Abstract:

Uruguay sits in the Rio de la Plata Grasslands, one of the biggest grassland biomes in the world, with a pronounced biodiversity. Nowadays, intensification of land use has led to the degradation of this ecosystem. A common practice for enhancing productivity of natural grasslands in the region is the overseeding of an exotic legume, alongside phosphorus fertilization. The objective of this study is to measure the impact of a phosphorous fertilization gradient on total plant species richness, the presence of exotic species and the net primary productivity. With the data obtained, we explored possible mechanisms that explain the changes in diversity observed. Two experimental sites were used: PA (in the geomorphological region ‘Sierras del Este’), and GL (in the geomorphological region ‘Cuesta Basáltica’). Eight plots were established, with different amounts of phosphorus fertilization, with overseeding of Lotus angustissimus. A vegetation sampling of each treatment was done on the third year and a half after the establishment of the experiment. Total and exotic species richness was evaluated, alongside the effect of each treatment on the total and the L. angustissimus net primary productivity of the first three years of the experiment. We found that total species richness decreased with increasing amounts of phosphorus fertilization, while exotic species richness increased with increasing amounts of phosphorus fertilization. The total and the L. angustissimus net primary productivity increased with fertilization for PA, but didn’t for GL. This suggests that, in GL, mechanisms regarding the ‘niche dimension’ and the ‘total competition’ hypothesis are taking place. In PA, it isn’t possible to distinguish which mechanisms are the predominant ones behind the species loss observed. We conclude that more studies need to be done on this technology, in search of fertilization thresholds that allow for greater productivity without putting biodiversity in jeopardy.