Effects of Elevated Co₂ and Nitrogen Fertiliser on Biomass Productivity, Community Structure and Species Diversity of a Semi-Natural Grassland in Ireland

The most common semi-natural grasslands in Ireland are 'neutral' communities belonging to the phytosociological association Centaureo-Cynosuretum. An experiment was established at Teagasc, Oak Park Research Centre, Co. Carlow, using a representative Centaureo-Cynosuretum grassland community, to investigate the long-term effects of exposure to ambient and elevated carbon dioxide (CO₂) and high- and low-nitrogen fertiliser applications on above-ground plant biomass production, community structure and species diversity. The experimental units were 0.4m × 0.4m × 0.4m grassland monoliths exposed to CO₂ in a miniFACE (free air CO₂ enrichment) system. The experiment was based on a randomised two-factorial block design of two CO₂ treatments (365μmol mol⁻¹ and 600μmol mol⁻¹), two N fertiliser treatments (50kg ha⁻¹ y⁻¹ and 250kg ha⁻¹ y⁻¹) and three replicates of each. CO₂ treatments began on 1 April 1999 and continued throughout the growing season. During the season, four species responded significantly in terms of biomass production to the treatments: Potentilla anserina, Plantago lanceolata, Trifolium repens and Holcus lanatus. When the species were classfied into monocotyledon and dicotyledon functional groups, significant changes in the structure of this mixed grassland community became apparent. Elevated CO₂ altered the relative monocot and dicot composition of the sward. Nitrogen fertiliser modified the CO₂ response, with concomitant effects on community biomass productivity and species diversity. The results may have long-term implications for the dominance and successional patterns in grasslands under increasing atmospheric CO₂ concentrations.