EFFECTS OF PHOSPHORUS AND NITROGEN NUTRIT ION STATUS AND OF BOTANICALCOMPOSITION OF PERMANENT PASTURES ON THEIR GROWTH IN SPRING

The objective of this study was to assess the effect of the botanical composition (BC), the nitrogen (IN) and phosphorus (IF) nutrition stat us on above-ground dry matter accumulation (DM) of permanent pastures in spring; these three factors are not always linked, particularly in relation to the cutting regime. In the central Pyrenees, one plant com munity dominated by grasses and used as reference and 17 other plant c ommunities well differentiated by their P and N nutrient status and th eir botanical composition were studied. We observed that the above-gro und dry matter accumulation rate decreased by over 50% when the IN dro pped from 100 (optimal N nutrition) to 60 (low N available), by nearly 30% when species characteristic of nutrient-rich habitats were replac ed with species characteristic of nutrient-poor habitats, and by about 10% when the IP dropped from 80 (optimal P nutrition) to 40 (very low P available). We discuss this data with reference to the literature. Then we show how the above-ground dry-matter changed over the years wi th the application of fertilizer to a meadow dominated by species that are characteristic oi nutrient-poor habitats. At the beginning the ab ove-ground dry-matter increases with no change in botanical compositio n. Then, after a few years, it keeps increasing as the original specie s are replaced by more productive ones.