Phosphate fertilization and liming in a trial conducted over 21 years: A survey for greater forage production and Pampa pasture conservation

International audience; Phosphorus fertilization and liming are alternatives used to increase forage production in Southern Brazil grasslands (Pampa grasslands) and to avoid its replacement by cultivated exotic crops. However, changes in natural soil fertility can significantly affect the equilibrium of natural vegetation communities. We hypothesized that triple and simple superphosphate and limestone using would lead to greater increase in dry matter production and to changes in the botanical composition and richness of Pampa grassland plant species than the use of lesser soluble P sources (phosphate rock) or no liming treatment. To test our hypotheses, we assessed a 21-year-old field trial to identify its forage production pattern, based on P sources and liming application. The vegetation was studied in 2009 and 2018, based on the following variables: Shannon and Pielou indices, species richness, dry matter and plant groups (species grouped based on growth form, life period, photosynthetic pathway, morphology and growing season). Soil chemical properties in the 0-10 cm layer were also evaluated in 2018. According to the long-term evaluation, two high dry matter yield periods were observed after P fertilization (55 % and 25 % higher dry matter yield, on average). Each of these periods was followed by decreased dry matter yield, regardless of the P source. Species richness did not show changes overtime, although there was high turnover of plant species. Soluble fertilizer usage resulted in increased dry matter production and in the replacement of large proportions of species, mainly in tussock grasses and forbs groups. The contribution of legume species for dry matter production increased due to the combined effect of phosphate fertilizers and limestone. However, maintaining forage production levels higher than without phosphorus fertilization requires regular supply of soluble phosphorus fertilizers, which, in the present case, cannot exceed three years.