Plant species vary in their growth response to arbuscular mycorrhizal (AM) fungi, with responses ranging from negative to positive. Differences in response to AM fungi may affect competition between plant species, influencing their ability to coexist. We hypothesized that positively responding species, whose growth is stimulated by AM fungi, will experience stronger intraspecific competition and weaker interspecific competition in soil containing AM fungi, while neutrally or negatively responding species should experience weaker intraspecific and stronger interspecific competition. We grew Plantago lanceolata, which responds positively to AM fungi, and Bromus inermis, which responds negatively to AM fungi, in an additive response surface competition experiment that varied the total density and relative frequency of each species. Plants were grown in sterilized background soil that had been inoculated with whole soil biota, which includes AM fungi, or a microbial wash, that contained other soil microbes but no AM fungi, or in sterilized soil that contained no biota. The positively responding P. lanceolata was more strongly limited by intraspecific than interspecific competition when AM fungi were present. By contrast, the presence of AM fungi decreased the strength of intraspecific competition experienced by the negatively responding B. inermis. Because AM fungi are almost always present in soil, strong intraspecific competition in positively responding species would prevent them from outcompeting species that respond neutrally or negatively to AM fungi. The potential for increased intraspecific competition to offset growth benefits of AM fungi could, therefore, be a stabilizing mechanism that promotes coexistence among plant species.

中文翻译：

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菌根生长反应的变化影响竞争相互作用和植物物种共存的机制。

植物种类对丛枝菌根（AM）真菌的生长反应各不相同，反应范围从阴性到阳性。对AM真菌的反应差异可能会影响植物物种之间的竞争，从而影响其共存能力。我们假设，由AM真菌刺激生长的正响应物种在含AM真菌的土壤中将经历更强的种内竞争和较弱的种间竞争，而中性或消极响应的物种将经历较弱的种内竞争和更强的种间竞争。在叠加响应表面竞争实验中，我们种植了对AM真菌呈阳性反应的车前草和对AM真菌呈阴性反应的无花布鲁姆，改变了每个物种的总密度和相对频率。将植物种植在已接种完整土壤生物区系的无菌背景土壤中，其中包括AM真菌或含有其他土壤微生物但不含AM真菌的微生物洗涤液，或在不含生物区系的无菌土壤中生长。当存在AM真菌时，阳性反应的lanceolata受到种内竞争比种间竞争的强烈限制。相比之下，AM真菌的存在降低了负反应的无定形芽孢杆菌所经历的种内竞争的强度。由于AM真菌几乎总是存在于土壤中，因此在积极响应的物种中进行强烈的种内竞争将阻止它们胜过对AM真菌具有中性或负面响应的物种。因此，可能会增加种内竞争以抵消AM真菌的生长益处。