A novel framework for quantifying past methane recycling by Sphagnum‐methanotroph symbiosis using carbon and hydrogen isotope ratios of leaf wax biomarkers

Abstract

The concentration of atmospheric methane is strongly linked to variations in Earth's climate.Currently, we can directly reconstruct the total atmospheric concentration of methane, but not individual terms of the methane cycle. Northern wetlands, dominated by Sphagnum, are an important contributor of atmospheric methane, and we seek to understand the methane cycle in these systems. We present a novel method for quantifying the proportion of carbon Sphagnum assimilates from its methanotrophic symbionts using stable isotope ratios of leaf-wax biomarkers. Carbon isotope ratios of Sphagnum compounds are determined by two competing influences, water content and the isotope ratio of source carbon. We disentangled these effects using a combined hydrogen and carbon isotope approach. We constrained Sphagnum water content using the contrast between the hydrogen isotope ratios of Sphagnum and vascular plant biomarkers. We then used Sphagnum water content to calculate the carbon isotope ratio of Sphagnum's carbon pool. Using a mass balance equation, we calculated the proportion of recycled methane contributed to the Sphagnum carbon pool, ''PRM.'' We quantified PRM in peat monoliths from three microhabitats in the Mer Bleue peatland complex. Modern studies have shown that water table depth and vegetation have strong influences on the peatland methane cycle on instrumental time scales. With this new approach, d 13 C of Sphagnum compounds are now a useful tool for investigating the relationships among hydrology, vegetation, and methanotrophy in Sphagnum peatlands over the time scales of entire peatland sediment records, vital to our understanding of the global carbon cycle through the Late Glacial and Holocene.

Stable carbon isotopic composition of peat columns, subsoil and vegetation on natural and forestry-drained boreal peatlands

“…4). In general, methane oxidation producing 13 C-depleted CO2 mostly happens in aerobic peat and inside Sphagnum cells [44], thus the signal of 13 C-depleted CH4 (δ 13 C -94 --38 ‰) [45] can be returned to the same peat column as plant biomass or as microbial biomass. Besides 410 methane oxidation, a plausible explanation for surface and basal peat δ 13 C correlation may be the continuous return of CO2 from peat respiration back to vegetation through photosynthesis, thus refixation of CO2 released from the same vertical location.…”

Holocene Vegetation, Climate, and Carbon History on Western Kodiak Island, Alaska

“…Hydrogen isotope ratios of individual n-alkanes were determined by continuous flow isotope ratio mass spectrometry. Eluent from a gas chromatograph (Thermo Trace GC) passes through a pyrolysis reactor (Thermo GC IsoLink) where organic compounds are converted to H 2 gas, which then is passed to the IRMS (Thermo Delta V) via a continuous flow device (Nichols et al, 2014a).…”

The δ13C of cellulose from modern plants and its responses to the atmosphere – From the peatland records of Dajiuhu, China

“…They showed 13 C-depleted values are related to low water table depths and vice versa. Furthermore, Nichols et al (2014) presented a novel method for quantifying of carbon Sphagnum assimilates symbionts using stable isotope ratios of leaf-wax biomarkers. They indicated that carbon ratios of Sphagnum compounds are determined by water content and the isotope ratios of source carbon competing influences.…”