Europ. J. Agronomy 20 (2004) 363–377 Effects of elevated CO 2 and cutting frequency on the productivity and herbage quality of a semi-natural grassland Catherine Picon-Cochard a , Florence Teyssonneyre a , Jean Michel Besle b , Jean-François Soussana a,∗ a INRA, Grassland Ecosystem Research, FGEP, 234 Avenue du Brézet, F-63039 Clermont-Ferrand, France b INRA, Herbivore Research Unit, 63122 St. Genès-Champanelle, France Received 10 September 2002; received in revised form 20 March 2003; accepted 21 April 2003 Abstract Monoliths of a fertile, although N limited, C 3 grassland community were subjected (or not) to an atmospheric CO 2 enrichment (600 mol mol -1 ), owing to the Mini-FACE system from August 1998 to June 2001, at two contrasting cutting frequencies (3 and 6 cuts per year). The present study reports the effects of elevated CO 2 on the above-ground productivity and on the herbage quality. Elevated CO 2 did not affect the dry matter (DM) yield of the swards in 1999. In 2000, the second year, there was a positive CO 2 effect (+26%) both on the DM and on the nitrogen yields (+30%). With the frequently cut monoliths, the DM of the legume component of the sward was strongly increased by elevated CO 2 . This effect became also significant in July 2000 for the low cutting frequency treatment. These results are in good agreement with the concept of an increased legume development and symbiotic N 2 fixation triggered by an increased ecosystem scale demand of N under elevated CO 2 . At a low cutting frequency, the DM of the forbs was strongly increased in elevated compared with ambient CO 2 . This increased development of the forbs apparently led to a competitive decline of the grasses. Therefore, the total DM yield response to CO 2 was smaller at a low (+15%) compared with a high (+36%) cutting frequency in 2000. An increase in the water soluble sugar content of the bulk forage under elevated CO 2 and a corresponding decline in cell wall contents (NDF) were observed. In June 1999, the decline in NDF was correlated with an increased in-vitro DM digestibility. The forage quality was also indirectly affected by elevated CO 2 through changes in leaf:stem ratio and in botanical composition. At a low cutting frequency, the increased forb content favoured the herbage quality because of a higher digestibility of the forb shoots and, indirectly, through the reduction in the mass of the grass stems. These results emphasise the role of species dynamics for elevated CO 2 impacts on semi-natural grassland productivity and herbage quality. © 2003 Elsevier B.V. All rights reserved. Keywords: Grassland; Elevated CO 2 ; FACE; Cutting frequency; Productivity; Digestibility; Climate change; Grasses; Legumes; Forbs ∗ Corresponding author. Tel.: +33-4-73-624423; fax: +33-4-73-624457. E-mail address: soussana@clermont.inra.fr (J.-F. Soussana). 1. Introduction Before the Industrial Revolution, the atmospheric CO 2 concentration was about 280 mol mol -1 but it has now risen by about 30% to reach 365 mol 1161-0301/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1161-0301(03)00040-6