Dynamics Of Top Soil Organic Carbon Density In The Greater Khingan Mountains

Due to the climate change and human activate,the carbon dioxide（CO₂）content has increased from anthropogenic emissions.Forests carbon stock plays a sink role in terrestrial ecosystems.Forest soils are the main carbon reservoirs of the carbon cycle of forest ecosystems,and the density and storage of organic carbon in their soils reflect the carbon sink capacity of forest ecosystems.Enhancing forests soil C stocks is an effective way to absorb CO₂ from anthropogenic emissions.The Greater Khingan Mountains（GKM）is the largest and contiguous state-owned forest area in China,with huge soil organic carbon reserves and sensitivity to climate change.Studying the dynamics and drivers of soil organic carbon density is essential for formulating appropriate forest management policies,and can provide scientific basis and theoretical guidance for the carbon sequestration potential and ecological contract restoration in the GKM.Due to the difficulty of mass sampling and the lack of survey data,the current research on soil organic carbon density in GKM mostly concentrated in local areas and the dynamic analysis of the overall area for a long time and the changes under future climate scenarios is less.There are still many uncertainties about the dynamics and drivers of soil organic carbon density in the GKM.Based on a large number of field survey and public data and normalized difference vegetation index（NDVI）,this study estimated the dynamics of surface soil organic carbon density（SOCD）of forest ecosystems in GKM in three periods（1989,2010 and 2020）,and analyzed the driving factors on spatial and temporal scales.The effects of different types and different disturbance intensities on the organic carbon density of surface soil in the GKM were further studied.Finally,the SOCD dynamics in 2050s and 2090s under future climate scenarios are analyzed.At the regional scale,the hypothesis that the topsoil carbon pool is used as a carbon sink in the GKM is verified.The main contents and conclusions of the study are as follows:（1）The spatial distribution of SOCD in GKM varied greatly(1.22-28.22 Kg C m^-2),and decreased with the altitude gradient.The average soil organic carbon density was highest among deciduous broadleaved forests,followed by deciduous needleleaf forests and broadleaved and needleleaf mixed forests.The main influencing factors are mean annual temperature,followed by mean annual precipitation and altitude in spatial variation.It was significantly correlated with the understory vegetation diversity in different forest types（P<0.05）.In different altitude gradients,with the increase of the elevation gradient,the influence of understory vegetation diversity decreases,and the influence of climate increases.Among them,the mean annual precipitation and mean growing season precipitation have the greatest impact,followed by the mean annual temperature and the mean growing season precipitation.（2）The topsoil organic carbon stock in the GKM is a function of carbon sink.Based on the collection data of 1980,2010 and field survey data in 2020,the average SOCD of 0-20 cm depth is 7.50±0.29 kg C m^-2,9.15±0.36 kg C m^-2 and 10.57±0.98 kg C m^-2,respectively.The soil organic carbon storage of 0-20 cm depthis 1.77±0.06 Pg C(Pg=10¹⁵g),2.16±0.08 Pg C and 2.50±0.23 Pg C.The SOCD of surface layer in GKM increased with an average increase rate of 0.17±0.07 kg C m^-2yr^-1 and 0.14±0.05 kg C m^-2yr^-1（P<0.05）,respectively.Based on NDVI estimates,the organic carbon density of the topsoil in the GKM in 2010 and 2020 was 11.60±0.0001 kg C m^-2,14.17±0.0001 kg C m^-2,respectively,with an average increase rate of 0.32±0.0001 kg C m^-2 yr^-1（P<0.05）.Over the past 40 years（1980-2020）,the restoration of forests in the GKM promoted the accumulation of topsoil organic carbon.The increase of SOCD is mainly concentrated in the northern section of the GKM,and small in the south central section.The northern part of the GKM is mainly deciduous needleleaf forest,while the central and southern parts are mostly deciduous broadleaved forest and broadleaved and needleleaf mixed forests.It is shown that broadleaf species can obtain more organic carbon in soil than needleleaf species.（3）By analyzing the driving factors of the dynamics of organic carbon density in the surface soil of GKM,the dynamics of soil organic carbon density estimated based on field data and NDVI methods,both are affected by precipitation fluctuations greater than temperature.In the context of warming and drying,the relationship between changes in soil organic carbon density and temperature is no longer strong and soil organic carbon density is more susceptible to changes in moisture.The response of soil organic carbon content to different fire interference intensities was different,and after mild fire interference,the soil surface carbon content increased significantly,the effectiveness of soil nitrogen increased,and the total phosphorus and total potassium content decreased.After 10 and 4 years of moderate and severe fire disturbance recovery,the nutrient profile in the soil has not yet returned to its initial level.The effects of Shelter wood logging and clear logging on the organic carbon content of the topsoil were different,and the Shelter wood logging significantly increased the soil carbon and nitrogen content in 0-10 cm（27.13%and 23.76%respectively）,which promoted the organic carbon accumulation and soil nutrient effectiveness of the topsoil soil.The loss of organic carbon and nutrients in the soil was caused by clear logging,and the organic carbon content of the soil in 0-10 cm and10-20 cm was significantly reduced by 18.03%and 59.42%.The two disturbances,in moderation,promote the accumulation of top soil organic carbon and nutrient cycling.（4）Based on the combination strategy weighting model of random forest,gradient boosting machines and K-nearest neighbor,this study analyzed that under the shared socio-economic pathways（SSPs）,the top soil organic carbon density of the GKM will be a carbon sink in the 2050s and 2090s.The largest increase is in the ssp126 scenario and the least increase in the ssp585 scenario.That is,the low-emission scenario can promote the accumulation of organic carbon in the topsoil.However,changes in spatial distribution is that the topsoil organic carbon density of the GKM in a larger range in the north,which was a carbon source.The increase of the topsoil organic carbon density in the 2050s was higher than that in the 2090s.That is to say,under the background of only considering climate change,the carbon sequestration potential of the topsoil carbon pool in the GKM may reach a maximum value after the carbon sequestration capacity of vegetation increases sharply,and then gradually decline.In the random forest prediction model,the topsoil organic carbon density in the GKM is most affected by the mean annual precipitation.That is,under the warm and dry climate in the future,the carbon sequestration ability of top soil organic carbon will be inhibited by precipitation.