Trophic status impacts both the magnitude and stable carbon isotope composition of methane flux from peatlands

Abstract

The influence of trophic status on CH4 production, emission and stable carbon isotope composition was investigated in two ombrogenous and two minerotrophic peatlands situated in Wales, UK. Methane production and emission rates were highest in the minerotrophic peatlands and CH4 in both pore water and emissions to the atmosphere were notably 13C‐enriched compared to the ombrogenous bogs. Highly negative δ13C values (−95 to −82‰) for CH4 flux from the acidic rainfed peatlands likely resulted from a combination of CO2/H2 methanogenesis and isotope effects associated with diffusion of CH4 through aerenchymatous tissue of vascular flora. The δ13C values presently attributed to CH4 flux from northern wetlands in isotope‐weighted mass balance budgets of the CH4 cycle may be too positive. Methane flux from northern bogs and fens should be assigned different δ13C values because CH4 is notably more 13C‐depleted in the former, albeit emission strength typically is weaker.

Methanotrophy potential versus methane supply by pore water diffusion in peatlands

“…The latter also are shown in Table 3 and have been reported previously in Bowes and Hornibrook (2006) and Hornibrook and Bowes (2007). The CH 4 fluxes to the atmosphere are due only to steady-state diffusion processes (i.e., pore water or plant-mediated transport).…”

“…The δ 13 C composition of CH 4 emissions from both plots are 13 C-depleted by ∼15 to 20‰ relative to pore water CH 4 , which eliminates the possibility that the small quantities of CH 4 emitted from station 1 are residual CH 4 that has survived transit across the unsaturated zone (Happell et al, 1994;Popp et al, 1999). Similarly, CH 4 emissions from Cors Caron, Crymlyn Bog and Gors Lwyd also are 13 C-depleted relative to the pore water CH 4 pool (Hornibrook and Bowes, 2007). These conclusions about transport processes based upon stable isotope data are consistent with the observation reported here that low affinity methanotrophs in the 3 cm thick zone where CH 4 first appears in the pore water pool (i.e., immediately below the depth [CH 4 ] 0 ) have a capacity for CH 4 consumption that significantly exceeds the upward CH 4 supply via pore water diffusion (Table 3).…”

“…Rates of CH 4 emission to the atmosphere from Blaen Fign and Cors Caron were the same order of magnitude as pore water CH 4 diffusion rates; however, it is unlikely that CH 4 transport by this mode contributed to atmospheric flux. The stable carbon isotope compositions (δ 13 C values) of CH 4 in pore water and surface flux have been used previously to demonstrate that diffusive emission of CH 4 to the atmosphere at all four peatlands occurs predominately via plant-mediated transport (Bowes and Hornibrook, 2006;Hornibrook and Bowes, 2007). For example, CH 4 emitted at a higher rate from sedge-rich station 2 at Blaen Fign has δ 13 C values that are statistically indistinguishable from CH 4 emissions from Sphagnum-rich station 1 (Bowes and Hornibrook, 2006).…”

“…The total potential rate of CH 4 oxidation in the 3 cm thick zone is based upon actual CH 4 concentrations measured in peat soils and the depth distribution of µ m and K s parameters determined experimentally for Crymlyn Bog and Cors Caron (Table 2). d Total diffusive CH 4 flux to the atmosphere measured using closed dynamic chambers and reported previously in Bowes and Hornibrook (2006) and Hornibrook and Bowes (2007). The number of chamber deployments is shown in brackets (i.e., n = 1, 2 or 3).…”

“…Collection methods and CH 4 flux data for all sites were reported previously in Bowes and Hornibrook (2006) and Hornibrook and Bowes (2007). Briefly, flux chambers and ground collars were constructed of polyvinyl chloride (PVC) and had a combined volume of either 11 or 15 l. The chambers were sealed onto the collars using large neoprene rubber o-rings coated with silicon grease and then covered with opaque lids also fitted with greased o-rings.…”