Changes In Organic Carbon Stocks In Permafrost Areas Of China Since The Last Glacial Maximum

Soil organic carbon stocks in permafrost areas account for about 50%of the global soil carbon pool,and are the largest carbon pool in terrestrial ecosystems.The Last Glacial Maximum（LGM;26～16 ka BP）and Holocene Megathermal Period（HMP;8.5-7～4-3 ka BP）were the coldest and warmest periods since the end of the Late Pleistocene,respectively.Climate change has an important impact on soil organic carbon density and storage in the permafrost,which is significant for studying the source-sink effect in the permafrost.However,there is a lack of evidence for estimating organic carbon stored in permafrost during the LGM period,and very few studies have been conducted on permafrost carbon storage in China since the LGM period,which limits the understanding of the response of permafrost carbon pools to climate change.This study takes the permafrost of the LGM period as the research object,including the present-day permafrost and the part of non-permafrost（which was permafrost during the LGM period）,and integrates the existing measured site data of the permafrost of the LGM in China from 1979 to 2014.According to the sampling time,divided them into 1980s（1979～1984）,2000s（2000～2009）,2010s（2010～2014）,normalized the data to 2010s using a random forest model,and spatially expanded the organic carbon density,then analyzed the main environmental factors affecting the distribution of organic carbon density;estimated the soil organic carbon density and carbon storage in the LGM permafrost zone between 1980s and 2010s,and analyzed its interdecadal spatial and temporal variation characteristics;through vegetation reconstruction,the estimation of soil carbon storage in the permafrost during the LGM and HMP periods,and explored the size and changes of soil organic carbon pools in the permafrost since the LGM period.The main results of the paper are as follows:（1）By comparing the simulation performance of three machine learning models:random forest,support vector machine,and gradient boosting decision tree,selecting random forest model with the best performance to temporal normalization and spatial expansion of organic carbon sites.The results showed that the spatial distribution pattern of soil carbon density in the permafrost zone of LGM in China is spatially heterogeneous.In the horizontal direction,the high organic carbon value areas are mainly distributed in the southeastern part of the Qinghai-Tibet Plateau,the Tianshan Mountains in Xinjiang,and the northeastern part of China;in the vertical direction,there are some differences in organic carbon density.The time-normalized results of organic carbon density show that the organic carbon densities of 0～20 cm,0～100 cm,and 0～200 cm depths in LGM permafrost zones are 4.64±0.08 kg C m^-2,10.90±0.24kg C m^-2,15.61±0.35 kg C m^-2,respectively.The carbon storages are 26.28±0.44 Pg,61.72±1.34 Pg,88.36±1.97 Pg respectively,among which,the carbon density of modern permafrost is 4.72±0.09 kg C m^-2,11.11±0.30 kg C m^-2,16.41±0.46 kg C m^-2,higher than those in non-permafrost.（2）Organic carbon density and carbon storage fluctuated from 1980s to 2010s,and the effect of carbon source-sink was different at different depths:0～20 cm,organic carbon density increased from 4.37±0.10 kg C m^-2in 1980s to 4.68±0.10 kg C m^-2in 2000s,and then decreased to 4.60±0.11 kg C m^-2in 2010s,it behaved as a carbon sink,and the carbon fixation rate was 7.92±0.22 g C m^-2yr^-1;0～100 cm,the organic carbon density increased from 13.33±0.30 kg C m^-2in 1980s decreased to10.82±0.26 kg C m^-2and then increased to 11.75±0.37 kg C m^-2,acting as a carbon source,decreasing at a rate of 52.02±2.44 g C m^-2yr^-1;0～200 cm,the organic carbon decreased from 18.75±0.47 kg C m^-2in 1980s to 13.83±0.38 kg C m^-2in 2000s than rose to 16.10±0.50 kg C m^-2in 2010s.The overall carbon source in 1980s～2010s,Organic carbon decreased at a rate of 88.02±0.90 g C m^-2yr^-1.（3）According to the results of vegetation reconstruction since the LGM period,vegetation types tend to move northward with climate warming,and the organic carbon density of different vegetation types in the permafrost is ranked as follows:forest>steppe>desert.0～200 cm,the carbon densities of forests,steppe,and deserts are 23.69±0.47 kg C m^-2,15.10±0.47 kg C m^-2,14.18±0.38 kg C m^-2,respectively.The estimated carbon storage of permafrost in the LGM period is 86.21±2.54 Pg,the carbon storage in the permafrost in the HMP period is 14.09±0.40 Pg,and the carbon storage in the permafrost at present is 26.54±0.75 Pg.The present-day carbon storage in the LGM permafrost zone is 88.36±1.97 Pg.It is shown that since the LGM period,the degradation of more than 3.91×10⁶km²of permafrost in the ancient permafrost region has resulted in a net increase of 2.15 Pg of carbon in present-day soils.This paper clarifies the spatial distribution pattern of soil organic carbon density and carbon storage in the LGM permafrost zone,elucidates the interdecadal variation characteristics of soil organic carbon density and carbon storage in the LGM permafrost from 1980s to 2010s,and estimates the changes of soil organic carbon density and carbon storage in the LGM,HMP and present-day permafrost.The research can provide a scientific basis for understanding the response of carbon pools in permafrost regions to climate change since the LGM period.