Nitrogen fertilization reduces Sphagnum production in bog communities

The effects of increased nitrogen influx on Sphagnum growth and on interspe cific competition between Sphagnum species were studied in a 3-yr experimen t in mires situated in two areas with different rates of airborne N deposit ion. Sphagnum growth was recorded after various supplementary N influxes (0 , 1, 3, 5 and 10 g m(-2) yr(-1)) in hummocks and lawn communities. Sphagnum biomass production decreased with increasing N influx in both areas. After the first season at the low-deposition site, Sphagnum showed an increased growth in length with the intermediate N treatment, but in the second and t hird seasons the control treatment had the highest growth in length. Capitu lum dry mass increased with increasing N influx. Sphagnum N concentration a nd N/P quotient were higher at the high- than at the low-deposition site. T he low quotient at the low-deposition site, together with the initial growt h increase with intermediate N supplements, indicates that growth was N-lim ited at this site, but our lowest N supplement was sufficient to reduce gro wth. The N treatments had no effect on interspecific competition between th e Sphagnum species. This indicates that the species have similar responses to N. The species studied all occur naturally on ombrotrophic, N-poor sites and show low tolerances to increased N influx. Reduced Sphagnum production mag. affect the carbon balance, changing the mires from C sinks to sources .