The response of perennial ryegrass/white clover swards to elevated atmospheric CO concentrations 1. Effects on competition and species composition and interaction with N supply.

The effects of long-term carbon dioxide enrichment on competition for nutrients and light in a ryegrass/clover association were determined for simulated swards of perennial ryegrass (Lolium perenne L. cv. Parcour) and white clover (Trifolium repens L. cv. Karina), which were grown as monocultures and in three mixtures (25/75, 50/50, 75/25), according to the replacement design, at two levels of nitrogen (N) supply (no additional N and 200 kg N ha ) and at season-long ambient (380 ppm) and elevated (670 ppm) CO concentrations, in open-top chambers. Stands were cut four times, at about monthly intervals, to a height of 5 cm. Plant material was separated into different species, fresh and dry weights were determined and the content of macroelements (N, P, K, S, Mg) in both species was measured. In addition, plant height of both species at harvest dates and during several regrowth periods was monitored. Results indicate that both species made demand on different resources and profited from growth in a mixed sward. Co related yield increase amounted to 16-4-2 % for white clover whereas the effect of high CO on ryegrass yield ranged between -33% and +9% depending on N supply, mixture and year. As a result the contribution of white clover to total yield in mixed swards was significantly enhanced by CO enrichment at many harvests in both N supply treatments. Without additional N supply, shoot competition for light was intensified by CO enrichment to the disadvantage of ryegrass, since clover petioles grew longer and ryegrass was shorter at elevated CO With N fertilization, no marked effect of CO enrichment on interspecific competition could be observed. Since clover and total yield were increased by CO enrichment, nutrient requirements were also increased and potassium deficiency and increased intraspecific competition of clover for K was observed in the mixtures under elevated CO which had the highest nutrient withdrawal. Although white clover profited much more from CO enrichment in both N fertilization treatments, the suppression of ryegrass in mixed swards could only be observed under low N conditions. Generally, the effect of N fertilization on competitive interference between both species was much greater than the effect of CO enrichment and it is suggested that the effect of elevated CO on the balance of species and the outcome of competition in a grass/clover sward is mainly dependent on the N status.