Effects Of Nutrient Level And N:P Ratio On The Competition Between Invasive Alternanthera Philoxeroides And Non-invasive Plants

Biological invasion has been an important part of global changes, and might aggravate under the influence of human disturbance and global climate change. Biological invasion might have serious influence on the ecosystem structure and functional stability, reducing the diversity of local species, hampering the economic development, and harming to people’s health. Alligator weed (Alternanthera philoxeroies (Mart.) Griseb) is a worldwide noxious invasive plant and has become a major invader of wetland habitats in China, causing serious ecological damages and economic losses. In the eutrophication wetland, A. philoxeroides could spread rapidly and form dominant species in large areas, which pose a serious threat to local biodiversity and wetland ecosystem service function. Nitrogen and phosphorus, considered as the most important nutrient elements for plant growth, has significant effect on the growth of A philoxeroies. The research on the effects of nutrient level and N:P ratio on growth of A. philoxeroies and the competition between A. philoxeroies and non-invasive species, could play an important role in revealing the invasive mechanism of A. philoxeroides and provide theoretical foundation and advice for the management of A. philoxeroides.A greenhouse experiment was conducted to study the effects of nutrient level and N:P ratio on growth of A. philoxeroides and competition between A. philoxeroides and two non-invasive plants. In the experiment, A. philoxeroides was grown in monoculture or in mixture with native plant Oenanthe javanica and non-invasive alien plant Iris pseudacorus at three N:P ratios (1,10and100) combined with three levels of total nutrient availability (low, medium, high).The experiment that A. philoxeroides was grown in monoculture indicated that both nutrient level and N:P ratio had significant effects on the biomass accumulation, morphological characteristics, photosynthetic fluorescence, leaf nitrogen and phosphorus concentration of A. philoxeroides. High nutrient level significantly increased the biomass accumulation, branch number, tiller number and the stolon length of A. philoxeroides. N:P ratio had significant influence on the aboveground part of A. philoxeroides, such as branch number, tiller number, length of maximum node and the stolon length, but had no significant effect on the biomass allocation. The SLA (Specific leaf area) of A. philoxeroides decreased significantly with the nutrient availability increasing, having lowest SLA under high nutrient level, and meanwhile under the same nutrient level, the SLA of A. philoxeroides increases with the N:P ratio decreasing, having highest SLA under N:P ratio1.The experiment that A. philoxeroides was grown in mixture with O.javanica and I. pseudacorus showed that A. philoxeroides had higher plasticity than other two non-invasive species, having higher adaptability to the different nutrient conditions. Invasive species A. philoxeroides had stronger response to the change of nutrient level and N:P ratio than other two species, promoting its the growth and competitive ability. And the SLA of A. philoxeroides had higher plasticity than that of O. javanica under different nutrient conditions. Higher leaf nitrogen and phosphorus concentration of A. philoxeroides indicated that species A. philoxeroides had higher ability to require nutrient, promoting its growth and invasion.When grown in mixture with two studied non-invasive plants, A. philoxeroides had absolute competitive advantage. High nutrient level could increase the competitive advantage of A. philoxeroides and low nutrient level could to some extent decrease its invasion. The effect of N:P ratio on the competitive advantage of A. philoxeroides changed with the nutrient level. Under medium nutrient level, phosphorus played an important role in the invasion of A. philoxeroides, while under low and high nutrient level, phosphorus had no effect on the competitive advantage of A. philoxeroides.In conclusion, both nutrient level and N:P ratio had significant effects on growth of A. philoxeroides and on the competition relationship between A. philoxeroides and two non-invasive plants. A. philoxeroides has more phenotypic plasticity than the other two non-invasive plants and adapts to high nutrient environment. These results would be of great importance for revealing the invasive mechanism of A. philoxeroides and predicting the effect of environment changing on its invasion. Meanwhile, the results could provide theoretical foundation for the scientific management of A. philoxeroides.