Effect Of Precipitation Reduction On Root Exudates And Soil Microbial Residue Carbon In Cunninghamia Lanceolata Forest

Global climate change is changing global precipitation patterns and water cycles,and changes in precipitation patterns will inevitably affect subsurface processes in ecosystems.When plants alter the amount and chemical composition of root exudates in response to changes in external climate,they may affect the accumulation of microbial residue carbon in the soil.Understanding the response and adaptation of the subsurface to reduced rainfall is important for maintaining and managing the productivity of natural ecosystems.Subtropical regions are facing rapid changes in climate in recent years,such as rising atmospheric temperature,changes in rainfall distribution patterns and extreme droughts.Therefore,relying on the National Field Scientific Observation and Research Station of Forest Ecosystem in Sanming,Fujian,this study selected Cunninghamia lanceolata plantations as the research object to explore the reduction of rainfall（50%reduction in penetration rain;PE）and three kinds of root and arbuscular mycorrhizal mycelia.The effects of isolation experiments（NT group:preserved root+mycelium;AM group:isolated root+preserved mycelium;NM group:isolated root+mycelium）on microbial community structure and soil microbial residue carbon were combined with in situ collected Cunninghamia lanceolata root exudates,revealing the changes of Cunninghamia lanceolata forests root exudates,soil microbial residue carbon and their correlations under the condition of rainfall reduction.Research indicates:（1）Responses of root exudates to reduced rainfall varied in different seasons.The decrease in summer rainfall increased the total organic carbon（TOC）content of root exudates into the soil by 74.6%,accelerate the exudate rate I of 106.7%calculated by root length and accelerate the exudate rate II of 50.7%calculated by root area.The reduction of winter rainfall decreased the TOC content of root exudates input into soil by35.4%,and decreased the exudate rate I and exudate rate II by 72.7%and 70.8%.Reduced rainfall increases organic acids（citric,myristic,caffeic）,fatty acids（palmitic,oleic,stearic）,methyl jasmonate,flavonoids（catechins）in root exudates,coumarin,niacin,glucose and other metabolites secretion,reduce the secretion of phenolic acids（ferulic acid,2,4,5-trimethoxybenzoic acid）,orange peel and naringenin in secretions secretion.（2）The rainfall reduction treatment significantly increased the mannose（Man N）and muramic acid（Mur A）of the isolated root+mycelium group（NM group）in the 10-20cm soil layer in summer,but had no significant effect on other amino sugars.Redundancy analysis（RDA）results showed that DOC was the most important explanatory variable for the variation of microbial residue carbon index under different treatments in winter,with an explanatory degree of 25.5%.Fungal biomass,MN and PLFAs explained 14.6%of microbial residue carbon,respectively,6.4%and 5.3%.In summer,DOC was the most important explanatory variable for the variation of microbial residue carbon index under different treatments,with an explanatory degree of 36.2%,and nitrate nitrogen（NO₃^-）explained 6.9%of microbial residue carbon.（3）The reduction of rainfall will promote the accumulation of bacterial residue carbon,fungal residue carbon and microbial residue carbon.The rainfall reduction treatment significantly increased the bacterial residue carbon（BRC）and microbial residue carbon（MRC）by 30.4%and 20.4%in the isolated root+mycelium group（NM group）in the 10-20cm soil layer in summer（July）,while the The retained root+mycelium group（NT group）and the isolated root+retained mycelium group（AM group）had no significant effect,indicating that decreased rainfall would promote the input and accumulation of bacterial residue carbon and microbial residue carbon.In the control treatments,the contents of fungal residue carbon（FRC）and MRC in the isolated root+retained mycelium group（AM group）and isolated root+mycelium group（NM group）were significantly lower,by 19.8%-27.9%,respectively and 22.3%-28.1%,while in the rainfall reduction treatments,there was no significant difference in FRC and MRC contents among the isolated root treatments,indicating that the rainfall reduction was beneficial to the input and accumulation of fungal residue carbon in the soil.（4）The input of root exudates increases the accumulation of fungal residue carbon and microbial residue carbon in the soil,but has no significant effect on bacterial residue carbon.There was no significant difference in FRC and MRC contents among the isolated root treatments in the rainfall reduction treatments,while the FRC and MRC contents in the preserved root+mycelium group（NT group）increased by 24.8%-38.7%and 21.7%-39.1%,respectively,in the control treatment.,PLFAs also had the same results,indicating that the input of root exudates from the preserved root+mycelium group can increase the biomass and activity of soil microorganisms（especially fungi）,thereby increasing the turnover rate of microorganisms.Reduced rainfall may also indirectly contribute to the accumulation of microbial residue carbon through effects on mycelial secretions.The results of Pearson correlation analysis also showed that there was a significant positive correlation between root exudate TOC and microbial residue carbon.（5）According to the results of structural equation model,it is concluded that the effect of rainfall reduction on the correlation between root exudates and microbial residue carbon is different in different seasons,and the winter（January）rainfall reduction control（positive effect）to affect the relationship between the indirect positive effect of NH₄⁺utilization by root exudates and the establishment of microbial residue carbon;while the summer rainfall reduction treatment was affected by mineral nitrogen（MN）and total microbial phospholipid fatty acids（PLFAs）control（positive effect）to influence the relationship between the indirect positive effect of root exudates utilization of MN,PLFAs and soluble organic carbon（DOC）and the establishment of microbial residue carbon.Therefore,the input of root exudates increases the accumulation of microbial residue carbon in the soil,and the decrease in rainfall affects the association between root exudates and microbial residue carbon through the control of NH₄⁺,MN and PLFA total biomass.Understanding the relationship between root exudates and soil microbial residue carbon and its changes under reduced rainfall provides a scientific basis for scientific prediction and adaptive management of carbon sequestration and productivity of Cunninghamia lanceolata plantations under the background of climate change.