Where old meets new: An ecosystem study of methanogenesis in a reflooded agricultural peatland

Abstract

Reflooding formerly drained peatlands has been proposed as a means to reduce losses of organic matter and sequester soil carbon for climate change mitigation, but a renewal of high methane emissions has been reported for these ecosystems, offsetting mitigation potential. Our ability to interpret observed methane fluxes in reflooded peatlands and make predictions about future flux trends is limited due to a lack of detailed studies of methanogenic processes. In this study we investigate methanogenesis in a reflooded agricultural peatland in the Sacramento Delta, California. We use the stable‐and radio‐carbon isotopic signatures of wetland sediment methane, ecosystem‐scale eddy covariance flux observations, and laboratory incubation experiments, to identify which carbon sources and methanogenic production pathways fuel methanogenesis and how these processes are affected by vegetation and seasonality. We found that the old peat contribution to annual methane emissions was large (~30%) compared to intact wetlands, indicating a biogeochemical legacy of drainage. However, fresh carbon and the acetoclastic pathway still accounted for the majority of methanogenesis throughout the year. Although temperature sensitivities for bulk peat methanogenesis were similar between open‐water (Q10 = 2.1) and vegetated (Q10 = 2.3) soils, methane production from both fresh and old carbon sources showed pronounced seasonality in vegetated zones. We conclude that high methane emissions in restored wetlands constitute a biogeochemical trade‐off with contemporary carbon uptake, given that methane efflux is fueled primarily by fresh carbon inputs.

Prompt rewetting of drained peatlands reduces climate warming despite methane emissions

“…Climatic effects of CO 2 from CH 4 oxidation should not be considered for CH 4 from biogenic sources 10 . However, although the large majority of CH 4 from peatlands stems from recent plant material (a biogenic source), the proportion of fossil CH 4 (from old peat) may be substantial in some cases 25 . Thus, we conservatively included the climatic effect of CO 2 from CH 4 oxidation in our analyses.…”

Temperature sensitivity of anaerobic methane oxidation versus methanogenesis in paddy soil: Implications for the CH4 balance under global warming

“…The modern global net emission to the atmosphere is ~580 Tg CH 4 year −1 (He et al, 2020) and increasing by ~5 Tg CH 4 year −1 (Saunois et al, 2016). CH 4 is produced in the last step of anaerobic organic carbon reduction (methanogenesis; Megonigal et al, 2004) by various methanogenic archaea (including hydrogenotrophs, formatotrophs, acetotrophs, methylotrophs, and alcoholotrophs; Bridgham et al, 2013;Chen et al, 2013;Jongejans et al, 2021;McNicol et al, 2020;Serrano-Silva et al, 2014). In turn, CH 4 itself can serve as a carbon and energy source for specialized aerobic and anaerobic microorganisms (methanotrophs) in sediments and soils (see Knittel & Boetius, 2009;Serrano-Silva et al, 2014 for reviews).…”

Gross Ecosystem Productivity Dominates the Control of Ecosystem Methane Flux in Rice Paddies

“…However, rice plants growing in water-saturated soils are also closely associated with the production and transport of CH 4 . Recently, several studies revealed that gross ecosystem productivity (GEP) is the dominant cause of the diel pattern of half-hourly CH 4 flux in rice paddies [9,10,12,13], and that GEP represents one of the most important factors regulating seasonal variations in daily CH 4 flux [9,[14][15][16]. However, in some sites, GEP are not that important as temperature for CH 4 flux [10,11].…”