Allelopathy is often defined as any direct or indirect, positive or negative effect by one plant species on another through the production and release of chemical compounds into the environment [1]. Plants release allelochemicals into the surrounding atmosphere and soil through volatilization, foliar leaching, root exudation, and residue decomposition [2]. As one group of important allelochemicals, volatile components enter the air and soil in similar ways: plants release volatiles into the atmosphere directly [3]; rainwater rinses these components (such as monoterpenes) out of the leaf secretory structures and surface waxes, providing the potential for volatile components into the soil [4]; plant roots could emit herbivore-induced and pathogen-induced volatiles into the soil [5]; these components in the plant litter are also released into the surrounding soil [6]. At present, volatile oils have been increasingly explored for their use in weed and pest management [7,8,9,10,11]. They are found to act by spreading in their gaseous state in the air and by transformation into other states into or onto the soil [3,12], playing an important role in inhibiting plant growth by interspecies interactions and altering the crop–weed plant community [13]. Several studies suggest that allelopathy may facilitate the establishment of dominance of plant species in natural ecosystems [14,15,16]. Therefore, dominant plant species can be targeted as potential sources of allelochemicals.
In recent years, allelopathic effects and allelochemicals have gradually received more and more attention from researchers for the purpose of identifying appropriate substitutes for synthetic herbicides [17,18,19,20]. In order to reduce agricultural losses, herbicides are increasingly used to control the growth of weeds. However, the indiscriminate application of synthetic herbicides has contributed to increased problems of weed resistance, the gradual degradation of the soil, and hazards to human health [21]. Natural allelopathic compounds from plants can offer considerable potential for the development of new herbicides, or as lead compounds toward identifying new, nature-derived herbicides [17,22].
Amomum villosum Lour. is a perennial herbaceous plant in the ginger family, growing to a height of 1.2–3.0 m in the shade of trees. It is widely distributed in South China, Thailand, Vietnam, Laos, Cambodia, and other Southeast Asian regions. The dry fruit of A. villosum is a kind of common spice because of its attractive flavor [23] and it represents a well-known traditional herbal medicine in China, which is widely used to treat gastrointestinal diseases. Several studies have reported that the volatile oils rich in A. villosum are the main medicinal components and aromatic ingredients [24,25,26,27]. Researchers found that essential oils of A. villosum exhibit contact toxicity against the insects Tribolium castaneum (Herbst) and Lasioderma serricorne (Fabricius), and strong fumigant toxicity against T. castaneum [28]. At the same time, A. villosum has a detrimental impact on the plant diversity, biomass, litterfall and soil nutrients of primary rainforests [29]. However, the ecological role of volatile oil and the allelopathic compounds are still unknown. In the light of previous studies into the chemical constituents of A. villosum essential oils [30,31,32], our objective is to investigate whether A. villosum releases compounds with allelopathic effects into the air and soil to help establish its dominance. Therefore, we plan to: (i) analyze and compare the chemical components of volatile oils from different organs of A. villosum; (ii) evaluate the allelopathy of volatile oils extracted and volatile compounds from A. villosum, and then identify the chemicals that had allelopathic effects on Lactuca sativa L. and Lolium perenne L.; and (iii) preliminarily explore the effects of oils from A. villosum on the diversity and community structure of microorganisms in the soil.
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