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 and bi‐species mixture in a Free Air Carbon Dioxide Enrichment (FACE) experiment at elevated (60 Pa) and ambient (35 Pa) CO 2 partial pressure (pCO 2 ) for three years. The effects of defoliation frequencies (4 and 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 N in 1994/95) on the growth response to pCO 2 were investigated. There were significant interspecific differences in the CO 2 responses during the first two years, while in the third growing season, these interspecific differences disappeared. Yield of T. repens in monocultures increased in the first two years by 20% when grown at elevated pCO 2 . This CO 2 response was independent of defoliation frequency and nitrogen fertilization. In the third year, the CO 2 response of T. repens declined to 11%. In contrast, yield of L. perenne monocultures increased by only 7% on average over three years at elevated pCO 2 . The yield response of L. perenne to CO 2 changed according to defoliation frequency and nitrogen fertilization, mainly in the second and third year. The ratio of root/yield of L. perenne increased under elevated pCO 2 , low N fertilizer rate, and frequent defoliation, but it remained unchanged in T. repens . We suggest that the more abundant root growth of L. perenne was related to increased N limitation under elevated pCO 2 . The consequence of these interspecific differences in the CO 2 response was a higher proportion of T. repens in the mixed swards at elevated pCO 2 . This was 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 . Based on these results, we conclude that the species proportion in managed grassland may change as the CO 2 concentration increases. However, an adapted management could, at least partially, counteract such CO 2 induced changes in the proportion of the species. Since the availability of mineral N in the soil may be important for the species’ responses to elevated pCO 2 , more long‐term studies, particularly of processes in the soil, are required to predict the entire ecosystem response.