Model Incorporation And Simulation For Nitrous Oxide Emissions From Global Forests And Grasslands

Recent studies show that the global warming is an indubitable fact.Studying the trend of greenhouse gas concentration in the atmosphere plays an important role to explore the global climate change rules.Most climate scientists agree that at present,the main cause of global warming is the increasing of"greenhouse effect".Nitrous Oxide?N2O?is a long-lived trace gas,it can not only participate in ozone decomposition by the photochemical reaction,but also has the intense radiation forcing effect,thus becoming one of the three major greenhouse gases which cause the climate change.At present,the research on N2O mainly focuses on the farmland and other agricultural ecosystems which are greatly influenced by human activities;however,the N2O emissions from natural ecosystems,which occupy a large proportion of total N2O budget,have been less studied and their estimations still have large uncertainties.Forests and grasslands are important natural ecosystems in the world.They play critical roles in maintaining ecological balance and ensuring material circulation in terrestrial ecosystems.Therefore,to enhance the simulation of N2O emission variation from natural forests and grasslands can help us to explore and quantify the relationship between N2O emissions and climate change,further provide data support for policy makers to deal with issues of laws or regulations which related to greenhouse gas emissions.Multiple biogeochemical processes,huge environmental factors and complex and delicate ecological conditions lead to a fact that N2O natural sources has a very strong spatial and temporal heterogeneity.The in-situ experiment cannot solve such problem,instead,ecological models become an efficient tool to estimate N₂O emissions at large scale and long-term time span.Based on this,this study has been carried out and main results are as follows:?1?Incorporating the nitrification and denitrification process into the biogeochemical sunmodule in original TRIPLEX-GHG model,thus made it possible to simulate N2O emission from global forest and grassland ecosystems.In this stduy,the main method of model coupling is to establish a two-way connection between the original TRIPLEX-GHG model and the nitrification and denitrification processes.On the one hand,soil organic carbon pools and soil mineral nitrogen pools provide substrates for nitrification and denitrification.On the other hand,soil organic carbon pools and soil mineral nitrogen pools are updated by calculating the consumption of soil nitrogen resulting from nitrification and denitrification.?2?Building a global N₂O emission flux database which was applied to model parameterization and model evaluation,and results show that the coupled model simulates well for global N2O fluxes.In this study,the sensitivity analysis of parameters shows that the maximum nitrification rate constant?COENR?is the most sensitive parameter related to N2O emission,revealing the important role of nitrification in N cycle,especially for quantifying N2O emission and providing reactants for denitrification.We have calibrated this parameter using measured data from 29 global forests and grasslands sites.The calibration results were gradually increased according to the order from tropical forests to grasslands,from temperate forests to northern forest types,with an average value of 0.009,0.03,0.04 and 0.09respectively.In addition,the model is able to capture seasonal dynamic changes of N₂O and simulating the magnitude of N2O emissions.However,the model also has certain limitations,for example,the model cannot capture emission peaks during the freeze-thawing periods,overestimated the N2O emissions level from places?usually in boreal regions?with N2O background emission patterns,and it not considered the N2O absorption under specific conditions etc..This study also validate the simulations,which use the average values of parameters in 52 corresponding points of the globe,against the observations.Result shows that the simulation of N₂O flux is significantly correlated with the observed values?R²=0.75;P<0.01?.?3?Simulating the temporal and spatial patterns of N2O emissions from global forest and grassland ecosystems,and further evaluate the impact of climate change and the extreme climate events on N₂O emissions and the relationship between them.In this study,using the land cover data?ESA-CCI-LC?that based on the remote sensing,the model simulated the temporal and spatial patterns of N2O emissions from global forests and grasslands.During the period of 1992-2015,the estimated average N2O emissions from forests and grasslands were3.62±0.16 Tg N yr^-1 and 1.40±0.03 Tg N yr^-1,respectively,and their annual N₂O emission showed a slight upward trend.Tropical and subtropical forests and grasslands generally contribute to large N2O emission?the contribution rate>80%?,but boreal forests and grasslands contribute to relatively small N2O emissions.Compared with the other studies,our results are in the reasonable range.Moreover,the correlation of soil N2O emissions and atmospheric N2O concentration results show that there is a positive but relatively low corrlation between N2O emissions from forest and grassland soils and the atmospheric N2O concentration?R²=0.26,P<0.01?,which means that although both showed a sustained rising trend,but the natural source of N₂O emissions is not the key factor leading to the rise of the atmospheric N2O concentration.In addition,we also found that N2O emissions increased or decreased in El Ni?o/La Ni?a year,which may be related to the significant negative correlation between precipitation and N2O flux in the tropics.Because the tropical grassland has obvious dry and wet seasonal periods,the El Ni?o/La Ni?a event has different effects on N2O fluxes in different periods,i.e.N2O will decrease/increase in wet seasons of El Ni?o/La Ni?a year,and performed opposite result in dry seasons.Based on this,because N2O emissions are closely related to extreme climate events?La Ni?a and El Ni?o?,this may become a breakthrough for studying the feedbacks between greenhouse gases and climate change.?4?Using the climate data simulated by CMIP5's climate models to predict future spatial and temporal patterns of N₂O emissions under three Representative Concentration Pathways?RCP?scenarios.In this study,to estimate future temporal and spatial variation of N₂O emissions,we also selected 15 CMIP5 climate model,and using their predicted climate data under RCP2.6,RCP4.5 and RCP8.5 scenarios to drive TRIPLEX-GHG model.The results show that total N₂O emissions from the global forest and grassland ecosystems in 2100 will reach at 13.74+0.89 Tg N under RCP2.6,16.12+0.83 Tg N under RCP4.5 and 19.6+1 Tg N under RCP8.5,which is a 164%,210%and 277%increase over the history period,respectively.In early 21th Century,emissions under three scenarios showed a similar growth trend.In the mid 21th Century,the rising trend of N₂O emissions under the RCP8.5 scenario is the largest,the rising trend under RCP2.6 scenario is the smallest and that under RCP4.5 is at a moderate level.However,by the end of the 21th Century,the total amount of N₂O emissions in RCP8.5 and RCP4.5 scenarios still showed a large growth rate,but it had been mitigate compared with before.For the RCP2.6 scenario,the interannual emissions of N₂O remained stable and no longer continued to grow.Furthermore,N₂O flux in tropical regions under 3 RCP scenarios all showed a higher level,the differences of spatial pattern between 3scenarios is not big,but compared with the RCP2.6 scenario,N₂O flux in most parts of southern north-temperate and boreal forest and grassland under the RCP8.5 scenario showed different degrees of increasing phenomenon,so we speculate that forests and grasslands in most parts of southern north-temperate zone and boreal areas are more sensitive to climate change on RCP8.5 scenarios.The uncertainty of the prediction of N₂O emissions under three RCP scenarios is mainly due to the uncertainty of future climate data,especially the precipitation and air mean temperature.Moreover,this uncertainty is mainly reflected in the prediction of N₂O emission fluxes in the tropics.Therefore,it is particularly important to develop the model specially for tropcial regions,and to create a unique future emission scenario for the tropics.