Phosphorus addition reduces the competitive ability of the invasive weed Solidago canadensis under high nitrogen conditions

Changes in resource supply ratios affect interspecific competition; however, the effects on the plant invasions are still relatively unknown. To determine the effects of phosphorus (P) availability on the competitive ability of the invasive herb Solidago canadensis in an elevated P environment, we conducted a greenhouse P addition experiment. The invasive herb S. canadensis and the native plant Pterocypsela laciniata were grown in both monoculture and mixture with P addition under an ambient N condition (P+N-) and under a high N condition (P+N+), using ambient N and P conditions (P-N-) as a control. The growth of P. laciniata was significantly inhibited by S. canadensis under the ambient conditions. The P+N- treatment promoted the growth of P. laciniata, but did not promote the growth of S. canadensis. The competition between S. canadensis and P. laciniata was alleviated under the ambient N condition after P addition. The N requirement of P. laciniata was higher than that of S. canadensis, and P. laciniata outcompeted S. canadensis under the P+N+ treatment. We concluded that P addition may reduce the competitive ability of the invasive weed S. canadensis, especially under a high N condition. The ratio of N and P might be a critical factor for determining the competitive abilities of the invasive herb S. canadensis and the native plant P. laciniata.