Limitations for Sphagnum biomass production
The lack of observed response of biomass production to increased atmospheric CO2 (Fig. 2B) is consistent with findings of several other studies on differentSphagnum species (Jauhiainen et al ., 1994, 1998; Van der Heijden et al ., 2000b; Berendse et al ., 2001; Heijmanset al ., 2001, 2002; Mitchell et al ., 2002; Toet et al ., 2006). In contrast, the decrease in photorespiration/photosynthesis ratio we detected clearly indicates increased C assimilation (assuming constant RuBP turnover rates) and thus increased NPP potential in response to increased atmospheric CO2. At low WT, increasing the CO2 level caused a slight but not significant increase in biomass production at high light, and a slight decrease at low light (Fig. 2B), suggesting that under field conditions, where light intensities are much higher, there might well be a CO2-driven increase in NPP. Thus, elevated CO2 might shift the light saturation point even above 500 µmol m-2 s-1 (Hajeket al ., 2009).
Furthermore, increases in atmospheric CO2 may only enhance biomass production of higher C3 plants if sufficient nutrients are available (Arp et al ., 1998; Poorter, 1998; Kirschbaum, 2011). Accordingly, we observed a decrease inSphagnum tissue N content and an increase in the C/N ratio at elevated CO2 (Fig. 3C, D), resulting in a high correlation with the D6S /D6R ratio. In higher C3 plants, acclimation to increased CO2 levels has been found to reduce leaf N and Rubisco contents due to reductions in demand for Rubisco (Drake et al ., 1997; Cotrufo et al ., 1998). This indicates reduced N investment in Rubisco under elevated CO2 and suggests that increasing CO2 reduces Rubisco limitation of C assimilation (Fig. 2A, 3D). Reductions in N contents associated with increases in CO2, which suppress photorespiration, have been observed in several Sphagnum species including S. fuscum, S. recurvum, S. palustre, and S. magellanicum (Jauhiainen et al ., 1998; Van der Heijden et al ., 2000b; Toet et al ., 2006; Heijmans et al ., 2001). This suggests similar effects of elevated CO2 on N content in most Sphagnumspecies.