Do plants of a semi-natural grassland community benefit from long-term CO2 enrichment?

Increasing atmospheric CO2 concentration ([CO2]) may alter plant community structure. The long-term responses of a semi-natural grassland community to elevated [CO2] and different cutting regimes were investigated. During four years the grassland was exposed in situ to a mean [CO2] Of 660 ppm using Free-Air CO2 Enrichment (FACE) and harvested once or twice per season. Under elevated [CO2], annual community biomass production was stimulated significantly only in the fourth year of investigation. Functional plant groups responded differentially to CO2 enrichment causing a clear shift in botanical composition from 1999 to 2002 towards a higher proportion of legumes under elevated [CO2] and two harvests per year, respectively. Photosynthetic capacity was not affected by higher [CO2] in the legume Lotus corniculatus but downregulated in the monocot Bromus erectus. Under elevated [CO2] the nitrogen content was lower in all functional plant groups, though C/N ratio was enhanced significantly only in grasses and non-leguminous dicots. In this nutrient-poor grassland community, legumes exhibit a higher competitiveness under elevated [CO2] due to their ability of symbiotic N-2-fixation.