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Nitrogen (N) fertilizer application and atmospheric N deposition will profoundly affect greenhouse gas (GHGs) emissions, especially nitrous oxide (N 2 O) and methane (CH 4) fluxes and ecosystem respiration (R e , i.e. CO 2 emissions). However, the impacts of long-term N inputs and the often associated N-induced soil acidification on GHG fluxes in arid and semi-arid ecosystems, especially temperate grasslands, are still uncertain. An in situ experiment was conducted to investigate the effect of long-term (13-years) N addition on N 2 O and CH 4 fluxes and R e from a temperate grassland in Inner Mongolia, northeast China, from April 2017 to October 2018. Soil pH values in the 0–5 cm layer receiving 120 (N 120) and 240 (N 240) kg N ha−1 decreased from 7.12 to 4.37 and 4.18, respectively, after 13 years of N inputs. Soil CH 4 uptake was significantly reduced, but N 2 O emission was enhanced significantly by N addition. However, N addition had no impact on R e. Structural Equation Modeling indicated that soil NH 4 +-N content was the dominant control of N 2 O emissions, but with less effect of the decreasing pH. In contrast, CH 4 uptake was generally controlled by soil pH and NO 3 −-N content, and R e by forb biomass. The measured changes in N 2 O and CH 4 fluxes and R e from temperate grassland will have a profoundly impact on climate change. An in situ experiment was conducted to investigate the effect of long-term (13-years) increased N addition on N 2 O and CH 4 fluxes and R e from a temperate grassland in Inner Mongolia, northeast China, from April 2017 to October 2018. Soil pH values in the 0–5 cm layer receiving 120 (N 120) and 240 (N 240) kg N ha−1 decreased from 7.12 to 4.37 and 4.18, respectively, after 13 years of N inputs. Soil CH 4 uptake was significantly reduced, but N 2 O emission was enhanced significantly by N addition. However, N addition had no impact on R e. Structural Equation Modeling indicated that soil NH 4 +-N content was the dominant control of N 2 O emissions, but with less effect of the decreasing pH. In contrast, CH 4 uptake was generally controlled by soil pH and NO 3 −-N content, and R e by forb biomass. The measured changes in N 2 O and CH 4 fluxes and R e from temperate grassland will have a profoundly impact on climate change. Image 1 • We studied the impacts of long-term N addition on a temperate grassland. • CH 4 uptake was significantly reduced by long-term N addition. • N 2 O emission was controlled mainly by soil ammonium N rather than soil pH. • CH 4 uptake was affected mainly by soil pH and nitrate N content. • Ecosystem respiration was impacted mainly by production of forbs. Long-term increased N addition significantly reduced CH 4 uptake and enhanced N 2 O emissions, but had no significant impact on ecosystem respiration in temperate grassland. [ABSTRACT FROM AUTHOR]

Paddy fields in China contributed to one third of the global cropland ammonia (NH 3) emission inventory, while rice accounted for half of cereal consumption, necessitating exhaustive considerations of the balance between NH 3 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 NH 3 emissions remains poorly understood. Here, by constructing novel crop-specific models for single rice production and NH 3 emissions in the Middle and Lower Yangtze River Basin (LYRB) as a case study, the relationships between fertilizer N application and yield-scaled NH 3 were estimated. Contrary to our hypothesis of a tipping point, our results showed that yield-scaled NH 3 curves could not directly identify optimal nitrogen (N) rates. However, the benefit of lower N fertilizer rate on NH 3 abatement consistently outweighed the risk of yield loss. The exponential relationships between yield-scaled NH 3 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, NH 3 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 NH 3 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 NH 3 emission while maintaining crop productivity. [Display omitted] • Yield-scaled NH 3 cannot directly guide N rate due to high abatement efficacy. • The Yield-scaled NH 3 –N surplus criterion reduces NH 3 emissions by 23–27%. • Five key factors contributing to spatial variations were unraveled. • Deep placement and optimal splitting frequency perform best in yield-scaled NH 3 mitigation. [ABSTRACT FROM AUTHOR]

Many Chinese croplands are suffering from soil acidification, defined as a decrease in soil acid neutralizing capacity (ANC), due to over-application of nitrogen (N) fertilizer and removal of base cations (BCs) in crop harvest. The contribution of fertilization and harvesting on soil acidification rates for different cropping systems is, however, seldom quantified under field conditions. Here, field experiments were conducted on a moderately acid silty clay soil in southwest China to quantify proton (H+) production rates under three fertilization management regimes, including control (non-N), optimized fertilization and farmers' practices (i.e. over fertilization), by measuring the inputs and outputs of major element in three cropping systems, i.e. rice-fallow, rice-wheat and maize-wheat. The maize-wheat cropping system had the highest proton production rates (2.3–10.2 kmol c H+ ha−1 yr−1), followed by rice-fallow (3.2–4.6 kmol c H+ ha−1 yr−1) and rice-wheat (2.2–4.5 kmol c H+ ha−1 yr−1) systems. Crop removal, dominated by straw removal, was the main driver of proton production in all three cropping systems, contributing over 50% of the total H+ production. Compared to the control, N fertilizer application enhanced soil acidification rates among three cropping systems, but had no significant effects on H+ production per kg grain in the rice-fallow and rice-wheat systems. The same was true for the maize-wheat system in case of optimized fertilization, but over fertilization increased both soil acidification rate and H+ production per kg grain, while grain yield was comparable to that with optimized fertilization. The change in soil ANC was dominated by accumulation of SO 4 2− (leading to potential soil acidification) that was mainly added to the soil with superphosphate containing high concentrations of both Ca2+ and SO 4 2− sometimes even causing an increase in the base cation pool, despite BC crop removal. The low contribution of N transformation processes to the total H+ production (3–47%) was due to high denitrification in this specific soil. The soil pH and base saturation did not change significantly among the three fertilization management regimes, largely due to the large BCs pools in the soil. Although the common farmers' practice does not strongly increase proton production in rice-fallow and rice-wheat systems, it still implies a waste of N fertilizer as the excess N input is either enhancing straw production or lost to air in this silty clay soil. Hence, optimized fertilization and localized BCs-rich straw return are suggested to reduce costs of N fertilizer without yield losses and mitigate soil acidification in those systems. [Display omitted] • Soil acidification rates were quantified in three cropping systems in China. • Maize-wheat rotations had higher acidification rates than rice-wheat and rice-fallow. • Optimized N fertilization reduced soil acidification rates without yield loss. • Base cation removal by crop harvesting was the main driver of soil acidification. • Low contribution of N cycle to soil acidification was due to high denitrification. [ABSTRACT FROM AUTHOR]

High ammonia (NH 3) emissions from fertilized soil in China have led to various concerns regarding environmental safety and public health. In response to China's blue skies protection campaign, effective NH 3 reduction measures need to consider both mitigation efficiency and food security. In this context, we conducted a meta-analysis (including 2980 observations from 447 studies) to select effective measures based on absolute (AV) and yield-scaled (YSAV) NH 3 volatilization reduction potential, with the aim of establishing a comprehensive NH 3 mitigation framework covering various crop production sectors, and offering a range of potential solutions. The results showed that manipulating crop density, using an intermittent irrigation regime for paddy field rice, applying N as split applications or partially substituting inorganic fertilizer N with organic N sources could achieve reductions in AV and YSAV reduction of 10–20 %; adopting drip irrigation regimes, adding water surface barrier films to paddy fields, or using double inhibitor (urease and nitrification), slow-release or biofertilizers could achieve 20–40 % mitigation; plastic film mulching, applying fertilizer by irrigation or using controlled-release fertilizers could yield 40–60 % reduction; use of a urease inhibitor, fully substituting fertilizer N with organic N, or applying fertilizer by deep placement could decrease AV and YSAV by over 60 %. In addition, use of soil amendments, applying suitable inorganic N sources, or adopting crop rotation, intercropping or a rice-fish production model all had significant benefits to control AV. The adoption of any particular strategy should consider local accessibility and affordability, direct intervention by local/government authorities and demonstration to encourage the uptake of technologies and practices, particularly in NH 3 pollution hotspot areas. Together, this could ensure food security and environmental sustainability. [Display omitted] • A comprehensive NH 3 mitigation framework for various crop production sectors was established. •Improved field and nutrient management reduces both NH 3 and yield-scaled NH 3 emissions. •Diversified cropping systems provide ecological options of controlling agricultural NH 3 loss. [ABSTRACT FROM AUTHOR]

Distinct cropland acidification has been reported in China due to nitrogen (N) fertilizer overuse. However, the impacts on food production and thereby on food security are largely unknown. Yield losses in the period 1980–2050 were therefore assessed by simulating soil pH changes combined with derived pH-yield relationships for wheat, maize and rice. If the N fertilizer input continues to increase at 1% annually, the predicted average soil pH decline is about one unit and relative yield losses are expected to increase from approximately 4%–24% during 2010–2050. If the N fertilizer increase stops in 2020 (N2020), the expected losses are approximately 16% in 2050, which is comparable to a scenario of 100% crop residue return (100%RR). However, if 30% of the N fertilizer is replaced by manure N (30%MR), the losses reduce to near 5% in 2050. Soil acidification was predicted to reverse and expected losses are only 2.5% in 2050 in a combined scenario of N2020, 100%RR and 30%MR. Our results illustrate the potential food insecurity induced by cropland acidification and address the necessity of mitigation. Image 1 • Yield losses due to cropland acidification were assessed in China. • Expected yield losses are about 24% at 2050 if current N fertilizer rates are continued. • The current policy of stopping N fertilizer increase is not enough to avert the yield losses. • Replacement of N fertilizer by manure is an effective way to reduce acidification. • Induced food insecurity can be eliminated by an appropriate fertilizer management strategy. Possible future cereal losses induced by cropland acidification and resulting food security were assessed on national level to address the necessity of mitigation. [ABSTRACT FROM AUTHOR]

Excessive N fertilizer application contributes to a surge in reactive N (Nr) emissions from agroecosystems, consequently threatening global ecosystems' quality and security. Adding functional microorganisms to agricultural soils, namely microbial inoculants (MI), is a promising method for ensuring sustainable agriculture, but general assessment of microorganism-driven N losses and mediating factors (such as climate conditions, soil properties, and field management practices) remains unclear. Therefore, a comprehensive meta-analysis on the effect of MI addition on NH 3 volatilization, N 2 O emissions, and N leaching in soil-crop systems was conducted by integrating 50 published articles (188 paired observations). The MI application significantly reduced N 2 O emissions and NH 3 volatilization by 29.8% and 24.9%, respectively. The boosted regression tree model showed that the soil organic carbon (SOC) and soil pH significantly affected the efficacy of MI application on both NH 3 and N 2 O loss mitigation. The effects of MI application on N leaching were not clear due to limited data points. Additionally, MI addition significantly increased the abundance of denitrification functional genes (nir K, nir S, and nos Z). In conclusion, MI application presented a high potential to reduce Nr from croplands, future adoption of MI techniques should consider the effects of soil properties and field practices on its efficacy in order to achieving sustainable N management in crop production systems. [Display omitted] • MI amendment mitigated N 2 O and NH 3 losses from fertilized soils. • N fertilizer management had great contribution to variation of MI efficacy. • Soil pH had significant impact on mitigation efficiency of N 2 O and NH 3 by MI. • MI amendment increased denitrification genes abundance. [ABSTRACT FROM AUTHOR]

• Soil pH decreased by 0.07 annually after 20 years of applying NPK fertilizers. • Long-term application of organic fertilizer increased soil SOC content. • Organic fertilizer resulted in higher grain yield than NPK alone. • Applying organic fertilizer alone prevents soil acidification. Long-term fertilization causes declines in soil quality and crop yield, hindering current agricultural development. This paper is about the effect of long-term fertilization (20 years) in Hunan's Qiyang County. It was found that chemical nitrogen, phosphorus, and potassium (NPK) fertilizer lowered soil pH by an annual average of 0.07, while organic fertilizer increased soil pH by about 0.04. Furthermore, organic fertilizer and chemical fertilizer both increased total soil organic carbon (SOC). Long-term chemical NPK and organic fertilizer increased straw and grain yield of wheat and maize to a greater extent than did other fertilization methods. Thus, our results suggest that mixing organic fertilizer with chemical NPK fertilizer should prevent soil acidification and elevate crop yield. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is the key nutrient for crop growth in most agricultural systems, but excessive N fertilizer applications have led to very large ammonia (NH 3) and greenhouse gas (GHG), especially nitrous oxide (N 2 O), emissions, resulting in severe air pollution and making a major contribution to global warming. To clarify the effects of improved N management on NH 3 and N 2 O emissions and crop production, a 2-year field experiment was carried out in a maize-wheat rotation in the North China Plain (NCP), testing four optimized N management practices including reducing the fertilizer (urea) N rate and using N 'synergists' with the fertilizer. NH 3 emissions were significantly reduced only by the urease inhibitor (UI) LIMUS®, with an average reduction of 66.9 % relative to emissions from the untreated urea. At the same N rate, N 2 O emissions were reduced by 30.8–76.7 % by all the synergists (UI, a nitrification inhibitor DMPP (NI) and urea with a bacterial agent (UB)), and crop yields were improved by 14.4 % and 15.2 % under the UI and UB treatments, respectively. An 15N tracer experiment showed that using N synergists decreased the total N loss by 10.9–58.7 %, and significantly increased the recovery efficiency of N (RE N) by 16.9–44.8 % over the whole rotation. Reducing the N rate reduced the GWP and yield-scaled GWP (GHGI) by 10.6 % and 8.4 %, respectively. Using the N synergists, GHGI was significantly decreased by 19.2–26.7 % (P < 0.05), while GWP was reduced by 13.2–18.1 % only by the application of DMPP or LIMUS®. Overall, the study showed that reducing N 2 O and NH 3 emissions is important for increasing N utilization and mitigating global warming. Reducing N rate and using synergists can address the joint problems of environmental pollution and agricultural efficiency. In particular, the application of the optimal amount of urea (33% reduction over rates typically applied by farmers) amended with the urease inhibitor LIMUS® to maize-wheat rotations in the NCP proved to be the most effective method for the 'green' and sustainable development of agriculture and the mitigation of global warming in that region of China. • Optimizing N rate decreased environmental impact while maintaining crop yield. • Cropland NH 3 emissions were halved by incorporating a urease inhibitor. • A nitrification inhibitor reduced cropland N 2 O emissions. • Both inhibitors increased the recovery efficiency of fertilizer N by 16.9–44.8 %. • Optimizing fertilizer N rate plus appropriate inhibitor mitigated GWP and GHGI. [ABSTRACT FROM AUTHOR]

• Three crop rotations were studied in Chongqing over five years. • Reduced fertilizer promoted grain yields and environmental and economic benefit. • The rice-wheat rotation produced the highest grain yield and economic benefit. • The maize-wheat rotation was not suitable for Chongqing. • The rice-flooded fallow rotation is the most suitable system for Chongqing. A suitable crop rotation with optimum fertilizer application can balance the need for high crop yields with minimal environmental risk. This has not yet been achieved in China, and the Chinese Government now requires rotations that include a fallow period with the aim of improving soil quality. As a step towards achieving these aims, a 5-year field experiment was carried out to evaluate the effects of different rates of nitrogen (N) fertilizer in three typical and acceptable crop rotations on grain yield, nitrogen use efficiency (NUE), N balance and economic benefit at Jiangjin, Chongqing, China. The rotations were rice-flooded fallow (RF), rice-wheat (RW) and maize-wheat (MW). Three fertilizer treatments were tested over five years. The RW rotation with reduced fertilizer application had the highest grain yield and generated the highest economic benefit. The MW rotation was not suitable for the region that we studied because high precipitation can cause serious disease problems for the wheat. The RF rotation with reduced fertilizer application proved to be the best system for ensuring food security, increasing economic benefit and reducing environmental risk. [ABSTRACT FROM AUTHOR]

The North China Plain (NCP) is an intensive agricultural area and ammonia (NH 3) emission hotspot in China. Urgent mitigation actions are needed to reduce its environmental impact. A field trial was conducted, and results combined with a synthesis of existing data to assess the NH 3 loss and mitigation potentials in a wheat-maize double-crop rotation, a dominant crop production system in the NCP. Ammonia volatilization losses from N fertilizer applied to summer maize and winter wheat were 9.1% and 5.0%, respectively, based on the data synthesis, and were significantly affected by fertilizer application method and N fertilizer type. From the field trial, optimized reduction in N application rate, compared with conventional practice, had great efficiency for NH 3 mitigation. Despite their high NH 3 reduction potential, the higher input costs associated with the use of controlled-release fertilizers, replacement of urea with calcium ammonium nitrate or inclusion of double inhibitors (urease and nitrification inhibitors) with urea limited their applicability from an economic perspective. The inclusion of a urease inhibitor with urea achieved over 70% reduction in NH 3 emission and also gave a significant improvement in grain yield, net income and N use efficiency. Future adoption of NH 3 mitigation options in the NCP should focus on establishing a reduction target and consider the cost-benefits of simultaneous implementation of multiple mitigation techniques. [Display omitted] • Placement of fertilizer and fertilizer types significantly influence NH 3 emissions. • High cost of fertilizer amendment compromised economic benefits. • Using urease inhibitor showed good economic and environmental performance. [ABSTRACT FROM AUTHOR]

Large uncertainties and challenges remain in the quantification of land-atmosphere exchanges of ammonia (NH 3), mainly due to limitations in measurement techniques. In this study, the eddy covariance (EC) technique based on an open-path quantum cascade laser (QCL) spectrometer was used to measure the NH 3 fluxes at a typical cropland in the North China Plain, one of the global NH 3 hotspots. It is the first attempt of measuring both NH 3 volatilization and deposition fluxes at an agricultural ecosystem using the open-path instrument-based EC technique. During the one-month experimental period, the NH 3 fluxes were characterized by enhanced emissions after fertilizer application and persistent dry depositions after ploughing practice, with the flux magnitude varying from −0.293 to 7.29 (median: −0.042) mg N m−2 h−1. Nearly 93% of the half-hourly fluxes (absolute values) exceeded the mean instrumental flux detection limit of 9.6 μg N m−2 h−1. Nitrogen loss rate from the wheat field through NH 3 volatilization was 0.57–0.71% relative to the applied nitrogen fertilizers. Ammonia deposition dominated the period after the ploughing practice, showing an average deposition rate of 0.014 kg N ha−1 d−1 with a significant diurnal pattern. Both the local emission sources and meteorological conditions influenced the variation of NH 3 dry deposition. The deposition fluxes measured by the EC technique were nearly one third the results modelled by the widely used inferential model method, but the difference needs to be validated by more observations that includes different surface and meteorological conditions. This study suggests that this open-path QCL-based EC instrument is an effective tool for long-term and high-frequency measurements of ecosystem NH 3 exchanges. It also enables cross-validation between different methods to improve the understanding in the atmospheric reactive nitrogen cycling. • The open-path QCL-based eddy covariance instrument can detect small NH 3 fluxes. • Nitrogen loss rate from the wheat field through NH 3 volatilization was 0.57–0.71%. • NH 3 dry deposition averaged to 0.014 kg N ha−1 d−1 with a significant diurnal pattern. • Depositions estimated by inferential model were two times larger than the observed. [ABSTRACT FROM AUTHOR]

Nitrogen (N) stabilizers such as urease inhibitors (UI) and nitrification inhibitors (NI) are promising tools to mitigate N losses from fertilized soils. To better understand the efficacy of UI and NI, a 2-year field trial was carried out in the North China Plain (NCP) to analyze the impact of UI and NI on NH 3 loss and N transformations following urea fertilizer application. Additionally, a laboratory study using an automated continuous-flow incubation system was conducted under controlled conditions, using the same soil, to evaluate the mitigation potential of the N stabilizers on NO, N 2 O and CO 2 emission. The results showed that a novel UI product, Limus® (BASF, Germany), containing active ingredients N-(n-butyl) thiophosphoric triamide (NBPT) and N-(n-propyl) thiophosphoric triamide (NPPT), limited ammonium formation rate and reduced NH 3 loss in both maize (by 85 and 96% for year 1 and 2, respectively) and wheat (by 41 and 64%, for year 1 and 2, respectively) seasons. The 3, 4-dimethypyrazole phosphate (DMPP) amendment significantly retarded nitrate formation rate but the effectiveness was compromised by high ambient temperature; additionally, the NI amendment increased NH 3 emission (by 16 and 24% for maize and wheat year 2, respectively). In the laboratory incubation experiment, addition of the UI reduced emissions of NO, N 2 O and CO 2 by 89, 73 and 19%, respectively, while the addition of the NI reduced the respective emissions by 95, 83 and 24% over the 16 d incubation period. These results suggest that N stabilizers can play a significant role in the mitigation of the environmental impacts of intensive agriculture, and, in particular, that the UI amendment may have the better potential to reduce N losses in the climatic and soil conditions of the NCP, and that the NI should be avoided under high ambient temperatures. Image 1 • Urease inhibitor amendment of urea reduced NH 3 , NO and N 2 O emissions. • Nitrification inhibitor amendment of urea reduced NO and N 2 O but increased NH 3 emissions. • Efficacy of the nitrification inhibitor was compromised by high ambient temperature. • Urease and nitrification inhibitor amendment of urea reduced CO 2 emissions. [ABSTRACT FROM AUTHOR]