Effects of nitrogen addition on the growth and competition of invasive plant Bidens Pilosa with different invasion proportions

The increasing levels of environmental nitrogen deposition can promote invasive plant species’ growth and competitive advantages, thereby expanding their invasion risk. Understanding the influence of nitrogen on the invasion of invasive plants and its response mechanism can provide an important theoretical basis for risk assessment and control of invasive plants under the background of global nitrogen deposition. Bidens pilosa was selected as the subject of investigation, and three levels of nitrogen 0, 2.5, and 5 g·(m²·a)⁻¹ and four invasion ratios (B.pilosa:native plants = 8:0, 4:4, 2:6, and 1:7) were set to study the growth characteristics and interspecific competition of B. pilosa. The results showed the following. (1) The plant height, specific leaf area, and chlorophyll of B.pilosa were positively correlated with nitrogen (P < 0.05). Furthermore, high nitrogen increased the phenotypic plasticity of B.pilosa morphological characteristics and biomass response to invasion ratio. (2) The morphological traits and biomass of B.pilosa were significantly affected by different invasion ratios (P < 0.05). Moreover, an allometric growth relationship was observed between stem biomass, leaf biomass, and total biomass across different invasion ratios (P < 0.05). (3) When competing with native plants and functional groups rich in species functional groups with similar characteristics, the competitive response based on the total biomass of B.pilosa showed a trend of first decreasing and then increasing at the negative half-axis (RII < 0), and there were significant differences between each nitrogen level and different invasion ratios (P < 0.05). The increase of nitrogen could stimulate B.pilosa to make full use of high phenotypic plasticity, and reduce the selection pressure brought by the new environment, and promote B.pilosa to adjust its resource allocation ratio and optimize the biomass allocation pattern to maximize its absorption and utilization of limited resources and increase its adaptability and invasion to heterogeneous environments. In addition, there was a strong interspecific competition for B.pilosa with the increase of community biodiversity. This weakened the dominance of B.pilosa invasion and promoted the competitive rejection of B.pilosa, especially when competing with indigenous species with similar characteristics. Therefore, global nitrogen deposition will promote the successful invasion of B.pilosa. However, the invasion could be effectively curbed to some extent by improving the diversity of local communities or adding indigenous species with characteristics similar to those of the invasive species.