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−1) and at season-long ambient (380 ppm) and
elevated (670 ppm) CO2 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.
CO2-related yield increase amounted to 16–42% for
white clover whereas the effect of high CO2 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
CO2 enrichment at many harvests in both N supply
treatments. Without additional N supply, shoot competition for light
was intensified by CO2 enrichment to the disadvantage of
ryegrass, since clover petioles grew longer and ryegrass was shorter
at elevated CO2. With N fertilization, no marked effect of
CO2 enrichment on interspecific competition could be
observed. Since clover and total yield were increased by
CO2 enrichment, nutrient requirements were also increased
and potassium deficiency and increased intraspecific competition of
clover for K was observed in the mixtures under elevated
CO2 which had the highest nutrient withdrawal. Although
white clover profited much more from CO2 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
CO2 enrichment and it is suggested that the effect of
elevated CO2 on the balance of species and the outcome of
competition in a grass/clover sward is mainly dependent on the N
status.