• Energy Research

AbstractGlobally, about 50% of all arable soils are classified as acidic. As crop and plant growth are significantly hampered under acidic soil conditions, many farmers, but increasingly as well forest managers, apply lime to raise the soil pH. Besides its direct effect on soil pH, liming also affects soil C and nutrient cycles and associated greenhouse gas (GHG) fluxes. In this meta‐analysis, we reviewed 1570 observations reported in 121 field‐based studies worldwide, to assess liming effects on soil GHG fluxes and plant productivity. We found that liming significantly increases crop yield by 36.3%. Also, soil organic C (SOC) stocks were found to increase by 4.51% annually, though soil respiration is stimulated too (7.57%). Moreover, liming was found to reduce soil N2O emission by 21.3%, yield‐scaled N2O emission by 21.5%, and CH4 emission and yield‐scaled CH4 emission from rice paddies by 19.0% and 12.4%, respectively. Assuming that all acid agricultural soils are limed periodically, liming results in a total GHG balance benefit of 633−749 Tg CO2‐eq year−1 due to reductions in soil N2O emissions (0.60−0.67 Tg N2O‐N year−1) and paddy soil CH4 emissions (1.75−2.21 Tg CH4 year−1) and increases in SOC stocks (65.7–110 Tg C year−1). However, this comes at the cost of an additional CO2 release (c. 624–656 Tg CO2 year−1) deriving from lime mining, transport and application, and lime dissolution, so that the overall GHG balance is likely neutral. Nevertheless, liming of acid agricultural soils will increase yields by at least 6.64 × 108 Mg year−1, covering the food supply of 876 million people. Overall, our study shows for the first time that a general strategy of liming of acid agricultural soils is likely to result in an increasing sustainability of global agricultural production, indicating the potential benefit of liming acid soils for climate change mitigation and food security.

Urban land use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to 1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N2O) and methane (CH4) fluxes, 2) quantify direct N2O emission factors (EFd) of fertilized urban soils used e.g., as lawns or forests, and 3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N2O emissions by 153%, to 3.0 kg N ha-1 yr-1, while rates of soil CH4 uptake are reduced by 50%, to 2.0 kg C ha-1 yr-1. The mean annual N2O EFd of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N2O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N2O emission by 0.46 Tg N2O-N yr-1 and decreased soil CH4 uptake by 0.58 Tg CH4-C yr-1. Urbanization-driven changes in soil N2O emission and CH4 uptake are associated with changes in soil properties (bulk density, pH, total N content and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N2O emissions and reduces the role of soils as a sink for atmospheric CH4. These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and restoring native ecosystems in urban landscapes.

# A global meta-analysis of yield-scaled N2O emissions and its mitigation efforts for maize, wheat and rice Author: Zhisheng Yao, Haojie Guo, Yan Wang, Yang Zhan, Tianli Zhang, Rui Wang, Xunhua Zheng, Klaus Butterbach-Bahl Any correspondence has to be send to ## Description of the Data and file structure Note: There are six sheets in the dataset. The variable name, unit, and description for each column of each sheet are shown below. The empty cells in this Excel file mean that data are not available. Variable List: #### Sheet1: Overview | Column | Name | Unit | Description | | :----- | :----------- | :--- | :----------------------------------- | | A | Full Name | None | Full name of collected parameters. | | B | Abbreviation | None | Abbreviation of collected parameter. | #### Sheet2: Maize | Column | Name | Unit | Description | | :----- | :----------------------------- | :--------- | :------------------------------------------------ | | A. | Number | None | Number of References used in the meta-analysis. | | B. | Reference | None | References used in the meta-analysis. | | C. | Location | None | Location of the experimental site. | | D. | Country | None | Country of the experimental site. | | E. | Continent | None | Continent of the experimental site. | | F. | Longitude | None | Longitude of the experimental site. | | G. | Latitude | None | Latitude of the experimental site. | | H. | Category | None | Category of the experimental site. | | I. | Observation period | None | Observation period. | | J. | Climate | None | Climate of the experimental site. | | K. | MAT | °C | Mean annual temperature. | | L. | Category | None | Category of Mean annual temperature . | | M. | MAP | mm | Mean annual precipitation. | | N. | Category | None | Category of Mean annual precipitation. | | O. | Sand | % | Sand content of soil. | | P. | Silt | % | Silt content of soil. | | Q. | Clay | % | Clay content of soil. | | R. | Soil texture | None | Soil texture of soil. | | S. | SOC | g C kg-1 | Description:Soil organic C of soil. | | T. | Category | None | Category of Soil organic C. | | U. | TN | g N kg-1 | Total N of soil. | | V. | Category | None | Category of Total N . | | W. | C/N ratio | None | C/N ratio of soil. | | X. | Category | None | Category of C/N ratio. | | Y. | Soil pH | None | pH of soil. | | Z. | Category | None | Category of Soil pH. | | AA. | BD | g cm-3 | Bulk density of soil. | | AB. | Category | None | Category of BD. | | AC. | Crop type | None | Crop type of experiment. | | AD. | Replicates | None | Replicates of experiment. | | AE. | N type | None | N type of experiment. | | AF. | Category | None | Category of N type. | | AG. | N rate | kg N ha-1 | N rate of experiment. | | AH. | Category | None | Category of N rate. | | AI. | N fertilizer management | None | N fertilizer management of experiment. | | AJ. | Optimized timing and placement | None | Optimized timing and placement. | | AK. | Straw return | Unit:None | Straw return. | | AL. | Water regime | None | Water regime. | | AM. | Tillage type | None | Tillage type. | | AN. | Plastic-film mulching | None | Plastic-film mulching | | AO. | Cumulative N2O fluxes | kg N ha-1 | Cumulative nitrous oxide fluxes of experiment. | | AP. | SE | kg N ha-1 | Standard error of Cumulative N2O fluxes. | | AQ. | SD | kg N ha-1 | Standard deviation of Cumulative N2O fluxes. | | AR. | Grain yield | Mg ha-1 | Grain yield of experiment. | | AS. | SE | Mg ha-1 | Standard error of Grain yield. | | AT. | SD | Mg ha-1 | Standard deviation of Grain yield. | | AU. | Yield-scaled N2O emission | g N Mg-1 | Yield-scaled N2O emission of experiment. | | AV. | SE | g N Mg-1 | Standard error of Yield-scaled N2O emission. | | AW. | SD | g N Mg-1 | Standard deviation of Yield-scaled N2O emission. | | AX. | PFPN | kg kg -1 | partial factor productivity of N. | | AY. | ANE | kg kg -1 | agronomic N efficiency. | | AZ. | EFd | % | Direct N2O emission factor. | #### Sheet3: Wheat | **Column** | **Name** | **Unit** | **Description** | | :--------- | :----------------------------- | :--------- | :------------------------------------------------ | | A. | Number | None | Number of References used in the meta-analysis. | | B. | Reference | None | References used in the meta-analysis. | | C. | Location | None | Location of the experimental site. | | D. | Country | None | Country of the experimental site. | | E. | Continent | None | Continent of the experimental site. | | F. | Longitude | None | Longitude of the experimental site. | | G. | Latitude | None | Latitude of the experimental site. | | H. | Category | None | Category of the experimental site. | | I. | Observation period | None | Observation period. | | J. | Climate | None | Climate of the experimental site. | | K. | MAT | °C | Mean annual temperature. | | L. | Category | None | Category of Mean annual temperature . | | M. | MAP | mm | Mean annual precipitation. | | N. | Category | None | Category of Mean annual precipitation. | | O. | Sand | % | Sand content of soil. | | P. | Silt | % | Silt content of soil. | | Q. | Clay | % | Clay content of soil. | | R. | Soil texture | None | Soil texture of soil. | | S. | SOC | g C kg-1 | Soil organic C of soil. | | T. | Category | None | Category of Soil organic C. | | U. | TN | g N kg-1 | Total N of soil. | | V. | Category | None | Category of Total N . | | W. | C/N ratio | None | C/N ratio of soil. | | X. | Category | None | Category of C/N ratio. | | Y. | Soil pH | None | pH of soil. | | Z. | Category | None | Category of Soil pH. | | AA. | BD | g cm-3 | Bulk density of soil. | | AB. | Category | None | Category of BD. | | AC. | Crop type | None | Crop type of experiment. | | AD. | Replicates | None | Replicates of experiment. | | AE. | N type | None | N type of experiment. | | AF. | Category | None | Category of N type. | | AG. | N rate | kg N ha-1 | N rate of experiment. | | AH. | Category | None | Category of N rate. | | AI. | N fertilizer management | None | N fertilizer management of experiment. | | AJ. | Optimized timing and placement | None | Optimized timing and placement. | | AK. | Straw return | None | Straw return. | | AL. | Water regime | None | Water regime. | | AM. | Tillage type | None | Tillage type. | | AN. | Plastic-film mulching | None | Plastic-film mulching | | AO. | Cumulative N2O fluxes | kg N ha-1 | Cumulative nitrous oxide fluxes of experiment. | | AP. | SE | kg N ha-1 | Standard error of Cumulative N2O fluxes. | | AQ. | SD | kg N ha-1 | Standard deviation of Cumulative N2O fluxes. | | AR. | Grain yield | Mg ha-1 | Grain yield of experiment. | | AS. | SE | Mg ha-1 | Standard error of Grain yield. | | AT. | SD | Mg ha-1 | Standard deviation of Grain yield. | | AU. | Yield-scaled N2O emission | g N Mg-1 | Yield-scaled N2O emission of experiment. | | AV. | SE | g N Mg-1 | Standard error of Yield-scaled N2O emission. | | AW. | SD | g N Mg-1 | Standard deviation of Yield-scaled N2O emission. | | AX. | PFPN | kg kg -1 | partial factor productivity of N. | | AY. | ANE | kg kg -1 | agronomic N efficiency. | | AZ. | EFd | % | Direct N2O emission factor. | #### Sheet4: Rice | **Column** | **Name** | **Unit** | **Description** | | :--------- | :----------------------------- | :--------- | :------------------------------------------------ | | A. | Number | None | Number of References used in the meta-analysis. | | B. | Reference | None | References used in the meta-analysis. | | C. | Location | None | Location of the experimental site. | | D. | Country | None | Country of the experimental site. | | E. | Continent | None | Continent of the experimental site. | | F. | Longitude | None | Longitude of the experimental site. | | G. | Latitude | None | Latitude of the experimental site. | | H. | Category | None | Category of the experimental site. | | I. | Observation period | None | Observation period. | | J. | Climate | None | Climate of the experimental site. | | K. | MAT | °C | Mean annual temperature. | | L. | Category | None | Category of Mean annual temperature . | | M. | MAP | mm | Mean annual precipitation. | | N. | Category | None | Category of Mean annual precipitation. | | O. | Sand | % | Sand content of soil. | | P. | Silt | % | Silt content of soil. | | Q. | Clay | % | Clay content of soil. | | R. | Soil texture | None | Soil texture of soil. | | S. | SOC | g C kg-1 | Soil organic C of soil. | | T. | Category | None | Category of Soil organic C. | | U. | TN | g N kg-1 | Total N of soil. | | V. | Category | None | Category of Total N . | | W. | C/N ratio | None | C/N ratio of soil. | | X. | Category | None | Category of C/N ratio. | | Y. | Soil pH | None | pH of soil. | | Z. | Category | None | Category of Soil pH. | | AA. | BD | g cm-3 | Bulk density of soil. | | AB. | Category | None | Category of BD. | | AC. | Crop type | None | Crop type of experiment. | | AD. | Replicates | None | Replicates of experiment. | | AE. | N type | None | N type of experiment. | | AF. | Category | None | Category of N type. | | AG. | N rate | kg N ha-1 | N rate of experiment. | | AH. | Category | None | Category of N rate. | | AI. | N fertilizer management | None | N fertilizer management of experiment. | | AJ. | Optimized timing and placement | None | Optimized timing and placement. | | AK. | Straw return | None | Straw return. | | AL. | Water regime | None | Water regime. | | AM. | Tillage type | None | Tillage type. | | AN. | Plastic-film mulching | None | Plastic-film mulching | | AO. | Cumulative N2O fluxes | kg N ha-1 | Cumulative nitrous oxide fluxes of experiment. | | AP. | SE | kg N ha-1 | Standard error of Cumulative N2O fluxes. | | AQ. | SD | kg N ha-1 | Standard deviation of Cumulative N2O fluxes. | | AR. | Grain yield | Mg ha-1 | Grain yield of experiment. | | AS. | SE | Mg ha-1 | Standard error of Grain yield. | | AT. | SD | Mg ha-1 | Standard deviation of Grain yield. | | AU. | Yield-scaled N2O emission | g N Mg-1 | Yield-scaled N2O emission of experiment. | | AV. | SE | g N Mg-1 | Standard error of Yield-scaled N2O emission. | | AW. | SD | g N Mg-1 | Standard deviation of Yield-scaled N2O emission. | | AX. | PFPN | kg kg -1 | partial factor productivity of N. | | AY. | ANE | kg kg -1 | agronomic N efficiency. | | AZ. | EFd | % | Direct N2O emission factor. | #### Sheet5: Potential mitigation strategies | Column | Name | Unit | Description | | :----- | :------------------------------- | :--------- | :--------------------------------------------------- | | A. | Number | None | Number of References used in the meta-analysis. | | B. | Reference | None | References used in the meta-analysis. | | C. | Crop type | None | Crop type of experiment. | | D. | C\_Cumulative N2O fluxes | kg N ha-1 | Cumulative N2O fluxes of control soil. | | E. | T\_Cumulative N2O fluxes | kg N ha-1 | Cumulative N2O fluxes of experimental soil. | | F. | ln RR\_Cumulative N2O fluxes | None | Response ratio of Cumulative N2O fluxes. | | G. | C\_Grain yield | Mg ha-1 | Grain yield of the control soil. | | H. | T\_Grain yield | Mg ha-1 | Grain yield of the experimental soil. | | I. | ln RR\_Grain yield | None | Response ratio of Grain yield. | | J. | C\_Yield-scaled N2O emission | g N Mg-1 | Yield-scaled N2O emission of the control soil. | | K. | T\_Yield-scaled N2O emission | g N Mg-1 | Yield-scaled N2O emission of the experimental soil. | | L. | ln RR\_Yield-scaled N2O emission | None | Response ratio of Yield-scaled N2O emission. | | M. | Potential mitagation strategies | None | Potential mitagation strategies. | #### Sheet6: References | Column | Name | Unit | Description | | :----- | :-------- | :--- | :----------------------------------- | | A | Number | None | Number of references. | | B | Reference | None | References used in the meta-analysis | ## Sharing/Access information Data was derived from the following sources: We identified a total of 608 published experimental studies covering 30 countries and 724 sites, including 236 experiments for maize, 190 for wheat and 298 for rice. Collect the yield-scaled N2O emissions, grain yields, total cumulative N2O emissions, and other key information, including geographical location (country, longitude and latitude), general climate (tropical, subtropical and temperate), mean annual air temperature (MAT) and precipitation (MAP), soil properties (e.g., soil texture, soil organic C (SOC), total N content (TN), soil C/N ratio, soil pH and bulk density (BD)), management practices (e.g., straw return, tillage and N fertilizer type and rate). And recorded in our full dataset. ## Code/Software None Maintaining or even increasing crop yields while reducing nitrous oxide (N2O) emissions is necessary to reconcile food security and climate change, while the metric of yield-scaled N2O emission (i.e., N2O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta-analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield-scaled N2O emissions for maize, wheat, and rice and associated potential mitigation options. Our results showed that the average yield-scaled N2O emissions across all available data followed the order wheat (322 g N Mg-1, with the 95% confidence interval (CI): 301-346) > maize (211 g N Mg-1, CI: 198-225) > rice (153 g N Mg-1, CI: 144-163). Yield-scaled N2O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield-scaled N2O emissions with increasing N use efficiency or in N2O emissions for production systems with cereal yields > 10 Mg ha-1 (maize), 6.6 Mg ha-1 (wheat) or 6.8 Mg ha-1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade-offs between crop production and N2O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced-efficiency N fertilizers significantly reduced yield-scaled N2O emissions at similar or even higher cereal yields. Our data-driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.

Atmospheric concentrations of the greenhouse gas nitrous oxide (N(2)O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle, with animal production being one of the main contributors. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N(2)O emissions. Internationally agreed methods to upscale the effect of increased livestock numbers on N(2)O emissions are based directly on per capita nitrogen inputs. However, measurements of grassland N(2)O fluxes are often performed over short time periods, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N(2)O fluxes remains limited. Here we report year-round N(2)O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N(2)O emission during spring thaw dominate the annual N(2)O budget at our study sites. The N(2)O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N(2)O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling and, hence, on N(2)O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze-thaw interactions, existing approaches may have systematically overestimated N(2)O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent.

AbstractUrban land‐use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta‐analysis to (1) assess the effects of urbanization‐induced land‐use conversion on soil nitrous oxide (N2O) and methane (CH4) fluxes, (2) quantify direct N2O emission factors (EFd) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N2O emissions by 153%, to 3.0 kg N ha−1 year−1, while rates of soil CH4 uptake are reduced by 50%, to 2.0 kg C ha−1 year−1. The global mean annual N2O EFd of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N2O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N2O emission by 0.46 Tg N2O‐N year−1 and decreased soil CH4 uptake by 0.58 Tg CH4‐C year−1. Urbanization driven changes in soil N2O emission and CH4 uptake are associated with changes in soil properties (bulk density, pH, total N content, and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta‐analysis shows that urbanization increases soil N2O emissions and reduces the role of soils as a sink for atmospheric CH4. These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.

The alpine meadow ecosystem is one of the major vegetation biomes on the Qinghai-Tibetan Plateau, which hold substantial quantities of soil organic carbon. Pronounced grassland degradations (induced by overgrazing/climate change and further exacerbated by the subterranean rodent activities) that have widely occurred in this ecosystem may significantly alter the non-growing season carbon turnover processes such as carbon dioxide (CO2) efflux, but little is known about how the non-growing season CO2 emissions respond to the degradation (particularly the exacerbated degradations by plateau zokor), as most previous studies have focused primarily on the growing season. In this study, the effects of four degradation levels (i.e., the healthy meadow (HM), degraded patches (DP), 2-year-old zokor mounds (ZM2), and current-year zokor mounds (ZM1)) on CO2 emissions and corresponding environmental and agronomic variables were investigated over the two non-growing seasons under contrasting climatic conditions (a normal season in 2013-2014 and a "warm and humid" season in 2014-2015). The temporal variation in the non-growing season CO2 emissions was mainly regulated by soil temperature, while increasing degradation levels reduced the temperature sensitivity of CO2 emissions due to a reduction in soil water content. The cumulative CO2 emissions across the non-growing season were 587-1283 kg C ha-1 for all degradation levels, which varied significantly (p < 0.05) interannually. The degradation of alpine meadows significantly (p < 0.05) reduced the vegetation cover and aboveground net primary productivity as well as the belowground biomass, which are typically thought to decrease soil CO2 emissions. However, the non-growing season CO2 emissions for the degraded meadow, weighted by the areal extent of the DP, ZM2, and ZM1, were estimated to be 641-1280 kg C ha-1, which was significantly higher (p < 0.05) as compared with the HM in the warm and humid season of 2014-2015 but not in the normal season of 2013-2014. Additionally, grassland degradation substantially increased the productivity-scaled non-growing season CO2 emissions, which showed an exponential trend with increasing degradation levels. These results suggest that there is a strong connection between grassland degradation and soil carbon loss, e.g., in the form of CO2 release, pointing to the urgent need to manage degraded grassland restoration that contributes to climate change mitigation.

AbstractMaintaining or even increasing crop yields while reducing nitrous oxide (N2O) emissions is necessary to reconcile food security and climate change, while the metric of yield‐scaled N2O emission (i.e., N2O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta‐analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield‐scaled N2O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield‐scaled N2O emissions across all available data followed the order wheat (322 g N Mg−1, with the 95% confidence interval [CI]: 301–346) > maize (211 g N Mg−1, CI: 198–225) > rice (153 g N Mg−1, CI: 144–163). Yield‐scaled N2O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield‐scaled N2O emissions with increasing N use efficiency or in N2O emissions for production systems with cereal yields >10 Mg ha−1 (maize), 6.6 Mg ha−1 (wheat) or 6.8 Mg ha−1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade‐offs between crop production and N2O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced‐efficiency N fertilizers significantly reduced yield‐scaled N2O emissions at similar or even higher cereal yields. Our data‐driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.