Wetland restoration and methanogenesis: the activity of microbial populations and competition for substrates at different temperatures

Abstract

Abstract. Ljubljana marsh in Slovenia is a 16 000 ha area of partly drained fen, intended to be flooded to restore its ecological functions. The resultant water-logging may create anoxic conditions, eventually stimulating production and emission of methane, the most important greenhouse gas next to carbon dioxide. We examined the upper layer (∼30 cm) of Ljubljana marsh soil for microbial processes that would predominate in water-saturated conditions, focusing on the potential for iron reduction, carbon mineralization (CO 2 and CH 4 production), and methane emission. Methane emission from water-saturated microcosms was near minimum detectable levels even after extended periods of flooding (>5 months). Methane production in anoxic soil slurries started only after a lag period of 84 d at 15 • C and a minimum of 7 d at 37 • C, the optimum temperature for methanogenesis. This lag was inversely related to iron reduction, which suggested that iron reduction out-competed methanogenesis for electron donors, such as H 2 and acetate. Methane production was observed only in samples incubated at 14-38 • C. At the beginning of methanogenesis, acetoclastic methanogenesis dominated. In accordance with the preferred substrate, most (91%) mcrA (encoding the methyl coenzyme-M reductase, a key gene in methanogenesis) clone sequences could be affiliated to the acetoclastic genus Methanosarcina. No methanogens were detected in the original soil. However, a diverse community of iron-reducing Geobacteraceae was found. Our results suggest that methane emission can remain transient and low if water-table fluctuations allow reoxidation of ferrous iron, sustaining iron reduction as the most important process in terminal carbon mineralization.

Flooding-related increases in CO<sub>2</sub> and N<sub>2</sub>O emissions from a temperate coastal grassland ecosystem

“…Longer-term flooding therefore increased, rather than reduced, the annual emissions by approximately 40 % and suggests that any increase in freshwater flooding in response to climate change could result in a significant increase in carbon dioxide emissions from these systems. The annual emissions of CO 2 found in this study are in line with those previously reported for floodplain wetlands (10.91 ± 0.54 Mg CO 2 -C ha −1 yr −1 ) (Batson et al, 2015), coastal plain wetlands (11.29 Mg CO 2 -C ha −1 yr −1 ) (Morse et al, 2012) and occasionally (9.7 Mg CO 2 -C ha −1 yr −1 ) or frequently flooded (13 Mg CO 2 -C ha −1 yr −1 ) riparian forests (Jacinthe, 2015). Our results are also in part agreement with other investigations on comparable ecosystems that examined the impact of flooding on GHGs (Morse et al, 2012;Jacinthe, 2015;Kim et al, 2015;Marín-Muñiz et al, 2015).…”

“…The annual emissions of CO 2 found in this study are in line with those previously reported for floodplain wetlands (10.91 ± 0.54 Mg CO 2 -C ha −1 yr −1 ) (Batson et al, 2015), coastal plain wetlands (11.29 Mg CO 2 -C ha −1 yr −1 ) (Morse et al, 2012) and occasionally (9.7 Mg CO 2 -C ha −1 yr −1 ) or frequently flooded (13 Mg CO 2 -C ha −1 yr −1 ) riparian forests (Jacinthe, 2015). Our results are also in part agreement with other investigations on comparable ecosystems that examined the impact of flooding on GHGs (Morse et al, 2012;Jacinthe, 2015;Kim et al, 2015;Marín-Muñiz et al, 2015). Jacinthe (2015) reported that CO 2 fluxes during summer were larger from a riparian forest affected by floods in winter and spring than from a flood protected area.…”

“…For example, Wilson et al (2011) showed significant changes in microbial structure and increases in soil enzymatic activity after short-term (24-day) inundation of floodplain soil. Nevertheless, an increase in microbial activity and GHG emissions as a response to short-term flooding (SFS) is not always found (Unger et al, 2009;Jacinthe, 2015), and the timing as well as the frequency of flooding events may be important in determining both the microbial community change and the associated GHG emissions. Organic substrate availability (via the breakdown of plant litter or the introduction of organic compounds in flowing water) is a key factor regulating the activities of soil microorganisms and the mineralisation and immobilisation of carbon and nitrogen (Badiou et al, 2011;Li et al, 2015).…”

Water-table height and microtopography control biogeochemical cycling in an Arctic coastal tundra ecosystem

“…Flooded, anoxic conditions also promote the growth and activity of methanogens, and lower diffusion rates can also trap more CH 4 in the soil. However, the increased availability of Fe(III) could competitively inhibit methane production (Roden and Wetzel, 1996;Bond and Lovley, 2002;Küsel et al, 2008;Jerman et al, 2009), weakening these effects (dotted flat arrow in lower left, Fig. 10).…”

“…Previous work at the Biocomplexity site showed that much of the basin is prone to anoxia, and that dissimilatory Fe(III) reduction is a particularly important anaerobic process that contributes significantly to the ecosystem C budget (Lipson et al, 2010). This process is generally thought to competitively inhibit methanogenesis by providing a more thermodynamically favorable respiratory pathway (Roden and Wetzel, 1996;Blodau, 2002;Bond and Lovley, 2002;Küsel et al, 2008;Jerman et al, 2009). And while CH 4 production has been studied in the Arctic Coastal Plain near Barrow, AK (Rhew et al, 2007;Zona et al, 2009;von Fischer et al, 2010), no studies have yet explored how Fe(III) reduction interacts with CH 4 production in this ecosystem.…”