Arbuscular mycorrhizal symbiosis and dark septate endophytes under contrasting grazing modes in the Magellanic steppe of Tierra del Fuego

The impact of the mode and intensity of sheep grazing on the co-occurrence of AM fungi and DSE in Poa rigidifolia and Deschampsia flexuosa roots and their associations with N and P in plant tissue was studied in a range of degraded grasslands from Tierra del Fuego (Argentina). Under different grazing modes, AM colonization was always higher in Deschampsia (36.8%) than in Poa (24.5%), whereas DSE colonization was similar in both species (28% in Deschampsia and 27% in Poa). High DSE colonization was associated with soils of low fertility and subjected to grazing associated with an intense grassland disturbance. In contrast, AM colonization was associated with more fertile soils and low grassland disturbance. An opposite response of AM and DSE colonization was observed in low or absence of grazing. Nutrients in the plant tissue were always higher in Deschampsia than in Poa. Nitrogen concentration in both plants was positively correlated with AM and negatively with DSE colonization, whereas P concentration showed no correlation with AM or DSE. The highest AM colonization indexes and nutrients in tissue may explain why Deschampsia is much better adapted to the acid, xeric conditions and low fertility of the soils of these grasslands than Poa.