Organic Carbon Dynamics Of The Forest Litter-Soil Continuum In Different Succession Stages Of Hailuogou Glacial Retreat

As a natural sink of atmospheric CO₂,soil organic carbon fixation plays an essential role in the stability of global carbon budget and climate system.Organic carbon derived from plant photosynthesis can be immobilized in soil through physical storage of soil aggregates or the formation of organic-inorganic complexes with active iron and aluminum minerals.However,the relative importance of these carbon sequestration mechanisms with soil succession is not clear.In this study,five succession years（40,55,90,130 and 190 years）of the soil-forest system in the Hailuogou Glacier retreat area were studied.Based on stable isotope technique,the dynamics of organic carbon in litter-soil continuum,the distribution of soil aggregate grain size,the source-sink relationship of organic carbon in aggregates,and the contribution rate of active iron-aluminum minerals bound carbon were analyzed.The results can provide important scientific information for soil carbon sink management and carbon neutralization.The main results are as follows:（1）The content of soil macroaggregates（>0.25 mm）at the initial stage of ecological succession（the 40-year succession stage）in the Hailuogou Glacier retreat area was significantly lower than that at the other four stages.Among them,the average mass fraction of soil macroaggregates at the initial stage of succession was（16.65±0.59）%.And the 90-year,130-year and 190-year succession stages tended to be steady,the mean values were（45.68±1.23）%,（41.89±0.98）%and（43.37±1.29）%,respectively.（2）Among the stability indexes of soil aggregate,the content of soil macroaggregates(R_0.25),the mean weight diameter（MWD）and the geometric mean diameter（GMD）all increased with the increase of succession years,whereas the fractal dimension（D）showed a completely opposite trend.Therefore,the soil agglomeration gradually increased with the succession,and the soil structure stability showed an upward trend.（3）The organic carbon content decreased from litter to soil layer,and theδ¹³C values were gradually positive.With the progress of succession,the difference between soil and litter organic carbonδ¹³C gradually decreased due to the increasing protective effect of soil on organic carbon.（4）The total organic carbon content（TOC）and storage(S_TOC)in 0-5 cm soil increased parabolically with the succession,and from the 40-year to 190-year succession stage,the total soil organic carbon content increased from 9.80 g·kg^-1 to 28.64 g·kg^-1,and the soil organic carbon storage increased by 170.58%.The organic carbon contribution rate of soil microaggregates（<0.25 mm）was 52.47%at the initial succession stage,and the contribution rate of macroaggregates was 47.50%.As the succession progressed,the protective effect of macroaggregates on organic carbon gradually became significant,and the contribution rate of macroaggregates increased to 68.17%at the top succession stage（the 190-year succession stage）,whereas the protective effect of microaggregates was relatively weakened.（5）For the primary stage of succession,the mobility of organic carbon between soil aggregates was weak,and the flow direction was mainly from finer aggregates to coarser aggregates.With the progress of succession,the connection between the aggregates of each particle size gradually became closer,the fluidity of organic carbon gradually increased,and the flow direction gradually changed from coarser particles to finer particles.（6）With the succession,the contribution rate of active iron-aluminum minerals to soil total organic carbon sequestration remained relatively stable at about 25%,but the contribution to microaggregate organic carbon sequestration became more and more significant.（7）The OC_Fe-Al/SOC was positively correlated with theδ¹³C_SOC（P<0.01）.In conclusion,because the soil OC_Fe-Al was steady and enriched more ¹³C component,theδ¹³C values of soil organic carbon with higher contribution rate of active iron-aluminum minerals was relatively positive.