Allelopathy of alien invasive plants (AIP) on plant germination performance is essential for their successful invasion. However, the allelopathy of AIP may be reformed or even strengthened under acid deposition. AIP in Asteraceae covers the uppermost number of AIP species at the family level presently in China. It is necessary to estimate the allelopathy of multiple Asteraceae AIP under acid deposition to address the mechanism driving their successful invasion, especially under acid deposition. However, research in this area is very restricted presently. This study purposes to estimate the allelopathy of four Asteraceae AIP, i.e., Conyza canadensis L. Cronq., Erigeron annuus (L.) Pers., Aster subulatus Michx., and Bidens pilosa L., on germination performance of the cultivated Asteraceae plant species Lactuca sativa L. which is sensitive to allelochemicals under acid deposition with different levels of acidity. Of the four Asteraceae AIP, C. canadensis, E. annuus, and B. pilosa create noticeable allelopathy on germination performance of L. sativa. The allelopathy of the four Asteraceae AIP decreases in the following order: E. annuus, C. canadensis, B. pilosa, and A. subulatus. Acid deposition with a low level of acidity reduces the allelopathy of C. canadensis, E. annuus, and B. pilosa. Inversely, acid deposition with a high level of acidity elevates the allelopathy of B. pilosa. The progressively growing level of acid deposition with high acidity may facilitate the invasion process of B. pilosa via the improved level of allelopathy.

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Compositae family comprises the largest number of species of invasive alien plants (IAPs) in China. Two IAPs can co-invade the same habitat. Drought can alter the litter decomposition of IAPs and soil enzyme activities. This study aims to estimate the independent and combined effects of two composite IAPs (Bidens pilosa L. and Solidago canadensis L.) on litter decomposition and soil enzyme activities under drought. A polyethylene litterbags-experiment (5 g litters of B. pilosa, 5 g litters of S. canadensis, or 5 g litters of B. pilosa and S. canadensis in an equal proportion per polyethylene litterbag) was performed. The polyethylene litterbags were treated with a gradient of drought, i.e., control, light level of drought, and heavy level of drought. The decomposition coefficient of the two composite IAPs and soil enzyme activities were determined. The co-invasion of the two composite IAPs poses a synergistic effect on urease activity. The mixed litters of two composite IAPs and the litters of B. pilosa have higher decomposition rates than S. canadensis. Urease activity increases in light droughts, but it is decreased in heavy drought. Heavy drought reduces the decomposition rate of the mixed litters of two composite IAPs and the litters of B. pilosa. Drought does not affect the decomposition rate of S. canadensis litters. Thus, heavy drought can decrease the nutrient cycling rate under the co-invasion of the two composite IAPs and the independent invasion of B. pilosa rather than the independent invasion of S. canadensis.

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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