Characteristics And Driving Mechanisms Of Soil Mineral Particle Organic Carbon Saturation Deficit During The Restoration Of Degraded Karst Forest

The degraded karst non-zonal forest community in the subtropical humid climate region of southern China can develop into zonal soil-yellow soil and non-zonal soil-lime soil after long enough time to recover and accumulation,due to the large amounts of calcium carbonate in calcareous soil input soil,and yellow soil with different carbon cycle characteristics,so the two kinds of soil carbon cycle process is of great significance under the double carbon context.The soil mineral particulate organic carbon saturation deficit（CSD）is an important index to reflect the carbon sequestration potential of soil organic carbon（SOC）,so discussing its characteristics and driving mechanism is the priority topic in the research field of terrestrial ecosystem carbon cycle and global carbon sequestration and emission reduction under the background of“double carbon”.In this study,the topsoil（0-20 cm）of the climax community of degraded karst forests restoration processes（10 nature reserves）in the subtropical humid climate region of southern China was taken as the research object,and the boundary effect method was used to construct the maximum saturated capacity model of soil mineral particle organic carbon（POC）in degraded karst forests restoration processes.The spatio-temporal evolution characteristics of CSD of degraded karst forests restoration processes in space（Maolan in southern Guizhou,Yuntai Mountain in the middle and Dashahe in the north）and time series（four vegetation restoration processes are herb stage,shrub stage,arbor stage,climax stage）were estimated by the model,and the main driving factors were determined.Using ¹³C isotope tracer technique,¹³C-labeled litters and calcium carbonate were added to yellow soil（zonal soil）and lime soil（non-zonal soil）in six treatments,which are as follows:Yellow soil control（CY）,yellow soil added litter（LY）,yellow soil added calcium carbonate（CCY）,lime soil control（CL）,lime soil added litter（LL）,and lime soil added calcium carbonate（CCL）were cultured continuously for 80 days.Before and 5,10,20,40,60 and80 days after culture,SOC mineralization and SOC fraction（>250μm,53-250μm,<53μm）and soil microorganisms were determined to study the effects of exogenous carbon addition on SOC accumulation and decomposition.Combined with the related ecological functions of soil microorganisms,the main driving microorganisms of CSD under exogenous carbon addition were identified.The main research results and conclusions are as follows:（1）Under the climax condition of degraded karst forests restoration processes,the average soil total organic carbon（TOC）of forest community is 77.61 g kg^-1,in which the>250μm fraction is 38.14 g kg^-1,53-250μm fraction is 25.03 g kg^-1,and that of<53μm fraction is 14.44 g kg^-1.There was no significant difference in the ratio of<53μm fraction to TOC in 10 climax communities,which was a stable soil carbon pool.Based on the correlation between the organic carbon content of<53μm fraction and the mass ratio of<53μm fraction in 10 climax communities,the maximum saturated capacity model of POC in Guizhou degraded karst forests restoration processes was established as y=0.66x（where y is the maximum saturated capacity of POC,x is<53μm organic carbon mass ratio）.（2）The distribution pattern of TOC in Guizhou degraded karst forests restoration processes is the highest in the east and south,higher in the west and middle,and low in the north.The results showed that the average TOC of climax community in the eastern was 75.79 g kg^-1,65.61 g kg^-1in the west,79.77 g kg^-1in the south,62.09 g kg^-1in the north,69.12 g kg^-1in the central Guizhou,and 94.13 g kg^-1,82.13 g kg^-1and 78.75 g kg^-1in the southern Maolan,central Yuntai mountain and northern Dashahe,respectively.The CSD in the three regions is Dashahe in the north（78.47%）>Yuntai mountain in the central（70.64%）>Maolan in the south（52.47%）.The development of carbon sequestration in degraded karst forests restoration processes is carried out from north to south as far as possible in spatial latitude to achieve the maximum relative carbon sequestration capacity.（3）In the degraded karst forests restoration processes,the<53μm fraction carbon in Maolan in the south of Guizhou,Yuntai mountain in the central and Dashahe in the north showed as follows:climax stage(16.26 g kg^-1)>arbor stage(10.60 g kg^-1)>shrub stage(5.62 g kg^-1)>herb stage(2.29g kg^-1).The CSD in herb stage,shrub stage and arbor stage was 87.67%,70.71%and43.21%,respectively.Therefore,the herb stage and shrub stage have the greatest carbon sequestration potential in the future,with a large potential excavation space,and it is of great significance to restore vegetation.（4）The core driving factor of CSD in degraded karst forests restoration processes was the amount of litter carbon input,and the established regression model was y=-0.39x+87.67（y is CSD,x is litter carbon input）.The main transformation mechanism is as follows:（1）Carbon transformation mechanism:Compared with calcium carbonate,the addition of litter promoted the positive priming effect of SOC,increased<53μm fraction carbon,and reduced the CSD,and the effect was more obvious in lime soil than in yellow soil.When¹³C-labeled litter and calcium carbonate were added to yellow soil and lime soil,respectively,it was found that the TOC content of litter-added treatment was the highest,followed by calcium carbonate treatment.With the extension of culture time,the TOC content of>250μm and 53-250μm decreased,while that of<53μm increased.During the whole incubation process,the mineralization rate and the cumulative mineralization amount of SOC were LL>CCL>LY>CL>CCY>CY.Exogenous carbon addition showed positive priming effect on SOC in different treatments and cultivation periods,and the priming effect of litter addition was greater than that of calcium carbonate,especially when litters were added to lime soil.The content of mineralized carbon derived from litter and calcium carbonate in yellow soil was higher than that in lime soil.The content of mineralized carbon derived from exogenous carbon in different exogenous carbon additives was CCY>LY>LL>CCL treatment,and that of native soil carbon was LL>CCL>LY>CCY.Among the different fractions,the content of SOC mainly comes from native soil carbon,and the content of SOC derived from native soil carbon and exogenous carbon is>250μm>53-250μm><53μm.In different culture periods,the content of SOC derived from native soil and exogenous carbon in<53μm fraction showed an increasing trend in LY,CCY,LL and CCL treatments,and the most obvious increase in LL treatment.The saturated deficit of soil mineral particulate organic carbon in yellow soil was smaller than that in limestone soil,and LY and LL treatments were the smallest in different soil types.With the extension of culture time,the saturated deficit of soil mineral particle organic carbon showed a decreasing trend with and without exogenous carbon in different soil types.（2）Microbial community driving mechanism:The main microorganisms affected by litter on CSD are Microbacteriaceae,Mycobacteriaceae,Pseudomonadaceae,Streptomycetaceae,Xanthomonadaceae,Cylindrocarpon,Leohumicola,Metarhizium,Neobulgaria,Neopestalotiopsis,Olpidium and Tetracladium.The diversity index of soil bacteria in different treatments was higher than that of fungi.With the extension of culture time,the Shannon index of soil bacteria in each treatment increased,while that of soil fungi decreased.There were 248 families of bacteria community and 189 genera of fungi community in each treatment.There was a significant negative correlation between CSD and soil bacterial Shannon diversity,and a significant positive correlation between soil fungal Shannon diversity and CSD.In conclusion,the CSD was the largest in herb and shrub stages during the restoration of degraded karst forests,and the main driving factor was litter carbon input,indicating that vegetation restoration is of great significance in coping with global climate change during the restoration of degraded karst forests.The results of this study will deepen and enrich the basic theory of carbon cycle in degraded karst forest ecosystem,and provide theoretical basis for sustainable development of ecosystem and increase of sink and emission reduction.