| Grasslands are one of the most common vegetation types in the world,account for nearly one fifth of the terrestrial surface.Since antiquity,grassland is closely related to human civilization,and is the most serious region affected by human activity.As an important component of the global vegetation system,grassland plays an important role in carbon cycle,exchange of material and energy,global climate change regulation,food and ecological security.Therefore,the relative contribution of fixed organic carbon,released CO2 and storage of grassland ecosystem in the carbon cycle is crucial for further understanding of climate change on terrestrial ecosystems feedbacks,global carbon budget and balance mechanism.Global grassland is the research object,IGBP data was employed to obtain global grassland area.In additional,model simulation is the main research methods,remote sensing and meteorological data was used to drive the model,study period is from 1982 to 2008.Based on these works,this study accomplished the following tasks:a.modeling the spatial distribution and variation of global grassland GPP and autotrophic respiration by using BIOME-BGC model,the CO2 fertilizer algorithm was incorporated in this model.Moreover,an inter-comparison analysis was performed to identify the difference of fraction of photosynthetically active radiation,which calculated by different data sources and algorithms.b.The spatial distribution and variation of global grassland NPP was simulated in this study,a comparative analysis was conducted to detect the spatial differences of NPP which modeled by three model,and its correlation with climate factors was also performed.Besides,the relative contribution of human activities and climate factors on grassland NPP dynamics by using scenario simulation method.c.The detachable carbon cycle model(DCC)was first proposed in this study,this model established based on a pool-and-flux scheme and contained 14 carbon pools,or carbon flow processes,each process could be detached from the main model and evaluated as an independent component.the spatial distribution of carbon storage and turnover time were modeled by using DCC model.Furthermore,the carbon storage and equilibrium time in response to key parameters of the model was analyzed in detail.d.The spatial distribution and variation of global grassland NEP and RH was simulated in this study,and the spatial distribution of carbon source/sink of global grassland was also identified.In additional,the correlation between NEP,RH and climate was analyzed,as well.e.the spatial distribution of carbon storage of 14 carbon pools in DCC model was analyzed,and also quantitative analyzed the carbon exchanges,turnover time and fluxes characteristics.The main conclusion as following:(1).Spatial-temporal distribution of global grassland GPP and AR.The total and mean global grassland GPP is 165.49 Pg/year and 937.81 g C m-2year-1,respectively;whereas,the total Autotrophic respiration is 48.41 Pg/year,and the average value is 887.40 g C m-2year-1.79.28%of global grassland area enhanced at a rate of 2.71 g Cm2year-1,by contrast,the 20.72%of global grassland area decreased at a rate of 1.12 g Cm-2year-1.50.73%of the global grassland AR exhibited an increasing trend,with an annual increment of 2.45 g C m-2,while the decreased area occupied 40.27%of total grassland area,the decrease rate is 2.01 g Cm-2year-1.The comparative analysis of FPAR indicates that the FPAR simulated by GLOPEM is higher than CASA and BIOME-BGC,and the FPAR of woody savanna is higher than savanna and grassland.(2).The comparative study of global grassland NPP and the quantitative analysis of influence factor.The BIOME-BGC results indicates that the global grassland NPP exhibits an increasing trend,the increase rate is 2.15 g Cm-2year-1 from 1982 to 2008.The NPP model revel that the grassland NPP in Outer Mongolia plateau exhibits decreasing trend.The contrastive analysis of GLOPEM,CASA and BIOME-BGC show that BIOME-BGC model have the highest accuracy,and the least difference of NPP observed between CASA and GLOPEM,whereas the largest difference of NPP is GLOPEM and CASA(GLOPEM higher than CASA),these differences were mainly identified in the mid-latitude and the southern hemisphere.In the view of vegetation types.the error in WS and SAV is greater than GRA.The analysis results of impact factors on NPP show that climate change is the dominate influence factor in global grassland NPP increase,whereas the combination of climate and human activity is the main factor that causes a decline in the global grassland NPP.Correlation analysis showed that the increases of annual accumulated temperature(>0°)and the decrease of mean annual precipitation are the main reason resulting in a decrease of grassland NPP,the increase of NPP is mainly attribute to the increasing mean annual radiation.(3).The carbon storage and turnover time of global grassland.The results of carbon storage simulation indicate that the total carbon storage in global carbon storage is 704.64 Pg C,and the carbon stored in vegetation,surface litter and soil is 132.17,4.64 and 567.83 Pg,respectively.the sensitivity analysis show that GRA is sensitive to 9 of 12 parameters,including environmental scalar,maximum decomposition rate of carbon pool 7,maximum decomposition rate of carbon pool 8,lignin content,the content of clay and silt.By contrast,WS and SAV only sensitive to 6 of 12 parameters(environmental scalar,maximum decomposition rate of carbon pool 7,maximum decomposition rate of carbon pool 8,NPP,carbon using efficiency of carbon pool 3 and the content of clay).The turnover time of three kind of grassland type is:WS>SAV>GRA.Overall,the turnover time of grassland has its own zonality,the turnover time of high latitudes and altitude regions are longer than others,tropics typically has short turnover time,therefore,temperature may be the main factor to promote carbon cycle.(4).The research results of global grassland NEP and RH.In general,global grassland presents carbon sink.The total NEP and RH is 116.90 and 342.00 Tg C.The central Brazilian plateau and most region of outer Mongolia grassland shows a decreasing trend in carbon sink function from 1982 to 2008.The correlation analysis indicates that NEP negative related with temperature(r=-0.21),and weak positive related with precipitation(r=0.08);In contrast,RH positive related with temperature(r=0.43),but negative related with precipitation(r=-0.10).Trend analysis shows that the synthetic effect of decreasing precipitation and increasing temperature might induce decrease in NEP,and temperature represent a strong positive effect on RH.(5).The carbon exchanges and fluxes characteristics of grassland carbon pools under equilibrium state.the results of carbon storage simulation show that the carbon stored in slow and passive pool greater than the others,account for 29.50%and 48.50%of total carbon storage,respectively.Overall,the carbon stored(Pg C)in these carbon pools is:carbon pool 8(342.73)>carbon pool 7(207.88)>carbon pool 11(51.27)>carbon pool 14(50.49)>carbon pool 13(26.89)>carbon pool 3(16.1 0)>carbon pool 10(2.14)>carbon pool 9(2.06)>carbon pool 2(1.80)>carbon pool 12(1.69)>carbon pool 1(1.62)>carbon pool 4(0.71)>carbon pool 5(0.14)>carbon pool 6(0.12).The characteristics of carbon pools indicates that carbon pool 7(89.90 g C/m2)released the most CO2 in the process of carbon decomposition,the largest carbon flux observed in carbon 13 transferred to carbon 6(127.80 g C/m2).The innovations of this study includes:1.Detachable carbon cycle model was established for the first time.Each process or carbon pool can be regard as independent component.The detachability of the model facilitates the identification of how uncertain parameters affect carbon storage,this feature also enables the correction and improvement of the carbon cycle model,because each carbon flow process can be assessed quantitatively.The equilibrium state of global grassland ecosystem was modeled by using DCC model,the carbon stored in vegetation,surface litter and soil was analyzed in detail,furthermore,14 carbon pool size and turnover time was also specified.2.The spatiotemporal variation of NEP was simulated by DCC model from 1982 to 2008,provides a reference for evaluate carbon sink function and carbon budget of global grassland ecosystem quantitatively.3.the carbon fluxes characteristics of grassland carbon pools under equilibrium state was analyzed in detail.To quantify the contribution of CO2 emissions in the process of carbon decomposition.This work provides a reference for further understanding the carbon cycle process of global grassland,the role of grassland ecosystem under global change and its response to the global climate change. |
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