Abstract: Ageratina adenophora, a noxious perennial forb, has invaded a large ar ea of open habitats and grasslands in tropical and subtropical areas of southwes tern China. To study the invasion mechanism of A. adenophora from the viewpo int of vegetation-soil interaction, a field experiment was conducted with the m onocultures of A. adenophora, Sida szechuensis, Brachiaria decumbens, and Se taria anceps, and the mixed cultures (50%/50%) of A. adenophora and other th ree species under three levels of nitrogen fertilization and planting density. T he results showed that the monocultures of four test species exhibited considera ble nitrogen-acclimation ability. With the increase of nitrogen fertilization l evel, their root/crown ratio decreased, while specific leaf area increased. The increment of biomass growth with nitrogen fertilization level was S. anceps>B. decumbens>A. adenophora>S. szechuensis. In mixed cultures, the same trends existed. Under medium planting density, A. adenophora had the greatest adver se effects on S. szechuensis; while under low planting density and high nitr ogen fertilization level, S. anceps and B. decumbens suppressed A. ade nophora. These results revealed that the intraspecific competition of the domi nant species in mixed cultures increased with planting density, resulting in the decrease of interspecific competition and the decreased inhibitory effects of A. adenophora on the mixed species under high planting density. In summary, A. adenophora inhibited the growth of other species via increasing soil fertility to promote itself. The positive feedback between A. ade-nophora and soil env ironment was related to its invasiveness, ensuring its competitive dominance in community. In addition, S. anceps could be more suitable to replacement control.

Key words: Metolachlor, S-Metolachlor (Dual Gold), Chiral pesticide, Catalase activity, Soil microorganisms