Model Incorporation And Simulation For Nitrous Oxide Emission From Global Cropland Ecosystems Based On TRIPLEX-GHG Model

Global warming effect induced by excessive green house gas（GHG）emission plays a dominant role in the significant rising air temperature after the industrial revolution.Nitrous oxide（N₂O）is one of the most important GHG which is also the only long-lived trace gas with a strong global warming potential.Comparing with natural soil,the cropland ecosystem is a hot spot of N₂O emission owing to extensive agricultural practices which have been widely investigated.However,there are still large uncertainties for the global N₂O budget,especially for the cropland because of the complicated mechanism of N₂O production and the large variety of model simulation.Therefore,further development of process-based models and better estimation of spatial and temporal variation of N₂O emission is essential for better understanding the climate and agricultural practices effects on GHG emission,thus reducing the current uncertainties of the global N₂O budget.We have incorporated the major agricultural practices process,including fertilization,irrigation and tillage,into different submodules in the original TRIPLEX-GHG model so that the new improved model is able to simulate N₂O emission from different cropland ecosystems under various agricultural practices.Hence,we quantified global cropland N₂O emission pattern and identified corresponding driving factors.First,we constructed a global cropland N₂O flux database including a large variety of agricultural practices and most of the dominated crop types for model calibration and validation.Sensitivity analysis for major parameters suggested that the coefficient of NO₃^-consumption rate for denitrification(COE_{d NO3})was the most sensitive parameter.Next,the model was calibrated with measured N₂O flux data from 39 sites across major global croplands and the model performance indices illustrated that the model can effectively simulate N₂O flux at daily time step.The results of COE_{d NO3} were divided into 6 continents,namely North America（NA）,Asia（AS）,Europe（EU）,Oceanic（AU）,South America（SA）and Africa（AF）region,and the continent-mean was used for model validation.Means of measured daily N₂O fluxes during experiment periods from 69 sites observation are highly correlated with that of modelled results（R²=0.87,p<0.001）.Consequently,our simulation results suggested that the new improved TRIPLEX-GHG model is capable to provide a reasonable estimation of N₂O emissions from croplands on a global scale.Afterwards,the spatial and temporal variation of historical N₂O emission from the global cropland ecosystems were provided in this study.Furthermore,we used scenario analysis to investigate the effects of major agricultural practices and environment factors on N₂O flux.To be specifically,simulated global cropland N₂O emission showed a generally increasing trend from 1901 to 2015.After the 21st century,the mean annual N₂O emission from cropland was3.06±0.18 Tg N yr^-1.As for spatial distribution of cropland N₂O emission,major hotspots for N₂O flux were EU,NA and AS regions.Chemical fertilizer and manure application account for the most important driving factors for N₂O emission from global croplands,contributing49.28%and 21.85%of the historical increase of total N₂O emission,respectively.Our modelled results were generally consistent with current estimations,especially for ensemble process-based model results.Finally,the mismatch between observed and simulated results probably resulted from the limited description of the agricultural effects on soil properties.The uncertainties of the global simulation were mostly derived from the over-estimated emission from EU,which was due to the detailed model structure of the N fertilizer input information and manure effects on N₂O production and emission.In this study,a new improved TRIPLEX-GHG model has been developed to effectively simulate the spatial and temporal changes of global cropland N₂O emissions.Further improvement for modeling of N₂O flux from rice-paddy,pasture and rangelands can provide a comprehensive solution for mitigation of GHG emissions and sustainable development of agriculture under global change.