Arbuscular mycorrhizas (AM) are particularly relevant in grasslands due to the high colonization in grasses, main constituent of this ecosystem. Natural grassland (NG) is the dominant ecosystem of Uruguay and it supports one of the main economic activities of the country: livestock. Available phosphorus (P) contents in NG soils of Uruguay are low, so phosphate fertilization is frequent. The aim of this work was to study the effect of phosphorus fertilization on arbuscular mycorrhizal fungi (AMF) diversity of two native grasses of Uruguay:Paspalum dilatatum and Coelorhachis selloana. Diversity and abundance of AMF spores in the rhizosphere of grasses were studied using morphological techniques, while AMF diversity in roots was studied through T-RFLP. The study was conducted in a long-term experiment of phosphorus fertilization in Uruguayan grasslands.The increase in available P did not affect the diversity of AMF in the roots or in the rhizosphere of the studied grasses. The richness of AMF biotypes in roots and spores abundance differed between host species, with higher values in C. selloana than in P. dilatatum. Differences in AMF diversity between seasons were observed, with greater number of biotypes in winter than in summer.

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Cynodon dactylon is one of the five most important invasive alien species worldwide. It is the invasive alien species with the broadest distribution range in Uruguay, and its expansion is frequently associated with disturbances. Since natural grasslands are facing processes of productive intensification, C. dactylon represents a threat as it could displace native species. However, the mechanisms that explain its invasion success remain unclear. The objective of this study was to analyse interspecific interactions under low nutrient conditions between C. dactylon and two species that are native to the Campo grasslands in Uruguay. Specifically, we assessed differences in the components of competitive ability effects and responses (or tolerance) as possible mechanisms involved in C. dactylon invasiveness. We performed a greenhouse experiment in pots with low‐nutrient substrate assessing pair‐wise interactions between C. dactylon, Mnesithea selloana and Paspalum notatum plus control pots consisting of single individual of each species. The invasive species showed greater competitive ability than both native grasses, as it reduced their below and above‐biomass. Conversely, the size of C. dactylon plants interacting with native species was similar to that of single C. dactylon plants growing alone (controls). This reveals that the greater competitive ability of the invasive species was due to a greater tolerance to grow with neighbouring plants. The reason underlying this tolerance was a marked increase in biomass allocation towards stolons and leaves, at the expense of roots. Conversely, native species barely changed their shoot‐root allocation pattern when interacting with neighbours. Furthermore, C. dactylon induced reproductive development solely when interacting with neighbours. Along with the fact that the potential growth rate of the invasive and native species was quite similar, these results suggest that sensitive and rapidly triggered shade avoidance responses could be one mechanism involved in the invasion success of C. dactylon.