Drivers of Methanogenesis Pathways in Subtropical Wetlands: Florida Everglades As a Case Study.

Methane production in WCA-2A regions of the subtropical freshwater Florida Wetlands has been shown to vary spatially along the nutrient gradient that extends from the eutrophic extrerior of the wetland to the oligotrophic interior. Although both acetoclastic and hydrogenotrophic pathways of methane production have been reported, their drivers are not fully understood. We investigated the effects of organic matter quality on methanogenesis pathways using manipulated soil laboratory incubations using CH3F. Soil samples were collected from three different sites in the Water Conservation Area 2A of the Everglades dominated by different plant species; Typha domingensis Pers., Nymphea odorata and Cladium jamaicense Crantz. We used biogeochemical properties and solid-state 13C-CPMAS-NMR spectroscopy to determine the organic matter quality. Methane and CO2 production decreased with soil depth from 0-5cm to 30-40cm and decreased with increasing alkyl, alkyl:o-alkyl ratio, dissolved organic carbon, and C:P ratio and with decreasing P, MBC, and MBN in all sites. Hydrogenotrophic methanogenesis dominated in all sites and depths apart from 5-10cm depth in F1 (63%) and U3S (57%) site. Methane 13C signatures ranged from -49 to -73. Acetoclastic methane production significantly decreased with increasing alkyl (P = 0.0117; R2 = 0.697) and alkyl: o-alkyl ratio (P = 0.0051; R2 = -0.749) but not significantly so for hydrogenotrophic methane production (P = 0.0892; R2 = 0.511) and (P = 0.2215; R2= - 0.381) respectively. Acetoclastic methanogenesis correlated positively with aromatic in F1 site (P= 0.0058; R2= 0.994) but negatively in U3S (P= 0.0232; R2= -0.977) and U3R (P= 0.0744; R2= -0.926). Organic matter recalcitrance appears to facilitate hydrogenotrophic pathway over acetoclastic pathway. Changes in organic matter quality will shift the dominant methanogenesis pathways and influence CH4 production.