Responses Of Terrestrial Ecosystem Carbon And Oxygen Cycles To Global Greening

Since 1980 s,as the development of the satellites observation methods,it has been shown that the global vegetation greenness experienced an drastic increase,which is called as “global greening” phenomenon.Global greening reveals that the human activities largely influenced the whole Earth system,along with the global warming,atmospheric carbon dioxide increase and sea level rise.The terrestrial ecosystem is one of the most important component of Earth carbon and oxygen cycles,therefore the global greening also has effect on the modern carbon and oxygen cycles,which leads to the change of Earth habitat.Combined with the previous studies,we studied the responses of terrestrial ecosystem carbon and oxygen cycles processes to the global greening,on global and regional scales,and predicted the variation in the future scenarios.One the one hand,we analyzed the spatial and temporal characteristic of global greening,and explored the cooperation between greening and changes in terrestrial carbon sinks.On the other hand,we established a long-term global terrestrial ecosystem oxygen production database for the first time,and investigated the possible mechanism of the future variation in terrestrial oxygen production.Finally,we carried out a field observation experiment on a semi-arid grassland throughout 2022,and explored the link between carbon and oxygen cycle in semi-arid regions.Also we analyzed the effecting factors of oxidative ratios and made corrections on the current and future terrestrial oxygen sources and carbon sinks.Through this study,our main conclusions are as below:(1)We revealed the changes of terrestrial ecosystem carbon sinks under global greening,and analyzed the modification effect of soil respiration on carbon sinks.Under the background of global greening,in the last decades,the terrestrial carbon sinks experienced a trend of increase.The afforestation and and ecological engineering,especially the planting of deciduous plantation,are beneficial to enhance the carbon sinks in terrestrial ecosystem.However,the regional analysis showed the simultaneous increases in soil heterotrophic respiration,counteracting most of the growth effects of net photosynthesis.With the ensemble results of coupled models,we found the global greening will continue,both in the SSP245 and SSP585 scenarios.But the synergistic analysis between net ecosystem production and leaf area index showed that there are a lot of regions experience greening but decreased carbon sinks.The situation will be even worse in the SSP245 scenario.This study confirms that vegetation greening is not equal as the increased carbon sinks.As the heterotrophic respiration will increase dramatically in the future,the whole carbon cycle will likely be influenced and it will threaten the regional carbon sinks.(2)We exhibited the current terrestrial oxygen production in modern oxygen cycle,and analyzed the future responses of production to global greening.The terrestrial ecosystem is the most important source of the modern oxygen cycle,producing about 7.10 ± 0.38 Gt per year.The largest oxygen sources are located in the tropics,and the production decreases along the latitudes increase.In the future scenarios,the global terrestrial ecosystem will provide more oxygen into atmosphere.In areas with more oxygen production in middle and low latitudes,terrestrial oxygen production will further increase;while in low oxygen production areas such as high latitudes and around deserts,terrestrial oxygen production will be significantly reduced and even become oxygen sink.In SSP245 and SSP585 scenarios,it shows the land ecosystem oxygen production capacity will experience a decline,or stagnation,trend.The analysis shows that the net photosynthetic oxygen production process after the 2050 s will gradually stagnate or even decline,but with the increase of soil temperature and humidity,oxygen consumption due to soil respiration will continue to increase in the future,which may threaten the land ecological security.(3)We designed a carbon and oxygen cycle link observation experiment on a semi-arid grassland throughout 2022,and analyzed the seasonal variations in elements of vegetation and soil samples.Also we calculated the oxidative ratios of grassland ecosystem.The observation indicates that the oxidative ratios of living vegetation,litter,roots and soil all have obvious seasonal changes,but the phases are different among samples.Among all the samples,the oxidative ratios of living,litter and soil reach the highest in summer,while the root samples reach their highest in winter.Finally,the calculated annual average value of the semi-arid grassland ecosystem oxidative ratio is 1.08 ± 0.02 mol/mol,higher than that in the previous studies.According to the seasonal variations of elements,the most consistent element with oxidative ratios is the nitrogen,which confirms that nitrogen is an important element in regulating the carbon and oxygen cycle,and the addition of nitrogen cycle is conducive to improve the terrestrial carbon sinks simulation results of the climate models.(4)We analyzed the relations among the oxidative ratios,environmental factors and vegetation factors,and established the relationship equation between the oxidation ratio and the environmental factors.The estimation of terrestrial oxygen production and carbon sinks are supplemented by observational data and relationship equations.After replacing the oxidation ratio in China’s semi-arid regions with observed data,the national total oxygen production increased by about 6.3%,while if the observed data were used in the global mid-latitude semi-arid regions,the global total oxygen production is estimated to increase by 8.9%.With atmospheric oxygen and carbon dioxide concentrations used to estimate land and ocean carbon sinks,land terrestrial carbon sink increases by 4.76% after the replacement of the new terrestrial ecological oxidation ratio.Temperature and precipitation are the main climate variables that regulate the oxidation ratio.Under the future global warming trend,the oxidation ratio will further increase,which will promote the oxygen production of terrestrial ecosystems in the future.In conclusion,based on the "characteristic analysis,mechanism exploration and observation supplement" framework,this paper systematically carried out the research on responses of the terrestrial ecosystem carbon oxygen cycle process to climate change and the variations in the future.Satellite,field observations and models simulations are analyzed to explore the processes and possible mechanisms that can modulate the carbon and oxygen cycles.This paper complements a better understanding of the modern carbon and oxygen cycles,contributing to the quantitative estimation of oxygen production and carbon deposition in terrestrial ecosystems,and especially for developing strategies that are more suitable to achieve the "carbon neutral" goal.