The Co-Invasion of the Three Asteraceae Invasive Plants Can Synergistically Increase Soil Phenol Oxidase Activity

The co-invasion of two or even more invasive plants in the same habitat can occur through invasion meltdown. Nevertheless, there is little progress in the ecological effects of the co-invasion of multiple invasive plants on soil enzyme activities. This study aimed to evaluate the effects of the co-invasion of three Asteraceae invasive plants (Conyza canadensis (L.) Cronq., C. sumatrensis (Retz.) Walker, and Solidago canadensis L.) on soil enzyme activities. In particular, part of the invasion status of the three Asteraceae invasive plants significantly increased soil urease and peroxidase activities, but significantly decreased soil phenol oxidase and protease activities compared with uninvaded plant communities. Hence, the invasion of the three Asteraceae invasive plants can enhance the urea hydrolysis capacity and the hydrogen peroxide oxidizing capacity, but decline the phenol oxidizing capacity and protein hydrolysis capacity. The species number of invasive plants was positively associated with soil phenol oxidase activity in the invaded communities. The contribution intensity of soil phenol oxidase activity to the species number of invasive plants was also obviously greater than that of the activities of soil urease, protease, and peroxidase. Consequently, the species number of invasive plants is a vital issue significantly affecting soil phenol oxidase activity in the invaded communities. Thus, the co-invasion of the three Asteraceae invasive plants can synergistically increase soil phenol oxidase activity in the invaded communities to a certain degree.