GROWTH-RESPONSE OF TRIFOLIUM-REPENS L AND LOLIUM-PERENNE L AS MONOCULTURES AND BI-SPECIES MIXTURE TO FREE-AIR CO2 ENRICHMENT AND MANAGEMENT

Trifolium repens L. and Lolium perenne L. were grown in monocultures a nd bi-species mixture in a Free Air Carbon Dioxide Enrichment (FACE) e xperiment at elevated (60 Pa) and ambient (35 Pa) CO2 partial pressure (pCO(2)) for three years. The effects of defoliation frequencies (4 a nd 7 cuts in 1993; 4 and 8 cuts in 1994/95) and nitrogen fertilization (10 and 42 g m(-2) y(-1) N in 1993; 14 and 56 g m(-2) y(-1) in 1994/9 5) on the growth response to pCO(2) were investigated. There were sign ificant interspecific differences in the CO2 responses during the firs t two years, while in the third growing season, these interspecific di fferences disappeared. Yield of T. repens in monocultures increased in the first two years by 20% when grown at elevated pCO(2). This CO2 re sponse was independent of defoliation frequency and nitrogen fertiliza tion. In the third year, the CO2 response of T. repens declined to 11% . In contrast, yield of L. perenne monocultures increased by only 7% o n average over three years at elevated pCO(2). The yield response of L . perenne to CO2 changed according to defoliation frequency and nitrog en fertilization, mainly in the second and third year. The ratio of ro ot/yield of L. perenne increased under elevated pCO(2), low N fertiliz er rate, and frequent defoliation, but it remained unchanged in T. rep ens. We suggest that the more abundant root growth of L. perenne was r elated to increased N limitation under elevated pCO(2). The consequenc e of these interspecific differences in the CO2 response was a higher proportion of T. repens in the mixed swards at elevated pCO(2). This w as evident in all combinations of defoliation and nitrogen treatments. However, the proportion of the species was more strongly affected by N fertilization and defoliation frequency than by elevated pCO(2). Bas ed on these results, we conclude that the species proportion in manage d grassland may change as the CO2 concentration increases. However, an adapted management could, at least partially, counteract such CO2 ind uced changes in the proportion of the species. Since the availability of mineral N in the soil may be important for the species' responses t o elevated pCO(2), more long-term studies, particularly of processes i n the soil, are required to predict the entire ecosystem response.