Effects Of Restoration Succession On Soil Organic Carbon Sequestration In A Black-Soil-Type Degraded Grassland

The unique climatic and geographical conditions of the Qinghai-Tibet Plateau have nurtured a fragile alpine grassland ecosystem,especially the increasing area of black-soil-type degraded grassland represented by the Yellow River source area,leading to a huge risk of loss of grassland soil carbon.Existing researches have focused on the impact of alpine grassland degradation and restoration succession on aboveground plant diversity and community construction,but the impact and mechanism of degraded grassland restoration succession on the composition and mineralization characteristics of soil organic carbon(SOC)are still unclear.In this study,the alpine meadow in Dari County,Guoluo State,located in the Yellow River source region,was selected as the research area.Two restoration succession models were selected:(1)Natural restoration succession selected using the spatial rather than temporal method(divided into four gradients: heavy degradation,moderate degradation,light degradation and non-degradation);(2)Artificial restoration succession(divided into four gradients: heavy degradation(control),three-year artificial restoration,seven-year artificial restoration and non-degradation(climax community)).Through soil structure analysis,nuclear magnetic resonance spectroscopy,high-throughput sequencing and mass spectrometry methods,this paper explores the impact of degraded grassland restoration succession on soil characteristics,SOC physical,chemical,biological components and composition of soil microbial community,revealing the mechanism by which soil physicochemical properties,storage and composition of SOC and soil microorganisms affect carbon mineralization rate.The main results are as follows:(1)The natural restoration succession of black-soil-type degraded grassland has significantly improved the SOC storage and soil quality,but short-term artificial restoration only improved the soil structure.The content of SOC in heavily degraded grassland(34.64 ± 1.71 mg/g)was significantly lower than those in moderately degraded grassland(69.85 ± 1.96 mg/g),lightly degraded grassland(69.25 ± 2.36mg/g)and non-degraded grassland(60.52 ± 1.67 mg/g).In the process of natural restoration succession,compared to heavy degradation,other restoration gradients significantly reduced soil p H and bulk density(P < 0.05),while soil moisture content and proportion of particulate soil significantly increased(P < 0.05).The establishment of artificial grassland has increased the proportion of mineral soil,especially in the seven-year artificial restoration grassland(62.92 ± 2.30%),which is significantly higher than the heavily degraded grassland(50.89 ± 3.24%).Both restoration succession models significantly increased the mean weight diameter of soil aggregates in grassland,indicating that restoration succession enhanced the stability of soil aggregates(P < 0.05).(2)The restoration succession of black-soil-type degraded grassland changed the composition of SOC but had no significant effect on its chemical molecular structure.Artificial restoration succession increased the contribution of soil particulate organic carbon(POC)to SOC,while maintaining a high contribution of mineral-associated organic carbon(MAOC)to SOC,indicating that the increase rate of POC during restoration succession was greater than that of MAOC,and MAOC may be in a saturated state.The chemical structure of SOC did not change significantly during the two restoration succession processes,but the contribution of micro-aggregate organic carbon to SOC decreased gradually,while the contribution of macro-aggregate organic carbon to SOC increased,indicating that the restoration succession process promoted the formation of macro-aggregates and SOC was mainly in the form of macro-aggregates rather than determined by chemical stability.(3)The restoration succession of black-soil-type degraded grassland significantly changed the composition of soil microbial communities,mainly due to changes in soil physicochemical properties and SOC components.Actinobacteriota(27.15%),Proteobacteria(18.33%)and Acidobacteria(18.27%)are the main bacteria in the soil,while Ascomycota(53.45%)is the main fungus in the soil.During the process of restoration succession,the diversity of bacteria showed a decreasing trend with restoration succession,while fungi showed the opposite trend,especially in the artificial restoration mode.(4)The different restoration succession models affected the rate of soil carbon mineralization.In natural restoration succession,soil physicochemical properties,microaggregate-organic carbon,(silty and clay)-organic carbon,iron bound organic carbon and microbial biomass carbon are key influencing factors for soil carbon mineralization rate.However,in artificial restoration succession,microaggregate-organic carbon and glomalin are the main factors affecting soil carbon mineralization rate.This study also found that soil microbial biomass,rather than its diversity,is a key influencing factor on soil carbon mineralization rate in natural restoration succession.In conclusion,both natural restoration and short-term artificial restoration succession models significantly improved soil structure and quality,increased the contribution of POC to SOC and enhanced the physical protection of soil aggregates to SOC.However,short-term artificial restoration succession did not promote the restoration of mineral chemical components(such as iron/calcium bound organic carbon)and biologically stable components(such as glomalin-related soil protein)of SOC.The two restoration succession models significantly changed soil microbial diversity and significantly increased soil carbon mineralization rates along the restoration process,but the key factors affecting soil carbon mineralization rates were significantly different among different restoration succession models.This study systematically analyzed the impact and mechanism of restoration succession of degraded grasslands in the Qinghai-Tibet Plateau on SOC sequestration from three aspects: SOC compositions,microbial community composition and soil carbon mineralization rate,highlighting the importance of restoration succession of degraded grasslands in the formation of soil carbon sequestration.