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.