Occurrence Of Organic Carbon In Alpine Soil And The Significance In Climate And Vegetation Change In Southern Area Of Three Rivers

Under the background of global change,the relationship between cycle of terrestrial carbon and climate change is one of the core content of the current environmental problems.Soil organic carbon?SOC?'s source and sink,the structure of the carbon pool and carbon cycle balance of land ecological system has always been a hot research area in both domestic and abroad.Soil organic carbon storage capacity is extremely large,and its minor changes will significantly affect the CO₂ content of the atmosphere,and affect the global climate change in the form of the greenhouse effect.The Qinghai-Tibetan Plateau is the highest and largest Eurasia geomorphic units,and is an extremely important part of the Earth's terrestrial ecosystems.SOC of Qinghai-Tibet plateau is as high as 335.1973×10⁸ t,which is about 2.4%of the global carbon pool,accounting for 23.44%of the national SOC,and is an important carbon sink of the earth ecosystem.Serving as a carbon sink,grassland ecosystem of Qinghai-Tibetan Plateau plays an important role not only in global climate change,but also is very sensitive to global climate change in itself.CO₂ emission of grassland soil by respiration is as high as 11.78×10⁸t C/a,accounting for about 28.3%of the total soil respiration,which is significantly higher than the national and even global averages.For the past 30 years since the 1970s,CO₂ emissions amounted to 30.23×108 t C due to changed land usage and grassland degradation,which is 9.1%of soil carbon emissions.Carbon emissions caused by grassland degradation in the form of "Black Beach"is 29.53×108 t,accounting for 97.68%,and is the major source of carbon emission.Compared with soil of most agricultural ecosystems in China,the biggest feature of soil in Qinghai-Tibetan Plateau ecological grassland is that human activities have less interference soil,thus making it possible to keep it a natural state.The XIALAXIU profile locates in the south area of "Three Rivers" region in Qinghai-Tibetan Plateau,which is at source area of the Lancang River geographically.This profile is differentiated into 16 layers of different thickness and different colors naturally.In this paper,we studied the XIALAXIU profile based on the inversion of the climate.Soil samples from 16 layers with different thickness and colors naturally were collected and SOC's?¹³C,oxidation stability,components of different layers were tested by infrared spectroscopy?IR?and nuclear magnetic resonance?NMR?to try to find and explore characteristics of soil organic carbon of this profile.The main conclusions of this paper listed as following:1.The oldest era of soil ¹⁴C median is 6688±32a B.P.,which is in Mid-Holocene.This ¹⁴C layer in XIALAXIU locates in middle or lower part of the profile rather than the lowest part because this profile is an accumulated overlay since Mid-Holocene.This profile experienced the warm,humid period?6000a B.P.before?;relatively warm and humid,changeable period?from 4500a B.P.to 6000a B.P.?;much colder and drier period?from 4500a B.P.to present?.In the latter stage,the period of grass growing layer is slightly colder and drier compared with the previous stage.2.SOC's?¹³C changed slightly in a range of-25.0%?-22.81%.?¹³C of grass growing layer is minimal.C4 plant abundance ranged from 10%?26%.C4 plants of grass growing layer were mainly composed of five plants of two families:grassy,bindweed.3.The oxidation stability of soil organic carbon characterized by higher content of easily oxidized organic carbon and changed in certain rules in the early stage;big and complicated changes in the middle period;higher content of organic carbon difficult oxidized.The oxidation stability coefficient?Kos?was less than 1 in the early period;bigger change in the middle period;Kos was greater than 1.5 in the late period.The oxidation stability of SOC responded to climate change.Warm and humid climate was conducive to the formation of SOC with low or unstable oxidation.However,cool and dry is beneficial to the formation of SOC with stable oxidation.4.The sum of fulvic and humic was less than humin only could be seen in only a several layers of early and middle periods,which was not sensitive to climate change.Warm and humid climate was conducive to the formation of fulvic;cold and dry climate was conducive to the formation of humic.Higher content of fulvic acid was observed in the early period,but content of humic acid increased in the mid or late period with the gradual climate change to cold and dry.The sum of iron bound humin and clay-combined humin was slightly higher in early stage than in later stage and was changeable in the middle.Iron bound humin was higher than clay-combined humin in some layers.Iron bound humin of current grass growing layer was slightly lower than it in the previous stage of the same period and was less than clay-combined humin.The sum of calcium bonding state soil organic carbon?Ca-C?and iron?aluminum?bonding state soil organic carbon?Fe?Al?-C?was slightly higher than residue state soil organic carbon?residue state-C?in early term;the sum of Ca-C and Fe?Al?-C was slightly higher than residue state-SOC at some layers in mid-term,however the sum of Ca-C and Fe?AI?-C was less than residue state-C in late-term,but not changed much compared with the mid-term.Fe?Al?-C was larger than Ca-C in all layers.5.Functional groups of soil organic carbon were mainly phenolic hydroxyl group.The content of aromatic carbon in hydrophobic organic carbon was greater than that of alkyl carbon,and the content of alkoxyl carbon in hydrophilic organic carbon was larger than carboxyl carbon,but content of amino carbon was minimal.Except PI layer,content of hydrophobic organic carbon content was greater than the content of hydrophilic organic carbon.Hydrophobic organic alkyl carbon was larger than aromatic carbon at grass layer,which was significantly different from paleosol layer,in which aromatic carbon was greater than the alkyl carbon.Warm and humid climate were conducive to form hydrophobic SOC,whereas cool and dry climate were beneficial to form hydrophilic SOC.The study showed that soil organic carbon can respond to climate change in a variety of angles,despite some variation and bias in response to some extent.But it is unquestionable and undoubted that climate can pose impact on soil organic carbon.High stability of soil organic carbon which formed under the weather conditions of relatively cold and dry is one of the prerequisites and key factor for the formation of "Black Beach"degraded grassland.