Simulated The Effects Of Nitrogen Deposition On Global Carbon Budget Using The IBIS Model

Since the 20th century,the consumptions of fossil fuels had been increasing,which directly increased the nitrogen releasing into the environment.The nitrogen re-entered into atmosphere and then settled into ecosystem through nitrogen deposition.Atmospheric nitrogen deposition had increased significantly over the past 100 years and it will likely be double in the next 25 years.Anthropogenic activities had greatly changed the ways and rate of nitrogen deposition from atmosphere to terrestrial ecosystems,thus many researchers focused on the issue of the effects of atmospheric nitrogen deposition on ecosystem carbon budget.This study aims to evaluate the effects of nitrogen deposition on global carbon budget using process based terrestrial ecosystem model.Firstly,the time variations of global atmospheric nitrogen deposition were estimated,and atmospheric nitrogen deposition in historical period were reconsturcted,based on the atmospheric NO2 column density remote sensing data with a long time series.Moreover,literatures on N deposition were collected to summarize the effects of nitrogen deposition on terrestrial ecosystem carbon cycling using meta-analysis,and then the results of the meta analysis were integrated into a terrestrial ecosystem model,IBIS(Integrated Biosphere Simulator).The long time series of atmospheric nitrogen deposition data and climate data were used as driving data into IBIS model to simulate global carbon,water and nitrogen cycling processing.Finally,to compare the results of multiple simulation experiments,the effects of nitrogen deposition on terrestrial ecosystem carbon budget,carbon and water coupling and carbon and nitrogen coupling were measured;and the effects of temperature,precipitation,atmospheric CO2 and nitrogen deposition were also compared.The results are listed as below.(1)In the global nitrogen deposition research,the principal component regression method was used to establish nitrogen deposition with long time series.NOZ column density,meteorological data and MOZART(Model for OZone and Related chemical Tracers)model outputs had been applied to establish the regression model.The results showed that atmospheric nitrogen deposition in most of the world were higher than the global nitrogen deposition background value.Moreover,the spatial distribution of nitrogen deposition showed high variations.The global nitrogen deposition increased by 38.35%from 1970 to 2009.China,India,Europe and the United States had the highest levels of nitrogen deposition.While,the fastest growing region is Asia,with the highest depostion of 1.58 g N m-2 yr-1 in India.(2)The effects of nitrogen saturation on photosynthesis,carbon allocation and litter decomposition had been incorporated into IBIS model which made nitrogen cycle module of IBIS model further improved.To validate the model simulation accuracy,the literature data,remote sensing data,sample observation data and Fluxnet data had been used to compare with IBIS simulation results in GPP(Gross Primary Productivity),NPP(Net Primary Productivity),NEP(Net Ecosystem Productivity)and ET(EvapoTranspiration).The validation results showed that IBIS simulation had high accuracy in global carbon and water cycling.(3)Both of the global total NPP and NEP had increased during the historical period.In the early first decade of this century,the total NPP increased to 52.8 Pg C yr-1 and total NEP increased to 3.5 Pg C yr-l.Comparing with the simulated results in 1970s,the total NPP and NEP had increased by 9.3%and 29.6%,respectively.Forest played an important roles both in the global total of NPP and NEP,among which,the tropical forest is the most important forest in global carbon budget.In the past 40 years,the global terrestrial nitrogen deposition had increased significantly,but it did not lead NPP and NEP change significantly.The average levels of nitrogen deposition during 2000-2009 was the highest among the past 40 years,which had increased by 53%compared with that in 1970s.At the meanwhile,the average NPP and NEP had only increased by 0.64%and 4.6%,respectively.The reasons for the weak effects of nitrogen deposition on carbon budget were the uneven distribution of carbon budget and nitrogen saturation.Most of the terrestrial ecosystem had not been affected by the intense nitrogen deposition.The high value of nitrogen deposition was mainly distributed in the area where have strong human activities.Forests are important to carbon assimilation and sequestration,but nitrogen deposition had not increased significant in forests generally,particularly in boreal forests.Moreover,the high level of nitrogen deposition lead nitrogen saturation which also limited nitrogen positive effects on carbon budget.(4)The increased nitrogen deposition declined NUE(Nitrogen Use Efficiency)at the global scale,with the average declined range of 2⒍4 g C/g N.On the spatial distribution,the highest decline of NUE caused by nitrogen deposition was located in subtropical forests in the east of China and temper forests in east Europe,where NUE declined by 20.3 g C/g N.And the highest increase of NUE was in some boreal forests in north of east Europe where NUE increased by 11.2g C/g N.The increase of nitrogen deposition and the decline of NUE had significant negative correlation which can be examined both around the world and in each biomes.From the change of this negative correlation,it can be found that coupling effects of climate change and rising atmospheric CO2 had made this negative correlation weaken.(5)WUE(Water Use Efficiency)had evident increasing in historical period.It had increased by about 10%from 1970 to 2009.The contribution of nitrogen deposition on the increase of WUE was weak.The other driving factors had made more contribution on WUE.In the coupling effects under global change,the increasing of temperature and the change of precipitation had taken negative effects on WUE,but the elevated nitrogen deposition and atmospheric CO2 had increased WUE.Among them,the coupling effects of elevated atmospheric CO2 had made the most significant positive impacts on WUE,on the contrary,temperature had take the maximum negative effects on WUE.The negative correlation of WUE and NUE was not significant from the view of global average value.But the negative correlation was widely distributed at the global and also significant in some areas.The effects of nitrogen deposition on this relationship was only limited in the areas with high nitrogen deposition level.