Mycorrhizae in Native Grasses: Long-term Effects of Phosphorus Fertilization

Natural grasslands (NG) are the major Uruguayan ecosystem and are relevant for extensive livestock production. They consist mainly of grasses with high mycorrhizal colonization. NG soils have low phosphorus (P) content, so fertilization practices are carried out. The impact of this practice on the vegetal community differs between species. The aim of this work was to study the effect of P on mycorrhizal colonization in two native grasses with differential responses to the addition of P: Paspalum dilatatum and Coelorhachis selloana. It was suggested that the two species would present different mycorrhizal response to P, which could partly explain its persistence in fertilized soil. We used a long-term experiment from INIA Palo a Pique, with three treatments: NG without fertilization, and with high and medium levels of P fertilization. We evaluated mycorrhizal colonization (total and by fungal structures) in P. dilatatum (which maintained its frequency in plots fertilized with P) and in C. selloana (which reduced its frequency). P fertilization negatively affected mycorrhizal interaction with both grasses, and this effect was dependent on the season. Since these results were independent of the response of the species in the plant community, we postulated that there would be other factors involved in determining the performance of these grasses in situations of increasing P. In environments with higher content of P there would be a phytocentric control that would limit the mycorrhization considering the energetic cost that this interaction has for plants.