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The allelopathy of alien plants is critical to their successful invasion. In nature, two alien plants can co-invade the same habitat. Changes in the forms of nitrogen may have the potential to alter the invasion process of alien plants by causing alterations in their allelopathy. This study aimed to evaluate the individual and combined allelopathy of two alien plants from the Asteraceae family, Solidago canadensis L. and Conyza canadensis (L.) Cronq. This study examined the effects of aqueous leaf extracts of the two alien plants with different nitrogen forms (NO3−N, NH4−N, and CO(NH2)2−N, and the mixed nitrogen forms at a 1:1:1 ratio) on the seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. using a germination bioassay. The allelopathy of the two alien plants significantly reduced the seed germination and seedling growth of L. sativa. Extracts from S. canadensis produced stronger allelopathy on the seed germination and seedling growth of L. sativa compared with those from C. canadensis. The mixture of extracts from the two alien plants produced an antagonistic effect when compared with the effects of extracts from each plant species. The addition of nitrogen intensified the allelopathy of the two alien plants on the seed germination and seedling growth of L. sativa. The degree of influence of nitrogen on the individual and combined allelopathy of the two alien plants was similar. Thus, nitrogen deposition may facilitate the independent invasion and co-invasion of the two alien plants via intensified allelopathy.

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Invasive and native plants can coexist in the same habitat; however, the decomposition process may be altered by the mixing of invasive and native leaves. Heavy metal contamination may further alter the co-decomposition of both leaf types. This study evaluated the effects of two concentrations (35 mg·L−1 and 70 mg·L−1) and three types (Pb, Cu, and combined Pb + Cu) of heavy metal contamination on the co-decomposition of leaves of the invasive tree Rhus typhina L. and the native tree Koelreuteria paniculata Laxm, as well as the mixed effect intensity of the co-decomposition of the mixed leaves. A polyethylene litterbag experiment was performed over six months. The decomposition coefficient of the two trees, mixed effect intensity of the co-decomposition, soil pH and enzymatic activities, soil bacterial alpha diversity, and soil bacterial community structure were determined. A high concentration of Pb and combined Pb + Cu significantly reduced the decomposition rate of R. typhina leaves. A high concentration of Pb or Cu significantly reduced the decomposition rate of the mixed leaves. In general, R. typhina leaves decomposed faster than K. paniculata leaves did. There were synergistic effects observed for the co-decomposition of the mixed leaves treated with combined Pb + Cu, regardless of concentration, but there were antagonistic effects observed for the co-decomposition of the mixed leaves treated with either Pb or Cu, regardless of concentration. A high concentration of Pb or Cu may increase antagonistic effects regarding the co-decomposition of mixed-leaf groups. Thus, heavy metal contamination can significantly affect the intensity of the mixed effect on the co-decomposition of heterogeneous groups of leaves.

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