Recent research indicates that the soil microbial community, particularly arbuscular mycorrhizal fungi (AMF), can influence plant invasion in several ways. We tested if 1) invasive species are colonised by AMF to a lower degree than resident native species, and 2) AMF colonisation of native plants is lower in a community inhabited by an invasive species than in an uninvaded resident community. The two tests were run in semiarid temperate grasslands on grass (Poaceae) species, and the frequency and intensity of mycorrhizal colonisation, and the proportion of arbuscules and vesicles in plant roots have been measured. In the first test, grasses representing three classes of invasiveness were included: invasive species, resident species becoming abundant upon disturbance, and non-invasive native species. Each class contained one C3 and one C4 species. The AMF colonisation of the invasive Calamagrostis epigejos and Cynodon dactylon was consistently lower than that of the non-invasive native Chrysopogon gryllus and Bromus inermis, and contained fewer arbuscules than the post-disturbance dominant resident grasses Bothriochloa ischaemum and Brachypodium pinnatum. The C3 and C4 grasses behaved alike despite their displaced phenologies in these habitats. The second test compared AMF colonisation for sand grassland dominant grasses Festuca vaginata and Stipa borysthenica in stands invaded by either C. epigejos or C. dactylon, and in the uninvaded natural community. Resident grasses showed lower degree of AMF colonisation in the invaded stand compared to the uninvaded natural community with F. vaginata responding so to both invaders, while S. borysthenica responding to C. dactylon only. These results indicate that invasive grasses supposedly less reliant on AMF symbionts have the capacity of altering the soil mycorrhizal community in such a way that resident native species can establish a considerably reduced extent of the beneficial AMF associations, hence their growth, reproduction and ultimately abundance may decline. Accumulating evidence suggests that such indirect influences of invasive alien plants on resident native species mediated by AMF or other members of the soil biota is 1 probably more the rule than the exception.Keywords: arbuscular mycorrhizal fungi (AMF), Calamagrostis epigejos, Cynodon dactylon, grasses, invasive plants, semiarid temperate grassland.Nomenclature: Tutin et al. (1964Tutin et al. ( -1993.Abbreviations: AMF -Arbuscular Mycorrhizal Fungi; a% -percentage arbuscule occurence of the AMF colonised root section; A% -percentage arbuscule occurence of the whole root; F% -frequency of root segments colonised by AMF; M% -intensity of mycorrhizal colonisation; v% -percentage vesicle occurence of the AMF colonised root section; V% -percentage vesicle occurence of the whole root;

Abstract. Symbiotic associations with the soil microbiota, particularly with arbuscular mycorrhizal fungi (AMF), might ameliorate the effects of environmental stress on plants, and this capacity may be different for resident and alien species. In a growth room pot experiment we tested if imposed water deficit leads to greater growth reduction in the absence of AMF than in the presence of AMF for two non-invasive resident (Danthonia alpina, Chrysopogon gryllus) and two invasive (Calamagrostis epigejos, Cynodon dactylon) grass species from semiarid temperate grasslands in Hungary. Both deficit watering and soil sterilization decreased biomass accumulation, but the non-invasive Danthonia and Chrysopogon performed better when grown in intact soil containing AMF than in sterilized soil. In contrast, the invasive Calamagrostis and Cynodon displayed mostly no difference in growth and biomass accumulation between intact and sterilized soil when subjected to water deficit. When plants were grown well-watered but deprived of AMF symbionts, both Danthonia and Chrysopogon achieved poorer growth than in AMF containing soil, while neither Calamagrostis nor Cynodon displayed any reduction. These results indicate that the influence of deficit watering was ameliorated by the presence of AMF in the soil for the resident non-invasive species, while not for the invasive species.