| Ectomycorrhiza(ECM)fungi can coexist with most coniferous trees and a few broadleaved trees on land to form ectomycorrhizae,which can effectively promote the absorption of water and nutrients by host plants and improve the stress resistance of host plants.Pinus tabuliformis,as a typical ectomycorrhizal conifer,is one of the most important afforestation tree species for ecosystem restoration on the Loess Plateau.Although ECM fungi can improve the drought resistance of host plants,the effects of different ECM fungi on host plants are significantly different,especially the actual effect of ECM fungi inoculation on the Loess Plateau is unknown.In this study,we investigated the composition of rhizosphere soil fungi and ECM fungal resources in P.tabuliformis plantations at different stand ages on the Loess Plateau in northern Shaanxi,and isolated and identified the ECM fungus(Suillus variegatus).On this basis,S.variegatus were inoculated on P.tabuliformis plantations and greenhouse seedlings,and the effects of ECM fungi on plant growth,rhizosphere soil physicochemical properties and fungal community structure of P.tabuliformis plantations were determined for 3 consecutive years.The purpose of this study is to clarify the effects of ECM fungi on the osmotic regulation and non-structural carbohydrates(NSC)of P.tabuliformis seedlings under drought stress,and on the improvement of the rhizosphere microenvironment of P.tabuliformis plantations on Loess Plateau,and to reveal the effect by which ECM fungi improve the drought resistance of P.tabuliformis.This study provides a theoretical basis for the ecological restoration of P.tabuliformis plantations using ECM fungal biotechnology on the Loess Plateau.The main results and conclusions are as below:1.Through research on the physicochemical properties of the rhizosphere soil and fungal community of P.tabuliformis plantations at different stand ages(10a,20 a and 30a)on the Loess Plateau,it was found that the P.tabuliformis plantation significantly increased soil ammonium nitrogen,nitrate nitrogen,total potassium,and organic matter content,significantly reducing soil available phosphorus content.The phosphorus element may be the biggest limiting factor for ecological restoration in this area.The Simpson index of rhizosphere soil fungi in P.tabuliformis plantations decreased significantly with the increase of stand age because of available phosphorus.Ascomycota and Basidiomycota were the dominant fungi in the rhizosphere of the 10 a P.tabuliformis plantation,and Ascomycota were the dominant fungi in the 20 a and 30 a plantations.The rhizosphere fungal community composition of these two stand ages was closer.With the increase of stand age,the fungal community in the rhizosphere soil of P.tabuliformis plantations tended to be stable,and all the relative abundances of ECM fungi,saprophytic fungi and plant pathogenic fungi increased significantly.On the one hand,the development of P.tabuliformis plantations can directly affect the community composition and diversity of rhizosphere soil fungi,and on the other hand,it can also indirectly affect soil fungal community composition and diversity by changing the content of ammonium nitrogen,nitrate nitrogen and available phosphorus in the rhizosphere soil.2.In the investigation of rhizosphere fungal resources and mycorrhizae in P.tabuliformis plantations at different stand ages on the Loess Plateau,13 fungi were successfully isolated,including 2 ECM fungi,2 dark septate endophytes,3 endophytic fungi;3 saprophytic fungi and 3 pathogenic fungi.The ECM fungus,S.variegatus,was simultaneously isolated from10 a,20a and 30 a P.tabuliformis plantations,which is an indigenous ECM fungus ubiquitous in local P.tabuliformis plantations.The mycorrhizal colonization rate of P.tabuliformis was significantly different at different stand ages,only 32.16% in 10 a,and 43.25% in 30 a.The morphology of mycorrhizal at different stand ages was also significantly different,which increased with the increase of stand age and was more diverse at 30 a P.tabuliformis plantation.A total of 20 ECM fungal operational taxonomic units were identified from the root tips of P.tabuliformis,belonging to 2 phyla,2 classes,7 orders,10 families and 11 genera.There were significant differences in the ECM dominant genera in the root system of P.tabuliformis at different stand ages.Suillus was the dominant ECM fungi in the root tip of P.tabuliformis in all stand ages,while Inocybe in 10 a and Mallocybe in 20 a,30a were the dominant genera in this area.3.The inoculation of S.variegatus had a significant effect on the growth and the physicochemical properties of the rhizosphere soil of 10 a P.tabuliformis in the field on the Loess Plateau of Fugu county in northern Shaanxi.There was a "hysteresis" in the effects of ECM fungi on host plants in the field compared with greenhouse experiments.In the first year after inoculation,the mycorrhizal colonization rate of P.tabuliformis did not change significantly,which increased significantly in second year and maintained a stable symbiosis in the subsequent time.This indicated that it took a longer time for ECM fungi to establish a stable symbiotic relationship with plants in the field.Compared with the uninoculated plot,the content of nitrate nitrogen and available phosphorus in the rhizosphere soil of P.tabuliformis increased significantly after one year of inoculation,and the content of chlorophyll and the height of new buds of P.tabuliformis were also significantly higher than those of the uninoculated plot.In the third year of inoculation,the increment of plant height of P.tabuliformis was significantly higher than that of the uninoculated plot.With the increase of inoculation time,the growth-promoting effect of S.variegatus on P.tabuliformis became more and more significant.These results indicate that inoculation of S.variegatus can improve soil nutrient status,increase the content of nitrate nitrogen and available phosphorus in rhizosphere soil,promote the growth of P.tabuliformis,and alleviate the degradation of plantations under natural drought conditions in the field in this area.4.The inoculation of S.variegatus could significantly affect the diversity and community composition of rhizosphere soil fungi of 10 a P.tabuliformis plantation under natural drought conditions on the Loess Plateau.Compared with the uninoculated plot,the richness of soil fungi in the rhizosphere of P.tabuliformis was significantly decreased in the first year after inoculation,but with the increase of inoculation time,the soil fungal diversity was significantly higher than that of the uninoculated plot after first year.At the same time,the relative abundance of Suillus in the rhizosphere soil increased significantly became of the inoculation of S.variegatus,while the relative abundances of Mortierella and Fusarium decreased significantly.The changes in soil physicochemical properties caused by the inoculation of S.variegatus were the main driving force for the changes in the fungal community structure in the rhizosphere soil of P.tabuliformis.These results indicate that the roles of ECM fungi,soil physicochemical properties and soil fungal communities are closely related.The inoculation of S.variegatus can significantly improve the rhizosphere soil microenvironment of P.tabuliformis,increase the diversity of rhizosphere soil fungi,and reduce the relative abundance of plant pathogenic fungi.5.The inoculation of S.variegatus significantly reduced the mortality of P.tabuliformis seedlings under drought stress in greenhouse.Compared with uninoculated seedlings,the mortality of inoculated seedlings was reduced by 44% and 25% under moderate and severe drought stress,respectively.The inoculation of S.variegatus could significantly increase the photosynthetic rate and transpiration rate of P.tabuliformis under drought stress(especially severe drought stress),which ensured the acquisition of carbon source and promoted plant growth to a certain extent.The inoculation of S.variegatus could alleviate the reduction of leaf water potential under drought,and increase root length and root surface area,which ensured water uptake and transport under drought stress.The inoculation of S.variegatus could significantly increase the starch content in coarse roots and the soluble sugar content in all tissues,which ensured the osmotic regulation of plants under low water potential by regulating the distribution of starch and soluble sugar.In addition,the inoculation of S.variegatus significantly increased the total content of NSC in P.tabuliformis,which further alleviated carbon starvation and greatly satisfied the carbon source required for growth.This study revealed that under drought stress,the inoculation of ECM fungus S.variegatus could improve soil nutrients and fungal community structure,promote the growth and water absorption of P.tabuliformis,enhance photosynthesis and osmotic regulation,alleviate carbon starvation,and enhanced the drought tolerance of P.tabuliformis.The mechanism of ECM fungi enhancing the drought tolerance of P.tabuliformis was further completed. |
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