Study On The Community Structure Of Arbuscular Mycorrhizal Fungi Under Different Afforestation Types And Their Effects On Litter Decomposition

In this study,different afforestation types including monocultures and mixture of Robinia pseudoacacia and Platycladus orientalis were selected in the Loess Plateau,China,to clarify the changes in arbuscular mycorrhizal fungal（AMF）attributes along a chronosequence of R.pseudoacacia plantations,analyze the effects of R.pseudoacacia mixed with P.orientalis on plant growth,nutrient status,soil properties and AMF community structure,and reveal the influence of AMF species（Rhizophagus irregularis）,which is dominant in the plantation area of the Loess Plateau,on litter decomposition and R.pseudoacacia growth under different water conditions.The main results are as follow:1.Growth status of different-aged R.pseudoacacia plantations and their affecting factorsThis study was conducted in Wangdonggou watershed region at Changwu County,Shaanxi Province,China.Soil,shoot and root samples were collected in a chronosequence of R.pseudoacacia plantations consisting of stands 0a（grassland）,11a,23a,35a and 46a.Soil physical-chemical properties,shoot and root nutrient contents,easily extracted glomalin contents,AMF colonization rate and spore density were measured.Along the chronosequence,AMF spore density and contents of soil NO₃-N and available K increased,and easily extracted glomalin content,dehydrogenase and alkaline phosphatase activities decreased,and soil catalase activity and the level of root colonization by arbuscules and hyphae first increased and then decreased.Stand age was significantly correlated with the nutrients status of R.pseudoacacia roots and shoots such as contents of K,Zn,Fe,Mn in roots and N,Cu,Zn,K,Mn,Ca,Mg in shoots.R.pseudoacacia growth was synthetically affected by soil physical-chemical properties,AMF and plant nutrient status,whereas the factors positively affecting R.pseudoacacia growth varied with stand age.Specially,the growth of 11 and 23a R.pseudoacacia was positively correlated with contents of shoot K,Mn,Ca,Mg,Fe,Zn and activities of soil alkaline phosphatase,catalase and the level of root colonization by arbuscules.The 35a R.pseudoacacia was positively correlated with contents of soil available K,soil catalase activity,shoot N and Cu,AMF colonization rate and spore density;the 46a R.pseudoacacia was positively correlated with AMF spore density and contents of root K and Fe,soil available K and NO₃-N and soil catalase activity.2.Effects of stand age on the community structure of soil-and root-inhabiting AMF under R.pseudoacaciaThe study on community composition of R.pseudoacacia plantations with different stand ages showed that:Roots and soils hosted different AMF diversity and communities.AMF diversity and community composition in R.pseudoacacia roots varied with stand age,but not in soils.In roots,the relative abundance of Claroideoglomus,together with Chao1richness and OTU richness,peaked at the intermediate stage（35a）and then declined,and the relative abundance of Glomus decreased linearly with stand age,whereas the relative abundance of the dominant genus Rhizophagus did not vary with stand age.Soil available K and NO₃-N largely explained the shift in the structure of the root-colonizing AMF community along the chronosequence.3.Effects of R.pseudoacacia and P.orientalis mixtures on plant growth and AMF attributesDifferent types of stands（i.e.monocultures and mixture of R.pseudoacacia and P.orientalis）were selected.The height and diameter at breast height of each tree species,soil physical-chemical properties,shoot and root nutrient contents,AMF colonization rate and AM fungal community composition were measured.The results showed as follows:the AMF attributes,shoot and root nutrient status,height and diameter of R.pseudoacacia were not influenced by the presence of P.orientalis,except AMF community in rhizosphere.For P.orientalis,however,growing in mixture increased height and diameter and reduced shoot Mn,Ca and Mg contents and changed AMF colonization,sporulation and communities in the roots and rhizosphere.The changes in AMF community structure detected in R.pseudoacacia rhizosphere soils,P.orientalis roots or P.orientalis rhizosphere soils were closely related with changes in soil properties such as soil moisture,available K,NO₃-N,Mn,catalase and alkaline phosphatase.The increase in soil water and nutrient in the presence of R.pseudoacacia not only directly stimulated P.orientalis growth,but also changed AMF community structure and further suppressed the development of AM symbiosis in P.orientalis roots,especially the production of AMF spores and vesicles,thus freeing up carbon to help P.orientalis growth.4.The roles of AMF in litter decompositionThe effects of AMF on litter decomposition of R.pseudoacacia and P.orientalis under different water conditions were studied using PVC boxes with various pore sizes.The results showed that:the inoculation of AMF could promote the photosynthetic rate of R.pseudoacacia leaves and their growth;AMF could enhance litter decomposition,especially under water stress;the effects of AMF on litter decomposition were dependent on litter composition,the highest effects were observed in R.pseudoacacia and P.orientalis litter mixture;the litter being added in the substrate has been found to decrease the photosynthetic rate of R.pseudoacacia leaves and AMF colonization rate.