Oxygen effects on methane production and oxidation in humid tropical forest soils

Abstract

We investigated the effects of oxygen (O 2 ) concentration on methane (CH 4 ) production and oxidation in two humid tropical forests that differ in long-term, time-averaged soil O 2 concentrations. We identified sources and sinks of CH 4 through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH 4 oxidized in situ.Complementary laboratory experiments were conducted to determine the effects of O 2 concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH 4 production occurred in soils that contained between 9 AE 1.1% and 19 AE 0.2% O 2 . For example, we observed CH 4 concentrations in excess of 3% in soils with 9 AE 1.1% O 2 . CH 4 emissions from the lower O 2 sites were high (22-101 nmol CH 4 m À2 s À1 ), and were equal in magnitude to CH 4 emissions from natural wetlands. During peak periods of CH 4 efflux, carbon dioxide (CO 2 ) emissions became enriched in 13 C because of high methanogenic activity. Gross CH 4 production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48-78% of the CH 4 produced was oxidized prior to atmospheric egress. O 2 availability influenced CH 4 oxidation more strongly than methanogenesis. Gross CH 4 production was relatively insensitive to O 2 concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH 4 production with increasing O 2 concentration, shifting the d 13 C composition of CH 4 to values that were more positive. Isotopic measurements suggested that CO 2 was an important source of carbon for methanogenesis in humid forests. The d 13 C value of methanogenesis was between À84% and À98%, which is well within the range of CH 4 produced from CO 2 reduction, and considerably more depleted in 13 C than CH 4 formed from acetate.

Spatial patterns in oxygen and redox sensitive biogeochemistry in tropical forest soils

“…The strong negative relationship between O 2 and CH 4 is not a surprise since methanogenesis is an anaerobic process. However, in the LEF soils, where some O 2 is always present, CH 4 production and consumption are both occurring (Teh et al 2005) with the net balance favoring production in the cloud forest and consumption in Bisley. Of all of the bivariate relationships with O 2 , the relationship between O 2 and N 2 O is least understood.…”

“…Although CH 4 effluxes typically exhibit high spatial variability, humid tropical forests are generally considered a net source of CH 4 (Frankenberg et al 2008). Given the spatial heterogeneity in O 2 availability and redox in soils, both production and consumption of CH 4 can occur simultaneously (Teh et al 2005). Humid tropical forests are the largest global source of N 2 O from natural ecosystems (Zhuang et al 2012).…”

“…Previous work described patterns in soil O 2, Fe species, P, N and C cycling along known redox gradients of topography and climate (Silver et al 1999, 2014, Teh et al 2005, Dubinsky et al 2010). Here we explored patterns within a given topographic zone in two tropical forest types that occupy the endpoints of a high rainfall gradient in the Luquillo Experimental Forest, Puerto Rico.…”

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

“…Of all these factors, fluctuation in soil redox conditions, as mediated by variations in water table depth, is perhaps most critical in regulating CH 4 dynamics (Couwenberg et al, 2010(Couwenberg et al, , 2011 because of the underlying physiology of the microbes that produce and consume CH 4 . 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).…”

“…Key factors include redox/water table depth (Couwenberg et al, 2010(Couwenberg et al, , 2011Silver et al, 1999;Teh et al, 2005;von Fischer and Hedin, 2007), plant productivity (von Fischer and Hedin, 2007;Whiting and Chanton, 1993), soil organic matter lability (Wright et al, 2011), competition for C substrates among anaerobes (Teh et al, 2008;von Fischer and Hedin, 2007), and presence of plants capable of facilitating atmospheric egress (Pangala et al, 2013). Of all these factors, fluctuation in soil redox conditions, as mediated by variations in water table depth, is perhaps most critical in regulating CH 4 dynamics (Couwenberg et al, 2010(Couwenberg et al, , 2011 because of the underlying physiology of the microbes that produce and consume CH 4 .…”