Priming Effect Of Root Exudates Of Fraxinus Mandshurica On Soil Organic Carbon

Root exudates are the main source of forest soil organic carbon and have an important impact on the priming effect（PE）of soil organic carbon in the rhizosphere,it may cause loss of soil organic carbon.Clarify the role of root exudates in the PE of soil organic carbon,it has great significance to understand the law of soil carbon cycle and improve the ability of soil carbon sequestration.Due to the complexity of soil PE,and the mechanism of the PE is not clear.In this study,the F.mandshurica of a precious timber species in northeast China,was selected as the research object.Through the qualitative and quantitative analysis of root exudates of F.mandshurica and the study on different root exudates on soil organic carbon PE,the stimulating effect and mechanism of root exudates of F.mandshurica on soil organic carbon were discussed,The main results from this study were as follows:（1）Qualitative and quantitative studies on the root exudates of F.mandshurica seedling collected at different times revealed 219 compounds.There were significant differences in secondary metabolites,but no significant differences in sugars,amino acids or organic acids.The quantitative results showed that the most abundant sugars,amino acids and organic acids in the root exudates of F.mandshurica were glucose（136.93μg/m L）,glycine（20.66 nmol/m L）and malonic acid（33.68μg/m L）,respectively,at 12 h.（2）After adding the three simulation root exudate compounds of F.mandshurica（glucose,glycine and malonic acid）into soil,the soil organic carbon significantly produced strong positive PE.The highest PE intensity induced by the three exudates occurred on the first day.The highest mineralization intensity induced by the glucose occurred on the first day,while glycine and malonic acid occurred on the third day.The initial intensity of the PE and mineralization of sugars was significantly greater than that of amino acids and organic acids,the cumulative PE effect showed the following trend:malonic acid>glycine>glucose,and the substrate cumulative mineralization effect showed the following trend:glycine>malonic acid>glucose.（3）The addition of the three simulation root exudates of F.mandshurica on the soil,increased soil CO₂respiration,produced strong positive PE and mineralization effects,caused the change of chemical indexes,such as dissolved organic carbon（DOC）,NH₄-N,NO₃-N,total inorganic N levels,MBN,MBC,but the significant differences among the three root exudates were observed.Glucose and malonic acid had no significant effect on p H,significantly increased the content of DOC and mineral nitrogen,glucose decreased the content of available phosphorous（AP）in the soil and malonic acid had no significant effect on AP.Glycine significantly increased the p H,the content of DOC and mineral nitrogen,decreased the content of AP,MBC and MBN in the soil.（4）The addition of the three simulation root exudates did not change the composition or structure of the dominant microbial groups in the soil,but did change the relative abundance and diversity of some bacterial groups in the soil microbial community at different culture stages.At the initial stage of culture,glucose and malonic acid caused a significant increase in the relative abundance of Actinobacteria,whereas glycine caused a significant increase in the relative abundance of Firmicutes.Midway through culturing,glucose caused a significant increase in the relative abundance of Firmicutes,while glycine and malonic acid caused a significant increase in the relative abundance of Proteobacteria.At the end of culture,glucose caused a significant increase in the relative abundance of Proteobacteria,Verracomicrobia,Chloroflexi and Acidobacteria,glycine caused a significant increase in the relative abundance of Firmicutes,and malonic acid caused a significant increase in the relative abundance of Actinobacteria.In addition,at the initial stage of culture,glucoseand malonic acid caused a significant increase in the relative abundance of Actinobacteria,whereas glycine caused a significant increase in the relative abundance of Firmicutes.Midway through culturing,glucose caused a significant increase in the relative abundance of Firmicutes,while glycine and malonic acid caused a significant increase in the relative abundance of Proteobacteria.At the end of culture,glucose caused a significant increase in the relative abundance of Proteobacteria,Verracomicrobia,Chloroflexi and Acidobacteria,glycine caused a significant increase in the relative abundance of Firmicutes,and malonic acid caused a significant increase in the relative abundance of Actinobacteria.Generally,in the process of soil organic carbon PE,Bacteria and Fungi cause PE through co metabolism,which was mainly dominated by Bacteria and assisted by Fungi.In conclusion,the addition of three types of F.mandshurica root exudates（glucose,glycine and malonic acid）to the soil produced strong positive PE,increased soil CO₂respiration.The addition of the three root exudates did not change the composition and structure of the dominant flora of soil microorganisms,but at different culture stages,the three root exudates changed the relative abundance and diversity of some flora in the soil microbial community,and had different stimulating effects on soil organic carbon.Taken together,the results showed that the root exudates of F.mandshurica could mediate the PE of soil organic carbon through changes in the microbial community,the mechanism may be:Energy-limited extracellular enzyme synthesis.