Effects of nitrogen fertilization on vegetation structure and dominance of Brachypodium pinnatum (L.) Beauv. in chalk grassland

During the last decade a sharp increase in dominance of Brachypodium pinnatum was observed in chalk grasslands. We hypothesize that the enhanced nitrogen input from the atmosphere caused this increase. Therefore, a N-fertilization experiment (with 120 kg N ha”1 in the first year, and 50 kg N ha ”1 in the second and third year) was carried out in two Dutch chalk grassland sites. Phytomass of Brachypodium increased significantly in the N-fertilized plots, while the dry weight of other graminoids and forbs decreased. Vertical distribution of phytomass was affected by N-fertilization, too: dry weight of Brachypodium increased enormously in the layers 10-40 cm above soil surface. As a consequence, it overtopped most other species and light penetration in the canopy was markedly reduced. Nutrient analyses showed that this increase in Brachypodium was possible without enlarged storage of phosphorus in the vegetation. It is concluded that increased nitrogen availability favours dominance of Brachypodium in chalk grasslands.