A new high-resolution N2O emission inventory for China in 2008

Influences of shallow groundwater depth on N2O diffusion along the soil profile of summer maize fields in North China Plain

The emissions of nitrous oxide (N2O) from agricultural fields are a significant contribution to global warming. Understanding the mechanisms of N2O emissions from agricultural fields is essential for the development of N2O emission mitigation strategies. Currently, there are extensive studies on N2O emissions on the surface of agricultural soils, while studies on N2O fluxes at the interface between the saturated and unsaturated zones (ISU) are limited. Uncertainties exist regarding N2O emissions from the soil-shallow groundwater systems in agricultural fields. In this study, a three-year lysimeter experiment (2019-2020, 2022) was conducted to simulate the soil-shallow groundwater systems under four controlled shallow groundwater depth (SGD) (i.e., SGD = 40, 70, 110, and 150 cm) conditions in North China Plain (NCP). Weekly continuous monitoring of N2O emissions from soil surface, N2O concentration in the shallow groundwater and the upper 10 cm of pores at the ISU, and nitrogen cycling-related parameters in the soil and groundwater was conducted. The results showed that soil surface N2O emissions increased with decreased shallow groundwater depth, and the highest emissions of 96.44 kg ha-1 and 104.32 kg ha-1 were observed at G2 (SGD = 40 cm) in 2020 and 2022. During the observation period of one maize growing season, shallow groundwater acted as a sink for the unsaturated zone when the groundwater depth was 40 cm, 70 cm, and 110 cm. However, when SGD was 150 cm, shallow groundwater became a source for the unsaturated zone. After fertilization, the groundwater in all treatment plots behaved as a sink for the unsaturated zone, and the diffusion intensity decreased with increasing SGD. The results would provide a theoretical basis for cropland water management to reduce N2O emissions.

Global nitrous oxide emissions from livestock manure during 1890-2020: An IPCC tier 2 inventory

Nitrous oxide (N2O) emissions from livestock manure contribute significantly to the growth of atmospheric N2O, a powerful greenhouse gas and dominant ozone-depleting substance. Here, we estimate global N2O emissions from livestock manure during 1890-2020 using the tier 2 approach of the 2019 Refinement to the 2006 IPCC Guidelines. Global N2O emissions from livestock manure increased by ~350% from 451 [368-556] Gg N year-1 in 1890 to 2042 [1677-2514] Gg N year-1 in 2020. These emissions contributed ~30% to the global anthropogenic N2O emissions in the decade 2010-2019. Cattle contributed the most (60%) to the increase, followed by poultry (19%), pigs (15%), and sheep and goats (6%). Regionally, South Asia, Africa, and Latin America dominated the growth in global emissions since the 1990s. Nationally, the largest emissions were found in India (329 Gg N year-1), followed by China (267 Gg N year-1), the United States (163 Gg N year-1), Brazil (129 Gg N year-1) and Pakistan (102 Gg N year-1) in the 2010s. We found a substantial impact of livestock productivity, specifically animal body weight and milk yield, on the emission trends. Furthermore, a large spread existed among different methodologies in estimates of global N2O emission from livestock manure, with our results 20%-25% lower than those based on the 2006 IPCC Guidelines. This study highlights the need for robust time-variant model parameterization and continuous improvement of emissions factors to enhance the precision of emission inventories. Additionally, urgent mitigation is required, as all available inventories indicate a rapid increase in global N2O emissions from livestock manure in recent decades.

Four decades of full-scale nitrous oxide emission inventory in China

China is among the top nitrous oxide (N2O)-emitting countries, but existing national inventories do not provide full-scale emissions including both natural and anthropogenic sources. We conducted a four-decade (1980-2020) of comprehensive quantification of Chinese N2O inventory using empirical emission factor method for anthropogenic sources and two up-to-date process-based models for natural sources. Total N2O emissions peaked at 2287.4 (1774.8-2799.9) Gg N2O yr-1 in 2018, and agriculture-developed regions, like the East, Northeast, and Central, were the top N2O-emitting regions. Agricultural N2O emissions have started to decrease after 2016 due to the decline of nitrogen fertilization applications, while, industrial and energetic sources have been dramatically increasing after 2005. N2O emissions from agriculture, industry, energy, and waste represented 49.3%, 26.4%, 17.5%, and 6.7% of the anthropogenic emissions in 2020, respectively, which revealed that it is imperative to prioritize N2O emission mitigation in agriculture, industry, and energy. Natural N2O sources, dominated by forests, have been steadily growing from 317.3 (290.3-344.1) Gg N2O yr-1 in 1980 to 376.2 (335.5-407.2) Gg N2O yr-1 in 2020. Our study produces a Full-scale Annual N2O dataset in China (FAN2020), providing emergent counting to refine the current national N2O inventories.

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.

Modeling denitrification nitrogen losses in China's rice fields based on multiscale field-experiment constraints

Denitrification plays a critical role in soil nitrogen (N) cycling, affecting N availability in agroecosystems. However, the challenges in direct measurement of denitrification products (NO, N2 O, and N2 ) hinder our understanding of denitrification N losses patterns across the spatial scale. To address this gap, we constructed a data-model fusion method to map the county-scale denitrification N losses from China's rice fields over the past decade. The estimated denitrification N losses as a percentage of N application from 2009 to 2018 were 11.8 ± 4.0% for single rice, 12.4 ± 3.7% for early rice, and 11.6 ± 3.1% for late rice. The model results showed that the spatial heterogeneity of denitrification N losses is primarily driven by edaphic and climatic factors rather than by management practices. In particular, diffusion and production rates emerged as key contributors to the variation of denitrification N losses. These findings humanize a 38.9 ± 4.8 kg N ha-1  N loss by denitrification and challenge the common hypothesis that substrate availability drives the pattern of N losses by denitrification in rice fields.

New emission inventory reveals termination of global dioxin declining trend

Accurate estimates of spatiotemporally resolved Polychlorinated dibenzo-p-dioxins (PCDD/Fs, or dioxins) emissions are critical for understanding their environmental fate and associated health risks. In this study, by utilizing an empirical regression model for PCDD/Fs emissions, we developed a global emission inventory for 17 toxic PCDD/Fs congeners from 8 source sectors with a spatial resolution of 1° × 1° from 2002 to 2018. The results show that PCDD/Fs emissions decreased by 25.7 % (12.5 kg TEQ) between 2002 and 2018, mostly occurring in upper- and lower-middle income countries. Globally, open-burning processes, waste incineration, ferrous and nonferrous metal production sectors and heat and power generation were the major source sectors of PCDD/Fs. Spatially, high PCDD/Fs emissions were mainly identified in East and South Asia, Southeast Asia, and part of Sub-Saharan Africa. We find that the declining trend of dioxin emissions over the past decades terminated from the early 2010s due to increasing significance of wildfire induced emissions in the total emission. The PCDD/Fs emission inventory developed in the present study was verified by inputting the inventory as initial conditions into an atmospheric transport model, the Canadian Model for Environmental Transport of Organochlorine Pesticides (CanMETOP), to simulate PCDD/Fs concentrations in air and soil. The predicted concentrations were compared to field sampling data. The good agreement between the modeled and measured concentrations demonstrates the reliability of the inventory.

Cropland nitrous oxide emissions exceed the emissions of RCP 2.6: A global spatial analysis

Nitrous oxide (N₂O), as a potent greenhouse gas, must be limited to prevent the global temperature increasing by >2 °C. Cropland is the largest source of anthropogenic N₂O emissions; however, earlier estimates for emissions and their exceedances still remain uncertainties. Here, we used a spatially explicit model to estimate cropland N₂O emission in 2014 by refined grid-level crop-specific EFs and considered the background emission. We also sought to determine where N₂O emissions exceed the "boundary" through analysis of spatial data from representative concentration pathway (RCP) 2.6. The global cropland N₂O emission was 2.92 ± 0.59 Tg N yr^-1, which far exceeds the 0.82 Tg N yr^-1 boundary, over 90 % of cropland areas exceeded the boundary. Western Europe, Southeastern China, Pakistan, and the Ganges Plain exceeded the boundary by >2 kg N ha^-1 yr^-1. The boundary exceedances showed a positive linear response with respect to total cropland emission and a quadratic response to GDP per capita at the country level. Our study highlights the necessity of accurate estimations of spatial variations in cropland N₂O emissions and evaluation of exceedances, to facilitate the development of more effective mitigation measures in different regions.

Aggravation of nitrogen losses driven by agriculture and livestock farming development on the Qinghai-Tibet Plateau

Nitrogen (N) losses from crop-livestock production is a major threat to the environment and human health at regional, national and global scales. A comprehensive understanding of the sources, spatiotemporal distribution and drivers of N losses is of great significance for mitigating its negative impacts and promoting N sustainable management. Here, we used the county-scale N flow model to quantitatively analyze the N losses and their driving forces of crop-livestock production on the Qinghai-Tibet Plateau (QTP). Between 2000 and 2018, the total N losses increased for more than 79% of counties on the QTP. The hotspot areas accounted for over 80% of total N losses, expanding from the east and south to the north and west of the QTP. NH₃ was the main source of atmospheric N losses (over 80%) while the direct discharge of manure was the main source of water N losses. Structural equation modeling (SEM) showed that chemical fertilizer caused the largest driving effect on atmospheric N losses, and the total output value of agriculture and forestry was the main driver of water N losses. Uneven distribution of crop production and livestock contributed to the aggravation of N losses. Over 70% of counties had grater manure N excretion than crops could take up, and large proportion of manure could not be returned to the field. More than 90% of the counties used grater amount of chemical fertilizer N than crops could take up, indicating that livestock manure has not yet fully replaced chemical fertilizer N. The results provide effective guidance and support for N utilization and management of livestock in agricultural and pastoral areas.

Delayed use of bioenergy crops might threaten climate and food security

The potential of mitigation actions to limit global warming within 2 °C (ref. ¹) might rely on the abundant supply of biomass for large-scale bioenergy with carbon capture and storage (BECCS) that is assumed to scale up markedly in the future^2-5. However, the detrimental effects of climate change on crop yields may reduce the capacity of BECCS and threaten food security^6-8, thus creating an unrecognized positive feedback loop on global warming. We quantified the strength of this feedback by implementing the responses of crop yields to increases in growing-season temperature, atmospheric CO₂ concentration and intensity of nitrogen (N) fertilization in a compact Earth system model⁹. Exceeding a threshold of climate change would cause transformative changes in social-ecological systems by jeopardizing climate stability and threatening food security. If global mitigation alongside large-scale BECCS is delayed to 2060 when global warming exceeds about 2.5 °C, then the yields of agricultural residues for BECCS would be too low to meet the Paris goal of 2 °C by 2200. This risk of failure is amplified by the sustained demand for food, leading to an expansion of cropland or intensification of N fertilization to compensate for climate-induced yield losses. Our findings thereby reinforce the urgency of early mitigation, preferably by 2040, to avoid irreversible climate change and serious food crises unless other negative-emission technologies become available in the near future to compensate for the reduced capacity of BECCS.

Responses of Soil N2O Emission and CH4 Uptake to N Input in Chinese Forests across Climatic Zones: A Meta-Study

Enhanced nitrogen (N) deposition has shown significant impacts on forest greenhouse gas emissions. Previous studies have suggested that Chinese forests may exhibit stronger N2O sources and dampened CH4 sinks under aggravated N saturation. To gain a common understanding of the N effects on forest N2O and CH4 fluxes, many have conducted global-scale meta-analyses. However, such effects have not been quantified particularly for China. Here, we present a meta-study of the N input effects on soil N2O emission and CH4 uptake in Chinese forests across climatic zones. The results suggest that enhanced N inputs significantly increase soil N2O emission (+115.8%) and decrease CH4 uptake (−13.4%). The mean effects were stronger for N2O emission and weaker for CH4 uptake in China compared with other global sites, despite being statistically insignificant. Subtropical forest soils have the highest emission factor (2.5%) and may respond rapidly to N inputs; in relatively N-limited temperate forests, N2O and CH4 fluxes are less sensitive to N inputs. Factors including forest type, N form and rate, as well as soil pH, may also govern the responses of N2O and CH4 fluxes. Our findings pinpoint the important role of Southern Chinese forests in the regional N2O and CH4 budgets.

Exploring the environmental impact of crop production in China using a comprehensive footprint approach

The carbon-nutrient-water cycles of farmland ecosystem not only provides support for crop production, but also has an impact on the environment. Comprehensively quantifying the impact of crop production on the environment can provide a basis for crop sustainable production. A series of environmental footprint approaches, including carbon footprint (CF), nitrogen footprint (NF) and water footprint (WF), were optimized to evaluate greenhouse gas (GHG) emissions, reactive nitrogen (Nr) loss and water resource consumption in crop production, and a comprehensive footprint method based on Endpoint modeling was proposed to evaluate the overall environmental impact of crop production in China. The CF, NF and WF of 28 forms of crop production varied from 1206.29 kg CO₂ equivalent (CO₂-eq) ha^-1 of oil crops to 7326.37 kg CO₂-eq ha^-1 of fiber crops, 16.07 kg Nr-eq ha^-1 of oil crops to 60.70 kg Nr-eq ha^-1 of sugar crops, and 4032.04 m³ ha^-1 oil crops to 12,476.28 m³ ha^-1 of sugar crops, respectively. The contribution of each component to footprints varied greatly among different crops, and fertilizer manufacture, NH₃ volatilization and green WF were generally the main contributors of CF, NF and WF, respectively. The total GHG emissions, Nr loss and water consumption were estimated to be 670.11 Tg CO₂-eq, 5.50 Tg Nr-eq and 837.06 G m³ for all crop production of China. The greenhouse vegetable with the highest area-scaled comprehensive footprint was 4.5 times that of the oil crops which had the lowest one. The contribution of crop production to the corresponding environmental impact in China was as low as 3.7%, of which NH₃ volatilization contributed 48% and grain production contributed 72%. Mineral N fertilization was the main driver of the variation of comprehensive footprint between provinces, with reduction of N fertilizer application as an important way to reduce the environmental impact of crop production.

Bottom-up estimates of reactive nitrogen loss from Chinese wheat production in 2014

Excessive use of synthetic nitrogen (N) for Chinese wheat production results in high loss of reactive N loss (Nr; all forms of N except N₂) into the environment, causing serious environmental issues. Quantifying Nr loss and its spatial variations therein is vital to optimize N management and mitigate loss. However, accurate, high spatial resolution estimations of Nr from wheat production are lacking due to limitations of data generation and estimation methods. Here, we applied the random forest (RF) algorithm to bottom-up N application rate data, obtained through a survey of millions of farmers, to estimate the Nr loss from wheat production in 2014. The results showed that the average total Nr loss was 52.5 kg N ha⁻¹ (range: 4.6-157.8 kg N ha⁻¹), which accounts for 26.1% of the total N applied. The hotspots for high Nr loss are the same as those high applied N, including northwestern Xinjiang, central-southern Hebei, Shandong, central-northern Jiangsu, and Hubei. Our database could guide regional N management and be used in conjunction with biogeochemical models.

Measurement(s)	reactive N loss
Technology Type(s)	random forest model
Sample Characteristic - Environment	cropland
Sample Characteristic - Location	China

Policy-enabled stabilization of nitrous oxide emissions from livestock production in China over 1978-2017

Mitigating livestock-related nitrous oxide (N₂O) emissions is key for China to meet its 2060 carbon neutrality target. Here we present a comprehensive analysis of the magnitude, spatiotemporal variation and drivers of Chinese livestock N₂O emissions from 1978 to 2017. We developed scenarios to explore emissions mitigation potential and associated marginal abatement costs and social benefits. The average growth rate of China's livestock N₂O emissions increased by 4.6% per year through 2006, falling sharply over 2007-2015 and gradually declining in 2017 due to a slowdown in population and meat-consumption growth rates. We estimate the technical mitigation potential of livestock N₂O emissions in 2030 to be 7-21% (or 23.1-70.9 Gg N₂O), with implementation costs of US$5.5 billion to US$6.0 billion. Priority regions for intervention were identified in the North China Plain, Northeast Plain and Lianghu Plain. Among mitigation opportunities, anaerobic digestion offers the greatest social benefit, while low crude protein feed is the most cost-effective option.

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

Gaseous reactive nitrogen (Nr) emissions from agricultural soils to the atmosphere constitute an integral part of global N cycle, directly or indirectly causing climate change impacts. The extensive use of N fertilizer in crop production will compromise our efforts to reduce agricultural Nr emissions in China. A national inventory of fertilizer N-induced gaseous Nr emissions from croplands in China remains to be developed to reveal its role in shaping climate change. Here we present a data-driven estimate of fertilizer N-induced soil Nr emissions based on regional and crop-specific emission factors (EFs) compiled from 379 manipulative studies. In China, agricultural soil Nr emissions from the use of synthetic N fertilizer and manure in 2018 are estimated to be 3.81 and 0.73 Tg N yr^-1 , with a combined contribution of 23%, 20% and 15% to the global agricultural emission total of ammonia (NH₃ ), nitrous oxide (N₂ O) and nitric oxide (NO), respectively. Over the past three decades, NH₃ volatilization from croplands has experienced a shift from a rapid increase to a decline trend, whereas N₂ O and NO emissions always maintain a strong growth momentum due to a robust and continuous rise of EFs. Regionally, croplands in Central south (1.51 Tg N yr^-1 ) and East (0.99 Tg N yr^-1 ) of China exhibit as hotspots of soil Nr emissions. In terms of crop-specific emissions, rice, maize and vegetable show as three leading Nr emitters, together accounting for 61% of synthetic N fertilizer-induced Nr emissions from croplands. The global warming effect derived from cropland N₂ O emissions in China was found to dominate over the local cooling effects of NH₃ and NO emissions. Our established regional and crop-specific EFs for gaseous Nr forms provide a new benchmark for constraining the IPCC Tier 1 default EF values. The spatio-temporal insight into soil Nr emission data from N fertilizer application in our estimate is expected to advance our efforts towards more accurate global or regional cropland Nr emission inventories and effective mitigation strategies.

Evaluation of variation in background nitrous oxide emissions: A new global synthesis integrating the impacts of climate, soil, and management conditions

Robust global simulation of soil background N₂ O emissions (BNEs) is a challenge due to the lack of a comprehensive system for quantification of the variations in their magnitude and location. We mapped global BNEs based on 1353 field observations from globally distributed sites and high-resolution climate and soil data. We then calculated global and national total BNE budgets and compared them to the IPCC-estimated values. The average BNE was 1.10, 0.92, and 0.84 kg N ha^-1  year^-1 with variations from 0.18 to 3.47 (5th-95th percentile, hereafter), 0.20 to 3.44, and -1.16 to 3.70 kg N ha^-1  year^-1 for cropland, forestland, and grassland, respectively. Soil pH, soil N mineralization, atmospheric N deposition, soil volumetric water content, and soil temperature were the principle significant drivers of BNEs. The total BNEs of three land use types was lower than IPCC-estimated total BNEs by 0.83 Tg (10¹²  g) N year^-1 , ranging from -47% to 94% across countries. The estimated BNE with cropland values were slightly higher than the IPCC estimates by 0.11 Tg N year^-1 , and forestland and grassland lower than the IPCC estimates by 0.4 and 0.54 Tg N year^-1 , respectively. Our study underlined the necessity for detailed estimation of the spatial distribution of BNEs to improve the estimates of global N₂ O emissions and enable the establishment of more realistic and effective mitigation measures.

Climate change may interact with nitrogen fertilizer management leading to different ammonia loss in China's croplands

Despite research into the response of ammonia (NH₃ ) volatilization in farmland to various meteorological factors, the potential impact of future climate change on NH₃ volatilization is not fully understood. Based on a database consisting of 1063 observations across China, nonlinear NH₃  models considering crop type, meteorological, soil and management variables were established via four machine learning methods, including support vector machine, multi-layer perceptron, gradient boosting machine and random forest (RF). The RF model had the highest R² of 0.76 and the lowest RMSE of 0.82 kg NH₃ -N ha^-1 , showing the best simulation capability. Results of model importance indicated that NH₃ volatilization was mainly controlled by total input of N fertilizer, followed by meteorological factors, human managements and soil characteristics. The NH₃ emissions of China's cereal production (paddy rice, wheat and maize) in 2018 was estimated to be 3.3 Mt NH₃ -N. By 2050, NH₃ volatilization will increase by 23.1-32.0% under different climate change scenarios (Representative Concentration Pathways, RCPs), and climate change will have the greatest impact on NH₃ volatilization in the Yangtze river agro-region of China due to high warming effects. However, the potential increase in NH₃ volatilization under future climate change can be mitigated by 26.1-47.5% through various N fertilizer management optimization options.

Uncertainty of nitrogen budget in China

The accuracy of the nitrogen (N) budget is of great importance for evidence-based decision-making to address both food security and environmental protection challenges. This study attempts to advance understanding of uncertainties in China's N budget using the Coupled Human And Natural Systems (CHANS) model and Monte Carlo simulation from 1980 to 2018. Results show that the spatial and temporal variations in agricultural and industrial activities and insufficient knowledge on N cycling parameterization are the two dominant causes of uncertainties in the N budget in China. Uncertainties of N inputs generally are <10%, while they are <30% for N outputs and >30% for N accumulations. Uncertainty of nitrogen oxides emission is more sensitive to energy consumption due to the large contributions from industry and transportation. While the uncertainty of ammonia emission is predominantly affected by agricultural activity. Combining surface measurements, satellite observations, and atmospheric simulation models enables cross-check of N fluxes in multiple systems and reduces uncertainties of N budget.

Global status of dioxin emission and China's role in reducing the emission

The global status of dioxin emissions across 150 countries/regions were compiled in this study. China, the major emitter of dioxin and the largest developing country, was chosen as an example to illustrate its emission reductions. The global dioxin emissions were about 97.0 kg TEQ/year, Asia and Africa emitted the most dioxins among the continents. Globally, open burning processes were the most important sources of dioxins. Dioxin emissions in developed countries have remained at low and stable level, while those in developing countries have remained at relatively high level or have continued to increase in recent years. It can be speculated that the global dioxin emissions will increase first and then decrease in the future. Chinese dioxin emissions were stable around 9 kg toxic equivalent (TEQ) in recent years, while 17 subcategories are the key sources of dioxin control in the future. Moreover, according to analysis toward China's dioxin emission trend and sources, there is a large space for dioxins reduction in industries such as metal production, waste incineration and disposal. The results indicated that there is at least 30-70% of reduction scope in China based on three scenarios, and this will reduce the world's annual dioxin emissions by 2.7-6.8%.

The underappreciated role of agricultural soil nitrogen oxide emissions in ozone pollution regulation in North China

Intensive agricultural activities in the North China Plain (NCP) lead to substantial emissions of nitrogen oxides (NOx) from soil, while the role of this source on local severe ozone pollution is unknown. Here we use a mechanistic parameterization of soil NOx emissions combined with two atmospheric chemistry models to investigate the issue. We find that the presence of soil NOx emissions in the NCP significantly reduces the sensitivity of ozone to anthropogenic emissions. The maximum ozone air quality improvements in July 2017, as can be achieved by controlling all domestic anthropogenic emissions of air pollutants, decrease by 30% due to the presence of soil NOx. This effect causes an emission control penalty such that large additional emission reductions are required to achieve ozone regulation targets. As NOx emissions from fuel combustion are being controlled, the soil emission penalty would become increasingly prominent and shall be considered in emission control strategies.

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.

Rainfall increasing offsets the negative effects of nighttime warming on GHGs and wheat yield in North China Plain

The effects of nighttime warming and rainfall increasing on crop productivity and soil greenhouse gas emissions are few studied. This study was conducted with a field experiment to investigate the effects of nighttime warming, rainfall increasing and their interaction on wheat grain yield, methane (CH₄) and nitrous oxide (N₂O) emissions during a winter wheat growing season in the North China Plain (NCP). The results showed that nighttime warming and rainfall increasing significantly altered soil temperature and moisture, and thus the CH₄ and N₂O emissions from the soil. Nighttime warming significantly promoted soil CH₄ uptake by 21.2% and increased soil N₂O emissions by 22.4%. Rainfall increasing stimulated soil N₂O emissions by 15.7% but decreased soil CH₄ uptake by 18.6%. Nighttime warming significantly decreased wheat yield by 5.5%, while rainfall increasing enhanced wheat yield by 4.0%. The results indicate that the positive effect of nighttime warming on CH₄ uptake and negative effect on wheat yield can be offset by rainfall increasing in the NCP. Generally, rainfall increasing significantly raised the global warming potential and greenhouse gas intensity induced by CH₄ and N₂O emissions. Overall, this study improves our understanding of agroecosystem C and N cycling in response to nighttime warming and rainfall increasing under future climate change.

Enhanced nitrous oxide emissions caused by atmospheric nitrogen deposition in agroecosystems over China

Atmospheric nitrogen (N) deposition in China has been the largest worldwide. Yet the impacts of atmospheric N deposition on soil N₂O emissions were often ignored by previous studies. Thus, we investigated how N deposition affected N₂O emissions over China using the process-based model (DNDC, DeNitrification-DeComposition). Total soil N inputs were 194 kg N ha^-1 in agricultural systems over China in 2010, including chemical N fertilizer (78%), atmospheric N deposition (12%), and crop residues N (10%). Annual N₂O emissions induced by N deposition were estimated at 97 Gg N, occupying 43% of total soil N₂O emissions (228 Gg N) in agricultural systems over China. In particular, the largest N₂O emissions caused by atmospheric N deposition were found in South China, followed by North China Plain and Southwest China. The efficiency of N deposition generating N₂O emissions (3.0%) over China was 4 times than that of N fertilizer (0.7%). N₂O emissions induced by N deposition increased from 81 Gg in 2000 to 93 Gg in 2014 (by 1% yr^-1), which was consistent with the long-term trend of N deposition. This suggests N deposition accelerated soil N₂O emissions largely contributing to global warming. Our results also indicated that 62% and 10% of soil N₂O emissions were reduced by applying a nitrification inhibitor and N fertilizer with 20% decrease. We highlight the significance of considering N deposition in determining total soil N₂O emissions over China. The results provide an important scientific basis for the prediction of greenhouse effect caused by N deposition over China.

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.

Assessment of Indirect N2O Emission Factors from Agricultural River Networks Based on Long-Term Study at High Temporal Resolution

Assessment of indirect emission factors (EF_5r) of nitrous oxide (N₂O) from agricultural river networks remains challenging, and results are uncertain due to limited data availability. This study compared two methods of assessing EF_5r using data from long-term observations at high temporal resolution in a typical agricultural catchment in subtropical central China. The concentration method (method 1) and the Intergovernmental Panel on Climate Change (IPCC) 2006 method (method 2) were employed to evaluate the emission factor. EF_5r estimated using method 1 (i.e., EF_5r1) was 0.00077 ± 0.00025 (0.00038-0.00097). EF_5r calculated using method 2 (i.e., EF_5r2) was lower than EF_5r1, with a mean value of 0.00004 (0.000015-0.00012). Both EF_5r1 and EF_5r2 were significantly lower than the IPCC 2006 default value of 0.0025, suggesting that N₂O emissions from China and world river networks may be grossly overestimated. A complex N₂O production pathway and diffusion mechanism were responsible for the transfer of N₂O from the sediment to river water and then to the atmosphere. These findings provide essential data for refining national greenhouse gas inventories and contribute evidence for downward revision of indirect emission factors adopted by the IPCC.

Deriving Emission Factors and Estimating Direct Nitrous Oxide Emissions for Crop Cultivation in China

Updating and refining the N₂O emission factors (N₂O-EFs) are vital to reduce the uncertainty in estimates of direct N₂O emissions. Based on a database with 1151 field measurements across China, the N₂O-EFs were established via three approaches including the maximum likelihood method, a linear regression with an intercept and a linear regression with the intercept set to 0 using 70% of the observations. The remaining 30% of the observations were then used to evaluate the predicted N₂O-EFs. The third method had the highest R² of 0.39 and the best model efficiency of 0.38 with no significant bias, showing the best calculation efficiency. The results showed that the N₂O-EFs varied with agroregions, crops, and management patterns. The agroregions of Huang-Huai-Hai and Yangtze River had the higher N₂O-EFs in maize and wheat seasons than other regions, and the highest N₂O-EFs of 0.66-0.92% in the rice season was found in the South and Southwest agroregions. Both fertilizer types and water regimes had the remarkable effects on N₂O-EFs. Based on the best estimation by the selected method, direct N₂O emissions from China's crop cultivation were estimated to be 194 Gg N₂O-N with a 95% confidence interval of 180-208 Gg N₂O-N in the year 2016.

Surface nitrous oxide concentrations and fluxes from water bodies of the agricultural watershed in Eastern China

Agriculture is one of major emission sources of nitrous oxide (N₂O), an important greenhouse gas dominating stratospheric ozone destruction. However, indirect N₂O emissions from agriculture watershed water surfaces are poorly understood. Here, surface-dissolved N₂O concentration in water bodies of the agricultural watershed in Eastern China, one of the most intensive agricultural regions, was measured over a two-year period. Results showed that the dissolved N₂O concentrations varied in samples taken from different water types, and the annual mean N₂O concentrations for rivers, ponds, reservoir, and ditches were 30 ± 18, 19 ± 7, 16 ± 5 and 58 ± 69 nmol L^-1, respectively. The N₂O concentrations can be best predicted by the NO₃^--N concentrations in rivers and by the NH₄⁺-N concentrations in ponds. Heavy precipitation induced hot moments of riverine N₂O emissions were observed during farming season. Upstream waters are hot spots, in which the N₂O production rates were two times greater than in non-hotspot locations. The modeled watershed indirect N₂O emission rates were comparable to direct emission from fertilized soil. A rough estimate suggests that indirect N₂O emissions yield approximately 4% of the total N₂O emissions yield from N-fertilizer at the watershed scale. Separate emission factors (EF) established for rivers, ponds, and reservoir were 0.0013, 0.0020, and 0.0012, respectively, indicating that the IPCC (Inter-governmental Panel on Climate Change) default value of 0.0025 may overestimate the indirect N₂O emission from surface water in eastern China. EF was inversely correlated with N loading, highlighting the potential constraints in the IPCC methodology for water with a high anthropogenic N input.

Emissions of non-CO2 greenhouse gases from livestock in China during 2000-2015: Magnitude, trends and spatiotemporal patterns

Livestock production, an important source for non-CO₂ greenhouse gases (GHGs) including methane (CH₄) and nitrous oxide (N₂O) in China, has changed remarkably over the past decades due to economic development and demand for livestock product. However, the variation of non-CO₂ GHGs from China's livestock have not received sufficient attention in existing literature. Here, we examine the spatiotemporal patterns of emissions of CH₄ and N₂O from main livestock in China as well as their long-term trends during the period 2000-2015. Results suggest that the livestock sourced emissions of non-CO₂ GHGs in China experienced three phases: a rapid increase from 2000 to 2006, followed by a sharp drop in 2007 and then a slow increase at a lower level from 2008 to 2015. The 2007 drop reflects the impact of macro-control policies on livestock development and extensive measures taken on livestock to control the flu outbreak that year, and the slower increase from 2008 to 2015 with respect to the period 2000-2006 reflects the changes in livestock categories and a general improvement in production efficiency. Spatiotemporal patterns demonstrate that traditional livestock provinces including Henan, Sichuan, Inner Mongolia, Shandong, Yunnan and Hunan stood out as top six provinces in emission of non-CO₂ GHGs in 2015. On the other hand, provinces like Jiangxi, Hubei, Hunan, Yunnan, Inner Mongolia, Liaoning and Xinjiang, identified as the emerging provinces, demonstrate the highest growth rates over the last decades. We find that different livestock categories dominated the difference in pattern of non-CO₂ GHG emissions in both provinces with high emissions and those with high growth rates. Mitigation measures and policies suggestions should not only focus on high non-CO₂ GHG emissions provinces, but also pay attention to the emerging new sources.

Nitrous oxide emissions from China's croplands based on regional and crop-specific emission factors deviate from IPCC 2006 estimates

Calculated N₂O emission factors (EFs) of applied nitrogen (N) fertilizer are currently based upon a single, universal value advocated by the IPCC (Inter-governmental Panel on Climate Change) even though EFs are thought to vary with climate and soil types. Here, we compiled and analyzed 151 N₂O EF values from agricultural fields across China. The EF of synthetic N applied to these croplands was 0.60%, on average, but differed significantly among six climatic zones across the country, with the highest EF found in the north subtropical zone for upland fields (0.93%) and the lowest in the middle subtropical zone for paddy fields (0.20%). Precipitation and soil pH, which showed non-linear relationships with EF, are among the factors governing it, explaining 7.0% and 8.0% of the regional variation in EFs, respectively. Annual precipitation was the key factor regulating N₂O emissions from synthetic N fertilizers. Among crop types, legume crops had the highest EFs, which were significantly (P < 0.05) higher than those of cereals. Total soil N₂O emissions from fertilized croplands with maize, rice, wheat, and vegetables in China, calculated using the climatic zone (regional) EFs, were estimated to be 239 Gg N yr^-1 with an uncertainty of 21%. Importantly, this value was substantially (33%) lower than that (357 Gg N yr^-1) derived from the IPCC default EF but close to the 253 Gg N yr^-1 estimated using crop-specific EFs. N₂O emissions from applied synthetic N fertilizer accounted for 66.5% of the total annual N₂O emissions from China's maize, rice, wheat and vegetable fields. Taken together, our study's results strongly suggest that regional EFs should be included for accurate N₂O inventories from croplands across China.

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

The IPCC assume a linear relationship between nitrogen (N) application rate and nitrous oxide (N₂O) emissions in inventory reporting, however, a growing number of studies show a nonlinear relationship under specific soil-climatic conditions. In the North China plain, a global hotspot of N₂O emissions, covering a land as large as Germany, the correlation between N rate and N₂O emissions remains unclear. We have therefore specifically investigated the N₂O response to N applications by conducting field experiments with five N rates, and high-frequency measurements of N₂O emissions across contrasting climatic years. Our results showed that cumulative and yield-scaled N₂O emissions both increased exponentially as N applications were raised above the optimum rate in maize ( Zea mays L.). In wheat ( Triticum aestivum L.) there was a corresponding quadratic increase in N₂O emissions with the magnitude of the response in 2012-2013 distinctly larger than that in 2013-2014 owing to the effects of extreme snowfall. Existing empirical models (including the IPCC approach) of the N₂O response to N rate have overestimated N₂O emissions in the North China plain, even at high N rates. Our study therefore provides a new and robust analysis of the effects of fertilizer rate and climatic conditions on N₂O emissions.

Magnitude and Pathways of Increased Nitrous Oxide Emissions from Uplands Following Permafrost Thaw

Permafrost thawing may release nitrous oxide (N₂O) due to large N storage in cold environments. However, N₂O emissions from permafrost regions have received little attention to date, particularly with respect to the underlying microbial mechanisms. We examined the magnitude of N₂O fluxes following upland thermokarst formation along a 20-year thaw sequence within a thermo-erosion gully in a Tibetan swamp meadow. We also determined the importance of environmental factors and the related microbial functional gene abundance. Our results showed that permafrost thawing led to a mass release of N₂O in recently collapsed sites (3 years ago), particularly in exposed soil patches, which presented post-thaw emission rates equivalent to those from agricultural and tropical soils. In addition to abiotic factors, soil microorganisms exerted significant effects on the variability in the N₂O emissions along the thaw sequence and between vegetated and exposed patches. Overall, our results demonstrate that upland thermokarst formation can lead to enhanced N₂O emissions, and that the global warming potential (GWP) of N₂O at the thermokarst sites can reach 60% of the GWP of CH₄ (vs ∼6% in control sites), highlighting the potentially strong noncarbon (C) feedback to climate warming in permafrost regions.

Hotspots for Nitrogen and Phosphorus Losses from Food Production in China: A County-Scale Analysis

Food production in China results in large losses of nitrogen (N) and phosphorus (P) to the environment. Our objective is to identify hotspots for N and P losses to the environment from food production in China at the county scale. To do this, we used the NUFER (Nutrient flows in Food chains, Environment and Resources use) model. Between 1990 and 2012, the hotspot area expanded by a factor of 3 for N, and 24 for P. In 2012 most hotspots were found in the North China Plain. Hotspots covered less than 10% of the Chinese land area, but contributed by more than half to N and P losses to the environment. Direct discharge of animal manure to rivers was an important cause of N and P losses. Food production was found to be more intensive in hotspots than in other counties. Synthetic fertilizer use and animal numbers in hotspots were a factor of 4-5 higher than in other counties in 2012. Also the number of people working in food production and the incomes of farmers are higher in hotspots than in other counties. This study concludes with suggestions for region-specific pollution control technologies for food production in China.

Projections of NH3 emissions from manure generated by livestock production in China to 2030 under six mitigation scenarios

China's rapid urbanization, large population, and increasing consumption of calorie-and meat-intensive diets, have resulted in China becoming the world's largest source of ammonia (NH₃) emissions from livestock production. This is the first study to use provincial, condition-specific emission factors based on most recently available studies on Chinese manure management and environmental conditions. The estimated NH₃ emission temporal trends and spatial patterns are interpreted in relation to government policies affecting livestock production. Scenario analysis is used to project emissions and estimate mitigation potential of NH₃ emissions, to year 2030. We produce a 1km×1km gridded NH₃ emission inventory for 2008 based on county-level activity data, which can help identify locations of highest NH₃ emissions. The total NH₃ emissions from manure generated by livestock production in 2008 were 7.3TgNH₃·yr^-1 (interquartile range from 6.1 to 8.6TgNH₃·yr^-1), and the major sources were poultry (29.9%), pigs (28.4%), other cattle (27.9%), and dairy cattle (7.0%), while sheep and goats (3.6%), donkeys (1.3%), horses (1.2%), and mules (0.7%) had smaller contributions. From 1978 to 2008, annual NH₃ emissions fluctuated with two peaks (1996 and 2006), and total emissions increased from 2.2 to 7.3Tg·yr^-1 increasing on average 4.4%·yr^-1. Under a business-as-usual (BAU) scenario, NH₃ emissions in 2030 are expected to be 13.9TgNH₃·yr^-1 (11.5-16.3TgNH₃·yr^-1). Under mitigation scenarios, the projected emissions could be reduced by 18.9-37.3% compared to 2030 BAU emissions. This study improves our understanding of NH₃ emissions from livestock production, which is needed to guide stakeholders and policymakers to make well informed mitigation decisions for NH₃ emissions from livestock production at the country and regional levels.

Urbanization-induced population migration has reduced ambient PM2.5 concentrations in China

Direct residential and transportation energy consumption (RTC) contributes significantly to ambient fine particulate matter with a diameter smaller than 2.5 μm (PM_2.5) in China. During massive rural-urban migration, population and pollutant emissions from RTC have evolved in terms of magnitude and geographic distribution, which was thought to worsen PM_2.5 levels in cities but has not been quantitatively addressed. We quantify the temporal trends and spatial patterns of migration to cities and evaluate their associated pollutant emissions from RTC and subsequent health impact from 1980 to 2030. We show that, despite increased urban RTC emissions due to migration, the net effect of migration in China has been a reduction of PM_2.5 exposure, primarily because of an unequal distribution of RTC energy mixes between urban and rural areas. After migration, people have switched to cleaner fuel types, which considerably lessened regional emissions. Consequently, the national average PM_2.5 exposure concentration in 2010 was reduced by 3.9 μg/m³ (90% confidence interval, 3.0 to 5.4 μg/m³) due to migration, corresponding to an annual reduction of 36,000 (19,000 to 47,000) premature deaths. This reduction was the result of an increase in deaths by 142,000 (78,000 to 181,000) due to migrants swarming into cities and decreases in deaths by 148,000 (76,000 to 194,000) and 29,000 (15,000 to 39,000) due to transitions to a cleaner energy mix and lower urban population densities, respectively. Locally, however, megacities such as Beijing and Shanghai experienced increases in PM_2.5 exposure associated with migration because these cities received massive immigration, which has driven a large increase in local emissions.

Evidence for the Importance of Atmospheric Nitrogen Deposition to Eutrophic Lake Dianchi, China

Elevated atmospheric nitrogen (N) deposition has significantly influenced aquatic ecosystems, especially with regard to their N budgets and phytoplankton growth potentials. Compared to a considerable number of studies on oligotrophic lakes and oceanic waters, little evidence for the importance of N deposition has been generated for eutrophic lakes, even though emphasis has been placed on reducing external N inputs to control eutrophication in these lakes. Our high-resolution observations of atmospheric depositions and riverine inputs of biologically reactive N species into eutrophic Lake Dianchi (the sixth largest freshwater lake in China) shed new light onto the contribution of N deposition to total N loads. Annual N deposition accounted for 15.7% to 16.6% of total N loads under variable precipitation conditions, 2-fold higher than previous estimates (7.6%) for the Lake Dianchi. The proportion of N deposition to total N loads further increased to 27-48% in May and June when toxic blooms of the ubiquitous non-N₂ fixing cyanobacteria Microcystis spp. are initiated and proliferate. Our observations reveal that reduced N (59%) contributes a greater amount than oxidized N to total N deposition, reaching 56-83% from late spring to summer. Progress toward mitigating eutrophication in Lake Dianchi and other bloom-impacted eutrophic lakes will be difficult without reductions in ammonia emissions and subsequent N deposition.

Linkage between N2O emission and functional gene abundance in an intensively managed calcareous fluvo-aquic soil

The linkage between N₂O emissions and the abundance of nitrifier and denitrifier genes is unclear in the intensively managed calcareous fluvo-aquic soils of the North China Plain. We investigated the abundance of bacterial amoA for nitrification and narG, nirS, nirK, and nosZ for denitrification by in situ soil sampling to determine how the abundance of these genes changes instantly during N fertilization events and is related to high N₂O emission peaks. We also investigated how long-term incorporated straw and/or manure affect(s) the abundance of these genes based on a seven-year field experiment. The overall results demonstrate that the long-term application of urea-based fertilizer and/or manure significantly enhanced the number of bacterial amoA gene copies leading to high N₂O emission peaks after N fertilizer applications. These peaks contributed greatly to the annual N₂O emissions in the crop rotation. A significant correlation between annual N₂O emissions and narG, nirS, and nirK gene numbers indicates that the abundance of these genes is related to N₂O emission under conditions for denitrification, thus partly contributing to the annual N₂O emissions. These findings will help to draw up appropriate measures for mitigation of N₂O emissions in this ‘hotspot’ region.

The effect of drip irrigation and drip fertigation on N2O and NO emissions, water saving and grain yields in a maize field in the North China Plain

N₂O and NO emissions, the water usage and grain yields of a maize field in the North China Plain (NCP) under traditional flood irrigation, drip irrigation and drip fertigation were compared. With respect to the flood irrigation treatment, N₂O emissions were reduced by 13.8% in the drip irrigation treatment and 7.7% in the drip fertigation treatment. NO emissions were reduced to 16.7% in the drip irrigation treatment but increased by 21.7% in the drip fertigation treatment. The molar ratios of NO/N₂O within 2days after each fertilization event were evidently greater from the drip fertigation treatment than from the flood irrigation treatment, indicating that nitrification was more intensive in the drip fertigation treatment than in the treatment of flood irrigation. Compared with the flood irrigation treatment, evident increase of the maize yields in the drip irrigation treatment (28%) and the drip fertigation treatment (3.7%) were found. Although the drip fertigation treatment could evidently increase NO emission, the 40% water reduction in drip fertigation is of great importance for the sustainable development of agriculture in the NCP where water resources are extremely limited. To mitigate NO emissions from agricultural fields in the NCP with drip fertigation, the addition of a nitrification inhibitor combined with N or nitrate fertilizer was recommended.

Long-Term Trends in Acid Neutralizing Capacity under Increasing Acidic Deposition: A Special Example of Eutrophic Taihu Lake, China

While North America and Europe have been recovering from acidification, China is experiencing impacts of acid deposition. The Taihu region is a seriously impacted area by acid rain in China, with the average rainfall pH < 5. However, the acid neutralizing capacity (ANC) and pH of Taihu Lake have significantly increased over the past 60 years (p < 0.05). Analyses showed that watershed neutralization by carbonates and in-lake alkalinization by algae activities were the two major reactions responsible for the increase. In the Taihu basin, the dominant carbonate bedrocks are the major source of base cations (particularly Ca²⁺ and Mg²⁺) and act as the acidification buffer. In addition, our field measurements across the lake showed that the pH values were significantly higher in algal bloom waters than in areas without blooms. This observation was further supported by our statistical analysis showing that the Taihu ANC and pH were significantly correlated with the chlorophyll increase (p < 0.05; 1985-2015). However, our regression analysis indicated that the base cations in the watershed would be depleted by the early 2040s if the acid deposition continues at the current rate. Our results suggest that interactions between human accelerated weathering, watershed geochemistry, and in-lake algae activities significantly impact the water chemistry of the lake. We urgently recommend an "integrated and balanced" recovery plan for the lake ecosystem.

The contribution of China's emissions to global climate forcing

Knowledge of the contribution that individual countries have made to global radiative forcing is important to the implementation of the agreement on "common but differentiated responsibilities" reached by the United Nations Framework Convention on Climate Change. Over the past three decades, China has experienced rapid economic development, accompanied by increased emission of greenhouse gases, ozone precursors and aerosols, but the magnitude of the associated radiative forcing has remained unclear. Here we use a global coupled biogeochemistry-climate model and a chemistry and transport model to quantify China's present-day contribution to global radiative forcing due to well-mixed greenhouse gases, short-lived atmospheric climate forcers and land-use-induced regional surface albedo changes. We find that China contributes 10% ± 4% of the current global radiative forcing. China's relative contribution to the positive (warming) component of global radiative forcing, mainly induced by well-mixed greenhouse gases and black carbon aerosols, is 12% ± 2%. Its relative contribution to the negative (cooling) component is 15% ± 6%, dominated by the effect of sulfate and nitrate aerosols. China's strongest contributions are 0.16 ± 0.02 watts per square metre for CO2 from fossil fuel burning, 0.13 ± 0.05 watts per square metre for CH4, -0.11 ± 0.05 watts per square metre for sulfate aerosols, and 0.09 ± 0.06 watts per square metre for black carbon aerosols. China's eventual goal of improving air quality will result in changes in radiative forcing in the coming years: a reduction of sulfur dioxide emissions would drive a faster future warming, unless offset by larger reductions of radiative forcing from well-mixed greenhouse gases and black carbon.