Soil Environment Changes Induced By Ageratina Adenophora Invasionon And Their Impacts On Different Plant Species

Biological invasion, a present focus of ecological research, severely threatensthe biodiversity of native ecosystems and has caused significant native ecosystemdeterioration and economic loss to the world. Ageratina adenophora, a noxiousperennial forb, has invaded a large area of open habitats and grasslands in tropicaland subtropical areas of southwestern China. A common and important managingpractice is to restore the habitats that have been invaded bythis exotic species usinghigh biodiversity of native species or to cultivate forage grasses with a lowbiodiversitybuthighyieldandeconomicgain.In the present dissertation, the interference of soil environment induced by A.adenophora invasion, including impacts on soil fertility and allelochemicalsremained in soil were examined. Moreover, the effects of these changes on thegrowth, morphological, biomass allocation and photosynthetic trails of this invasivespecies itself, native forbs and grass species were investigated. The experimentdesignsandmainresultsareasfollows:(1) The comparison was made on the activities of six enzymes and 12 physicaland chemical factors in four layers of soil within 0~30 cm depth of four differentcommunities: a grassland invaded by A. adenophora for more than ten years, asecondary grassland that was abandoned from a maize field in November of 2001, aprimary grassland of Themeda yunnanensis, and a two years old artificial grasslandof S. anceps, respectively. The results showed that the activities of polyphenoloxidase, alkaline phosphatase and urase changed significantly among communitiesand soil layers, and were closely related to most of the variables affecting soilfertility, indicatingthat these enzymes can be used as indicator of soil fertility. Theactivities of these three enzymes, content of organic matter, total N, total Ca, totalMg, hydrolyzable N, active P, and pH value were higher in the soil under A.adenophora community compared to the soils under the other communities. These resultsindicatethatlong-termA.adenophorainvasioncanincreasesoilfertility.(2) In the laboratory, seeds of two native forbs (Sida szechuensis and Artemisiamyriantha) and one grass (Setaria anceps) were germinated in the soil that had beeninfested by A. adenophora for more than ten years. Activated carbon (AC) wasmixed into the soil to eliminate allelopathic effects of A. adenophora. The ACtreatment significantly increased the seed germination rate, shoot length and shootdry weight of the seedlings of these species, indicating the allelopathic effectremainedinthesoil.In the grassland that had been infested by A. adenophora for more than tenyears,twonativeforbs (S.szechuensis andA.myriantha)andtwointroducedgrasses(Brachiaria decumbens and S. anceps) were grown separately for two years. As awhole, activated carbon (AC) treatment increased the total biomass, plant height,branch number, total leaf area, maximum net photosynthetic rate and nitrogenutilization efficiency of the four species, and decreased their root / crown ratios.These results revealed that allelochemicals of A. adenophora remained in soil for along time and suppressed other plant species but promoted itself growth. Amongthe tested species, the least allelopathic effects by A. adenophora was found on S.anceps, indicating this grass species is a suitable candidate to be used for theecologicalrestoration.(3) Another field experiment, lasting for two years, was carried out withmonocultures of A. adenophora, S. szechuensis, B. decumbens and S. anceps, andmixed cultures (50/50%) with A. adenophora and other three species in threedifferent nitrogen and densitylevels. The results showed that monocultures of fourspecies exhibited considerable nitrogen-acclimation abilities. With the increase ofnitrogen levels, their root mass fraction and root / crown mass ratio decreased, buttheir leaf mass fraction and leaf area ratio increased. The extent of increase inbiomass growth with the nitrogen level was: S. anceps > B. decumbens > A.adenophora > S. szechuensis. Inmixedcultures,theresultsshowedthesametrend.Under intermediate density level, the greatest adverse effect by A. adenophora wasfound on S. szechuensis, while under low density and high nitrogen level, S. anceps and B. decumbens suppressed A. adenophora. These results revealed that theintraspecificcompetitionofthedominantspecies inthemixedcultureincreasedwithdensity, resulting in decrease of interspecific competition and the decreasedinhibitingeffectsonthemixedspeciesunderhighdensitylevelbyA.adenophora.Insummary,thepresentstudyindicatesthatA.adenophorainhibitedthegrowthof other species via increasing soil fertility and allelochemicals remained in the soilto promote itself. The positive feedback between A. adenophora and soilenvironment is related to its invasiveness, ensuring its competitive dominance incommunity. Therefore, it is difficult to restore a habitat invaded heavily by thisspeciesbysimplycleaningmethod. It isnecessarytorestoresoilenvironmentsandtousethereplacementmethodwithsuitableplantspecies.