Evolution Characteristics And Influencing Factors Of Soil Aggregates In The Process Of Secondary Succession In The Loess Plateau

Soil aggregate stability is an important basis to measure soil structure.However,due to the complexity of soil,the research on the influence of long-term vegetation succession on aggregate stability and its factors is still controversial.This thesis takes the succession process of natural secondary vegetation in Ziwuling forest area of the Loess Plateau as the research object,including five stages: farmland,grassland,shrub,pioneer forest and top forest.Using the combination of field investigation and indoor analysis,this paper studies the evolution characteristics of soil aggregate stability in the process of different vegetation succession,expounds the influence of plant soil microorganisms on soil aggregate stability,and reveals the driving mechanism affecting aggregate stability in the process of long-term vegetation succession,to provide a scientific basis of soil quality evaluation and sustainable development of ecosystem in the context of plant ecological restoration in the Loess Plateau.The main research progress is as following:(1)Effects of long-term vegetation succession on the stability of soil aggregatesWith the long-term vegetation succession from farmland to top forest,the stability of aggregates and the proportion of large aggregates increased with the growth of vegetation succession time.The organic carbon(SAOC)of aggregates with each particle size from farmland succession to top forest shows a steady upward trend,of which the change range of 0.25-0.053 mm SAOC is the largest(56.3%)and that of <0.053 mm SAOC is the smallest(45.6%).After long-term vegetation succession,the proportion of large aggregates(> 0.25 mm)gradually increased(11.69%),and the average weight diameter and geometric average diameter gradually increased,indicating that the stability of aggregates gradually improved with plant succession.Therefore,vegetation restoration is helpful to increase the long-term stability of aggregates.(2)Effects of vegetation succession on plant biomass,soil nutrients and microbial activitySoil nutrient content,plant biomass,microbial and extracellular enzyme activities increased significantly under vegetation succession.In the long-term succession from farmland to top forest,the vegetation coverage reached the maximum in Pioneer forest(88.67%),and the plant biomass increased significantly(litter biomass: 621%;root biomass: 652%).Total soil nutrients(organic carbon,total nitrogen,total phosphorus)and available nutrients(soluble organic carbon,available phosphorus)increased significantly during vegetation succession(35.34%,20.30%,81.08%,20.27%,116.89%).With vegetation succession,soil microbial biomass and extracellular enzyme activity gradually increased,the relative abundance of basidiomycetes gradually increased and ascomycetes gradually decreased.(3)Effects of plant soil microorganism on the stability of soil aggregates in the process of vegetation successionIn the process of long-term vegetation succession,the aboveground and underground biomass of plants from farmland to top forest,soil organic carbon and soil total phosphorus,microbial biomass carbon,nitrogen and phosphorus were significantly positively correlated with aggregate stability,and soil available nutrients,bacteria and fungi OTUs were significantly negatively correlated with aggregate stability.The main factors affecting aggregate stability are vegetation restoration time,vegetation coverage,soil available nutrients,microorganisms,aggregate nutrients and their stoichiometric ratio,microbial carbon limitation and phosphorus limitation.In 0-60 years,water is the key factor affecting the stability of aggregates(0.443),and in 60-160 years,microorganisms adapt to the stability of aggregates(0.243).In conclusion,vegetation succession can significantly improve the stability of soil aggregates,soil nutrient content,microbial biomass and extracellular enzyme activity,but there is little difference in nutrient content in litter and roots at different successive stages.In the process of aggregate turnover,the factors that have the greatest impact on the stability of aggregates are the years of vegetation restoration,the stoichiometric ratio of soil nutrients and the relevant indicators of microorganisms(MBC,MBN,MBP).Different vegetation succession stages also have different paths to affect the stability of aggregates.The key factor affecting the stability of aggregates in 0-60 years is soil water content,and the factors affecting the stability of aggregates in 60-160 years are soil available nutrients and plant biomass,indicating that the mechanisms affecting the stability of aggregates in different succession stages are different.Therefore,the secondary succession of vegetation can effectively improve soil structure and quality,soil fertility and anti-corrosion ability.