Peatlands are a globally important store of soil carbon (C) (Gorham 1991; Dise 2009), and as they occur primarily in the high latitudes, they are likely to be one of the more sensitive ecosystems to climate change (IPCC 2007). These two factors combined mean that it is particularly important to understand what drives C cycling and thus C storage in these systems. Although the major driver of high C storage in peatlands is low temperature and water-logged conditions (Baird et al. 2009), it is increasingly recognized that the composition of peatland plant species can also significantly influence ecosystem C cycling (De Deyn, Cornelissen & Bardgett 2008; Ward et al. 2009). A major component of peatland vegetation is formed by mosses, which can contribute a high proportion of total plant biomass and NPP (Ward et al. 2007; Lang et al. 2009; Lindo & Gonzalez 2010) and often have a high species richness (Limpens et al. 2008) that encompasses a wide variety of traits (Skre & Oechel 1981; Cornelissen et al. 2007; Lang et al. 2009). Despite the potential for a significant contribution of moss species to C cycling we have, compared with vascular plants, a poor understanding of their functional role in ecosystems (Cornelissen et al. 2007). This is partly because many studies to date have focused on single genera (e.g. Malmer et al. 2003; Gunnarsson 2005) or treat mosses as a single functional group (e.g. Verville et al. 1998; Ward et al. 2009), although several more recent studies have begun to compare comprehensive ranges of species (Marschall & Proctor 2004; Turetsky et al. 2008; Lang et al. 2009). Most studies have also examined the impact of moss species on single C processes (e.g. decomposition rates or photosynthetic rates), limiting the depth of our understanding of the role of mosses in overall ecosystem C cycling.
The influence of moss species on peatland ecosystem C cycling is likely to occur through multiple direct and indirect processes. Moss species directly affect gross and net peatland ecosystem CO2 fluxes as a result of their photosynthesis and respiration and through the quality and quantity of their litter inputs (Lang et al. 2009). They also have a wide range of indirect effects on C cycling as they influence soil temperature, hydrology and chemistry (Turetsky 2003; Cornelissen et al. 2007; Gornall et al. 2007; Lindo & Gonzalez 2010), which together regulate the activity of soil decomposers and C loss through microbial respiration. The the exact effect of a given plant species on C cycling can vary depending on growth stage or other environmental contexts such as soil moisture, disturbance regimes and climate (Skre & Oechel 1981; Sveinbjornsson & Oechel 1983; Jackson et al. 2011). Gaining further understanding of both the impact of moss species on C cycling and whether these effects are consistent regardless of environmental context is essential to fully understand their role in C cycling and what impact changes in their composition may have on feedbacks to climate change.
Here, we use a unique, long-term peatland fire frequency experiment (>50 years) at Moor House National Nature Reserve in the UK to examine the effects of three dominant moss species [i.e. Sphagnum capillifolium (Ehrh.) Hedw., Plagiothecium undulatum Hedw. and Hypnum jutlandicum Holmen & Warncke; Fig. 1] on ecosystem C cycling. This fire frequency experiment provides an ideal opportunity to test in detail how environmental context affects the impact of species on C cycling: as fire frequency decreases from every 10 years to every 20 years and finally to not being burnt for >50 years, the fungal/bacterial ratio of the microbial community increases, vegetation cover increases and vascular plant composition changes from Eriophorum dominated to Calluna dominated (Hobbs 1984; S.E. Ward, N.J. Ostle, S. Oakley, H. Quirk, A. Stott, P.A. Henrys, W.A. Scott & R.D. Bardgett, unpublished data). These changes are likely to affect underlying belowground respiration, soil temperature, light availability and the ability of mosses to intercept nutrients in rainfall (Malmer et al. 2003), which are in turn likely to influence moss species effects on C cycling. We aimed to test the hypotheses that (i) coexisting moss species have different effects on ecosystem C cycling, as assessed by measurements of net ecosystem exchange (NEE), moss litter decomposition rates and the effect of mosses on the decomposition environment, and (ii) that moss species effects on ecosystem C cycling are dependent on environmental context, in this instance the effects of fire frequency.