Net greenhouse gas balance in response to nitrogen enrichment: perspectives from a coupled biogeochemical model

Spatial-temporal differentiation and control strategies of nitrogen environmental loss in China's coastal regions based on flow analysis

Nitrogen pollution emissions from human production and living activities in coastal regions are important topics in the management of environmental pollution in coastal waters. However, to date, there has been relatively little research systematically assessing the environmental loss of nitrogen (NEL) from human activities that negatively affect marine ecosystems. This study categorised emission sources into five subsystems, namely livestock, farming, aquatic, industrial, and residential. Through flow analysis, the anthropogenic emissions of nitrogen in the gas, liquid, and solid phases from 11 coastal provinces in China in 2011, 2015, and 2020 were determined. A nitrogen cost index was constructed by combining the social indicators of each province. The effectiveness of nitrogen emission control since the land-sea coordination and the future challenges for the coastal region were discussed from various perspectives. The results of the study showed that the total NEL that poses a potential threat to marine ecosystems in coastal areas of China has decreased from 18.93 TgN to 14.66 TgN since the proposal of land-sea coordination, with livestock systems and aquatic systems emitting the most. The Bohai and Yellow Seas area were most threatened by nitrogen pollution. Among the three oceanic pathways, liquid-phase nitrogen discharge from each subsystem was effectively controlled, and the control of gas-phase nitrogen emissions is still the most numerous NEL state, although it has had a significant effect. The results of the correlation analysis suggest that NEL flow can characterize the regional management of nutrient-based organic pollutants. Past management tools and environmental investments in China have been more effective in controlling emissions from point and line sources involving artificial facilities, but less direct effect on mariculture. How to control surface source pollution from livestock and aquaculture will be an important challenge to reduce reactive nitrogen emissions in the future.

Long-term phosphorus addition alleviates CO2 and N2O emissions via altering soil microbial functions in secondary rather primary tropical forests

Tropical forests, where the soils are nitrogen (N) rich but phosphorus (P) poor, have a disproportionate influence on global carbon (C) and N cycling. While N deposition substantially alters soil C and N retention in tropical forests, whether P input can alleviate these N-induced effects by regulating soil microbial functions remains unclear. We investigated soil microbial taxonomy and functional traits in response to 10-year independent and interactive effects of N and P additions in a primary and a secondary tropical forest in Hainan Island. In the primary forest, N addition boosted oligotrophic bacteria and phosphatase and enriched genes responsible for C-, P-mineralization, nitrification and denitrification, suggesting aggravated P limitation while N excess. This might stimulate P excavation via organic matter mineralization, and enhance N losses, thereby increasing soil CO₂ and N₂O emissions by 86% and 110%, respectively. Phosphorus and NP additions elevated C-mining enzymes activity mainly due to intensified C limitation, causing 82% increase in CO₂ emission. In secondary forest, P and NP additions reduced phosphatase activity, enriched fungal copiotrophs and increased microbial biomass, suggesting removal of nutrient deficiencies and stimulation of fungal growth. Meanwhile, soil CO₂ emission decreased by 25% and N₂O emission declined by 52-82% due to alleviated P acquisition from organic matter decomposition and increased microbial C and N immobilization. Overall, N addition accelerates most microbial processes for C and N release in tropical forests. Long-term P addition increases C and N retention via reducing soil CO₂ and N₂O emissions in the secondary but not primary forest because of strong C limitation to microbial N immobilization. Further, the seasonal and annual variations in CO₂ and N₂O emissions should be considered in future studies to test the generalization of these findings and predict and model dynamics in greenhouse gas emissions and C and N cycling.

The Effects of N Enrichment on Microbial Cycling of Non-CO2 Greenhouse Gases in Soils-a Review and a Meta-analysis

Terrestrial ecosystems are typically nitrogen (N) limited, but recent years have witnessed N enrichment in various soil ecosystems caused by human activities such as fossil fuel combustion and fertilizer application. This enrichment may alter microbial processes in soils in a way that would increase the emissions of methane (CH₄) and nitrous oxide (N₂O), thereby aggravating global climate change. This review focuses on the effects of N enrichment on methanogens and methanotrophs, which play a central role in the dynamics of CH₄ at the global scale. We also address the effects of N enrichment on N₂O, which is produced in soils mainly by nitrification and denitrification. Overall, N enrichment inhibits methanogenesis in pure culture experiments, while its effects on CH₄ oxidation are more complicated. The majority of previous studies reported that N enrichment, especially NH₄⁺ enrichment, inhibits CH₄ oxidation, resulting in higher CH₄ emissions from soils. However, both activation and neutral responses have also been reported, particularly in rice paddies and landfill sites, which is well reflected in our meta-analysis. In contrast, N enrichment substantially increases N₂O emission by both nitrification and denitrification, which increases proportionally to the amount of N amended. Future studies should address the effects of N enrichment on the active microbes of those functional groups at multiple scales along with parameterization of microbial communities for the application to climate models at the global scale.

Century-long changes and drivers of soil nitrous oxide (N2 O) emissions across the contiguous United States

The atmospheric concentration of nitrous oxide (N₂ O) has increased by 23% since the pre-industrial era, which substantially destructed the stratospheric ozone layer and changed the global climate. However, it remains uncertain about the reasons behind the increase and the spatiotemporal patterns of soil N₂ O emissions, a primary biogenic source. Here, we used an integrative land ecosystem model, Dynamic Land Ecosystem Model (DLEM), to quantify direct (i.e., emitted from local soil) and indirect (i.e., emissions related to local practices but occurring elsewhere) N₂ O emissions in the contiguous United States during 1900-2019. Newly developed geospatial data of land-use history and crop-specific agricultural management practices were used to force DLEM at a spatial resolution of 5 arc-min by 5 arc-min. The model simulation indicates that the U.S. soil N₂ O emissions totaled 0.97 ± 0.06 Tg N year^-1 during the 2010s, with 94% and 6% from direct and indirect emissions, respectively. Hot spots of soil N₂ O emission are found in the US Corn Belt and Rice Belt. We find a threefold increase in total soil N₂ O emission in the United States since 1900, 74% of which is from agricultural soil emissions, increasing by 12 times from 0.04 Tg N year^-1 in the 1900s to 0.51 Tg N year^-1 in the 2010s. More than 90% of soil N₂ O emission increase in agricultural soils is attributed to human land-use change and agricultural management practices, while increases in N deposition and climate warming are the dominant drivers for N₂ O emission increase from natural soils. Across the cropped acres, corn production stands out with a large amount of fertilizer consumption and high-emission factors, responsible for nearly two-thirds of direct agricultural soil N₂ O emission increase since 1900. Our study suggests a large N₂ O mitigation potential in cropland and the importance of exploring crop-specific mitigation strategies and prioritizing management alternatives for targeted crop types.

Emerging weed resistance increases tillage intensity and greenhouse gas emissions in the US corn-soybean cropping system

Tillage is a common agricultural practice that helps prepare the soil and remove weeds. However, it remains unknown how tillage intensity has evolved and its effect on net greenhouse gas (GHG) emissions. Here, using a process-based modelling approach with a multi-source database, we examined the change in tillage intensity across the US corn-soybean cropping systems during 1998-2016 and the impact of tillage intensity on soil GHG emissions. We found that tillage intensity first decreased and then, after 2008, increased, a trend that is strongly correlated with the adoption of herbicide-tolerant crops and emerging weed resistance. The GHG mitigation benefit (-5.5 ± 4.8 TgCO₂e yr^-1) of decreasing tillage intensity before 2008 has been more than offset by increased GHG emissions (13.8 ± 5.6 TgCO₂e yr^-1) due to tillage reintensification under growing pressure of weed resistance. As weed resistance persists or grows, tillage intensity is anticipated to continue rising, probably increasing GHG emissions. Our results imply that farmers' choices in managing herbicide resistance may help mitigate agricultural GHG emissions, underscoring the importance of an alternative strategy to control weeds.

Experimental evidence shows minor contribution of nitrogen deposition to global forest carbon sequestration

Human activities have drastically increased nitrogen (N) deposition onto forests globally. This may have alleviated N limitation and thus stimulated productivity and carbon (C) sequestration in aboveground woody biomass (AGWB), a stable C pool with long turnover times. This 'carbon bonus' of human N use partly offsets the climate impact of human-induced N₂ O emissions, but its magnitude and spatial variation are uncertain. Here we used a meta-regression approach to identify sources of heterogeneity in tree biomass C-N response (additional C stored per unit of N) based on data from fertilization experiments in global forests. We identified important drivers of spatial variation in forest biomass C-N response related to climate (potential evapotranspiration), soil fertility (N content) and tree characteristics (stand age), and used these relationships to quantify global spatial variation in N-induced forest biomass C sequestration. Results show that N deposition enhances biomass C sequestration in only one-third of global forests, mainly in the boreal region, while N reduces C sequestration in 5% of forests, mainly in the tropics. In the remaining 59% of global forests, N addition has no impact on biomass C sequestration. Average C-N responses were 11 (4-21) kg C per kg N for boreal forests, 4 (0-8) kg C per kg N for temperate forests and 0 (-4 to 5) kg C per kg N for tropical forests. Our global estimate of the N-induced forest biomass C sink of 41 (-53 to 159) Tg C yr^-1 is substantially lower than previous estimates, mainly due to the absence of any response in most tropical forests (accounting for 58% of the global forest area). Overall, the N-induced C sink in AGWB only offsets ~5% of the climate impact of N₂ O emissions (in terms of 100-year global warming potential), and contributes ~1% to the gross forest C sink.

Nitrogen deposition accelerates greenhouse gas emissions at an alpine steppe site on the Tibetan Plateau

The increase in atmospheric N deposition can alter the exchange of greenhouse gas (GHG) between the biosphere and atmosphere. The effect of N deposition on GHG is poorly understood in alpine steppe. Therefore, we conducted a 4-year experiment with multilevel N addition of 0, 10, 20, 40, 80, 160 kg N ha^-1 using the static chamber and gas chromatography method to quantify responses of GHG fluxes and characterize the relationship between GHG fluxes and N addition rate in Namco located in the alpine steppe in the central Tibetan Plateau (TP). The mean CO₂ efflux, CH₄ uptake, N₂O emission during growing seasons across four years were 33.59 ± 16.41 mg C m^-2 h^-1, -56.42 ± 29.20 μg C m^-2 h^-1, 0.67 ± 4.40 μg N m^-2 h^-1 respectively. CO₂ efflux increased 1.55-22.6%, CH₄ uptake decreased 5.96-20.1%, N₂O emission increased 37.3-133.5% with N addition. We also observed a critical threshold of 40 kg N ha^-1 a^-1 of N addition above which the increase rate of N₂O emission would diminish. Our current results implied that under the background of future N deposition, N induced C sequestration might be offset by GHG balance causing a heating effect to the climate.

Increased greenhouse gas emissions intensity of major croplands in China: Implications for food security and climate change mitigation

Balancing crop production and greenhouse gas (GHG) emissions from agriculture soil requires a better understanding and quantification of crop GHG emissions intensity, a measure of GHG emissions per unit crop production. Here we conduct a state-of-the-art estimate of the spatial-temporal variability of GHG emissions intensities for wheat, maize, and rice in China from 1949 to 2012 using an improved agricultural ecosystem model (Dynamic Land Ecosystem Model-Agriculture Version 2.0) and meta-analysis covering 172 field-GHG emissions experiments. The results show that the GHG emissions intensities of these croplands from 1949 to 2012, on average, were 0.10-1.31 kg CO₂ -eq/kg, with a significant increase rate of 1.84-3.58 × 10^-3  kg CO₂ -eq kg^-1  year^-1 . Nitrogen fertilizer was the dominant factor contributing to the increase in GHG emissions intensity in northern China and increased its impact in southern China in the 2000s. Increasing GHG emissions intensity implies that excessive fertilizer failed to markedly stimulate crop yield increase in China but still exacerbated soil GHG emissions. This study found that overfertilization of more than 60% was mainly located in the winter wheat-summer maize rotation systems in the North China Plain, the winter wheat-rice rotation systems in the middle and lower reaches of the Yangtze River and southwest China, and most of the double rice systems in the South. Our simulations suggest that roughly a one-third reduction in the current N fertilizer application level over these "overfertilization" regions would not significantly influence crop yield but decrease soil GHG emissions by 29.60%-32.50% and GHG emissions intensity by 0.13-0.25 kg CO₂ -eq/kg. This reduction is about 29% and 5% of total agricultural soil GHG emissions in China and the world, respectively. This study suggests that improving nitrogen use efficiency would be an effective strategy to mitigate GHG emissions and sustain China's food security.

Long-term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N-limited temperate forests. In N-rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old-growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low-N), 100 (Medium-N), and 150 (High-N) kg N ha^-1  year^-1 . Soil organic carbon (SOC) content increased under High-N, corresponding to a 33% decrease in CO₂ efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N₂ O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High-N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N₂ O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.

Impact of 13-years of nitrogen addition on nitrous oxide and methane fluxes and ecosystem respiration in a temperate grassland

Nitrogen (N) fertilizer application and atmospheric N deposition will profoundly affect greenhouse gas (GHGs) emissions, especially nitrous oxide (N₂O) and methane (CH₄) fluxes and ecosystem respiration (R_e, i.e. CO₂ 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 N₂O and CH₄ 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 (N₁₂₀) and 240 (N₂₄₀) kg N ha^-1 decreased from 7.12 to 4.37 and 4.18, respectively, after 13 years of N inputs. Soil CH₄ uptake was significantly reduced, but N₂O emission was enhanced significantly by N addition. However, N addition had no impact on R_e. Structural Equation Modeling indicated that soil NH₄⁺-N content was the dominant control of N₂O emissions, but with less effect of the decreasing pH. In contrast, CH₄ uptake was generally controlled by soil pH and NO₃^--N content, and R_e by forb biomass. The measured changes in N₂O and CH₄ fluxes and R_e from temperate grassland will have a profoundly impact on climate change.

Evaluation of four modelling approaches to estimate nitrous oxide emissions in China's cropland

Process-based models are useful tools to integrate the effects of detailed agricultural practices, soil characteristics, mass balance, and climate change on soil N₂O emissions from soil - plant ecosystems, whereas static, seasonal or annual models often exist to estimate cumulative N₂O emissions under data-limited conditions. A study was carried out to compare the capability of four models to estimate seasonal cumulative N₂O fluxes from 419 field measurements representing 65 studies across China's croplands. The models were 1) the DAYCENT model, 2) the DNDC model, 3) the linear regression model (YLRM) of Yue et al. (2018), and 4) IPCC Tier 1 emission factors. The DAYCENT and DNDC models estimated crop yields with R² values of 0.60 and 0.66 respectively, but both models showed significant underestimation for all measurements. The estimated seasonal N₂O emissions with R² of 0.31, 0.30, 0.21 and 0.17 for DAYCENT, DNDC, YLRM, and IPCC, respectively. Based on RMSE, modelling efficiency and bias analysis, YLRM performed well on N₂O emission prediction under no fertilization though bias still existed, while IPCC performed well for cotton and rapeseed and DNDC for soybean. The DAYCENT model accurately predicted the emissions with no bias across other crop and fertilization types whereas the DNDC model underestimated seasonal N₂O emissions by 0.42 kg N₂O-N ha^-1 for all observed values. Model evaluation indicated that the DAYCENT and DNDC models simulated temporal patterns of daily N₂O emissions effectively, but both models had difficulty in simulating the timing of the N₂O fluxes following some events such as fertilization and water regime. According to this evaluation, algorithms for crop production and N₂O emission should be improved to increase the accuracy in the prediction of unfertilized fields both for DAYCENT and DNDC. The effects of crop types and management modes such as fertilizations should also be further refined for YLRM.

Global ammonia emissions from synthetic nitrogen fertilizer applications in agricultural systems: Empirical and process-based estimates and uncertainty

Excessive ammonia (NH₃ ) emitted from nitrogen (N) fertilizer applications in global croplands plays an important role in atmospheric aerosol production, resulting in visibility reduction and regional haze. However, large uncertainty exists in the estimates of NH₃ emissions from global and regional croplands, which utilize different data and methods. In this study, we have coupled a process-based Dynamic Land Ecosystem Model (DLEM) with the bidirectional NH₃ exchange module in the Community Multiscale Air-Quality (CMAQ) model (DLEM-Bi-NH₃ ) to quantify NH₃ emissions at the global and regional scale, and crop-specific NH₃ emissions globally at a spatial resolution of 0.5° × 0.5° during 1961-2010. Results indicate that global NH₃ emissions from N fertilizer use have increased from 1.9 ± 0.03 to 16.7 ± 0.5 Tg N/year between 1961 and 2010. The annual increase of NH₃ emissions shows large spatial variations across the global land surface. Southern Asia, including China and India, has accounted for more than 50% of total global NH₃ emissions since the 1980s, followed by North America and Europe. Rice cultivation has been the largest contributor to total global NH₃ emissions since the 1990s, followed by corn and wheat. In addition, results show that empirical methods without considering environmental factors (constant emission factor in the IPCC Tier 1 guideline) could underestimate NH₃ emissions in context of climate change, with the highest difference (i.e., 6.9 Tg N/year) occurring in 2010. This study provides a robust estimate on global and regional NH₃ emissions over the past 50 years, which offers a reference for assessing air quality consequences of future nitrogen enrichment as well as nitrogen use efficiency improvement.

Carbon footprint of main crop production in China: Magnitude, spatial-temporal pattern and attribution

Agricultural management practices that increase carbon (C) sequestration and reduce greenhouse gas (GHG) emissions have been identified as attractive mitigation strategies. In this study, we evaluated the spatial-temporal dynamics of carbon footprints (CFs) in China and their driving factors aimed to analyze potential mitigation strategies. Our CF data comprised the GHG emissions and C sequestration of main crop production. The results indicated that from 2000 to 2015, the GHG emissions of main crop production reached 133.3 Tg CO₂-C equivalents (Ce) year^-1 and the soil C sequestration was 25.3 Tg Ce year^-1, resulting in a CF of 108.0 Tg Ce year^-1. In addition, the CF increased by only 1.9%, and 90.9% of the increase in GHG emissions was offset by C sequestration from straw return and fertilization. Moreover, the farm CF (FCF) and product CF (PCF) decreased by 9.7% and 30.4%, respectively, because the proportion of cropland practicing straw return increased by 26.4% and the improved efficiency of fertilization resulting from decrease in fertilizer use of 8 kg nitrogen (N) ha^-1 year^-1. At the regional scale, GHG emissions increased by 9.1-65.0%, and the primary drivers underlying this change were fertilization, machinery operation and rice paddy CH₄ flux in the northeast-northwest, north and south, respectively. However, the PCF decreased by 15.0-47.0% as soil C sequestration increased because of the decreased use of fertilizer and the implementation of straw return. The expansion of straw return and promotion of fertilization efficiency significantly mitigated 25.3 Tg Ce year^-1 emissions, indicating that these measures are important mitigation strategies and should be implemented specifically in different regions combined with other management practices. Our study provided a comprehensive method of calculating the CF of croplands and its driving factors in China and indicated potential mitigation strategies that would benefit policy selection for reduced emissions without influencing crop yields.

Half-Century Ammonia Emissions From Agricultural Systems in Southern Asia: Magnitude, Spatiotemporal Patterns, and Implications for Human Health

Much concern has been raised about the increasing threat to air quality and human health due to ammonia (NH3) emissions from agricultural systems, which is associated with the enrichment of reactive nitrogen (N) in southern Asia (SA), home of more than 60% the world's population (i.e., the people of West, central, East, South, and Southeast Asia). Southern Asia consumed more than half of the global synthetic N fertilizer and was the dominant region for livestock waste production since 2004. Excessive N application could lead to a rapid increase of NH3 in the atmosphere, resulting in severe air and water pollution in this region. However, there is still a lack of accurate estimates of NH3 emissions from agricultural systems. In this study, we simulated the agricultural NH3 fluxes in SA by coupling the Bidirectional NH3 exchange module (Bi-NH3) from the Community Multi-scale Air Quality model with the Dynamic Land Ecosystem Model. Our results indicated that NH3 emissions were 21.3 ± 3.9 Tg N yr-1 from SA agricultural systems with a rapidly increasing rate of ~0.3 Tg N yr-2 during 1961-2014. Among the emission sources, 10.8 Tg N yr-1 was released from synthetic N fertilizer use, and 10.4 ± 3.9 Tg N yr-1 was released from manure production in 2014. Ammonia emissions from China and India together accounted for 64% of the total amount in SA during 2000-2014. Our results imply that the increased NH3 emissions associated with high N inputs to croplands would likely be a significant threat to the environment and human health unless mitigation efforts are applied to reduce these emissions.

Review and analysis of strengths and weaknesses of agro-ecosystem models for simulating C and N fluxes

Biogeochemical simulation models are important tools for describing and quantifying the contribution of agricultural systems to C sequestration and GHG source/sink status. The abundance of simulation tools developed over recent decades, however, creates a difficulty because predictions from different models show large variability. Discrepancies between the conclusions of different modelling studies are often ascribed to differences in the physical and biogeochemical processes incorporated in equations of C and N cycles and their interactions. Here we review the literature to determine the state-of-the-art in modelling agricultural (crop and grassland) systems. In order to carry out this study, we selected the range of biogeochemical models used by the CN-MIP consortium of FACCE-JPI (http://www.faccejpi.com): APSIM, CERES-EGC, DayCent, DNDC, DSSAT, EPIC, PaSim, RothC and STICS. In our analysis, these models were assessed for the quality and comprehensiveness of underlying processes related to pedo-climatic conditions and management practices, but also with respect to time and space of application, and for their accuracy in multiple contexts. Overall, it emerged that there is a possible impact of ill-defined pedo-climatic conditions in the unsatisfactory performance of the models (46.2%), followed by limitations in the algorithms simulating the effects of management practices (33.1%). The multiplicity of scales in both time and space is a fundamental feature, which explains the remaining weaknesses (i.e. 20.7%). Innovative aspects have been identified for future development of C and N models. They include the explicit representation of soil microbial biomass to drive soil organic matter turnover, the effect of N shortage on SOM decomposition, the improvements related to the production and consumption of gases and an adequate simulations of gas transport in soil. On these bases, the assessment of trends and gaps in the modelling approaches currently employed to represent biogeochemical cycles in crop and grassland systems appears an essential step for future research.

Molecular mechanisms of water table lowering and nitrogen deposition in affecting greenhouse gas emissions from a Tibetan alpine wetland

Rapid climate change and intensified human activities have resulted in water table lowering (WTL) and enhanced nitrogen (N) deposition in Tibetan alpine wetlands. These changes may alter the magnitude and direction of greenhouse gas (GHG) emissions, affecting the climate impact of these fragile ecosystems. We conducted a mesocosm experiment combined with a metagenomics approach (GeoChip 5.0) to elucidate the effects of WTL (-20 cm relative to control) and N deposition (30 kg N ha^-1  yr^-1 ) on carbon dioxide (CO₂ ), methane (CH₄ ) and nitrous oxide (N₂ O) fluxes as well as the underlying mechanisms. Our results showed that WTL reduced CH₄ emissions by 57.4% averaged over three growing seasons compared with no-WTL plots, but had no significant effect on net CO₂ uptake or N₂ O flux. N deposition increased net CO₂ uptake by 25.2% in comparison with no-N deposition plots and turned the mesocosms from N₂ O sinks to N₂ O sources, but had little influence on CH₄ emissions. The interactions between WTL and N deposition were not detected in all GHG emissions. As a result, WTL and N deposition both reduced the global warming potential (GWP) of growing season GHG budgets on a 100-year time horizon, but via different mechanisms. WTL reduced GWP from 337.3 to -480.1 g CO₂ -eq m^-2 mostly because of decreased CH₄ emissions, while N deposition reduced GWP from 21.0 to -163.8 g CO₂ -eq m^-2 , mainly owing to increased net CO₂ uptake. GeoChip analysis revealed that decreased CH₄ production potential, rather than increased CH₄ oxidation potential, may lead to the reduction in net CH₄ emissions, and decreased nitrification potential and increased denitrification potential affected N₂ O fluxes under WTL conditions. Our study highlights the importance of microbial mechanisms in regulating ecosystem-scale GHG responses to environmental changes.

Manipulating Planting Density and Nitrogen Fertilizer Application to Improve Yield and Reduce Environmental Impact in Chinese Maize Production

Relatively low nitrogen (N) efficiency and heavy environmental costs caused by excessive N fertilizer applications with outdated fertilization techniques are current cultivation production problems with maize among smallholders in North China Plain. Although many studies have examined agronomical strategies for improving yields and N use, the integrated effects of these measures and the associated environmental costs are not well understood. We conducted a 2-year field study with two densities (67,500 plants ha^-1, which was similar to local farmers' practices, and 90,000 plants ha^-1) and three N rates (0, 180, and 360 kg ha^-1, the rate local farmers' commonly apply) to test the integrated effects for maize production at Wuqiao experimental station in North China Plain. The higher planting density produced significant increases in grain yield (GY), N use efficiency (NUE), agronomic N efficiency (AEN), and N partial productivity (PFP_N) by 6.6, 3.9, 24.7, and 8.8%, respectively; in addition, N₂O emission and greenhouse gas intensity decreased by 7.3 and 4.3%, respectively. With a lower N application rate, from 360 to 180 kg ha^-1, GY was unchanged, and NUE, AEN, and PFP_N all significantly increased by 6.2, 96.0, and 98.7%, respectively; in addition, N₂O emission and greenhouse gas intensity decreased by 61.5 and 46.2%, respectively. The optimized N rate (180 kg N ha^-1) for the 90,000 plants ha^-1 treatment achieved the highest yield with only 50% of the N fertilizer input commonly employed by local farmers' (360 kg N ha^-1), which contributed to the increased N-uptake and N-transfer capacity. Therefore, our study demonstrated that agronomical methods such as increasing planting density with reasonable N application could be useful to obtain higher GY along with efficient N management to help lower environmental costs of maize production.

The effects of nitrogen fertilization on N2O emissions from a rubber plantation

To gain the effects of N fertilizer applications on N₂O emissions and local climate change in fertilized rubber (Hevea brasiliensis) plantations in the tropics, we measured N₂O fluxes from fertilized (75 kg N ha⁻¹ yr⁻¹) and unfertilized rubber plantations at Xishuangbanna in southwest China over a 2-year period. The N₂O emissions from the fertilized and unfertilized plots were 4.0 and 2.5 kg N ha⁻¹ yr⁻¹, respectively, and the N₂O emission factor was 1.96%. Soil moisture, soil temperature, and the area weighted mean ammoniacal nitrogen (NH₄⁺-N) content controlled the variations in N₂O flux from the fertilized and unfertilized rubber plantations. NH₄⁺-N did not influence temporal changes in N₂O emissions from the trench, slope, or terrace plots, but controlled spatial variations in N₂O emissions among the treatments. On a unit area basis, the 100-year carbon dioxide equivalence of the fertilized rubber plantation N₂O offsets 5.8% and 31.5% of carbon sink of the rubber plantation and local tropical rainforest, respectively. When entire land area in Xishuangbanna is considered, N₂O emissions from fertilized rubber plantations offset 17.1% of the tropical rainforest’s carbon sink. The results show that if tropical rainforests are converted to fertilized rubber plantations, regional N₂O emissions may enhance local climate warming.

Nitrogen deposition and greenhouse gas emissions from grasslands: uncertainties and future directions

Increases in atmospheric nitrogen deposition (Ndep) can strongly affect the greenhouse gas (GHG; CO2, CH4, and N2O) sink capacity of grasslands as well as other terrestrial ecosystems. Robust predictions of the net GHG sink strength of grasslands depend on how experimental N loads compare to projected Ndep rates, and how accurately the relationship between GHG fluxes and Ndep is characterized. A literature review revealed that the vast majority of experimental N loads were higher than levels these ecosystems are predicted to experience in the future. Using a process-based biogeochemical model, we predicted that low levels of Ndep either enhanced or reduced the net GHG sink strength of most grasslands, but as experimental N loads continued to increase, grasslands transitioned to a N saturation-decline stage, where the sensitivity of GHG exchange to further increases in Ndep declined. Most published studies represented treatments well into the N saturation-decline stage. Our model results predict that the responses of GHG fluxes to N are highly nonlinear and that the N saturation thresholds for GHGs varied greatly among grasslands and with fire management. We predict that during the 21st century some grasslands will be in the N limitation stage where others will transition into the N saturation-decline stage. The linear relationship between GHG sink strength and N load assumed by most studies can overestimate or underestimate predictions of the net GHG sink strength of grasslands depending on their N baseline status. The next generation of global change experiments should be designed at multiple N loads consistent with future Ndep rates to improve our empirical understanding and predictive ability.

North American terrestrial CO2 uptake largely offset by CH4 and N2O emissions: toward a full accounting of the greenhouse gas budget

The terrestrial ecosystems of North America have been identified as a sink of atmospheric CO₂ though there is no consensus on the magnitude. However, the emissions of non-CO₂ greenhouse gases (CH₄ and N₂O) may offset or even overturn the climate cooling effect induced by the CO₂ sink. Using a coupled biogeochemical model, in this study, we have estimated the combined global warming potentials (GWP) of CO₂, CH₄ and N₂O fluxes in North American terrestrial ecosystems and quantified the relative contributions of environmental factors to the GWP changes during 1979-2010. The uncertainty range for contemporary global warming potential has been quantified by synthesizing the existing estimates from inventory, forward modeling, and inverse modeling approaches. Our "best estimate" of net GWP for CO₂, CH₄ and N₂O fluxes was -0.50 ± 0.27 Pg CO₂ eq/year (1 Pg = 10¹⁵ g) in North American terrestrial ecosystems during 2001-2010. The emissions of CH₄ and N₂O from terrestrial ecosystems had offset about two thirds (73 %±14 %) of the land CO₂ sink in the North American continent, showing large differences across the three countries, with offset ratios of 57 % ± 8 % in US, 83 % ± 17 % in Canada and 329 % ± 119 % in Mexico. Climate change and elevated tropospheric ozone concentration have contributed the most to GWP increase, while elevated atmospheric CO₂ concentration have contributed the most to GWP reduction. Extreme drought events over certain periods could result in a positive GWP. By integrating the existing estimates, we have found a wide range of uncertainty for the combined GWP. From both climate change science and policy perspectives, it is necessary to integrate ground and satellite observations with models for a more accurate accounting of these three greenhouse gases in North America.

Complex spatiotemporal responses of global terrestrial primary production to climate change and increasing atmospheric CO2 in the 21st century

Quantitative information on the response of global terrestrial net primary production (NPP) to climate change and increasing atmospheric CO₂ is essential for climate change adaptation and mitigation in the 21^st century. Using a process-based ecosystem model (the Dynamic Land Ecosystem Model, DLEM), we quantified the magnitude and spatiotemporal variations of contemporary (2000s) global NPP, and projected its potential responses to climate and CO₂ changes in the 21^st century under the Special Report on Emission Scenarios (SRES) A2 and B1 of Intergovernmental Panel on Climate Change (IPCC). We estimated a global terrestrial NPP of 54.6 (52.8–56.4) PgC yr⁻¹ as a result of multiple factors during 2000–2009. Climate change would either reduce global NPP (4.6%) under the A2 scenario or slightly enhance NPP (2.2%) under the B1 scenario during 2010–2099. In response to climate change, global NPP would first increase until surface air temperature increases by 1.5°C (until the 2030s) and then level-off or decline after it increases by more than 1.5°C (after the 2030s). This result supports the Copenhagen Accord Acknowledgement, which states that staying below 2°C may not be sufficient and the need to potentially aim for staying below 1.5°C. The CO₂ fertilization effect would result in a 12%–13.9% increase in global NPP during the 21^st century. The relative CO₂ fertilization effect, i.e. change in NPP on per CO₂ (ppm) bases, is projected to first increase quickly then level off in the 2070s and even decline by the end of the 2080s, possibly due to CO₂ saturation and nutrient limitation. Terrestrial NPP responses to climate change and elevated atmospheric CO₂ largely varied among biomes, with the largest increases in the tundra and boreal needleleaf deciduous forest. Compared to the low emission scenario (B1), the high emission scenario (A2) would lead to larger spatiotemporal variations in NPP, and more dramatic and counteracting impacts from climate and increasing atmospheric CO₂.

Simulating greenhouse gas mitigation potentials for Chinese Croplands using the DAYCENT ecosystem model

Understanding the potential for greenhouse gas (GHG) mitigation in agricultural lands is a critical challenge for climate change policy. This study uses the DAYCENT ecosystem model to predict GHG mitigation potentials associated with soil management in Chinese cropland systems. Application of ecosystem models, such as DAYCENT, requires the evaluation of model performance with data sets from experiments relevant to the climate and management of the study region. DAYCENT was evaluated with data from 350 cropland experiments in China, including measurements of nitrous oxide emissions (N2 O), methane emissions (CH4 ), and soil organic carbon (SOC) stock changes. In general, the model was reasonably accurate with R(2) values for model predictions vs. measurements ranging from 0.71 to 0.85. Modeling efficiency varied from 0.65 for SOC stock changes to 0.83 for crop yields. Mitigation potentials were estimated on a yield basis (Mg CO2 -equivalent Mg(-1) Yield). The results demonstrate that the largest decrease in GHG emissions in rainfed systems are associated with combined effect of reducing mineral N fertilization, organic matter amendments and reduced-till coupled with straw return, estimated at 0.31 to 0.83 Mg CO2 -equivalent Mg(-1) Yield. A mitigation potential of 0.08 to 0.36 Mg CO2 -equivalent Mg(-1) Yield is possible by reducing N chemical fertilizer rates, along with intermittent flooding in paddy rice cropping systems.