Shoot growth and phosphorus-nitrogen relationship of grassland swards in response to mineral phosphorus fertilization

The sustainable P management of grassland ecosystems require a better understanding of the effect of P fertilization on the growth of grassland swards and the relationship between shoot P and N concentrations with the overall goal of developing improved methods of predicting P requirements of grassland swards. Our objectives were to determine the importance of P supply during the primary growth in spring, to analyze the optimal relationship between shoot P and N concentrations for timothy (Phleum pratense L.) and multi-species swards under a wide range of P nutrition, and conditions of soil and climate, and to confirm for grassland swards the decrease in the N:P ratio during crop growth previously reported for annual crops. Experiments with varying rates of P fertilization were conducted for two to five consecutive years at sites with timothy swards in Canada (Lévis, Normandin, Charlottetown, and Quebec) and Finland (Maaninka), and at sites with multi-species swards from long-term P fertilization experiments in Switzerland (Les Verrières) and France (Ercé). Dry matter (DM) yield, and N and P concentrations were measured on four dates with one-week intervals from the vegetative to late heading stages of development during primary growth. Our results (i) indicate that a P deficiency mostly affects the early season growth of grassland swards corresponding to shoot biomass less than approximatively 1 Mg DM ha−1, (ii) confirm the optimal relationship between shoot P and N concentrations for timothy and multi-species swards but with variations of the relationship with the level of N nutrition and between timothy and multi-species swards, and (iii) confirm for grassland swards the decrease in the N:P ratio during the primary growth and the close relationship of the N:P ratio to shoot biomass.