EFFECTS OF NUTRIENT ENRICHMENT IN DUTCH CHALK GRASSLAND

(1) The sharp increase in dominance of Brachypodium pinnatum in Dutch chalk grasslands during the past decade was hypothesized to have resulted from increased atmospheric nitrogen inputs. To test this, N, P, K were applied at 10, 3 and 10 g m-2 year-1, respectively, in a 3-year experiment at two Dutch chalk grassland sites, plus an NPK addition. (2) After 3 years, peak standing crop increased significantly with added N and NPK. Phytomass of legumes increased after P addition and B. pinnatum mass more than doubled with added N. NPK addition also sharply increased the dry weight of other graminoids. (3) Total below-ground phytomass increased only with added NPK. Addition of N increased the below-ground dry wt of B. pinnatum, but decreased that of other species. (4) B. pinnatum acquired N and P efficiently, and produced more phytomass per unit time after N application even without enhanced P uptake, while its shoot N:P ratio increased to > 30. (5) The vertical structure of the vegetation was affected by N and NPK additions, and light penetration in the canopy was considerably reduced, decreasing species density and diversity. (6) It is concluded that B. pinnatim benefits more from atmospheric N inputs than other chalk grassland species. Its ability to cope with decreasing concentrations of P caused by N-induced stimulated growth largely explains its success with long-term N enrichment.