Potassium limits potential growth of bog vegetation under elevated atmospheric CO2 and N deposition

The free air carbon dioxide enrichment (FACE) and N deposition experiments on four ombrotrophic bogs in Finland, Sweden, the Netherlands and Switzerland, revealed that after three years of treatment: (1) elevated atmospheric CO2 concentration had no significant effect on the biomass growth of Sphagnum and vascular species; and (2) increased N deposition reduced Sphagnum growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum, while vascular plant biomass growth was not affected. This paper focuses on water chemistry, plant nutrient content, and litter decomposition rates. Potassium limitation, or low supply of K and P, may have prevented a significant increase of Sphagnum growth under elevated CO2 and N deposition. Vascular plant growth under elevated CO2 and N deposition was also limited by K, or by K in combination with P or N (N in CO2 experiment). Elevated CO2 and N deposition had no effect on decomposition rates of Sphagnum and vascular plant litter. Aside from a possible effect of N deposition on light competition between species, we expect that elevated atmospheric CO2 and N deposition concentrations will not affect Sphagnum and vascular plant growth in bogs of north-west Europe due to K-, or K in combination with N- or P-, limited growth. For the same reason we expect no effect of elevated CO2 and N deposition on litter decomposition. Net primary production of raised ombrotrophic bogs that are at or close to steady state, is regulated by input of nutrients through atmospheric deposition. Therefore, we hypothesize that the expected increase of plant growth under elevated CO2 and N deposition is diminished by current levels of K (and to some extent P and N) in atmospheric deposition.