremendous fertilizer consumption in crop production, regional decoupling between livestock and crop production often results in fertilizer overuse and environmental pollution. However, city-level coupling analysis between livestock and crop production is rare, and its impact on fertilizer usage also remains unclear. Here, we evaluated the nitrogen (N) nutrient supply from the livestock breeding sector and the N nutrient demand of cropland during the 2007-2020 period in a typical agricultural region in China. The city-level coupling degree of livestock and crop production and the effect on fertilizer usage were explored with spatial analysis and regression methods. Our results show that the province level has a relatively high coupling degree. However, significant spatial heterogeneity was found at the city level, especially in western Sichuan Province due to the highly unbalanced distribution of livestock and crop production, and this decoupling phenomenon may hinder fertilizer reduction. Furthermore, we reveal that technological development is not an effective way to achieve sustainable agriculture without other policy instruments, such as livestock spatial relocation, which must be considered when formulating crop-livestock integration policies. The findings expand city-level knowledge of the livestock-crop system and provide important implications for adjusting agricultural practices to realize sustainable agricultural development.

Effects of microplastics exposure on soil inorganic nitrogen: A comprehensive synthesis

Microplastics, a growing environmental concern, impact soil inorganic nitrogen (N) transformation, specifically affecting water-extractable nitrate N (NO3--N) and ammonium N (NH4+-N). However, inconsistencies among relevant findings necessitate a systematic analysis. Accordingly, the present meta-analysis addresses these discrepancies by evaluating the effects of microplastics on soil inorganic N and identifying key influencing factors. Our meta-analysis of 216 paired observations from 47 studies demonstrates microplastics exposure causes an overall significant reduction of 7.89% in soil NO3--N concentration, but has no significant impact on NH4+-N concentration. Subgroup analysis further revealed effects of microplastics on soil inorganic N were modulated by microplastics characteristics, experimental conditions (exposure time, experimental temperature, plant effects), and soil properties (soil texture, initial soil pH, initial soil organic carbon, soil total N concentration). We found that microplastics exposure above 27 ℃ enhances soil NO3--N concentration, a finding linked to specific soil properties and conditions, underscoring the impacts of global warming. Importantly, the microplastics polymer type was the most influential predictor of effects on soil NO3--N concentration, while soil NH4+-N concentration was primarily affected by soil texture and microplastics type. These findings illuminate the complex effects of microplastics on soil inorganic N, informing soil management amid increasing microplastics pollution.

Data-driven estimation of nitric oxide emissions from global soils based on dominant vegetation covers

Soils are a major source of global nitric oxide (NO) emissions. However, estimates of soil NO emissions have large uncertainties due to limited observations and multifactorial impacts. Here, we mapped global soil NO emissions, integrating 1356 in-situ NO observations from globally distributed sites with high-resolution climate, soil, and management practice data. We then calculated global and national total NO budgets and revealed the contributions of cropland, grassland, and forest to global soil NO emissions at the national level. The results showed that soil NO emissions were explained mainly by N input, water input and soil pH. Total above-soil NO emissions of the three vegetation cover types were 9.4 Tg N year-1 in 2014, including 5.9 Tg N year-1 (1.04, 95% confidence interval [95% CI]: 0.09-1.99 kg N ha-1  year-1 ) emitted from forest, 1.7 Tg N year-1 (0.68, 95% CI: 0.10-1.26 kg N ha-1  year-1 ) from grassland, and 1.8 Tg N year-1 (0.98, 95% CI: 0.42-1.53 kg N ha-1  year-1 ) from cropland. Soil NO emissions in approximately 57% of 213 countries surveyed were dominated by forests. Our results provide updated inventories of global and national soil NO emissions based on robust data-driven models. These estimates are critical to guiding the mitigation of soil NO emissions and can be used in combination with biogeochemical models.

Ammonia mitigation campaign with smallholder farmers improves air quality while ensuring high cereal production

Reducing cropland ammonia (NH3) emissions while improving air quality and food supply is a challenge, particularly in China where there are millions of smallholder farmers. We tested the effectiveness of a tailored nitrogen (N) management strategy applied to wheat-maize cropping systems in 'demonstration squares' across Quzhou County in the North China Plain. The N-management techniques included optimal N rates, deep fertilizer placement and application of urease inhibitors, implemented through cooperation between government, researchers, businesses and smallholders. Compared with conventional local smallholder practice, our NH3 mitigation campaign reduced NH3 volatilization from wheat and maize by 49% and 39%, and increased N-use efficiency by 28% and 40% and farmers' profitability by 25% and 19%, respectively, with no detriment to crop yields. County-wide atmospheric NH3 and fine particulate matter (with aerodynamic diameter <2.5 μm) concentrations decreased by 40% and 8%, respectively. County-wide net benefits were estimated at US$7.0 million. Our demonstration-square approach shows that cropland NH3 mitigation and improved air quality and farm profitability can be achieved simultaneously by coordinated actions at the county level.

Surface Enhancement Effects of Tiny SnO2 Nanoparticle Modification on α-Fe2O3 for Room-Temperature NH3 Sensing

The development of a gas sensor capable of detecting ammonia with high selectivity and rapid response at room temperature has consistently posed a formidable challenge. To address this issue, the present study utilized a one-step solvothermal method to co-assemble α-Fe2O3 and SnO2 by evenly covering SnO2 nanoparticles on the surface of α-Fe2O3. By controlling the morphology and Fe/Sn mole ratio of the composite, the as-prepared sample exhibits high-performance detection of NH3. At room temperature conditions, a gas sensor composed of α-Fe2O3@3%SnO2 demonstrates a rapid response time of 14 s and a notable sensitivity of 83.9% when detecting 100 ppm ammonia. Experiments and density functional theory (DFT) calculations suggest that the adsorption capacity of α-Fe2O3 to ammonia is enhanced by the surface effect provided by SnO2. Meanwhile, the existence of SnO2 tailors the pore structure and effective surface area of α-Fe2O3, creating multiple channels for the diffusion and adsorption of ammonia molecules. Additionally, an N-N heterostructure is formed between α-Fe2O3 and SnO2, which enhances the potential energy barrier and improves the ammonia sensing performance. Demonstration experiments have proved that the sensor shows significant advantages over commercial sensors in the process of ammonia detection in agricultural facilities. This work provides new insights into the perspectives on ammonia detection at room temperature.

Magnitude and efficiency of straw return in building up soil organic carbon: A global synthesis integrating the impacts of agricultural managements and environmental conditions

Enhancing soil organic carbon (SOC) through straw return (SR) has been widely recommended as a promising practice of climate-smart agriculture. Many studies have investigated the relative effect of straw return on SOC content, while the magnitude and efficiency of straw return in building up SOC stock remain uncertain. Here, we present an integrative synthesis of the magnitude and efficiency of SR-induced SOC changes, using a database comprising 327 observations at 115 sites globally. Straw return increased SOC by 3.68 ± 0.69 (95 % Confidence Interval, CI) Mg C ha^-1, with a corresponding C efficiency of 20.51 ± 9.58 % (95 % CI), of which <30 % was contributed directly by straw-C input. The magnitude of SR-induced SOC changes increased (P < 0.05) with increasing straw-C input and experiment duration. However, the C efficiency decreased significantly (P < 0.01) with these two explanatory factors. No-tillage and crop rotation were found to enhance the SR-induced SOC increase, in both magnitude and efficiency. Straw return sequestrated larger amount of C in acidic and organic-rich soils than in alkaline and organic-poor soils. A machine learning random forest (RF) algorithm showed that the amount of straw-C input was the most important single factor governing the magnitude and efficiency of straw return. However, local agricultural managements and environmental conditions were together the dominant explanatory factors determining the spatial differences in SR-induced SOC stock changes. This entails that by optimizing agricultural managements in regions with favorable environmental conditions the farmer can accumulate more C with minor negative impacts. By clarifying the significance and relative importance of multiple local factors, our findings may aid the development of tailored region-specific straw return policies integrating the SOC increment and its environmental side costs.

Nanobiochar Associated Ammonia Emission Mitigation and Toxicity to Soil Microbial Biomass and Corn Nutrient Uptake from Farmyard Manure

The unique properties of NB, such as its nano-size effect and greater adsorption capacity, have the potential to mitigate ammonia (NH₃) emission, but may also pose threats to soil life and their associated processes, which are not well understood. We studied the influence of different NB concentrations on NH₃ emission, soil microbial biomass, nutrient mineralization, and corn nutrient uptake from farmyard manure (FM). Three different NB concentrations i.e., 12.5 (NB1), 25 (NB2), and 50% (NB3), alone and in a fertilizer mixture with FM, were applied to corn. NB1 alone increased microbial biomass in soil more than control, but other high NB concentrations did not influence these parameters. In fertilizer mixtures, NB2 and NB3 decreased NH₃ emission by 25% and 38%, respectively, compared with FM alone. Additionally, NB3 significantly decreased microbial biomass carbon, N, and soil potassium by 34%, 36%, and 14%, respectively, compared with FM. This toxicity to soil parameters resulted in a 21% decrease in corn K uptake from FM. Hence, a high NB concentration causes toxicity to soil microbes, nutrient mineralization, and crop nutrient uptake from the FM. Therefore, this concentration-dependent toxicity of NB to soil microbes and their associated processes should be considered before endorsing NB use in agroecosystems.

Biochar amendments and climate warming affected nitrification associated N2O and NO production in a vegetable field

Soil nitrification driven by ammonia-oxidizing microorganisms is the most important source of nitrous oxide (N₂O) and nitric oxide (NO). Biochar amendment has been proposed as the most promising measure for combating climate warming; both have the potential to regulate the soil nitrification process. However, the comprehensive impacts of different aged biochars and warming combinations on soil nitrification-related N₂O and NO production are not well understood. Here, 1-octyne and acetylene were used to investigate the relative contributions of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential nitrification-mediated N₂O and NO production from the fertilized vegetable soil with different aged biochar amendments and soil temperatures in microcosm incubations. Results demonstrated that AOB dominated nitrification-related N₂O and NO production across biochar additions and climate warming. Biochar amendment did not significantly influence the relative contribution of AOB and AOA to N₂O and NO production. Field-aged biochar markedly reduced N₂O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N₂O yield while fresh- and lab-aged biochar produced negligible effects on AOB-dependent N₂O yield. Climate warming significantly increased N₂O production and AOB-dependent N₂O yield but less so on NO production. Notably, the relative contribution of AOB to N₂O production was enhanced by climate warming, whereas AOB-derived NO showed the opposite tendency. Overall, the results revealed that field-aged biochar contributed to mitigating warming-induced increases in N₂O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N₂O yield. Our findings provided guidance for mitigating nitrogen oxide emissions in intensively managed vegetable production under the context of biochar amendments and climate warming.

Biochar with Inorganic Nitrogen Fertilizer Reduces Direct Greenhouse Gas Emission Flux from Soil

Agricultural waste can have a catastrophic impact on climate change, as it contributes significantly to greenhouse gas (GHG) emissions if not managed sustainably. Swine-digestate-manure-derived biochar may be one sustainable way to manage waste and tackle GHG emissions in temperate climatic conditions. The purpose of this study was to ascertain how such biochar could be used to reduce soil GHG emissions. Spring barley (Hordeum vulgare L.) and pea crops in 2020 and 2021, respectively, were treated with 25 t ha^-1 of swine-digestate-manure-derived biochar (B₁) and 120 kg ha^-1 (N₁) and 160 kg ha^-1 (N₂) of synthetic nitrogen fertilizer (ammonium nitrate). Biochar with or without nitrogen fertilizer substantially lowered GHG emissions compared to the control treatment (without any treatment) or treatments without biochar application. Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) emissions were directly measured using static chamber technology. Cumulative emissions and global warming potential (GWP) followed the same trend and were significantly lowered in biochar-treated soils. The influences of soil and environmental parameters on GHG emissions were, therefore, investigated. A positive correlation was found between both moisture and temperature and GHG emissions. Thus, biochar made from swine digestate manure may be an effective organic amendment to reduce GHG emissions and address climate change challenges.

Nitrogen management to minimize yield-scaled ammonia emission from paddy rice in the Middle and Lower Yangtze River Basin: A meta-analysis

Paddy fields in China contributed to one third of the global cropland ammonia (NH₃) emission inventory, while rice accounted for half of cereal consumption, necessitating exhaustive considerations of the balance between NH₃ emissions abatement and food demand. The concept of yield-scaled emission intensity (emissions per unit crop production) has the potential to guide sustainable intensification strategies, yet its application to NH₃ emissions remains poorly understood. Here, by constructing novel crop-specific models for single rice production and NH₃ emissions in the Middle and Lower Yangtze River Basin (LYRB) as a case study, the relationships between fertilizer N application and yield-scaled NH₃ were estimated. Contrary to our hypothesis of a tipping point, our results showed that yield-scaled NH₃ curves could not directly identify optimal nitrogen (N) rates. However, the benefit of lower N fertilizer rate on NH₃ abatement consistently outweighed the risk of yield loss. The exponential relationships between yield-scaled NH₃ and N surplus allowed us to estimate the N surplus criterion as 15.6 kg N ha^-1 (or 190 kg N ha^-1 fertilizer N rate) for the LYRB. Under the N surplus criterion, NH₃ emissions can be reduced by 23-27% without severely impacting rice yield, compared to the N rate required for the highest yield. Moreover, five major controlling factors for yield-scaled NH₃ were estimated by random forest models, ranked in order of importance as N rate, total N, K rate, mean annual precipitation, and soil organic carbon. Among the agricultural practices (irrigation, tillage, and fertilizer management), deep placement was the most effective measure to reduce yield-scaled NH_3, showing 48% reduction potential, followed by proper N splitting frequency (43%). Overall, this study highlights the efficacy of N application optimization and targeted farm management in mitigating NH₃ emission while maintaining crop productivity.

Enhanced-Efficiency Fertilizers Impact on Nitrogen Use Efficiency and Nitrous Oxide Emissions from an Open-Field Vegetable System in North China

Open vegetable fields in China are a major anthropogenic source of nitrous oxide (N₂O) emissions due to excessive nitrogen (N) fertilization. A 4 yr lettuce experiment was conducted to determine the impacts of controlled-release fertilizers (CRFs) and nitrification inhibitors (NIs) on lettuce yield, N₂O emissions and net economic benefits. Five treatments included (i) no N fertilizer (CK), (ii) conventional urea at 255 kg N ha^-1 based on farmers' practice (FP), (iii) conventional urea at 204 kg N ha^-1 (OPT), (iv) CRF at 204 kg N ha^-1 (CU) and (v) CRF (204 kg N ha^-1) added with NI (CUNI). No significant differences were found in the lettuce yields among different N fertilization treatments. Compared with FP, the cumulative N₂O emissions were significantly decreased by 8.1%, 38.0% and 42.6% under OPT, CU and CUNI, respectively. Meanwhile, the net benefits of OPT, CU and CUNI were improved by USD 281, USD 871 and USD 1024 ha^-1 compared to CN, respectively. This study recommends the combined application of CRF and NI at a reduced N rate as the optimal N fertilizer management for the sustainable production of vegetables in China with the lowest environmental risks and the greatest economic benefits.

Climate-smart planting for potato to balance economic return and environmental impact across China

Potato production plays an important role in safeguarding food security in China since the central government implemented the 'Potato-as-Staple-Food' policy in 2015. However, a key challenge facing China's potato production is to realize a tradeoff between economic return and environmental impact. Effective strategies for reducing carbon emission without compromising potato yield remain to be developed. This study conducted a comprehensive assessment by integrating climate, soil, crop, and agricultural input data, crop model and life cycle impact assessment model to quantify potato yields, GHG emission amounts and intensities (GHGI), and economic benefits under the conventional planting pattern (CPP), the lowest GHG emission pattern (LEP), and the highest yield pattern (HYP) across China's potato planting regions including four sub-regions, i.e., North Single planting region (NS), Central Double planting region (CD), South Winter planting region (SW), and Southwest Mixed planting region (SWM). Averaged fresh potato yield, GHG emission amount, and GHGI under the CPP were 21.7 t ha^-1, 2815.1 kg CO₂eq ha^-1, and 137.3 kg CO₂eq t^-1, respectively, in China's potato planting region. Compared with the CPP, averaged GHG emission amount and GHGI under the LEP could be decreased by 48.2 % and 51.5 % respectively while the fresh potato yield and economic benefit could be enhanced by 8.1 % and 18.5 %, respectively. For the HYP, averaged GHG emission amount and GHGI could be decreased by 24.2 % and 39.8 % respectively while the fresh potato yield and economic benefit could be enhanced by 18.7 % and 39.6 %, respectively, compared with the CPP. Across the four potato planting regions, SW had the largest potential in reducing GHG emissions owing to a high reduction amount of nitrogen application rate. Our study demonstrates that optimizing agronomic management could reduce environmental impact without compromising economic benefit and provides a scientific method for assessing crop potential to realize the climate-smart planting.

Greenhouse gas fluxes (CO2, N2O and CH4) of pea and maize during two cropping seasons: Drivers, budgets, and emission factors for nitrous oxide

Agriculture contributes considerably to the increase of global greenhouse gas (GHG) emissions. Hence, magnitude and drivers of temporal variations in carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes in croplands are urgently needed to develop sustainable, climate-smart agricultural practices. However, our knowledge of GHG fluxes from croplands is still very limited. The eddy covariance technique was used to quantify GHG budgets and N2O emission factors (EF) for pea and maize in Switzerland. The random forest technique was applied for gap-filling N2O and CH4 fluxes as well as to determine the relevance of environmental, vegetation vs. management drivers of the GHG fluxes during two cropping seasons. Environmental (i.e., net radiation, soil water content, soil temperature) and vegetation drivers (i.e., vegetation height) were more important drivers for GHG fluxes at field scale than time since management for the two crop species. Both crops acted as GHG sinks between sowing and harvest, clearly dominated by net CO2 fluxes, while CH4 emissions were negligible. However, considerable N2O emissions occurred in both crop fields early in the season when crops were still establishing. N2O fluxes in both crops were small later in the season when vegetation was tall, despite high soil water contents and temperatures. Results clearly show a strong and highly dynamic microbial-plant competition for N driving N2O fluxes at the field scale. The total loss was 1.4 kg N2O-N ha-1 over 55 days for pea and 4.8 kg N2O-N ha-1 over 127 days for maize. EFs of N2O were 1.5 % (pea) and 4.4 % (maize) during the cropping seasons, clearly exceeding the IPCC Tier 1 EF for N2O. Thus, sustainable, climate-smart agriculture needs to consider crop phenology and better adapt N supply to crop N demand for growth, particularly during the early cropping season when competition for N between establishing crops and soil microorganisms modulates N2O losses.

Rice-crayfish pattern in irrigation-drainage unit increased N runoff losses and facilitated N enrichment in ditches

The rice-crayfish (RC) integrated pattern has been developed vigorously in China, but how it affects the nitrogen (N) runoff loss and distribution status during rice production is still poorly studied. Based on this, we selected two types of irrigation and drainage units (IDUs), which adopted the traditional rice-wheat (RW) rotation pattern and burgeoning RC rotation pattern separately, to investigate the effect of the RC pattern on N runoff loss, inorganic N distribution and N balance of the IDU. The results showed that there was a 241 kg ha^-1 yr^-1 and 135 kg ha^-1 yr^-1 N surplus achieved under RW and RC, respectively. Among these, the N surplus of RC was 53 % lower than that of RW during the rice growing season and was 37 % lower at other times. The NH₄⁺-N contents of paddy field soils, rice yields and productive traits were not affected by rotation patterns. Nevertheless, the total nitrogen (TN), dissolved organic nitrogen (DON) and NH₄⁺-N concentrations of RC field water were significantly higher (P < 0.01), and the N runoff losses of the RC pattern increased by 103 % to 855 % compared with the RW pattern. In addition, the NH₄⁺-N reserved in RC ditch sediments substantially increased regardless of the dynamic changes during the rice growing season or from the vertical distribution at depths of 0-40 cm. Our results indicated that the RC pattern was beneficial for decreasing the N surplus without impacting the rice yield. However, larger N runoff losses and more available N flowing into crayfish farming ditches still pose great environmental risks. Therefore, more efficient and cleaner measures should be applied for the N management of IDU under the RC pattern.

Dissolved greenhouse gas emissions from agricultural groundwater irrigation in the Guanzhong Basin of China

While evidence indicates that groundwater is a potential source for greenhouse gas (GHG) emissions, information for such emissions in groundwater used for irrigation is lacking. Based on 23 wells in the mid-western Guanzhong Basin of China, we investigated the dissolved CO₂, N₂O, and CH₄ distributions in groundwater, their relationships with water indicators, and emission fluxes during flood irrigation. We found zero methane, but CO₂ and N₂O were 30 and 25 times, respectively, supersaturated compared to atmospheric concentrations. Dissolved N₂O in groundwater was positively correlated with NO₃^--N (P = 0.009), while CO₂ depended mainly on low pH and high dissolved inorganic carbon. The CO₂ and N₂O emission fluxes detected in wellheads, especially in shallow wells, implied potential emissions. Flood irrigation experiments showed that 24.55% of dissolved CO₂ and 36.81% of dissolved N₂O in groundwater was degassed immediately (within 12 min of irrigation) to the atmosphere. Our study demonstrates that direct GHG emissions from groundwater used for agricultural irrigation in the Guanzhong Basin are potentially equivalent to about 2-4% of the GHG emissions from 3 years of fertilizer use on these farmlands, so further research should focus on optimizing irrigation strategies to mitigate GHG emissions.

Optimizing nitrogen management to mitigate gaseous losses and improve net benefits of an open-field Chinese cabbage system

The excessive and inappropriate application of nitrogen (N) fertilizer in open vegetable fields is a major anthropogenic source of gaseous N losses including nitrous oxide (N₂O) and ammonia (NH₃) emissions in China. A 2-yr Chinese cabbage (Brassica pekinensis L.) experiment was carried out to explore the impacts of optimized N management (reduced N application rate, controlled-release urea [CRF] and nitrification inhibitor [NI]) on cabbage yield, soil inorganic N, and N₂O and NH₃ emissions, and to assess their economic benefits by a cost-benefit analysis. Six treatments including i) no N fertilizer (CK), ii) conventional urea fertilizer at 400 kg N ha^-1 based on farmers' practices (CN), iii) conventional urea at 320 kg N ha^-1 (RN), iv) conventional urea (320 kg N ha^-1) with the addition of NI (RN + NI), v) CRF at 320 kg N ha^-1 (CR) and vi) CRF (320 kg N ha^-1) with the addition of NI (CR + NI) were implemented in an open Chinese cabbage field. No significant differences were found in the cabbage yields and soil NH₄⁺-N contents under different N fertilization treatments. Only CR + NI treatment had significantly lower soil NO₃^--N contents than CN by 17.6%-34.6% at the early growing stages of cabbage in both years. Compared with CN, the N₂O emissions were significantly decreased by 8.61%, 34.4%, 37.8% and 46.6% under RN, RN + NI, CR and CR + NI, respectively, indicating that CR + NI favors N₂O abatement especially when NH₃ has been suppressed by other 4 R practices. Meanwhile, the NH₃ volatilization was 20.6% higher under RN + NI and 30.8% and 17.3% lower under CR and CR + NI compared to CN, respectively, which implied that CR was the most effective treatment in reducing the NH₃ volatilization and total gaseous N loss in high NH₃-N loss scenarios. Moreover, the net benefit of RN decreased by $945 USD ha^-1 and those of RN + NI, CR and CR + NI treatments increased by $855, $930 and $1004 USD ha^-1 compared to CN, respectively. This study recommends CR + NI as the optimal N fertilizer management for the sustainable production of vegetables with the lowest environmental risks and the greatest economic benefits.

Spatiotemporal changes in greenhouse gas emissions and soil organic carbon sequestration for major cropping systems across China and their drivers over the past two decades

Chinese agricultural systems have experienced dramatic changes in crop planting area, cropping system, irrigation and fertilization managements, and crop yields in recent decades. These changes can substantially affect greenhouse gases (GHG) emissions and soil organic carbon (SOC) sequestration in croplands. However, the spatiotemporal patterns, as well as their driving factors and mechanisms, have not been well understood. Here, the Denitrification-Decomposition model is calibrated and validated to estimate nitrous oxide (N₂O) and methane (CH₄) emissions and SOC sequestration for seven major cropping systems in China during 2001-2020. The Logarithmic Mean Divisia Index method is further applied to attribute the net GHG emissions (NGEs) trend to various drivers. The results show that the total N₂O emissions, CH₄ emissions, and SOC sequestration were approximately 23.7, 182.0, and 177.6 Tg CO₂-eq/year in the croplands across China. The national average NGEs per unit area ranged from -8705 to 8431 kg CO₂-eq ha^-1 year^-1 across the major cropping systems. During 2001-2020, the trend in national annual NGEs was 0.66 kg CO₂-eq ha^-1 year^-2, ranging from -78.9 to 82.2 kg CO₂-eq ha^-1 year^-2 across the major cropping systems. The paddy lands were mainly a carbon source due to the large amount of CH₄ emissions while the uplands could be a carbon sink owing to SOC sequestration. As a whole, the cropland in China was a carbon source with the NGEs equal to 28.4 Tg CO₂-eq/year, and the NGEs increased by 0.047 Tg CO₂-eq/year² in the past 20 years. Nationally, changes in crop planting area and yields reduced the NGEs whereas changes in nitrogen use efficiency and cropping systems increased them, although the major factors and their impacts varied greatly among regions. Optimizing cropping systems and nitrogen fertilization based on the local genotype, environment and management should be the most effective method to reduce the NGEs in croplands.

A synthesis of nitric oxide emissions across global fertilized croplands from crop-specific emission factors

Nitrogen (N) fertilizer application to agricultural soils results in substantial emissions of nitric oxide (NO), a key substance in tropospheric chemistry involved in climate forcing and air pollution. However, the estimates of global cropland NO emissions remain uncertain due to a lack of information on direct NO emission factors (EF_d s) of applied N for various cropping systems at seasonal or annual scales. Here we quantified the crop-specific seasonal and annual-scale NO EF_d s through synthesizing 1094 measurements from 125 field-based studies worldwide. The global mean crop-specific seasonal EF_d was 0.53%, with the highest for vegetables (0.75%). Among cereal crops, the EF_d of maize (0.45%) or wheat (0.47%) was about three times higher than for rice (0.12%). At annual scale, the mean EF_d across all cropping systems was 0.58%, with tea plantations having the highest (1.54%). For other cropping systems, the annual-scale EF_d s ranged from 0.02% to 1.07%. Besides crop type, also soil organic carbon, total N, and pH as well as N fertilizer type were the main factors explaining the variations of NO EF_d s. Based on obtained specific EF_d s for each crop type, we estimated that NO emissions due to the use of synthetic fertilizers from global croplands are about 0.42-0.62 Tg N year^-1 . Our budgets are relatively lower if compared to estimates derived by the use of IPCC defaults for NO emissions (0.72-1.66 Tg N year^-1 ) or reported elsewhere (0.67-1.04 Tg N year^-1 ). In our estimates, cash crops (vegetable, tea and orchard), which cover only 9% of the world cropland area, contributed about 31% to total NO emissions from global fertilized croplands. Overall, our meta-analysis provides improved crop-specific NO EF_d s reflecting current stage of knowledge. The work also highlights the relative importance of cash crop production as sources for atmospheric NO, that is, agricultural systems on which mitigation efforts may focus.