Effects Of Ectomycorrhizal Fungal Diversity On Rhizosphere Soil Physical And Biochemical Characteristics And Growth Of Spruce Seedlings

A single ectomycorrhizal（ECM）fungal host plant in the subalpine spruce forest in western Sichuan resulted in a decrease in ecosystem structure stability and soil ecological function,which affected plant regeneration and growth.Many studies have shown that the diversity of aboveground host plants is significantly correlated with the diversity of mycorrhizal fungi,and mycorrhizal fungi are known to play an important role in the process of forest ecological restoration.However,whether a higher diversity of mycorrhizal fungal community can promote seedlings’growth by improving soil nutrient availability has not received much attention and research.Therefore,the scientific question to be explored in this study is:Is the decrease of underground mycorrhizal fungal diversity caused by a single aboveground host plant the main reason for the difficulty of spruce seedling renewal and slow growth?In view of this,this research used pot experiments to inoculate spruce seedlings with different diversity gradients(0 species（CK）,1 species（S₁）,2 species（S₂）,and 8 species（S₈）of ECM fungal communities to study the effects of ECM fungal diversity on rhizosphere soil physical and biochemical properties and the growth of spruce seedlings,and to explore the mechanisms between them.The results are shown as follows:1.Effects of ECM fungal diversity on physical and biochemical characteristics of Spruce rhizosphere soil.（1）The diversity of ECM fungi had no significant effect on soil water content.The contents of soil organic matter and organic carbon decreased with the increase of ECM fungal diversity,and the contents of soil organic matter and organic carbon were significantly higher in CK than in S₈.Soil nitrate-nitrogen（S₈was significantly higher than CK,S₁）and inorganic nitrogen（S₈was significantly higher than CK）showed an increasing trend with the increase of ECM fungal diversity.（2）S₈significantly increased the activities ofβ-glucosidase（βG）,β-glucuronidase（βLU）（carbohydrate decomposition enzyme）,and leucine aminopeptidase（LAP）in soil.S₂and S₈significantly increased soil phenoloxidase（PHE）,peroxidase（PRO）,and acid phosphatase（AP）enzyme activities.Stepwise regression analysis showed that soilβG,βLU,LAP,AP,and PRO enzyme activities were mainly driven by the diversity of ECM fungi.（3）Inoculation of ECM fungi with different diversity gradients had no significant effect on the bacterial community structure in the rhizosphere soil of spruce seedlings,but inoculation of ECM fungi with higher diversity could significantly change the relative abundance of some bacteria.The relative abundance of Patescibacteria increased with the increase of ECM fungal diversity（S₂and S₈were significantly greater than CK）.At the order level,the relative abundance of Betaproteobacteriales decreased with the increase of ECM fungal diversity（S₂and S₈were significantly lower than CK）.The relative abundance of Saccharimonadales（S₁,S₂,and S₈were significantly greater than CK）,Xanthanomonas（S₈was significantly greater than CK and S₁）,and Rhizobiales（S₈was significantly greater than CK）showed the opposite trend.2.Effects of ECM fungal diversity on the growth of Spruce seedlings（1）Compared with other treatments,S₈seedlings had a higher rate of ECM fungal infection.The difference in total root enzyme activity was mainly reflected in the decomposition potential of carbon,nitrogen,phosphorus,and lignocellulose.Overall,S₈had significantly higher total root tipβLU and NAG enzyme activities than CK and other treatments,while S₈had only significantly higher AP andβG enzyme activities than CK.（2）The results of the nonparametric multivariate analysis of variance showed that S₁,S₂,and S₈were significantly different from CK.The growth rate of the seedling height of S₈was significantly higher than that of CK.The growth rate of seedling biomass of S₈was significantly higher than that of other treatments,while the growth rate of seedling taproot length of S₂was significantly higher than that of S₁and S₈.The results of regression analysis showed that the growth rate of biomass and plant height was significantly correlated with the diversity of ECM fungi,and the growth rate of taproot length was significantly correlated with the infection rate of ECM fungi.（3）The nitrogen content of spruce S₈was significantly higher than CK and S₁.The results of regression analysis showed that the difference in nitrogen content was mainly determined by ECM diversity（P<0.001）.The potassium content of S₁spruce plants was significantly higher than CK and other treatments.The sodium content of S₁and S₈was significantly higher than S₂.The magnesium content of S₂is significantly higher than that of CK.3.Correlation between ECM fungal diversity,soil physical and biochemical characteristics,and spruce growth（1）The content of sodium in spruce was positively correlated with the diversity of ECM fungi.There was a significant positive correlation between potassium and sodium and magnesium content in spruce.（2）The enzyme activities ofβG,βLU,and AP in total root tips of spruce were significantly correlated with the enzyme activities ofβG,βLU,and AP in soil,respectively.The results of the structural equation model showed that,in terms of carbon-related metabolic pathways,ECM diversity mainly affected the growth rate of biomass by increasing the activities of total root enzymes and soil enzymes（βG,βLU,PRO,and PHE）.In terms of nitrogen-related metabolic pathways,the increase in biomass growth rate was mainly attributed to changes in soil enzymes and total root enzymes related to nitrogen degradation（NAG and LAP）and plant nitrogen content.In conclusion,with the increase in diversity of the ECM fungal community,the functional complementary effect of ECM fungi species was strengthened,and the potential enzyme activity of plant roots and soil was enhanced,which promoted the decomposition of organic matter and the availability of N,and further promoted the growth of seedlings.Our findings help to establish a mechanistic link between plant growth and ECM fungal diversity,which can further deepen our understanding of forest ecosystem carbon sequestration and soil nutrient cycling.