THE IMPACT OF DIFFERENCES IN NITROGEN-CONTENT, NITROGEN-UTILIZATION AND LOSS FROM LAMINAE ON COMPETITION BETWEEN 4 GRASS SPECIES IN AN OLD PASTURE

The concentration of nitrogen (N) within the emerging, youngest fully expanded and the youngest dead leaf laminae were examined in the grass es Lolium perenne, Agrostis stolonifera, Holcus lanatus and Poa trivia lis in Devon, UK, in 1986 and 1987. Lamina growth, appearance interval and lamina utilization were also examined in each species. Marked til lers were measured in situ over 14-21 day periods in a continuously gr azed permanent pasture under steady state management on plots receivin g either zero (ON) or 400 kg nitrogen (400N) fertilizer/ha per annum. The concentration of N tended to be greatest in the distal half and le ast in the basal part of each lamina in each species. Total mass showe d an opposite trend due in part to the shape of the lamina. Less than 40% of the lamina N was lost through grazing either in the ON or 400N plots in the four species, except once in H. lanatus when more was los t. In absolute terms, because L. perenne and H. lanatus maintained lar ger and longer laminae than either A, stolonifera or P. trivialis, the y lost more N through grazing. The four grass species recycled N from the senescing lamina with the same apparent efficiency; this meant tha t differences in lamina N concentration and carbon:nitrogen ratios wer e present in the dead laminae of the four species. L. perenne achieved the highest tissue growth rate per unit of N in the lamina in the ON plot, not only in comparison with the three other grasses but also com pared with the 400N plot. This high N-use efficiency in L. perenne was not translated into an ability to either expand or maintain its popul ation in the ON plot. L. perenne had a lower leaf appearance rate than the other species in both the 400N and ON plots, but this inherent ch aracteristic of the species was particularly pronounced in the ON plot . A slower leaf appearance rate would limit the potential tillering ca pacity of L. perenne compared with the other species. A reduced tiller ing capacity, exacerbated by N deficiency, was probably the principal factor limiting the ability of L. perenne to exploit available niches in the ON pasture.