Invasive Mechanism Of Ageratina Adenophora Sprengel (Compositae): Underground Self-reinforced Capacity By Changing Soil Biota

The rapid expansion of invasive plants in agricultural and natural ecosystem is threateningbiodiversity, productivity and ecosystem health throughout the world. Understanding the strategies usedby invasive plant for rapid proliferation is one of the most important problems in ecology, because thesuccessful invasion of particular species is neither mechanistically uniform nor predictable. Positivefeedback of soil biota may facilitate exotic plant invasion which attracts increasing attention recently.Ageratina adenophora (Sprengel) R. King & H. Robinson (Synonym: Eupatorium adenophorum) is anotorious worldwide invasive weed and one of the most unwanted in China. A. adenophora may alterunderground microbial communities, and the feedback of modified soil biota in invaded ranges maypromote its invasivibility. To examine this possible underground invasion mechanism, effects of A.adenophora invasion on soil microbial communities were analyzed, dominant rhizosphere microbes ofA. adenophora and their antibiosis activity were identified and assessed, and plant-soil biota feedbackexperiments were designed to measure the effect of invasion-induced changes of soil biota on A.adenophora growth and competition. The main results are as follows:1. Effects of A. adenophora invasion on native soil biota and soil nutritionSoil biota and soil nutrition were analyzed in the four sites (Heavily invaded, Newly invaded,Non-invaded and Native plant), and the results showed that soil microbial community structure wasclearly separated in all the four sites, and A. adenophora significantly increased soil fungi, azotobacteria,ammonia oxidizing bacteria and soil VAM (Vesicular-arbuscular mycorrhizal fungi) abundance as wellas the fungi/bacteria ratio. Soil NO₃^--N, NH₄⁺-N, available P and K content were significantly higher inheavily invaded site as compared to the newly invaded site. Soil fungi, azotobacteria and ammoniaoxidizing bacteria were strongly associated with most of the measured soil nutrition characters. Itindicates that A. adenophora changed soil microbial communities, especially the soil nutrition cyclingrelated soil microbe groups and VAM, probably creating favorable soil environment to benefit itself.2. Screening and identification of A. adenophora dominant rhizosphere microbesTotally 25 strains of dominant rhizosphere bacteria of A. adenophora were isolated and identified,of which 8 strains were assessed of their antibiosis activity. The results showed that Bacillus andPseudomonas genus were in high abundance in A. adenophora rhizosphere soil, of which Bacillussubtilis and Bacillus megaterium were the most abundant and consist 55.6% of the total identifiedbacterium. These dominant identified bacteria showed antibiosis activity to Fusarium oxysporum andRalstonia solanacearum at different level and Bacillus subtilis BS-5 and Bacillus thuringiensis BT-1metabolic products had the strongest inhibitory effects to Fusarium oxysporum with antibiosis activity85.5% and 83.8% respectively. The abundant antagonistic bacteria around A. adenophora rhizospheremay help A. adenophora resist harmful soil-borne disease and provide a way to escape natural enemies.3. Effects of A. adenophora leachates on isolated dominant bacterium Soil biota incubation experiments were conducted by using A. adenophora leachates to imitatefield invasion processes. The results indicate that soil microbial community was significantly changedafter A. adenophora root and aerial part leachates treatment in 2 and 3 weeks, respectively. The newlyinvaded soil biota treated by 100% A. adenophora leachates was significantly different from the controland much closer to the heavily invaded soil. Growth of 8 strains dominant rhizosphere bacterium waspromoted by A. adenophora leachates. Effects of A. adenophora leachates on soil biota were in linearwith concentration of leachates and root leachates had stronger effects than aerial part leachates. A.adenophora can change soil microbial community through nutritional and chemical communication.4. Feedback of invasion-induced changes of soil biota to A. adenophora and nativesThe greenbouse experiment indicated that the soil biota in heavily invaded site had more inhibitoryeffects on native plant species than A. adenophora; and the soil biota in native plant site inhibited thegrowth of native plant species, but not of A. adenophora. In pots with soil from heavily invaded site,sterilization improved the biomass of Lolium perenne by 23%, of Eupatorium fortunei by 93%, ofMedicago sativa by 73%, and of A. adenophora by 11%. Soil biota in all of the four sites increased A.adenophora relative dominance as compared to each of the three native plant species, where soil biotain the heavily invaded site had stronger beneficial effects on A. adenophora relative dominance index(24% higher on average) than soil biota in the non-invaded site. Our results suggest that Ageratinaadenophora modified soil microbial communities to facilitate its growth and inhibit natives.5. SummaryA. adenophora is more positively affected by the soil biota in invaded sites than resident natives,and earns its competition superiority directly or indirectly through the beneficial soil biota andantagonistic rhizosphere microbes. Once it establishes in new ranges it further alters the soil communityin a way that favors itself and inhibits natives helping promote invasion. Soil biota alteration followingA. adenophora establishment may be an important part of its invasion process, which is used as anunderground self-reinforcing invasion mechanism to strengthen its invasiveness.