Allelopathic Effect And Decompositon Dynamics Of Leaf Litter Of An Invasive Plant: Ageratina Adenophora Sprengel (Compositae)

Invasive alien plants (IAP) has a profound implication to ecosystem not only because they will cause great economical loss in the invaded regions, but also due to their competition advantage over native plants, they successfully form their monocultures and threaten greatly to the environment and divesity of ecosystem. The influences of IAP on ecological processes will in turn affect the interaction between IAP and native plants. To uncover the invasion mechanisms for IAP is of vital importance. Recently, Novel Weapons Hypothesis (NWH) has been put forward to emphasize the explanatory role of allelopathy in IAP invasion. There are a lot of pathways for IAP to generate allelopathic effect, among which release of allelochemicals by decomposition of leaf litter of IAP is an important one. To reveal the influences and feedbacks of allelopathic effect of IAP leaf litter on native plants and soil ecological process will not only enrich our understanding of NWH, but provide as a theoretical guideline for the ecological restorations of the destructed habitats.Ageratina adenophora (Sprengel) R. King & H. Robinson (Synonym: Eupatorium adenophorum) is a typically noxious invasive alien plant in the world. In China, it has caused great economical loss and damage to ecosystem in southwestern part. Based on previous study, by choosing leaf litter of A. adenophora as a entry point, we aimed to further examine the interaction among allelopathic effect of IAP, soil ecological process and native plants according to review the allelopathic mechanism for IAP and the theory of population expansion for A. adenophora. On the one hand, laboratory bioassay and greenhouse pot experiment was conducted to testify the allelopathic effect of A. adenophora leaf litter. On the other hand, through greenhouse pot experiment and indoor analysis, we tested the effects and feedbacks of A. adenophora leaf litter on soil ecological process in the invaded region. Finally, allelopathic decomposition dynamics of leaf litter waterextract was studied via inoculating both A. adenophora and native plant rhizosphere soil solutions into waterextract. Our destination in this research focused on the determination of allelopathic effect of A. adenophora leaf litter and was to elucidate its influence on soil ecological processes and their feedbacks to the invisibility of A. adenophora and the regulation of allelopathic decomposition for leaf litter. Our main results were as follows:1. Determination of allelopathic effect of A. adenophora leaf litterLaboratory bioassay of effects of leaf litter waterextract on seed germination rate and seedling growth of Medicago sativa, Trifolium repens, Lolium perenne and Galinsoga parviflora found that responses of these 4 receiver plants to different concentrations of waterextract varied from species to species. Except for L. perenne, waterextract showed a significant inhibitory effect on other 3 plants, among which M. sativa was the insensitive plant while the T. repens was the most sensitive one. Meanwhile, there was a positive correlation between the concentration and the inhibition effect. Results from greenhouse pot experiment indicated that a significant inhibitory effect of leaf litter in soil on the growth of T. repens had been figured out. Single plant biomass of T. repens was reduced significantly by adding 5% leaf litter into soil. However, subsequent mixing of active carbon into the same soil had reversed this inhibition effect and the single plant biomass of T. repens was increased by 71.3%.Our results indicate that by allelopathic effect of leaf litter, A. adenophora inhibited the seed germination and seedling growth of native plants, which testified the directly allelopathic inhibition role of leaf litter in the invasion mechanism for A. adenophora.2. Effects and feedbacks of leaf litter on soil ecological processA phenomenon that A. adenophora created a favorable soil micro-environment to strengthen its invisibility through employing its leaf litter to generate allelopathic effects on soil microbial community and thus caused changes in soil enzymatic activity and chemical property was obtained by the methods of greenhouse pot experiment assisted by indoor cultivation and detection analysis.1) Results from traditional cultivation method for estimating numbers of cultivable microorganisms showed that the number of bacteria, fungi, actinomyces, auto-nitrogen fixation bacteria, inorganic phosphobacteria and potassium bacteria was increased significantly after leaf litter decomposition in soil. The number of the above 6 microorganisms was 8.4, 6.5, 4.0, 5.4, 3.8 and 6.1 times that of the control after mixing 5% leaf litter into soil, which was in line with the results from PLFAs analysis. PLFAs results demonstrated that there were significant differences in fatty acid content among functional microbe between different concentration treatments, among which the fatty acid content of fungi was 1.8 times that of control in 5% leaf litter treatment. In addition, the ratios of G (-) / G (+) and fungi / bacteria were also improved obviously. Results from cluster analysis and PCA implied that a significant difference in microbial community among high concentration, low and medium concentration and control was determined. However, there were no obvious differences between low and medium concentration treatments because a overlap phenomenon was found.2) Indoor chemical analysis for soil enzymatic activity and chemical property indicated that the activity of soil invertase, acid phosphotase and urase was significantly improved, among which the activity of invertase in 1.6% treatment was 1.5 times that of control and acid phosphatase and urase in 5% treatment were 1.9 and 3.3 times that of control. A positive correlation between the concentrations and enzymatic activity was also fond. Soil chemical property almost showed the same trend and the contents of soil available N, P, K in non-sterilized soil were 1.4, 2.8, and 2.0 times that of control.3) Mutual effects of soil microbe, enzymatic activity and chemical property in turn affected the invisibility of A. adenophora. Feedback effect from greenhouse pot experiment showed that single plant biomass of A. adenophora was increased significantly while for T. repens decreased obviously after leaf litter decomposition in soil. When soil unsterilized, single plant biomass of A. adenophora in 1.6% treatment was 2.5times that of control while for T. repens was only 0.3 times compared to control. When A. adenophora was cultivated with T. repens and L. perenne in 1.6%leaf litter treatment, the relative dominant percentage of A. adenophora were 1.21 and 1.16 times compared to control respectively.Our results implied that an indirect influence of allelopathic leaf litter on soil ecological processes was determined to improve the invisibility and competition advantage of A. adenophora. 3. Allelopathic decomposition dynamics of leaf litter waterextractAllelopathic decomposition dynamics of leaf litter waterextract was studied using seed relative germination rate and seedling relative length of T. repens as testing indictors through inoculating both A. adenophora and native plant rhizosphere soil solutions into waterextract under both aerobic and anaerobic conditions. Results from three way ANOVA indicated that decomposition conditions and time alone affected the decomposition performance of waterextract but the soil solution origin did not. However, mutual effects of soil solution origin with condition and time respectively and the mutual effect of the three factors significantly affected the decomposition performance. When choosing seedling relative length as indicator, a phenomenon that the allelopathic effect was reinforced after 1 d and subsequently attenuated gradually with 10 d as its decomposition peak was observed. It seemed that allelopathic effect of waterextract disappeared much faster under aerobic condition than anaerobic. When the condition was anaerobic, it seemed that the allelopathic effect was more endurable and inoculating of A. adenophora rhizosphere soil solution showed a relatively faster decomposition type. Finally, the pH value of waterextract was improved along with the decomposition time.Our findings demonstrated that certain kinds of functional microbe which are specific to rhizosphere soil of A. adenophora accelerated the decomposition process of allelopathic effect of leaf litter compared to natives under anaerobic condition. This might serve as another explanation for better survival ability of A. adenophora under stressful environment such as in soils that oxygen availability is severely limited.4. ConclusionOur conclusion was that by leaf litter decomposition in invaded soil, A. adenophora exerted both direct allelopathic inhibition effect on neighboring plants and indirect allelopathic influence on soil ecological process to create a more favorable soil microbial surrounding to strengthen its invisibility. Furthermore, some microbe that were specific to the rhizosphere soil of A. adenophora was responsible for relatively faster decomposition mechanism of allelopathic effect produced by leaf litter and this was in turn a unique advantage for A. adenophora to survive in barren and oxygen unfavorable habitats.