INTRODUCTION
Only 3% of the earth’s land surface is covered by peatlands, but more than a third of global soil carbon (C) is stored in boreal mires (Frolking et al ., 2011; Loisel et al ., 2014). Most of those mires are dominated bySphagnum peat mosses, which hence contribute substantially to global peatland C sequestration (Gunnarsson, 2005; Wu et al ., 2012; Laing et al ., 2014; Loisel et al ., 2014). Thus, reliable prediction of future C sequestration and storage in peatlands requires profound understanding of Sphagnum C acquisition and accumulation (Frolking et al ., 2011; Charman et al ., 2013; Loisel, et al ., 2014; Wu & Roulet 2014), but the mosses’ responses to increases in atmospheric CO2 concentrations are still not well understood.
Sphagnum physiology strongly depends on local environmental conditions, including local weather, hydrology and nutritional constraints. Distinct Sphagnum species favor specific mire microhabitats, and form associated structures, including hummocks, lawns and hollows that reflect characteristic average water table (WT) levels (Rydin et al ., 2006; Weston et al ., 2015; Moor et al ., 2017; Nijp et al ., 2017). Key elements of the species’ adaptation to these microhabitats are based on physiological and anatomical traits that govern their capillary water retention (Rydin & Clymo, 1989; McCarter & Price, 2014; Weston et al ., 2015).Sphagnum fuscum is an abundant globally distributed hummock species (Klinggräff, 1872) that is capable of growing up to 50 cm above the WT (Hogg, 1993; Gunnarsson, 2005; Rydin et al ., 2006). In contrast, Sphagnum majusinhabits lawns and hollows and grows generally less than 10 cm above the WT (Nijp et al. , 2014).
The ongoing increase in atmospheric CO2 from preindustrial levels of 280 ppm to the contemporary ~400 ppm and concomitant changes in climate, such as higher temperatures and precipitation in the northern hemisphere, are expected to have major consequences for mire vegetation and C sequestration (Hilbert et al ., 2000; Belyea & Malmer, 2004; Limpens et al ., 2008; Frolking et al ., 2011; Gallego-Sala et al ., 2018). CO2 ‘fertilization’ on higher plants has been extensively studied, but its magnitude and underlying mechanisms remain unclear (IPCC, 2013; Ainsworth & Long, 2005; DeLucia et al ., 2005; Schimel et al ., 2015). Even less is known about Sphagnum mosses, which lack certain anatomical features of higher plants, such as the cuticle, stomata and roots, so they can only regulate CO2 and water fluxes indirectly (Hayward & Clymo, 1982; Williams & Flanagan, 1998; Price & Whittington, 2010).
In previous manipulation experiments, conflicting results have been obtained concerning the photosynthetic response of Sphagnum to increases in atmospheric CO2 levels. They found enhanced net photosynthesis, but not biomass, of S. fuscum (Jauhiainenet al ., 1994; Jauhiainen & Solviola, 1999). Most studies of other Sphagnum species also did not detect any increase in biomass (Van der Heijden et al ., 2000b; Berendse et al ., 2001; Heijmans et al ., 2001, 2002; Mitchell et al ., 2002; Toet et al ., 2006), except for Van der Heijden et al . (2000a). However, in most of these studies, effects of confounding factors such as temperature, moisture, light intensity and nutrient availability may have masked effects of changes in atmospheric CO2 levels on C assimilation.
On the molecular level, C assimilation in C3 plants such as Sphagnum is initiated by the reaction of CO2with ribulose 1,5-bisphosphate (RuBP, carboxylation), catalyzed by the enzyme Rubisco. Due to Rubisco’s enzymatic properties, this reaction is accompanied by a massive side reaction of RuBP with O2(oxygenation). This process gives rise to the photorespiration pathway, which leads to loss of C as CO2; therefore, this oxygenation reaction decreases net CO2 fixation (Lainget al ., 1974). Models predict that the anthropogenic increase in atmospheric CO2 (from 280 to over 400 ppm) will generally result in suppressed C3 photorespiration rates (under given conditions) and contribute to an increase in net photosynthesis of ~35% (Ehlers et al ., 2015). Thus, robust understanding of the ratio of photorespiration to gross photosynthesis and its determinants is crucial for predicting overall net C balances of terrestrial biomass, and particularly for modeling responses of photosynthetic processes to changes in environmental conditions (Weston et al ., 2015; Pugh et al ., 2016).
The above-mentioned conflicting responses in Sphagnum C assimilation were observed in manipulation experiments, with CO2 increases above ambient (> 350 ppm). Another question is how to constrain the response that has occurred over the 20th century. To date there are no methods to trace if there has been a suppression of photorespiration during the 20th century. However, accurate estimates of the ratio of photorespiration to gross photosynthesis, at the time of formation of both current and historical plant tissues, can be obtained using deuterium (D) isotopomers (Ehlers et al ., 2015). In this method, the abundance ratio of the D isotopomers named D6S and D6R in the C6H2 group of glucose derived from hydrolyzed cell wall carbohydrates is measured by NMR spectroscopy (Fig. 1).
The D6S /D6R ratio indicates the Rubisco oxygenation to carboxylation flux ratio, and thus the photorespiration to gross photosynthesis ratio. For simplicity, in the following text we will refer to this ratio as photorespiration/photosynthesis ratio. Analysis with this method confirmed that the photorespiration of many C3 plants was suppressed and their net photosynthesis rates increased by the last century’s increase in atmospheric CO2 (Ehlers et al ., 2015). In the cited study, comparison of S. fuscumherbarium material formed at ≤ 300 ppm atmospheric CO2with modern plants showed that net photosynthesis rates of peat-formingSphagnum also increased during the last century. However, the extent to which photorespiration of Sphagnum plants is suppressed at increased atmospheric CO2 may be influenced by other factors, particularly water content, temperature and light intensity. Thus, the overall aim was to understand variations and potential effect of suppressed photorespiration (and associated changes in C assimilation rates) in relation to biogeophysical conditions at peatland surfaces and climatic changes. This will allow us to explain D isotopomer data of historical Sphagnum tissues. To address this aim, we investigated the response of S. fuscum ’s photorespiration/photosynthesis ratio to the recent increase in atmospheric CO2 under various combinations of different atmospheric CO2, WT, temperature and light intensity levels. To do so, we used D isotopomers as well as δ13C, δ15N and elemental analysis (C and N). The response was further tested for the lawn speciesS. majus at different CO2 and WT levels.