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Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non‐CO2 GHGs (CH4 and N2O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom‐up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH) in both forests and grasslands, while a significant N‐induced increase in soil CO2 fluxes (RS, soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4, suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production‐based method (forest, 1.28 Pg CO2‐eq year−1 vs. grassland, 0.58 Pg CO2‐eq year−1) was greater than those estimated using the SOC‐based method (forest, 0.32 Pg CO2‐eq year−1 vs. grassland, 0.18 Pg CO2‐eq year−1) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%–4.8%) by the combined effects of its stimulation of N2O emissions together with the reduced soil uptake of CH4. [ABSTRACT FROM AUTHOR]

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 (N2O) 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 N2O 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 N2O 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. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) emissions from fertilized soils to the atmosphere and the subsequent deposition to land surface exert adverse effects on biogeochemical nitrogen (N) cycling. The region‐ and crop‐specific emission factors (EFs) of N fertilizer for NH3 are poorly developed and therefore the global estimate of soil NH3 emissions from agricultural N fertilizer application is constrained. Here we quantified the region‐ and crop‐specific NH3 EFs of N fertilizer by compiling data from 324 worldwide manipulative studies and focused to map the global soil NH3 emissions from agricultural N fertilizer application. Globally, the NH3 EFs averaged 12.56% and 14.12% for synthetic N fertilizer and manure, respectively. Regionally, south‐eastern Asia had the highest NH3 EFs of synthetic N fertilizer (19.48%) and Europe had the lowest (6%), which might have been associated with the regional discrepancy in the form and rate of N fertilizer use and management practices in agricultural production. Global agricultural NH3 emissions from the use of synthetic N fertilizer and manure in 2014 were estimated to be 12.32 and 3.79 Tg N/year, respectively. China (4.20 Tg N/year) followed by India (2.37 Tg N/year) and America (1.05 Tg N/year) together contributed to over 60% of the total global agricultural NH3 emissions from the use of synthetic N fertilizer. For crop‐specific emissions, the NH3 EFs averaged 11.13%–13.95% for the three main staple crops (i.e., maize, wheat, and rice), together accounting for 72% of synthetic N fertilizer‐induced NH3 emissions from croplands in the world and 70% in China. The region‐ and crop‐specific NH3 EFs of N fertilizer established in this study offer references to update the default EF in the IPCC Tier 1 guideline. This work also provides an insight into the spatial variation of soil‐derived NH3 emissions from the use of synthetic N fertilizer in agriculture at the global and regional scales. [ABSTRACT FROM AUTHOR]

Soils are among the important sources of atmospheric nitric oxide ( NO) and nitrous oxide (N2O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer-reviewed publications with 520 field measurements were synthesized using meta-analysis procedure to examine the N fertilizer-induced soil NO and the combined NO+N2O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO-N fluxes were estimated to be 4.06 kg ha−1 yr−1, with the greatest (9.75 kg ha−1 yr−1) in vegetable croplands and the lowest (0.11 kg ha−1 yr−1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO ( EFNO) and combined NO+N2O ( EFc) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71-1.61% and 1.81-3.35%, respectively. Forests had the greatest EFNO (2.39%). Within the croplands, the EFNO (1.71%) and EFc (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EFNO (2.93%) and EFc (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands. [ABSTRACT FROM AUTHOR]

Nitrogen fertilizer-induced direct nitrous oxide (N2O) emissions depend on water regimes in paddy fields, such as seasonal continuous flooding (F), flooding–midseason drainage–reflooding (F-D-F), and flooding–midseason drainage–reflooding–moist intermittent irrigation but without water logging (F-D-F-M). In order to estimate the changes in direct N2O emission from paddy fields during the rice-growing season in Mainland of China between the 1950s and the 1990s, the country-specific emission factors of N2O-N under different water regimes combined with rice production data were adopted in the present study. Census statistics on rice production showed that water management and nitrogen input regimes have changed in rice paddies since the 1950s. During the 1950s–1970s, about 20–25% of the rice paddy was continuously waterlogged, and 75–80% under the water regime of F-D-F. Since the 1980s, about 12–16%, 77%, and 7–12% of paddy fields were under the water regimes of F, F-D-F, and F-D-F-M, respectively. Total nitrogen input during the rice-growing season has increased from 87.5 kg N ha−1 in the 1950s to 224.6 kg N ha−1 in the 1990s. The emission factors of N2O-N were estimated to be 0.02%, 0.42%, and 0.73% for rice paddies under the F, F-D-F, and F-D-F-M water regimes, respectively. Seasonal N2O emissions have increased from 9.6 Gg N2O-N each year in the 1950s to 32.3 Gg N2O-N in the 1990s, which is accompanied by the increase in rice yield over the period 1950s–1990s. The uncertainties in N2O estimate were estimated to be 59.8% in the 1950s and 37.5% in the 1990s. In the 1990s, N2O emissions during the rice-growing season accounted for 8–11% of the reported annual total of N2O emissions from croplands in China, suggesting that paddy rice development could have contributed to mitigating agricultural N2O emissions in the past decades. However, seasonal N2O emissions would be increased, given that saving-water irrigation and nitrogen inputs are increasingly adopted in rice paddies in China. [ABSTRACT FROM AUTHOR]

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO 2 exchange and the emission or uptake of non-CO 2 GHGs (CH 4 and N 2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH 4 and N 2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (R H ) in both forests and grasslands, while a significant N-induced increase in soil CO 2 fluxes (R S , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N 2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH 4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH 4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO 2 -eq year -1 vs. grassland, 0.58 Pg CO 2 -eq year -1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO 2 -eq year -1 vs. grassland, 0.18 Pg CO 2 -eq year -1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N 2 O emissions together with the reduced soil uptake of CH 4 ., (© 2023 John Wiley & Sons Ltd.)

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 (NH 3 ), nitrous oxide (N 2 O) and nitric oxide (NO), respectively. Over the past three decades, NH 3 volatilization from croplands has experienced a shift from a rapid increase to a decline trend, whereas N 2 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 N 2 O emissions in China was found to dominate over the local cooling effects of NH 3 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., (© 2021 John Wiley & Sons Ltd.)

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N 2 O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer-reviewed publications with 520 field measurements were synthesized using meta-analysis procedure to examine the N fertilizer-induced soil NO and the combined NO+N 2 O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO-N fluxes were estimated to be 4.06 kg ha -1  yr -1 , with the greatest (9.75 kg ha -1  yr -1 ) in vegetable croplands and the lowest (0.11 kg ha -1  yr -1 ) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF NO ) and combined NO+N 2 O (EF c ) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71-1.61% and 1.81-3.35%, respectively. Forests had the greatest EF NO (2.39%). Within the croplands, the EF NO (1.71%) and EF c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF NO (2.93%) and EF c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands., (© 2016 John Wiley & Sons Ltd.)