Temperature-induced increase in methane release from peat bogs: a mesocosm experiment

Abstract

Peat bogs are primarily situated at mid to high latitudes and future climatic change projections indicate that these areas may become increasingly wetter and warmer. Methane emissions from peat bogs are reduced by symbiotic methane oxidizing bacteria (methanotrophs). Higher temperatures and increasing water levels will enhance methane production, but also methane oxidation. To unravel the temperature effect on methane and carbon cycling, a set of mesocosm experiments were executed, where intact peat cores containing actively growing Sphagnum were incubated at 5, 10, 15, 20, and 25°C. After two months of incubation, methane flux measurements indicated that, at increasing temperatures, methanotrophs are not able to fully compensate for the increasing methane production by methanogens. Net methane fluxes showed a strong temperature-dependence, with higher methane fluxes at higher temperatures. After removal of Sphagnum, methane fluxes were higher, increasing with increasing temperature. This indicates that the methanotrophs associated with Sphagnum plants play an important role in limiting the net methane flux from peat. Methanotrophs appear to consume almost all methane transported through diffusion between 5 and 15°C. Still, even though methane consumption increased with increasing temperature, the higher fluxes from the methane producing microbes could not be balanced by methanotrophic activity. The efficiency of the Sphagnum-methanotroph consortium as a filter for methane escape thus decreases with increasing temperature. Whereas 98% of the produced methane is retained at 5°C, this drops to approximately 50% at 25°C. This implies that warming at the mid to high latitudes may be enhanced through increased methane release from peat bogs.

Figure 1. Potential methane oxidation rates (grey bars) and production rates (white bars).

Sphagnum plants and peat from a pool-site and a hummock-site were analysed. Sphagnum plants were divided in three parts. Rates are expressed in µg.g dw⁻¹.day⁻¹ and are means of triplicate incubations ± s.d. Letters indicate statistically significant groups of data (P<0.05).

Figure 2. Methane cycling at different temperatures.

A) Diffusive methane flux, with and without Sphagnum, B) methane consumption, the difference in methane flux before and after removal of Sphagnum, C) methane retention. Fluxes are measured on small peat cores after two months of incubation and values are expressed in µg.cm⁻².day⁻¹. Methane retention is expressed in % of the initial flux measured without Sphagnum. Values represent means of triplicate incubations ± s.d. Letters indicate statistically significant groups of data (P<0.05). Diffusive methane flux data with and without Sphagnum were not compared to each other.

Figure 3. Growth rates of Sphagnum at different temperatures.

Growth rates are measured after two months of incubation. Values are expressed in cm and represent means of four replicates ± s.d. Letters indicate statistically significant groups of data (P<0.05).