Effects of elevated CO2 and cutting frequency on the productivity and herbage quality of a semi-natural grassland

Monoliths of a fertile, although N limited, C3 grassland community were subjected (or not) to an atmospheric CO2 enrichment (600 micromol 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 CO2 on the above-ground productivity and on the herbage quality. Elevated CO2 did not affect the dry matter (DM) yield of the swards in 1999. In 2000, the second year, there was a positive CO2 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 CO2. 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 N2 fixation triggered by an increased ecosystem scale demand of N under elevated CO2. At a low cutting frequency, the DM of the forbs was strongly increased in elevated compared with ambient CO2. This increased development of the forbs apparently led to a competitive decline of the grasses. Therefore, the total DM yield response to CO2 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 CO2 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 CO2 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 CO2 impacts on semi-natural grassland productivity and herbage quality.