Most allelopathic studies have focused only on the allelopathy of invasive plants on the growth performance of native plants, while neglecting the allelopathy of native plants on the growth performance of invasive plants, especially species that can co-invade the same ecosystem. This study aimed to evaluate the allelopathy of the native plant Pinus massoniana Lamb. on the seed germination and seedling growth of two invasive Amaranthaceae species, Celosia argentea L. and Amaranthus retroflexus L. using a germination bioassay. Pinus massoniana showed significant allelopathy on the seed germination and seedling growth of the two invasive Amaranthaceae species. The allelopathy of P. massoniana on the seed germination and seedling growth of the two invasive Amaranthaceae species may be attributed to the effects of the generated stress on their germination activity. Pinus massoniana exhibited stronger allelopathy on the seed germination and seedling growth of A. retroflexus than on that of C. argentea. Thus, the invasiveness of A. retroflexus is expected to be lower than that of C. argentea in contact with P. massoniana. The allelopathy of P. massoniana on the seed germination and seedling growth of the two invasive Amaranthaceae species under the co-cultivated conditions was stronger than that under the monocultural conditions. Accordingly, the aqueous extract of P. massoniana can be considered a potential bioherbicide for the control of the two invasive Amaranthaceae species, particularly when both species co-occuring.
Soil N-fixing bacterial (NFB) community may facilitate the successful establishment and invasion of exotic non-nitrogen (N) fixing plants. Invasive plants can negatively affect the NFB community by releasing N during litter decomposition, especially where N input from atmospheric N deposition is high. This study aimed to quantitatively compare the effects of the invasive Rhus typhina L. and native Koelreuteria paniculata Laxm. trees on the litter mass loss, soil physicochemical properties, soil enzyme activities, and the NFB. Following N supplementation at 5 g N m−2 yr−1 in four forms (including ammonium, nitrate, urea, and mixed N with an equal mixture of the three individual N forms), a litterbag-experiment was conducted indoors to simulate the litter decomposition of the two trees. After four months of decomposition, the litter cumulative mass losses of R. typhina under the control, ammonium chloride, potassium nitrate, urea, and mixed N were 57.93%, 57.38%, 58.69%, 63.66%, and 57.57%, respectively. The litter cumulative mass losses of K. paniculata under the control, ammonium chloride, potassium nitrate, urea, and mixed N were 54.98%, 57.99%, 48.14%, 49.02%, and 56.83%, respectively. The litter cumulative mass losses of equally mixed litter from both trees under the control, ammonium chloride, potassium nitrate, urea, and mixed N were 42.95%, 42.29%, 50.42%, 46.18%, and 43.71%, respectively. There were antagonistic responses to the co-decomposition of the two trees. The litter mass loss of the two trees was mainly associated with the taxonomic richness of NFB. The form of N was not significantly associated with the litter mass loss in either species, the mixing effect intensity of the litter co-decomposition of the two species, and NFB alpha diversity. Litter mass loss of R. typhina was significantly higher than that of K. paniculata under urea. The litter mass loss of the two trees under the control and N in four forms mainly affected the relative abundance of numerous NFB taxa, rather than NFB alpha diversity.
This study aimed to clarify the differences in the decomposition rates, soil carbon and nitrogen contents, soil enzyme activities, and the structure of the soil bacterial community between the four Asteraceae invasive plants (AIPs), Bidens pilosa L., Conyza canadensis (L.) Cronq., Solidago canadensis L., and Symphyotrichum subulatum (Michx.) G.L. Nesom, and the native plant Pterocypsela laciniata (Houtt.) Shih under the artificially modeled nitrogen with four forms (including nitrate, ammonium, urea, and the mixed nitrogen forms with an equal mixture of three individual nitrogen forms). The mixed nitrogen forms significantly increased the decomposition rate of the four AIPs and P. laciniata. The positive effects of the mixed nitrogen forms on the decomposition rate of the four AIPs and P. laciniata were obviously greater than those of individual nitrogen forms. Nitrogen with four forms visibly up- or down-regulated the dominant role of predominant soil bacterial biomarkers, and significantly increased the species number, richness, and phylogenetic diversity of the soil bacterial community, as well as the number of most of the functional gene pathways of the soil bacterial communities involved in the decomposition process. The decomposition rate of the four AIPs was similar to that of P. laciniata. The leaves of C. canadensis decomposed more easily than those of S. subulatum. The decomposition process of the four AIPs caused remarkable changes in the relative abundance of several taxa of the soil bacterial community and soil bacterial beta diversity, and caused apparent up- or down-regulation in the dominant role of predominant soil bacterial biomarkers and the number of several functional gene pathways of the soil bacterial communities involved in the decomposition process.
The functional differences between invasive plants and coexisting native plants can affect the invasion process of the former because invasive plants and coexisting native plants are exposed to similar or even identical environmental pressures. Acid deposition is an important component of atmospheric pollution, and acid deposition with different sulfur–nitrogen ratios may affect the invasion process of invasive plants by shifting the functional differences and differences in the growth performance between the invasive and coexisting native plants. It is crucial to analyze the functional indices and growth performance of these plants when exposed to acid deposition with different chemical compositions to assess the ecological impacts of atmospheric pollution on the growth performance of invasive plants. This study aimed to evaluate the functional differences and growth performance between the invasive plant Amaranthus spinosus L. and the native plant A. tricolor L. in mono- and mixed culture when exposed to an acid deposition with different sulfur–nitrogen ratios, including sulfur-rich acid deposition (sulfur–nitrogen ratio = 5:1), nitrogen-rich acid deposition (sulfur–nitrogen ratio = 1:5), and mixed acid deposition (sulfur–nitrogen ratio = 1:1). The acidity of the three types of simulated acid deposition was set at pH = 5.6 and pH = 4.5, respectively, with distilled water as a control (pH = 7.0). The competition experiment between A. spinosus and A. tricolor was conducted in the greenhouse. Amaranthus spinosus exhibited a strong growth performance over A. tricolor in the mixed culture, mainly via the increased leaf photosynthetic capacity. The competitiveness for light acquisition, leaf photosynthetic capacity, and enzymatic defense capacity under stress of A. spinosus may be vital to its growth performance. The lower pH acid deposition had imposed a greater reduction in the growth performance of both Amaranthus species than the higher pH acid deposition. Sulfur-rich acid deposition was more toxic to the growth performance of both Amaranthus species than nitrogen-rich acid deposition. Amaranthus spinosus was more competitive than A. tricolor, especially when exposed to acid deposition, compared with just distilled water. Thus, acid deposition, regardless of the sulfur–nitrogen ratio, may facilitate the invasion process of A. spinosus via the stronger growth performance.
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