EFFECTS OF ELEVATED ATMOSPHERIC CO2 AND NITROGEN-FERTILIZATION ON YIELD OF TRIFOLIUM-REPENS AND LOLIUM-PERENNE

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 two years. The effects of nitrogen fertilisation (10 and 42 g N m(-2) a(-1) in 1993; 14 and 56 g N m(-2) a(-1) in 1994) on the growth response to pCO, were investigated in frequently defoliated (7 cuts in 1993; 8 cuts in 1994) swards. The yield of Trifolium in monoc ultures increased by 22% when grown at elevated pCO(2). In contrast, t he yield of Lolium monocultures was not affected (2%) by elevated pCO( 2), whereas Lolium increased its root mass considerably. The consequen ce of these interspecific differences in the CO2 response was an incre ase in the proportion of Trifolium in the mixed swards from 39% at amb ient to 50% at elevated pCO(2). However, the proportion of the species was more strongly affected by N fertilisation than by elevated pCO(2) . Based on these 2' results, we conclude that the species proportion i n managed grassland may change as the CO2 concentration increases. How ever, an adapted management may, at least partially, counteract such C O2 induced changes in the proportion of the species.