Carbon Sequestration Characteristics And Microbial Influencing Mechanisms Within Soil Aggregates Of Different Vegetation Communities In The Source Zone Of The Yellow River

Under the context of global warming,promoting carbon sequestration and stabilization in terrestrial ecosystems has garnered widespread recognition by the international community as the primary way to mitigate climate change.Especially,grassland ecosystem,which is one of the most important parts of global terrestrial ecosystems,it has been considered as the most promising reservoir for storing additional carbon and reducing CO₂ load except for forest ecosystems for their dominant position in the global carbon balance.The Qinghai-Tibet plateau,which serves as a crucial element of grassland ecosystem and a key source of global greenhouse gases emisssions,accounts for approximately 49.1%of the total carbon stock in China.The impact of variation in vegetation communities and topography,such as sunny slope（MS）,alpine shrubs on a shady slope（SS）and alpine wetlands on the riverside（WR）on carbon sequestration in alpine mountains is mainly in the form of dynamic turnover of organic-inorganic carbon pools driven by biotic and abiotic within the aggregates.However,the mechanisms of dynamic transformation across different aggregate scales and the relationship with aggregate formation remain poorly understood.Therefore,four secondary watersheds with distinct vegetation communities（MS,SS and WR）were selected in Henan-Mongolian Autonomous County,Qinghai Province,where slope aspect and slope position factors were integrated,and samples were randomly collected at the upper,middle and bottom of the same altitude of MS and SS and WR,respectively.The aggregates were separated by the modified dry-sieving method to obtain four size fractions:>2 mm,2–0.25 mm,0.25–0.053mm and<0.053 mm.And the biotic and abiotic indicators within the aggregates were measured to systematically analyze the composition of soil aggregates,carbon pool fractions,carbon–cycle associated enzyme activities and microbial community structure distribution characteristics within aggregates across different vegetation communities from the spatial scale.In addition,we have explored the SOC mineralization characteristics and the temperature sensitive within each particle–size aggregate through indoor constant temperature incubation.The goals of our study were:（1）to reveal the mechanisms of carbon sequestration and turnover in the microdomain environment from the aggregate scale,and（2）to elucidate the primary roles of different microorganisms（bacteria and fungi）in carbon sequestration and soil structure stability maintenance.It is expected to supplement data for a comprehensive understanding of carbon sequestration mechanisms as well as provide theoretical basis and technical support for the construction of eco-pastures and the sustainable development of alpine grassland ecosystem in China.The specific conclusions are as follows:（1）Spatial distribution differences of vegetation communities in alpine mountains affect the composition of soil aggregates and the distribution of carbon fraction,enzyme activity and microbial community structure within the aggregates.The aggregate size gradually shifted towards lower size classes from the upper to the bottom slope positions,reducing the MWD of the soil structure.The SOC and STC within the aggregates mainly increased in the 0.25–0.053 mm fraction at different positions on the MS and SS,but those values decreased with decreasing particle size in the WR.TheβG andαG activities decreased with increasing soil aggregate size at different positions of all sampled areas.However,the SIC and DOC showed the opposite pattern.The MBC of the different aggregate sizes was mainly enriched in the 2–0.25 mm fraction,with the lowest content in the<0.053 mm fraction at different positions of all sampled areas.The diversity of bacteria within aggregates decreased with decreasing particle size,while the diversity of fungi increased with decreasing particle size,and bacterial diversity was greater than fungal diversity and less sensitive to environmental factors,while dominant phyla abundances were unevenly distributed.（2）Spatial differences of vegetation communities in alpine mountains regulate enzyme activity and microbial community structure by p H,which in turn change the dynamic transformation efficiency of carbon pools and ultimately affect the carbon sequestration capacity of soil aggregates and soil structural stability.The organic carbon turnover efficiency in the alpine mountains ranked as MS>WR>SS,and silt+clay fraction>microaggregates>macroaggregates.The results of Monte Carlo substitution test showed that soil p H was the major abiotic regulator of the changes in enzyme activity,bacterial and fungal community composition and diversity,and organic-inorganic carbon pool dynamics within aggregates,which indirectly inhibited enzyme activity,reduced bacterial community composition and diversity,and reduced fungal community composition,while increasing the fungal community diversity to promote the conversion of MBC,DOC,and SIC to SOC and increased aggregate associated STC during the spatial variation with vegetation communities,and eventually promoting macroaggregation and stability of the soil structure.（3）Spatial distribution differences of vegetation communities in alpine mountains affect the SOC mineralization and temperature sensitivity within the aggregates,which ultimately change the carbon sequestration capacity of the aggregates and the dynamic transformation of organic-inorganic carbon pools.The SOC mineralization rate and cumulative mineralization within aggregates in alpine mountain followed the trend of WR>MS>SS at the spatial scale,and the SOC mineralization rate within aggregates decreased with decreasing particle size,thus confirming that the carbon sequestration capacity（C₀ and C₀/SOC values）of soil aggregates in alpine mountain varied from SS>WR>MS,and gradually increased with decreasing grain size and temperature.The temperature sensitivity(Q₁₀)were generally decreasing with increasing temperature across different aggregate size in MS,SS and WR,and Q₁₀ of macroaggregates were greater than those of microagglomerates and silt+clay fraction under low temperatures,while the opposite trend was observed under high temperatures.（4）Indoor carbon mineralization incubation experiments confirmed that p H is the dominant factor influencing carbon sequestration and organic-inorganic dynamic transformation in natural vegetation communities of alpine mountains.The SOC,MBC,βG,andαG enzyme activities of each particle-size aggregates decreased in MS,SS and WR when the SOC mineralization rate stabilized,and the decrease gradually decreased with decreasing incubation temperature and particle size.the DOC of each particle-size aggregates increased with increasing temperature and particle size;the SIC and p H of each particle-size aggregates increased in MS and WR,but the opposite trend was observed in SS.Structural equation modeling showed that increasing p H could indirectly decrease the SOC mineralization and DOC increment by inhibitingβG andαG enzyme activities and microbial biomass,and accelerating the rate of SOC to SIC conversion;while decreasing p H could directly promote microbial biomass andβG andαG enzyme activities to indirectly increase SOC mineralization and DOC,while inhibiting SOC to SIC conversion and decreasing SIC.