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

Abstract

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 (NP(max)), maximum efficiency of photosystem II [variable fluorescence (F (v))/maximum fluorescence yield (F (m))] 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. NP(max) 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. F (v)/F (m) did not differ between N treatments. Increased temperature (+3.6 degrees C) had a small negative effect on N concentration, but had no significant effect on NP(max) or F (v)/F (m). Addition of 2 g S m(-2) year(-1) showed a weak negative effect on NP(max) and F (v)/F (m). Our results suggest a unimodal response of NP(max) 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.

Symbiosis revisited: phosphorus and acid buffering stimulate N<sub>2</sub> fixation but not <i>Sphagnum</i> growth

“…appears to be high compared to a range of Sphagnum N contents for different N deposition sites (Lamers et al, 2000). Optimal growth conditions for Sphagnum balticum were found at an N content of 12.9 mg g −1 (Granath et al, 2009), suggesting that Sphagnum in our experiment is around the saturation point. Indeed, high amounts of inorganic N were still taken up from rainwater by Sphagnum spp., leaving the surface water nearly depleted of N (Table 2).…”

Photosynthetic responses of two pleurocarpous mosses to low-level nitrogen addition: a study in an old-growth fir forest

“…These results suggest that mosses may respond with an increased photosynthetic rate to low doses of N addition or deposition (e.g. less than 1 g N/m 2 /year; Granath et al, 2009b). No significant effect of N addition on F v /F m of Sphagnum spp.…”

“…Thus, mosses are expected to be sensitive to N addition, especially physiologically (Manning et al, 2006;Granath et al, 2012). Granath et al (2009b) found a unimodal response of photosynthesis with a N concentration optimum for photosynthetic capacity at around 1.3% for Sphagnum balticum. Positive relationships were also observed in a greenhouse experiment (Granath et al, 2012) and along a latitudinal nitrogen deposition gradient (Granath et al, 2009a).…”

“…The physiological effects of N enrichment on photosynthesis have been extensively studied (e.g. Granath et al, 2009bGranath et al, , 2012. The photosynthetic rate should theoretically increase after N addition, since more N can be allocated to the photosynthetic apparatus (e.g.…”

Variations in nitrogen, phosphorus, and δ15N in Sphagnum mosses along a climatic and atmospheric deposition gradient in eastern Canada

“…Atmospheric N deposition increases N content in Sphagnum mosses (Aerts et al 1992;Bragazza et al 2005;Granath et al 2009;Jiroušek et al 2011). Less is known about rates of atmospheric P deposition, which have a high degree of spatial variation (Tipping et al 2014) and are thus more difficult to assess in terms of their impact on Sphagnum P concentration.…”