Interactions Between Three Invasive Composite Plants (Ageratina Adenophora, Ambrosia Artemisiifolia, Flaveria Bidentis) And Soil Biota

The interaction between invasive plants and soil microorganisms has been paid more attention by scientists in the area of the invasion mechanisms of alien plants.Three Chinese typical invasive composite plants,i.e.Ageratina adenophora(Sprengel),Ambrosia artemisiifolia L.and Flaveria bidentis were taken as examples to study their effects on the diversity and structure of invaded-soil biota and the feedbacks caused by them to the growth of receptor plants.Meanwhile,common garden experiments were conducted in order to elucidate the interaction between staple invasive composite plants and soil microbial communities.The main results were as follows:The micro-habitat of the invaded-soil was changed greatly and the content of absorbable NO₃^--N,NH₄⁺-N,available P and K,organic matter in soil were increased sharply.The highest amount of NH₄⁺-N in heavily invaded soil reached 53.00mg/kg,which was almost 14.1,9.9 and 5.9 times each as high as bare soil,multi-native-plants soil and single-native-plant soil,respectively.The content of NO₃^--N in soil had a tendency to increasing with the intensification of invasion and was different significantly among soils with various invasion degree.The amount of NO₃^--N in heavily invaded soil,with the content of 91.98mg/kg,was as nearly 7.3 times as that in bare soil.There was an obvious correlation between the change of soil enzyme activity and the alteration of soil physical and chemical properties and the change of soil microbial communities and enzyme activity in turn might lead to the modification of soil nutrient condition.The number of soil fungi,azotobacteria and ammonia oxidizing bacteria in heavily invaded site was comparably high as testified by the results of the DGGE analysis.The diversity of AMF was reduced and the structure of AMF communities in native plants was damaged following the invasion of A.adenophora,which could explain the advantages of A.adenophora in competition with native plants via greenhouse pot experiments controlled by fungicide. The micro-habitat of soil with different level of invasion by A.artemisiifolia also altered greatly.The content of soil available N,P and K content reached 12.79 mg.kg^-1,32.98 mg.kg^-1 and 93.8mg.kg^-1 each in heavily invaded sites,which were almost 2.4,1.9 and 2.7 times as high as soil collected from native region.The PLFA fingerprint spectrum indicated that not only the amout of PLFA extracted from soil microorganisms in invaded site was changed significantly,the structure of soil microbial communities was altered obviously,with the decreasing of soil fungi.Greenhouse sterilized pot experiments found that the altered microbial communities in invaded soil by A.artemisiifolia facilitated the growth of A. artemisiifolia but inhibited native species.The soil microbial communities in various invasive habitats of F.bidentis were compared through the method of PLFA analysis.The result showed that the structure of soil microbial communities was changed significantly in invaded area,with improving the content of 16:1ω5 fatty acid of token AMF.The result of the competition between F.bidentis and native plants Chenopodium serotinum L.and Digitaria chinensis Homem.simulated by common garden experiment demonstrated that the biomass of D.chinensis in competion with F.bidentis was 61%that of D.chinensis grown alone,which indicated that the presence of F.bidentis inhibited the growth of D.chinensis greatly and the inhibition effect on the growth of C. serotinum was obvious.The results simulated of common garden simulating experiments suggested that three invasive composite plants and native plants had exerted different influences on soil micro-habitat.The results from pot feedback experiment showed that the soil invaded by the three invasive plants had inhibited the growth of oryza sativa.The plant height of oryza sativa increased by 113%,83%and 17%respectively when it was cultivated in sterilized soils invaded by F.bidentis,A.artemisiifolia and A.adenophora.Soil invaeded by invasive plants also had an inhibitive effect on the growth of Medicago sativa L.After sterilization,root length of M.sativa increased by 36%and 14%,respectively,when it was grown in A. adenophora and F.bidentis invading soils but influence was not significant while grown in A. artemisiifolia invading soil(F₁=2.96,P=0.1018). In conclusion,the above results suggested that the invasive composite plants might destroy the soil micro-habitat of native plants and create new one favourable to the gowth of the invasive plants through alteration of soil microbial communities.