An Isotopic Analysis of Carbon Dynamics in Sphagnum Peat

Abstract

Methane and carbon dioxide produced in peatlands can be held temporarily in dissolved surface waters and accessible to aquatic plants. The primary peat building plant in northern peatlands, Sphagnum moss, grows in these dissolved carbon rich waters. Understanding Sphagnum carbon cycles is key for radiocarbon dating, peatland restoration, and modelling of methane release from warming peatlands globally. This combined surface and multi‐proxy palaeoenvironmental study challenges common assumptions about the dissolved gas cycling in this semi‐aquatic environment. The Sphagnum associated methanotrophic communities within the water filled hyaline cells have commonly been linked to unusual 13C‐depleted measurements of Sphagnum carbon isotope ratios. The distinctly 13C‐depleted methane dissolved in surface pool water is traced using isotopic analysis as it is oxidised and travels into the interstitial Sphagnum water, through to being photosynthetically captured in α‐cellulose that makes up living Sphagnum, and finally to the carbon stored in layers of peat. Evidence is presented of a carbon supply available to Sphagnum from both symbiotic methanotrophs and heterotrophs. Mass balance calculations showed that Sphagnum associated methanotrophic communities only contribute a small proportion of the carbon fixed by Sphagnum growing in wet environments despite significant local oxidation. Evidence is presented for the interstitial Sphagnum water, rich in dissolved carbon dioxide, often being bypassed in favour of the much faster diffusion of carbon dioxide through air. Combined Sphagnum δ13C and hopanoid biomarkers in core material showed evidence of a brief period of substantial methane uptake, however, it is proposed that these events are only detectable in rare instances whereby uptake is sustained for periods of months to years. This study has demonstrated that an evolving body of research regarding Sphagnum associated methanotrophs can be applicable in blanket bog habitats. In the context of methane emissions from peatlands globally, this study indicates that previous estimates of methane uptake by Sphagnum applied broadly in these landscapes would likely overestimates the true extent of this process. Nonetheless, the demonstration of methane oxidation to carbon dioxide in these recently restored sites indicates a resilience of Sphagnum associated methanotrophic community activity and further evidences of the value in Sphagnum reintroduction as a host for methane oxidising bacteria in degraded landscapes undergoing rewetting.