Study On NPP, R_h And SOC Of Northern Grassland Ecosystems Responding To Climate Change

Natural grassland as an important component of terrestrial ecosystem plays an essential role in theprocess of absorption and fixation atmospheric CO₂. However, as the result of overgrazing,misadministration, climate change and lack of investment, grassland ecosystem has been suffering withgrassland degradation, declining productivity, biodiversity reduction and weakening ecological function.Therefore, understanding the carbon balance of grassland ecosystem responding to climate change ispractically meaningful for scientific evaluation on the contributions to the sources or sinks ofatmospheric CO₂, and its adaptation to the climate change.Based on the calibrated and validated CENTURY model, dynamic characteristics of net primaryproduction （NPP）, heterotrophic respiration （R_h）, soil organic carbon （SOC） in four types of grasslandecosystems, namely alpine meadow （AM）, temperate meadow steppe （TM）, temperate steppe （TS） andtemperate desert （TD）, were simulated and analyzed by using observed climatic data （1961-2010） andprojected climatic data （1961-2100） by PRECIS model with or without considering the enhancedatmospheric CO₂concentration under A2and B2scenarios, respectively. These main research resultsand conclusions achieved as follows:（1） In the past50years, dynamic characteristics of NPP, R_hand SOC in four types of grasslandecosystems showed that: NPP in four types of grassland ecosystems showed large inter-annualvariation and the coefficient of variation was in sequence by TD>TS>TM>AM. However, the averageof NPP in four types of grassland ecosystems was AM>TM>TS>TD, and the trend of NPP in AMascended significantly. R_hhas the same characteristics of fluctuation and trend as NPP in four types ofgrassland ecosystems. Actually, they were in the synchronous changing relationship. Total SOC infour types of grassland ecosystems showed a little inter-annual variation and the coefficient of variationwas mainly affected by active and slow SOC. The average of total SOC in four types of grasslandecosystems was AM>TS>TM>TD. The trend of total SOC in TD declined significantly because of thedecrease in slow and passive SOC, and that trend of slow or passive SOC in AM, TM and TS alsodeclined significantly. According to the analysis of partial correlation, NPP, R_hand SOC areinterrelated in four types of grassland ecosystems. The synchronous changing relationship between NPPand R_his reached through the crucial factor of active SOC, while the results of dynamic balancebetween NPP and R_hcan be reflected by total SOC.（2） Without considering the effects of elevation of atmospheric CO₂concentration, the dynamicfeedback results of four types of grassland ecosystems responding to the changes of temperature andprecipitation in2020s,2050s and2080s under A2and B2scenarios showed that: The averagechanges of NPP in four types of grassland ecosystems in2020s,2050s and2080s compared to baseline（1961-1990） would increase by3.6%,9.6%,14.8%and1.6%,4.6%,6.5%under A2and B2scenarios,respectively. Among them, the increase rate of NPP would be TS>TD>TM>AM. The averagechanges of R_hin four types of grassland ecosystems in2020s,2050s and2080s would increase by3.4%, 9.2%,14.3%and1.9%,4.3%,6.7%under A2and B2scenarios, respectively. Among them, the increaserate of R_hwould be the same as NPP in sequence by TS>TD>TM>AM. However, the averagechanges of SOC in four types of grassland ecosystems in2020s,2050s and2080s would decrease by-1.0%,-2.4%,-4.4%and-1.3%,-2.6%,-4.1%under A2and B2scenarios, respectively. Among them,the decrease rate of SOC would be TM>AM>TD>TS.（3） With considering the effects of elevation of atmospheric CO₂concentration, the dynamicfeedback results of four types of grassland ecosystems responding to the changes of temperature andprecipitation in2020s,2050s and2080s under A2and B2scenarios showed that: Changes of NPP in2020s,2050s and2080s would be19.0%,29.9%,42.4%and7.7%,12.0%,16.0%, which wouldsignificantly increase by15.5%,20.3%,27.6%and6.1%,7.3%,9.4%compared to without CO₂enhanced effect under A2and B2scenarios, respectively. Thus, NPP of grassland ecosystems wouldbenefit from atmospheric CO₂enhancement. Changes of R_hin2020s,2050s and2080s would be15.3%,21.6%,32.6%and5.9%,9.5%,12.9%, which would significantly increase by11.9%,12.4%,18.3%and3.9%,5.1%,6.3%compared to without CO₂enhanced effect under A2and B2scenarios,respectively. Thus, heterotrophic respiration of grassland ecosystem would be stimulated by CO₂enhancement. Changes of SOC in2020s,2050s and2080s would be-5.4%,-8.5%,-11.9%and-2.2%,-3.7%,-5.5%, which would significantly decrease by-4.4%,-6.1%,-7.5%and-0.9%,-1.1%,-1.3%compared to without CO₂enhanced effect under A2and B2scenarios, respectively. The main reasonsfor causing more decrement of SOC would be that R_hof grassland ecosystem would be increased underhigher atmospheric CO₂concentration.