Species Number of Invasive Plants Negatively Regulates Carbon Contents, Enzyme Activities, and Bacterial Alpha Diversity in Soil

The leaves of multiple invasive plants can coexist and intermingle within the same environment. As species number of invasive plants increases, variations may occur in decomposition processes of invasive plants, soil nutrient contents, soil enzyme activities, and soil microbial community structure. Existing progress have predominantly focused on the ecological effects of one species of invasive plant compared to native species, with limited attention paid to the ecological effects of multiple invasive plants compared to one species of invasive plant. This study aimed to determine the differences in the effects of mono- and co-decomposition of four Asteraceae invasive plants, horseweed (Erigeron canadensis (L.) Cronq.), Guernsey fleabane (E. sumatrensis Retz.), daisy fleabane (E. annuus (L.) Pers.), and Canada goldenrod (Solidago canadensis L.), on litter decomposition responses, soil carbon contents, soil enzyme activities, and soil bacterial community structure. Species number of invasive plants did not significantly affect on the decomposition rate of mixed leaves or mixed-effect intensity of co-decomposition. Soil pH and electrical conductivity enhanced as species number of invasive plants increased. Soil carbon contents (including soluble organic carbon content and microbial carbon content), soil enzyme (including polyphenol oxidase, FDA hydrolase, and sucrase) activities, soil bacterial alpha diversity (including the OTU species, Chao1 richness, ACE richness, and Phylogenetic diversity indexes), and the number of pathways of most functional genes of soil bacterial communities closely related to decomposition processes declined as species number of invasive plants increased. Hence, soil pH and electrical conductivity significantly increased with increasing species number of invasive plants, but soil carbon contents, soil enzyme activities, soil bacterial alpha diversity, and the number of pathways of most functional genes of soil bacterial communities closely related to decomposition processes significantly reduced with growing species number of invasive plants.