The competition between peat mosses (Sphagnum) and vascular plants as affected by raised CO2 and increased N deposition was studied in a glasshouse experiment by exposing peat monoliths with monocultures and mixtures of Sphagnummagellanicum and Eriophorumangustifolium to ambient (350 ppmv) or raised (560 ppmv) atmospheric CO2 concentrations, combined with low (no N addition) or high (5 g m−2 yr−1 added) N deposition. Growth of the two species was monitored for three growing seasons.
The presence of Eriophorum did not affect Sphagnum biomass, because Eriophorum density did not become high enough to severely shade the moss surface. In contrast, Sphagnum had a negative effect on Eriophorum biomass, particularly on the number of flowering stems. Possibly, the presence of a living Sphagnum layer decreased nutrient availability to Eriophorum by immobilising nutrients mineralised from the peat.
Raised CO2 and/or increased N deposition did not change these competitive relationships between Sphagnum and Eriophorum, but had independent effects. Raised CO2 had a positive effect both on Sphagnum and Eriophorum biomass, though on Eriophorum the effect was transient, probably because of P limitation. Nitrogen addition had a direct negative effect on Sphagnum height growth in the first growing season, but by the third year an increased shoot density had cancelled this out, so no N effect on Sphagnum biomass was present at the end of the experiment. The response of Eriophorum to N addition was small; N availability appeared not to limit its growth.