Physiological responses to nitrogen and sulphur addition and raised temperature in Sphagnum balticum

Sphagnum, the main genus which forms boreal peat, is strongly affected by N and S deposition and raised temperature, but the physiological mechanisms behind the responses are largely unknown. We measured maximum photosynthetic rate (NPmax), maximum efficiency of photosystem II [variable fluorescence (Fv)/maximum fluorescence yield (Fm)] and concentrations of N, C, chlorophyll and carotenoids as responses to N and S addition and increased temperature in Sphagnum balticum (a widespread species in the northern peatlands) in a 12-year factorial experiment. NPmax did not differ between control (0.2 g N m−2 year−1) and high N (3.0 g N m−2 year−1), but was higher in the mid N treatment (1.5 g N m−2 year−1). N, C, carotenoids and chlorophyll concentration increased in shoot apices after N addition. Fv/Fm did not differ between N treatments. Increased temperature (+3.6°C) had a small negative effect on N concentration, but had no significant effect on NPmax or Fv/Fm. Addition of 2 g S m−2 year−1 showed a weak negative effect on NPmax and Fv/Fm. Our results suggest a unimodal response of NPmax to N addition and tissue N concentration in S. balticum, with an optimum N concentration for photosynthetic rate of ~13 mg N g−1. In conclusion, high S deposition may reduce photosynthetic capacity in Sphagnum, but the negative effects may be relaxed under high N availability. We suggest that previously reported negative effects on Sphagnum productivity under high N deposition are not related to negative effects on the photosynthetic apparatus, but differences in optimum N concentration among Sphagnum species may affect their competitive ability under different N deposition regimes.