Anaerobic oxidation of methane in tropical and boreal soils: Ecological significance in terrestrial methane cycling

Abstract

[1] Anaerobic oxidation of methane (AOM) is a considerable sink for the greenhouse gas methane (CH 4 ) in marine systems, but the importance of this process in terrestrial systems is less clear. Lowland boreal soils and wet tropical soils are two hot spots for CH 4 cycling, yet AOM has been essentially uncharacterized in these systems. We investigated AOM in soils from sites in Alaska and Puerto Rico. Isotope tracers were utilized in vitro to enable the simultaneous quantification of CH 4 production and consumption without use of biological inhibitors. Boreal peat soil and tropical mineral soil oxidized small but significant quantities of CH 4 to CO 2 under anoxic conditions (p < 0.001). Potential AOM rates were 21 AE 2 nmol g dw À1 d À1 and 2.9 AE 0.5 nmol g dw À1 d À1 for the boreal and tropical soils, respectively. The addition of terminal electron acceptors (NO 3 À , Fe(III), and SO 4 2À ) inhibited AOM and methanogenesis in both soils. In all incubations, CH 4 production occurred simultaneously with AOM, and CH 4 production rates were always greater than AOM rates. There was a strong correlation between the quantity of CH 4 produced and the amount of CH 4 oxidized under anoxic conditions (Alaska: r = 0.875, p < 0.0001; Puerto Rico: r = 0.817, p < 0.0001). CH 4 oxidation under anoxic conditions was biological and likely mediated by methanogenic archaea. While only a small percentage of the total CH 4 produced in these soils was oxidized under anoxic conditions (0.3% and 0.8% for Alaskan and Puerto Rican Soils), this process is important to understand since it could play a measurable role in controlling net CH 4 flux.

Changes in peat chemistry associated with permafrost thaw increase greenhouse gas production

“…Anaerobic CH 4 oxidation (48,49), if present, may be more pronounced in collapsed palsa and bog peat than in fen peat, but because CH 4 oxidation tends to enrich the δ 13 C of residual CH 4 (50) whereas the δ 13 C CH4 values in bog and collapsed palsa peat were relatively depleted ( Fig. 2A), this explanation seems unlikely.…”

Effects of alternative electron acceptors on the activity and community structure of methane-producing and consuming microbes in the sediments of two shallow boreal lakes

“…The effects of the isotopic fractionation associated with CO 2 dissolution and bicarbonate formation (Stumm and Morgan 1981) were considered to be negligible in the calculations, since the incubations were labelled with 13 C. The CH 4 oxidation in this study was determined solely based on the transfer of 13 C from CH 4 to TIC, and hence, the proportion of CH 4 -C bound to the biomass was not taken into account. As in the study by Blazewicz et al (2012), it was assumed that the biogenic CH 4 produced during the incubation had δ 13 C = -50‰ (F bCH4 = 0.01051). The chosen value represents the typical values of boreal lakes, since it is in the middle of the total range, -20‰ to -81‰, as measured previously from the water columns of two boreal lakes (Kankaala et al 2007;Nykänen et al 2014).…”

“…This was calculated for each time point using a modification of the equation used by Blazewicz et al (2012) and Moran et al (2005):…”

Seasonal variability in methane and nitrous oxide fluxes from tropical peatlands in the western Amazon basin

“…Methanogenic archaea are obligate anaerobes that only produce CH 4 under anoxic conditions (Conrad, 1996); as a consequence, they are only active in stably anoxic soil microsites or soil layers, where they are protected from the effects of strong oxidants such as oxygen or where competition for reducing equivalents (e.g., acetate, H 2 ) from other anaerobic microorganisms is eliminated (Teh et al, 2005(Teh et al, , 2008von Fischer andHedin, 2002, 2007). CH 4 oxidation, in contrast, is thought to be driven primarily by aerobic methanotrophic bacteria in tropical soils (Hanson and Hanson, 1996;Teh et al, 2005von Fischer andHedin, 2002, 2007), with anaerobic CH 4 oxidation playing a quantitatively smaller role (Blazewicz et al, 2012). Thus, fluctuations in redox or water table depth play a fundamental role in directing the flow of C among different anaerobic pathways (Teh et al, 2008;von Fischer and Hedin, 2007), and shifting the balance between production and consumption of CH 4 (Teh et al, 2005;von Fischer and Hedin, 2002).…”