Microbial Regulation Mechanism Of Soil Organic Carbon In Grassland Of Different Restoration Years On The Loess Plateau

Promoting C storage by soils is an important way for reducing atmospheric CO₂.Natural restoration of abandoned land increases soil carbon storage.Soil microorganisms are the major drivers of the turnover of soil organic matter.Thus,it is a scientific problem that the microbial regulation mechanism of soil organic carbon storage in the restoration of ecosystem.Nevertheless,there is a dearth of in-depth study on the investigation of microbial regulatory mechanisms governing soil organic carbon storage,due to the high species diversity and complex community structure of soil microorganisms.Therefore,grasslands with varying years of natural recovery（2,8,20,30 and 50 years）located in the Loess Plateau,a pivotal area for the Grain for Green afforestation,were selected as the research subject for this study.To reveal the microbial regulation mechanism of soil carbon dynamics during the restoration of grassland ecosystems,this study employed a regional scale sample strip survey and utilized soil microbial Illumina Mi Seq high-throughput and metagenome sequencing techniques to investigate the linkages between microbial community properties and functional genes related to carbon cycling and soil organic carbon storage.The main results are as followed:（1）The results showed that the restoration year had significant impacts on grassland plant communities,soil physicochemical properties,and soil organic carbon content（P<0.05）.Specifically,restoration years significantly increased plant species richness and above-ground biomass（P<0.05）,and altered the plant community composition（P=0.001）from one dominated by annual/biennial forbs and annual grasses to one dominated by semi-shrubs,perennial grasses,and perennial forbs.Additionally,soil clay,Ca²⁺,and Mg²⁺,as well as total nitrogen content,increased significantly along the restoration chronosequence（P<0.05）.Furthermore,the restoration year significantly increased soil organic carbon content（P<0.05）.（2）This study revealed that the restoration year significantly affected the properties of soil microbial communities and functional genes of grasslands（P<0.05）.Specifically,the restoration years resulted in a significant decrease in soil bacterial diversity along the restoration chronosequence（P<0.05）,and the structure of soil archaea,bacteria,and fungi communities varied between restoration years（P=0.001）.Moreover,the properties of microbial co-occurrence network modules significantly changed along the restoration chronosequence.the richness and abundance of module 2,which had more nodes,increased and then stabilized over the restoration years（P<0.01）,while the richness of module 3increased and then stabilized over the restoration years,but the abundance of module 3decreased and then stabilized over the restoration years（P<0.001）.Furthermore,restoration years had a significant impact on the relative abundance and composition of soil functional gene.The relative abundance of genes involved both labile and recalcitrant carbon decomposition showed an overall increasing trend,but was lowest in the 20th year of recovery（P<0.01）.Additionally,the relative abundance of genes involved hemicellulose and cellulose,which decompose the labile carbon fraction,and lignin decomposition genes,which decompose the recalcitrant carbon,increased significantly along the restoration chronosequence（P<0.01）.We found that soil microbial co-occurrence network modules had different functional metabolic potential.Specifically,module 2 and module 4 attributes were significantly positively correlated with functional genes（P<0.05）,while module 1 and module 3 attributes were significantly negatively correlated with functional genes（P<0.05）.（3）The impact soil microbial functional genes were found to be more closely related to soil organic carbon content than to soil microbial community attributes in restored grasslands.While soil microbial richness and community composition were not significantly correlated with soil organic carbon content（P>0.05）,the richness and abundance of module 2 and module 3,as well as microbial functional genes,significantly affected soil organic carbon content（P<0.05）.The variance partition analysis（VPA）showed that that soil microbial functional genes explained up to 57%of the variation in soil organic carbon concerning plant community attributes and soil attributes,which was higher than soil microbial richness（22%）,microbial community composition（29%）and microbial module attributes（54%）concerning plant community attributes and soil attributes.Structural equation model（SEM）further indicated that soil microbial community properties indirectly affect soil organic carbon by impacting microbial functional genes.However,after considering both plant community and soil attributes,the effect of the relative abundance of genes involved cellulose decomposition on soil organic carbon storage changed from positive（P<0.05）to irrelevant（P>0.05）,and the effect of the relative abundance of genes involved hemicellulose decomposition on soil organic carbon storage changed from positive to negative（P<0.05）.Plant species richness and soil total nitrogen content played a crucial role in regulating the relationship between microorganisms and soil organic carbon content.They could directly or indirectly affect soil organic carbon content by influencing other key factors of microorganisms.In summary,the results suggested that conversion of grassland to agricultural land was an effective strategy for restoring grassland plant communities on the Loess Plateau,improving soil physical and chemical properties,and increasing soil organic carbon content.In parallel,it could alter soil microbial communities and functional genes.Compared to the soil microbial community properties,the relative abundance of microbial functional genes was highly correlated with soil organic carbon content.Soil microbial community properties had an indirect influence on soil organic carbon,primarily through their impact on microbial functional genes.Moreover,attention should be paid to the regulatory effects of plant species richness and soil total nitrogen content on the relationship between microorganisms and soil organic carbon.The results demonstrated the crucial role of microbial functional genes in soil organic carbon during the restored grassland ecosystems,which was significant in revealing the microbial regulation mechanism.